From the journal Environmental Science: Atmospheres Peer review history

31-Oct-2022

Dear Dr Yuan:

Manuscript ID: EA-ART-08-2022-000110
TITLE: Emerging investigator series: Assessment of Long Tubing in Measuring Atmospheric Trace Gases: Applications on Tall Towers

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Reviewer 1

Review of the manuscript “Assessment of Long Tubing in Measuring Atmospheric Trace Gases: Applications on Tall Towers” by Xiao-Bing Li et al.

The article presents a study on technical aspects of measuring atmospheric trace gases (CO2, NOx, O3 and some selected organic compounds) through long 400m PFA tubing. In the laboratory, the authors quantify the delay times for 400m tubing using standard gas set-ups. In the field, authors compare co-located measurements through 5m and 400m-long tubing as well as show some vertical profile results from 5, 40, 70, 120, 220 and 335 m measurement heights. The authors explain very well and in great detail what they have done using also high-quality figures. However, I have some reservations regarding novelty and further usability of the results. I make some suggestions below.

General comments:

The conclusions of the first part of the manuscript (delay due to tubing) are somewhat similar to Pagonis et al. 2017, but for a much longer tubing with a larger diameter and for a bigger array of compounds. Similarly, the authors find that tubing delay is a function of a compound’s saturation vapor concentration with delay increasing with increased tube length and decreasing with increased flow (provided laminarity is preserved).

The second part of the manuscript discusses the change between ambient and measured concentrations due to modified chemistry when the sample air enters the tubing that is shielded from the sunlight. The authors present two box model runs to simulate this effect. The model output, however, is presented as stand-alone result and only qualitatively compared to the measurements. It would be good if authors could discuss the reason for choosing these particular starting conditions and if model run could be quantitatively compared to the measurements through 5 and 400m tubing. Does the model predict well the changes in NO, NO2 and O3 concentrations (in 5/400m comparison or in the actual vertical profile measurements)? If someone does not have in-situ vertical measurements, can they use similar model to correct for the changes?

In the last part, authors present vertical profile measurements of CO2, NOx and ozone as well as acetone and toluene. Only two volatile organic species are presented as “representative species” (lines 323-324). It would be beneficial in context of this manuscript to look also at some other organic species with lower C*. Are there any challenges associated with their measurements? Could the authors discuss more about the choice of switching interval between the heights?

Specific comments:

Abstract: There are some inconsistencies between conclusions in main text and in abastract. Abstract says “Some highly reactive species (e.g., α-pinene) may have detectable losses …”. On the other hand, in lines 282-284, authors say “However, insignificant discrepancies in concentrations of monoterpenes measured using the two tubing were observed in both daylight and dark environments.” Please make conclusions more in-line with the results.

Line 186-187: Can you indicate somewhere which compounds were excluded, i.e. had less than 30s delay?

Lines 195-199: Could authors please explain what is meant here? It seems that the results of this study are in line with literature, i.e. that increasing flow decreases the delay (in laminar regime), while increasing the tube length increases the delay (at fixed flow rate). The referred study of Pagonis et al. 2017 presents only two flow rates and two tube lengths, so naturally the result is a line.

Line 226: There could be also inorganic species with small C*, so maybe just list species here without specifying “inorganic”.

Lines 248-249: To my mind, this statement is self-evident, as reduction in NO and ozone is compensated by increase in NO2, so NOx (NO+NO2) and Ox (O3+NO2) concentrations will change less or not at all. Please rephrase.

Line 253: What is meant by “dark environments”? Maybe “in absence of (sun)light”?

Lines 272-273: The numbers in brackets appear before it is mentioned what they are.

Lines 282-289: You suggest that other monoterpenes are present instead of ɑ- and β-pinene? I think a lot of other monoterpenes have very similar reaction rate coefficient with NO3, so it somehow seems unplausible explanation.
Figure 1: Please explain all figure panels. In panel a what does (HA) stand for? In caption, it says that some points that are excluded are shown in “grey hollow circles” – I didn’t see any on the plot. Please check.

Figure 2: I assume in Figure 2, it is y=kx+b, but it could be mentioned somewhere.

Figure 4: Were concentrations in the plots determined using the corresponding sensitivities from Figure S15 or the sensitivity without tubing? Panel e reports many masses as concentrations in ppb. How was the conversion made for uncalibrated masses? I would suggest report here in counts per second. What is the resolution of data in panel e, 10min or across whole period?

Figure S15: It seems the instrument had higher sensitivity with the longer tubing. Could the authors check if the axes were labelled correctly?

Figure S21 is not referred to anywhere in the text. Can more context or explanation be added to the caption?

References:
Pagonis, Demetrios, Jordan E. Krechmer, Joost de Gouw, Jose L. Jimenez, and Paul J. Ziemann. “Effects of Gas–Wall Partitioning in Teflon Tubing and Instrumentation on Time-Resolved Measurements of Gas-Phase Organic Compounds.” Atmospheric Measurement Techniques 10, no. 12 (December 4, 2017): 4687–96. https://doi.org/10.5194/amt-10-4687-2017.

Reviewer 2

This paper presents an analysis of the effect of sampling through long tubing on the integrity of trace gas measurements. This a potentially useful study that can probably eventually be acceptable. This version of the manuscript has some serious problems that will need to be taken care of before it is suitable for publication. I have the following general and specific comments and issues that need to be dealt with.

General Comments

The abstract says that studies on the impacts of sampling through long inlets have not been reported. This is wrong, there have been a number of studies. See for example Helmig et al., 2006, 2007, 2014 and Johnson et al., 2008 for just a few studies. I suggest the author spend time doing a more thorough literature search as there are certainly more studies out there.

The authors need to read the Karion et al., 2010 study that describes using long tubing to collect samples that contain altitude profiles. This paper describes the effects of diffusion and dispersion on gases that are sampled through a tube. This is an essential starting point that gives the fundamental time constant, or equilibration time, that will happen in the absence of any chemical partitioning to the tubing walls. This analysis needs to be done up front at the beginning of the paper. The Karion et al., analysis is for laminar flow, so the authors need to show upfront that their system operates under laminar flow. With this analysis done, then the authors need to show us profiles (signal vs time) for one or two species that they think are not retained on the tubing walls. Figure S8 would be an example of that, but the time scales for the signal in the “passivation” and “depassivation” need to be enlarged so we can actually see what the signals look like, and whether or not the signals match what would be expected from the diffusion and dispersion happening over the time scale of the transit through the tubing.

Throughout the paper the authors use confusing terms like “delay time” and it is not clear what they mean. The transit time down 400m of tubing is substantial, 155 sec or so. So how can species have a “delay time” of only a few seconds as described in several places in the paper? I think what the authors mean is that the effective time constant of concentration changes of a given species is only a few seconds. Here is where the diffusion and dispersion analysis described above can really help.

Specific Comments

Line 27. Give the range of equilibration times for the compounds studied.

Lines 29-31. Is the problem the change in light levels or is the problem reactions with O3 and NO3?

Line 31-32. A simple residence time analysis, and your own data, show that NO gets titrated to NO2 in the tube due to reaction with ambient O3. This needs to be stated here and throughout the paper.

Lines 47-53. The references for this paragraph have a distinct Chinese-centric bias. If the authors took a broader view here they would have seen that there are many more studies out there and would have found the Helmig et al., references for example. Clearly the authors cannot cite every study, but there are key summary papers that could be included: for example, Andreae et al., 2015 and Brown et al., 2013.

Lines 55-56. Minor grammatical point, “targeting” should be “targeted”.

Line 62. Leave out the word “were”.

Line 69. “inertia property” should be “inert properties”. I assume the author mean chemical inertness, so maybe say “chemically inert properties”.

Lines 81-82. The statement that these effects have not been investigated so far is not correct, see General Comment above.

Line 103-104. The typical chemiluminescence method for NOx uses a catalyst and actually responds to more than just NO and NO2, so we need more details here.

Line 116. The “delay time” has to be at least 155 sec because that is the residence time in the tubing, so you really mean something else here. See the General Comments.

Line 118. “of’ should be “on”.

Lines 122-123. This explanation of how tubing delays were quantified is not sufficient, as this topic is not well known. So, the equations need to be given here and the authors also need to explain what they mean by the terms “normalized tubing delay” and “normalized concentration”

Line 127. “of “ should be “on”

Line 130. “amount” should be “mount” and “tubing” should be “tubes”

Lines 139, 141, 152. “tubing” should be “tube”

Line 163. The first thing that should be shown in this section is a plot of signal versus time for a number of compounds – in other words plots of what the basic data look like on time scales that we can see the effects of the diffusion/dispersion, and the chemical absorption/desorption.

Line 165. I’m not completely sure how to interpret the plots in Figure1 because I don’t know what “normalized tubing delay” is.

Line 167-168. How were the delay times of the PTR and auxiliary tubing “excluded”? Show us how that was done.

Line 174. I assume this 200s is after subtracting the residence time in the tubing?
Line 182-183. What do you mean by “is closely associated with the residence time of the air stream”? Certainly the diffusion/dispersion effect is associated with that, but the chemical effects are associated with the surface area of the tubing.

Lines 187-190. I don’t understand the reason for excluding these data, and what is meant by “stochastic errors”.

Line 191-192. More rapidly than what?

Line 195. More rapidly than what?

Line 200. What does viscosity have to do with this effect? I think the author mean highly absorptive compounds.

Lines 201-204. The conditions used by the author are close to the transition between laminar and turbulent flow (this needs to be discussed as noted above), so turbulent flow might be responsible for some of the effects observed here.

Line 211-212. I think the author mean smaller than the time between concentration steps.

Line 226-228. This statement is misleading, if there is NO and O3 in the sample line, NO will be lost due to titration, all the ambient data show this. Putting in NO alone isn’t a very useful test.

Lines 246-252. All these statements about NO, NOx and O3 depend on the relative amounts of NO and O3. The only reason NO at night doesn’t look so far off is that there is so much of it relative to O3, another site or another time where NO is lower relative to O3 would show more serious problems. In fact, that is what the authors see with their system.

Line 266. I think “estimated” is a more appropriate word than “determined”.

Line 272. What is “k”?

Line 276. “excepted” should be “expected”.

Line 279, 283. “tubing” should be “tubes”

Line 289, 308. “due to that the” should be “due to the fact that”.

Line 298, 302. What is “k”?

Line 303. I disagree, there is clearly loss of NO at the lower ranges of NO.

Line 353. I don’t know what “Except for tall towers” means. Do they mean “in addition to tall towers”?

Line 356. “targeting” should be “targeted”.

I believe the journal policy requires that the data need to be made available.

Supporting Information

Page 5, What are the variables and their units in this equation? The equation should have a slight correction for pressure.

Figure S1, there needs to be an expanded time scale plot so we can actually see what the signal versus time looks like. My guess is that the 5 second timescale used for the PTR will not capture the rise of the signal very well.

What is the difference between measured and calculated residence time due to, is it temperature and pressure differences (you haven’t specified the units of Q). Could the difference be due to the residence times in the auxiliary tubing and associated parts of the system?

We need to see the units in Equation S3.

Page S7. What is the time resolution of the PTR based on? Is this the averaging time? Could you change it to get better resolution on some of the faster profiles?

Equation S4 is an exponential (actually double exponential), not hyperbolic fit. Show us some fits. In the main paper you say delay times were based on time to reach 90% of the change, are you now says that they are based on tau 1 and tau 2? This is not clear.

Figure S9. Show us some actual fits to Equation S4!

Box Model section. Does the model have O3 + alkenes producing OH radicals?

Figure S11 shows that there is something seriously wrong with the box model. OH radical concentrations of 5 x10¹⁰ molec/cm3 are a factor of 10⁴ too high!

Figure S12. Your box model shows you that the NO measurements are completely compromised by using the long inlet. This needs to be explained in the main text.

Figure S17. Your data show that you cannot measure NO properly with long inlets

In summation, this paper needs major revisions if it is to be acceptable for publication.


References

Andreae, M.O., et al., The Amazon Tall Tower Observatory (ATTO): overview of pilot measurements on ecosystem ecology, meteorology, trace gases, and aerosols, Atmos Chem. Phys., 15, 10723-10776, 2015, doi: 10.5194/acp-15-10723-2015.

Helmig, D., Johnson, B., Oltmans, S.J., Neff, W., Eisele, F., Davis, D.D., Elevated boundary-layer ozone at South Pole. Atmospheric Environment, 42, 2788-2803, 2008 doi:10.1016/j.atmosenv.2006.12.032.

Helmig, D., Johnson, B., Warshawsky, M., Morse, T., Neff, W., Eisele, F., Davis, D.D., Nitric oxide in the boundary-layer at South Pole during the Antarctic Tropospheric Chemistry Investigation (ANTCI). Atmospheric Environment, 42, 2817-2830, 2008 doi:10.1016/j.atmosenv.2007.03.061.

Helmig, D., Thompson, C.R., Evans, J., Boylan, P., Hueber, J., and J.-H. Park, Highly elevated atmospheric levels of volatile organic compounds in the Uintah Basin, Utah, Enivron. Sci. Technol., 48, 4707-4715, 2014, doi.10.1021/es405046r

Johnson, B.J., Helmig, D., and Oltmans, S.J., Evaluation of ozone measurements from a tethered balloon-sampling platform at South Pole Station in December 2003, Atmos. Environ, 42, 2780-2787, 2008,

Karion, A., Sweeney, C., Tans, P., and Newberger, T.: AirCore: An innovative atmospheric sampling system, J. Atmos. Ocean. Technol., 27, 1839-1853, 2010.

Steven S. Brown, et al., Nitrogen, Aerosol Composition and Halogens on a Tall Tower (NACHTT): Overview of a Wintertime Air Chemistry Field Study in the Front Range Urban Corridor of Colorado, J. Geophys. Res., 118, 10.1002/jgrd.50537, 2013.

This text has been copied from the PDF response to reviewers and does not include any figures, images or special characters.

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Response to Reviewer #1
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The article presents a study on technical aspects of measuring atmospheric trace gases (CO2, NOx, O3 and some selected organic compounds) through long 400m PFA tubing. In the laboratory, the authors quantify the delay times for 400m tubing using standard gas set-ups. In the field, authors compare co-located measurements through 5m and 400m-long tubing as well as show some vertical profile results from 5, 40, 70, 120, 220 and 335 m measurement heights. The authors explain very well and in great detail what they have done using also high-quality figures. However, I have some reservations regarding novelty and further usability of the results. I make some suggestions below.
Reply: We appreciate the reviewer for the valuable comments and suggestions, which are very important for the improvement of the manuscript.
General comments:
The conclusions of the first part of the manuscript (delay due to tubing) are somewhat similar to Pagonis et al. 2017, but for a much longer tubing with a larger diameter and for a bigger array of compounds. Similarly, the authors find that tubing delay is a function of a compound’s saturation vapor concentration with delay increasing with increased tube length and decreasing with increased flow (provided laminarity is preserved).
Reply: We appreciate the reviewer for the comments. Our study can be considered an important extension of the works by Pagonis et al. 2017 and some other previous works. As summarized by the reviewer, only short tubes and a small group of chemical species were assessed in these previous studies. With increasing demands for the utilization of long tubes in measuring atmospheric trace gases (particularly for various organic trace gases), it is quite important to conduct a more detailed investigation on the performance of long tubes.
The second part of the manuscript discusses the change between ambient and measured concentrations due to modified chemistry when the sample air enters the tubing that is shielded from the sunlight. The authors present two box model runs to simulate this effect. The model output, however, is presented as stand-alone result and only qualitatively compared to the measurements. It would be good if authors could discuss the reason for choosing these particular starting conditions and if model run could be quantitatively compared to the measurements through 5 and 400 m tubing. Does the model predict well the changes in NO, NO2 and O3 concentrations (in 5/400 m comparison or in the actual vertical profile measurements)? If someone does not have in-situ vertical measurements, can they use similar model to correct for the changes?
Reply: We appreciate the reviewer for the valuable comments and suggestions. The key purpose of the box model simulations is to investigate potential maximum changes in chemical reaction conditions between ambient air and inside the tubing in daytime. Concentrations of organic compounds were set as the same for the sake of evaluating their changes after traversing the long tubing. In addition, OH concentrations are the highest at noon time and are closely associated with ozone concentrations. The time with high concentrations of ozone and OH radicals in a day is also the time when reactive chemical species may suffer from significant losses after traversing the long tubing. Therefore, the time period of LT 11:00-14:00 was selected to perform the simulation.
We believe that the changes in concentrations of various trace gases can be well predicted by model simulations. This can be achieved by simulating more scenarios with different concentrations of trace gases. However, we don’t think this is a necessary work because most of the trace gases have very tiny losses after traversing the 400 m tubing according to the present model results. In addition, ambient concentrations of most reactive species are very low in daytime and keep changing over time. As a result, it is highly difficult to identify the losses of reactive species after traversing the tubing in real environments. For example, the mean concentration of monoterpenes was only ~0.1 ppb in daytime during the campaign. Therefore, a <10% loss of monoterpenes concentrations (~10 ppt) after traversing the 400 m tubing is very hard to be clearly identified.
In the last part, authors present vertical profile measurements of CO2, NOx and ozone as well as acetone and toluene. Only two volatile organic species are presented as “representative species” (lines 323-324). It would be beneficial in context of this manuscript to look also at some other organic species with lower C*. Are there any challenges associated with their measurements? Could the authors discuss more about the choice of switching interval between the heights?
Reply: We appreciate the reviewer for the valuable comments. As suggested by the reviewer, we have provided time series and vertical profiles of four organic compounds (namely phenol, furfural, guaiacol, and naphthalene) with smaller C* values in SI (Figure S22). Results show that measurements of the vertical observation system can also well characterize their concentrations and vertical distributions. More results and discussions for gradient measurements of the vertical observation system will be presented in another paper and thus are not displayed in this manuscript.
During the campaign, all the six tubes of the vertical observation system were simultaneously and continuously drawn with large flow rates. This design helped the trace gases maintaining at the equilibrium of gas-tubing wall partitioning, which minimize the effects of the delay time from the long inlets. According to the results of our tests, we find that the cross-interferences of different heights on the measurements will disappear in the first 30 s after the switch of inlet height. Therefore, the switching interval between the heights was set to 2 min to perform as many vertical measurement cycles as possible. In this condition, the vertical observation system can obtain at least eight effective concentrations at each height and perform five vertical measurement cycles in one hour. As suggested by the reviewer, we have provided more detailed discussions about the selection of switching intervals between the inlet heights in the manuscript. [P: 8-9; L: 204-212]
“As introduced above, all the tubes were continuously flushed by ambient air to minimize the effects of the delay time from the long inlets. Therefore, the selection of switching intervals between the inlet heights should follow two principles: ⅰ) obtaining as many effective samples as possible at each height; ⅱ) performing as many vertical measurement cycles as possible in a certain amount of time. According to our tests, the measurements of all trace gases made in the first 30 s and the last 10 s of a switch should be discarded to remove cross interferences from different inlet heights. In this study, the solenoid valve group was set to switch a new inlet height every two minutes during the field campaign.”

Figure S22. (a-d) Time series of mixing ratios of the selected chemical species with small values of C* measured by the vertical observation system at the SMT site. (e-h) Mean vertical profiles of the selected chemical species (mean ± 0.5 standard deviations) for daytime (LT 10:00-16:00) and nighttime (LT 22:00-05:00) from January 16 to 29, 2021.
Specific comments:
Abstract: There are some inconsistencies between conclusions in main text and in abstract. Abstract says “Some highly reactive species (e.g., α-pinene) may have detectable losses …”. On the other hand, in lines 282-284, authors say “However, insignificant discrepancies in concentrations of monoterpenes measured using the two tubing were observed in both daylight and dark environments.” Please make conclusions more in-line with the results.
Reply: We appreciate the reviewer for the valuable suggestions. In the revised manuscript, we put more faith in the results of our field validations and believe that concentrations of some reactive organic compounds can be also well measured using the long tubing even in daytime. Accordingly, we have revised the abstract as follows. [P: 2; L: 18-38]
“Perfluoroalkoxy alkane (PFA) Teflon tubing has been widely used to draw air samples for analyzing atmospheric trace gases. However, impacts of long tubing on measurements of atmospheric trace gases were rarely reported so far, especially for various organic trace gases. In this study, interactions between long tubing and various trace gases were assessed using a combination of laboratory tests, field experiments, and modeling techniques. A tower-based observation system of trace gases was also established using long tubes. Results show that measured concentrations of organic compounds required varying amounts of time (e.g., 10-474 s for a 400 m-long tubing at a flow rate of 13 standard liters per minute) to stabilize after traversing the tubing. Tubing delays of organic compounds were highly dependent on their saturation concentrations and the residence time in tubing. In lab tests, there are no detectable losses of the targeted chemical species after traversing a 400 m-long tubing. In real applications, concentrations of nitric oxide (NO) cannot be well measured through long tubes in daytime due to its low ambient concentrations and rapid consumption by ozone. In addition, negligible losses were observed for most of the other targeted species when measured using long tubes. Measurements of various trace gases made by the vertical observation system can well characterize their concentrations and vertical distributions. In addition to the usage on tall towers, long PFA Teflon tubes can be also used on mobile platforms (e.g., tethered balloons) for vertical measurements of atmospheric trace gases or to surveil emissions of targeted chemical species at multiple sites.”
Line 186-187: Can you indicate somewhere which compounds were excluded, i.e. had less than 30s delay?
Reply: We appreciate the reviewer for the valuable suggestions. To avoid misunderstandings, this sentence has been removed and all the species were taken into account in the revised manuscript.
Lines 195-199: Could authors please explain what is meant here? It seems that the results of this study are in line with literature, i.e. that increasing flow decreases the delay (in laminar regime), while increasing the tube length increases the delay (at fixed flow rate). The referred study of Pagonis et al. 2017 presents only two flow rates and two tube lengths, so naturally the result is a line.
Reply: We appreciate the reviewer for the valuable comments. The results reported by Pagonis et al. 2017 and some other related works all highlighted that tubing delays of organic compounds are proportional to tubing length and inversely proportional to the flow rate. Thus, only a short tube was used to investigate tubing delays of various organic compounds. However, the results of our study show that tubing delays of organic compounds exhibited slight non-linear responses to changes in tubing length and flow rate. This is also the reason why measurements of various trace gases made using long tubes should be also systematically assessed.
Line 226: There could be also inorganic species with small C*, so maybe just list species here without specifying “inorganic”.
Reply: We appreciate the reviewer for the valuable suggestion and this sentence has been revised accordingly. [P: 13; L: 329-331]
“In contrast to organic compounds, concentrations of NO, NO2, O3, and CO2 measured at both ends of the 400 m long tubing agreed well within 5%, as shown in Figure 3(d).”
Lines 248-249: To my mind, this statement is self-evident, as reduction in NO and ozone is compensated by increase in NO2, so NOx (NO+NO2) and Ox (O3+NO2) concentrations will change less or not at all. Please rephrase.
Reply: We appreciate the reviewer for the valuable suggestions. To avoid redundant statements, this sentence has been removed from the manuscript.
Line 253: What is meant by “dark environments”? Maybe “in absence of (sun)light”?
Reply: We appreciate the reviewer for the valuable comments. Dark environments refer to the absence of sunlight and this sentence has been rephrased in the manuscript. [P: 13-14; L: 354-356]
“In the absence of sunlight, the primary degradation pathways of hydrocarbons (e.g., NO3- and ozone-initiated reactions) are quite different from those in daytime due to the rapid decline in concentrations of OH radicals.”
Lines 272-273: The numbers in brackets appear before it is mentioned what they are.
Reply: We appreciate the reviewer for the valuable comments. In our study, the linear fitting method (y=kx+b) was mainly used to perform the comparison analysis between two datasets. Thus, the numbers (k and R2) in brackets refer to the slope of the linear fitting line and the determination coefficient, respectively. We have provided their definitions in the manuscript. [P: 6; L: 127-131]
“Impacts of long tubing on measurements of trace gases can be quantitatively assessed according to changes in their concentrations at the two ends of the tubing. In this study, scatter plots and linear fittings (y=kx+b; k is the slope and b is the intercept) were mainly used to perform the comparison analysis. The coefficient of determination (R2) was used to evaluate the goodness of fit.”
Lines 282-289: You suggest that other monoterpenes are present instead of ɑ- and β-pinene? I think a lot of other monoterpenes have very similar reaction rate coefficient with NO3, so it somehow seems unplausible explanation.
Reply: We appreciate the reviewer for the valuable comments. We agree with the reviewer’s opinion that the insignificant differences in monoterpenes concentrations measured by the two tubes with different lengths were not caused by the presence of unknown isomers. The box model results only estimated the potential maximum changes in concentrations of monoterpene species after traversing the tubing. In real environments, the situation is much more complicated than the settings of model simulations. We reanalyzed the data and believe that the insignificant differences between the two measurements of monoterpenes are mainly caused by two reasons. First, the maximum ozone concentration on January 30, 2021, was only 55.1 ppb, leading to much lower concentrations of NO3 radicals than those in model simulations. Therefore, smaller losses of reactive monoterpene species were expected in comparison to those when the ambient ozone concentration was 100 ppb. Second, the ambient concentrations of monoterpenes are usually very low (~0.1 ppb) in the daytime and keep changing over time. Therefore, the small losses of monoterpenes concentrations (only several ppts) after traversing the tubing cannot be well identified by comparisons of their ambient measurements. We have rephrased related sentences and provided additional discussions in the manuscript. [P: 14-15; L: 383-399]
“The most concerned organic compounds are monoterpenes due to their large NO3 reactivities and notable potential losses after traversing the 400 m long tubing in daytime. However, insignificant discrepancies in concentrations of monoterpenes measured using the two tubes were observed in both daytime and nighttime, as shown in Figure 5(d). The maximum ozone concentration at ground level before LT 13:00 on January 30, 2021 was only 55.1 ppb, resulting in lower mixing ratios of NO3 radicals and smaller losses of reactive monoterpene species in the tubing (Figure S16) than those when ambient ozone concentration was 100 ppb (Figure 4). It should be noted that the box model results only estimated the potential maximum changes in concentrations of the targeted species after traversing the tubing. In real environments, the situation is much more complicated than the settings of model simulations. The expected changes in concentrations of the targeted trace gases may be lower than those predicted by the model results. In addition, the ambient concentrations of monoterpenes were relatively low (~0.1 ppb) in daytime and changed over time. As a result, the small losses of monoterpenes concentrations (only several ppt) after traversing the tubing cannot be well identified by comparisons of their ambient measurements.”
Figure 1: Please explain all figure panels. In panel a what does (HA) stand for? In caption, it says that some points that are excluded are shown in “grey hollow circles” – I didn’t see any on the plot. Please check.
Reply: We appreciate the reviewer for the valuable suggestions. In Panel (a), HA is short for higher alkanes that were measured by PTR-ToF-MS in NO+ mode, which has been explained in the revised caption. In addition, to avoid misunderstandings, tubing delays of all the species in the gas standard were considered in calculating the averages (denoted by grey hollow circles). In the original caption, the grey hollow circles did not indicate the excluded species but the mean values of the selected species. We have replotted the figure and explained all the panels in Figure 1 (it is Figure 2 in the revised manuscript). [P: 27; L: 768-774]
“Figure 2. Tubing delays of various organic compounds as a function of (a) C*, (b) residence time, (c) tubing length, and (d) flow rate; In panel (a), HA is the abbreviation for higher alkanes that were measured by PTR-ToF-MS in NO+ mode. In panels (b-c), tubing delays of organic compounds were normalized to those measured for the 400 m long tubing at the flow rate of 6 SLPM. In panel (d), tubing delays were normalized to those measured for the 400 m long tubing at the flow rate of 13 SLPM. In panels (b-d), circles are color coded by C*.”
Figure 2: I assume in Figure 2, it is y=kx+b, but it could be mentioned somewhere.
Reply: We appreciate the reviewer for the valuable comments. In this study, the linear fitting method (y=kx+b) was mainly used to perform the comparison analysis between two datasets. k and is the slope and intercept of the linear fitting line, respectively. We have provided their definitions in the manuscript. [P: 6; L: 127-131]
Figure 4: Were concentrations in the plots determined using the corresponding sensitivities from Figure S15 or the sensitivity without tubing? Panel e reports many masses as concentrations in ppb. How was the conversion made for uncalibrated masses? I would suggest report here in counts per second. What is the resolution of data in panel e, 10min or across whole period?
Reply: We appreciate the reviewer for the valuable comments and suggestions. The PTR-ToF-MS was automatically calibrated using the gas standards three times daily for the H3O+ mode and twice daily for the NO+ mode. Therefore, concentrations of organic compounds were calculated using these calibrations without the long tubing. [P: 8; L: 197-199]
In addition to the chemical species contained in the gas standard, concentrations of the other organic compounds were calculated using the method based on the reaction kinetics of the PTR-ToF-MS. In PTR-ToF-MS, the VOC species that have higher proton affinities than water (H2O) can be ionized via reactions with H3O+ to produce product ions (proton-transfer reactions). Thus, sensitivities of the VOC species that were not contained in the gas standard can be estimated using the rate constants for the proton-transfer reactions. Detailed information on the calculation of the sensitivities for uncalibrated species has been provided in our previous papers (Yuan et al., 2017; Wu et al., 2020; He et al., 2022). To make this point clearer, we have provided a brief introduction in the revised manuscript. [P: 15; L:402-404]
“In addition to the organic compounds contained in the gas standard, we also compared mean mixing ratios of 207 species measured by PTR-ToF-MS using the two different tubes. Concentrations of these species were calculated using the method based on reaction kinetics of the PTR-ToF-MS, as detailed in our previous study 45.”
References:
Wu, et al., Measurement report: Important contributions of oxygenated compounds to emissions and chemistry of volatile organic compounds in urban air, Atmos. Chem. Phys., 20, 14769-14785, https://doi.org/10.5194/acp-20-14769-2020, 2020.
He, X., Yuan, B., Wu, C., Wang, S., Wang, C., Huangfu, Y., Qi, J., Ma, N., Xu, W., Wang, M., Chen, W., Su, H., Cheng, Y., and Shao, M.: Volatile organic compounds in wintertime North China Plain: Insights from measurements of proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS), J Environ Sci (China), 114, 98-114, 10.1016/j.jes.2021.08.010, 2022.Yuan, B., Koss, A. R., Warneke, C., Coggon, M., Sekimoto, K., and de Gouw, J. A.: Proton-Transfer-Reaction Mass Spectrometry: Applications in Atmospheric Sciences, Chem Rev, 117, 13187-13229, https://doi.org/10.1021/acs.chemrev.7b00325, 2017.
In Figure 4(e) (it is Figure 5 in the revised manuscript), concentrations of the 207 species measured by the two tubes of different lengths were compared using the averages over the whole measurement period. We have provided an explanation in the caption of the figure in the revised manuscript. [P: 30; L:788-792]
“Figure 5. (a) Time series of ozone and NO2 mixing ratios along with the photolysis rate of NO2, denoted by jNO2, measured at ground level at the SMT site. (b-d) Intercomparisons of species mixing ratios that were averaged in time intervals of 10 min. (e) Intercomparison of mixing ratios of various organic compounds that were averaged over the whole measurement period.”

Figure S15: It seems the instrument had higher sensitivity with the longer tubing. Could the authors check if the axes were labelled correctly?
Reply: We appreciate the reviewer for pointing out this mistake and we have provided a new figure in the revised SI file. [P: 16 in SI]

Figure S15. Intercomparison of species signals measured before and behind the 400 m long tubing (two 200 m long tubes for sampling air samples at 120 m on the SMT) at a flow rate of ~15 SLPM.
Figure S21 is not referred to anywhere in the text. Can more context or explanation be added to the caption?
Reply: We appreciate the reviewer for pointing out this mistake. The Figure S21 in the original manuscript was used as a supplement to show time series of the chemical species made by the vertical observation system. In the revised manuscript, Figure S21 is replaced with a new figure (Figure 22) to show measurements of the organic compounds with smaller C* values.
References:
Pagonis, Demetrios, Jordan E. Krechmer, Joost de Gouw, Jose L. Jimenez, and Paul J. Ziemann. “Effects of Gas–Wall Partitioning in Teflon Tubing and Instrumentation on Time-Resolved Measurements of Gas-Phase Organic Compounds.” Atmospheric Measurement Techniques 10, no. 12 (December 4, 2017): 4687–96. https://doi.org/10.5194/amt-10-4687-2017.
Reply: We appreciate the reviewer for the valuable suggestions.


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Response to Reviewer #2
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This paper presents an analysis of the effect of sampling through long tubing on the integrity of trace gas measurements. This a potentially useful study that can probably eventually be acceptable. This version of the manuscript has some serious problems that will need to be taken care of before it is suitable for publication. I have the following general and specific comments and issues that need to be dealt with.
Reply: We appreciate the reviewer for the valuable comments and suggestions that are very helpful for the improvement of the manuscript.
General Comments
The abstract says that studies on the impacts of sampling through long inlets have not been reported. This is wrong, there have been a number of studies. See for example Helming et al., 2006, 2007, 2014 and Johnson et al., 2008 for just a few studies. I suggest the author spend time doing a more thorough literature search as there are certainly more studies out there.
Reply: We appreciate the reviewer for recommending these valuable papers and pointing out this mistake. These papers provided valuable basis for our study in the assessment of using long tubing to conduct vertical profile measurements of atmospheric trace gases. We have done a long-time literature survey before conducting the analysis presented in this study, however, we must admit that we miss these papers. According to the results of our revised literature survey and reviewers’ suggestion, assessment of long tubes in measuring atmospheric trace gases have been rarely reported so far and we have rephrased this sentence in the abstract. [P: 2; L: 19-21]
“Perfluoroalkoxy alkane (PFA) Teflon tubing has been widely used to draw air samples for analyzing atmospheric trace gases. However, impacts of long tubing on measurements of atmospheric trace gases were rarely reported so far, especially for various organic trace gases.”
The authors need to read the Karion et al., 2010 study that describes using long tubing to collect samples that contain altitude profiles. This paper describes the effects of diffusion and dispersion on gases that are sampled through a tube. This is an essential starting point that gives the fundamental time constant, or equilibration time, that will happen in the absence of any chemical partitioning to the tubing walls. This analysis needs to be done up front at the beginning of the paper. The Karion et al., analysis is for laminar flow, so the authors need to show upfront that their system operates under laminar flow. With this analysis done, then the authors need to show us profiles (signal vs time) for one or two species that they think are not retained on the tubing walls. Figure S8 would be an example of that, but the time scales for the signal in the “passivation” and “depassivation” need to be enlarged so we can actually see what the signals look like, and whether or not the signals match what would be expected from the diffusion and dispersion happening over the time scale of the transit through the tubing.
Reply: We appreciate the reviewer for the valuable comments. As suggested by the author, we have meticulously read the work by Karion et al., 2010. The AirCore system described in Karion et al., 2010 is a passive sampling system, which is totally different from the observation system developed in our study. Air samples collected by the AirCore system at different altitudes stayed a relatively long time in the tube before being analyzed. In this condition, molecular diffusion/dispersion is a key factor that affects the identification of altitude information for air samples in the tube. In addition, the method proposed by Karion et al., 2010 may be suitable for making vertical profile measurements of greenhouse gases (CO2 and CH4) but are not suitable for measurements of organic compounds if their interactions with tubing wall were not elucidated. In our study, to reduce the residence time of air samples in tubing, all the tubes were continuously drawn at large flow rates (12-15 SLPM). The residence time of air samples in a tube of the vertical observation system is far less than that in the AirCore system. Thus, molecular diffusion/dispersion may play insignificant roles in causing delay times of trace gas concentrations after passing through in a long tube.
According to the method in Karion et al., 2010, the longitudinal mixing length of benzene and O-xylene concentrations caused by molecular diffusion/dispersion are estimated to be ~10.8 and 9.5 cm after traversing the 400 m long tube at the flow rate of 13 SLPM. As a result, the time scales affected by molecular diffusion/dispersion for benzene and O-xylene measurements are estimated to be ~3.5×10-2 and 3.1×10-2 s, respectively, which are approximately two orders of magnitude smaller than their tubing delays (11 and 77 s) observed in this study. Therefore, we believe that gas-surface interactions, rather than molecular diffusion/dispersion, was the key reason for causing tubing delays of trace gas concentrations after passing through in a long tube.
As also suggested by the reviewer, we have provided additional discussions and a new figure (Figure 1) with enlarged depassivation curves (Figure 1(c)) in the revised manuscript to show the effects of molecular diffusion/dispersion on measured tubing delays of targeted chemical species. [P: 10-11; L: 250-266]
“As shown in Figures 1, concentrations of organic compounds have significant but differentiated delay times after traversing the 400 m long tubing at a flow rate of 13 SLPM. In addition to gas-surface interactions, the work by Karion et al., (2010)54 also proposed that the longitudinal mixing of trace gases, caused by molecular diffusion/dispersion, can also have effects on their concentrations after traversing a long tubing. However, the time scale affected by molecular diffusion/dispersion for measurements in a long tubing with large flow rates are generally much lower than those caused by gas-surface interactions. For example, the longitudinal mixing lengths of benzene and O-xylene concentrations caused by molecular diffusion/dispersion are estimated 54 to be ~10.8 and 9.5 cm, respectively, when passing through the 400 m long tubing at the flow rate of 13 SLPM. As a result, the effects of molecular diffusion/dispersion on measurements of benzene and O-xylene will last for ~3.5×10-2 and 3.1×10-2 s, respectively, which are approximately two orders of magnitude smaller than their tubing delays (11 and 77 s) observed in this study. Therefore, the effects of molecular diffusion/dispersion on measured concentrations of various organic compounds could be neglected in comparison to those cause by gas-surface interactions after traversing a long tubing.”

Figure 1. (a) Passivation and (b) depassivation curves of four selected ion signals measured by PTR-ToF-MS for the 400 m long tubing at a flow rate of 13 SLPM. Ion signals were normalized to those measured at the start time (t=0 s) of the step-function change; Grey solid lines in panel (b) are double exponential fitting lines. (c) Depassivation curves of m/z69 for the 400 m long tubing at different flow rates.
Throughout the paper the authors use confusing terms like “delay time” and it is not clear what they mean. The transit time down 400m of tubing is substantial, 155 sec or so. So how can species have a “delay time” of only a few seconds as described in several places in the paper? I think what the authors mean is that the effective time constant of concentration changes of a given species is only a few seconds. Here is where the diffusion and dispersion analysis described above can really help.
Reply: We appreciate the reviewer for the valuable comments and suggestions. As for delay times of trace gases measured at the outlet end of a tubing, it refers to the amount of time required for the instruments to measure stable concentrations of targeted species in response to a change in species concentrations at the tubing inlet. This definition has been mentioned in the Introduction [P: 4; L: 74-76] and were also widely used in the literature. As also recommended in the literature, the tubing delay of a trace gas is usually estimated as the amount of time required to reach 90% of the change in its concentration before entering the tubing. [P: 6-7; L: 139-160] By contrast, the residence time is defined as the ratio of the tubing length to the flow velocity, which is clearly distinguished from tubing delay time. For a tubing with fixed length and inside flow rate, all chemical species have the same residence time but have different tubing delay times.
References:
Pagonis, et al., Effects of gas–wall partitioning in Teflon tubing and instrumentation on time-resolved measurements of gas-phase organic compounds, Atmos. Meas. Tech., 10, 4687-4696, 10.5194/amt-10-4687-2017, 2017.
Liu, et al., Effects of gas–wall interactions on measurements of semivolatile compounds and small polar molecules, Atmos. Meas. Tech., 12, 3137-3149, 10.5194/amt-12-3137-2019, 2019.
Specific Comments
Line 27. Give the range of equilibration times for the compounds studied.
Reply: We appreciate the reviewer for the valuable suggestions. We have provided delay times (10-474 s) of the organic compounds measured for a 400-long tubing at a flow rate of 13 SLPM in this sentence. [P: 2; L: 24-27]
“Results show that measured concentrations of organic compounds required varying amounts of time (e.g., 10-474 s for a 400 m-long tubing at a flow rate of 13 standard liters per minute) to stabilize after traversing the tubing.”
Lines 29-31. Is the problem the change in light levels or is the problem reactions with O3 and NO3?
Reply: We appreciate the reviewer for the valuable comments. The potential losses of these organic compounds after traversing a tubing in daytime were mainly caused by reactions with NO3, the concentrations of which were highly increased due to the absence of sunlight. In addition, the chemical reactions between ozone and NO, the reactions between ozone and other trace gases are not important at such a short time scale. According to results of the field experiments, we did not observe significant losses of the organic compounds when measured using a 400 m long tubing. Thus, we have removed this sentence to avoid misunderstandings and mainly highlight the loss of NO by reactions with ozone after traversing the tubing in the revised manuscript. [P: 2; L: 30-33]
“In real applications, concentrations of nitric oxide (NO) cannot be well measured through long tubes in daytime due to its low ambient concentrations and rapid consumption by ozone. In addition, negligible losses were observed for most of the other targeted species when measured using long tubes.”
Line 31-32. A simple residence time analysis, and your own data, show that NO gets titrated to NO2 in the tube due to reaction with ambient O3. This needs to be stated here and throughout the paper.
Reply: We appreciate the reviewer for the valuable suggestions and we have rephrased this sentence in the revised manuscript. [P: 2; L: 30-31]
“In real applications, concentrations of nitric oxide (NO) cannot be well measured through long tubes in daytime due to its low ambient concentrations and rapid consumption by ozone.”
Lines 47-53. The references for this paragraph have a distinct Chinese-centric bias. If the authors took a broader view here they would have seen that there are many more studies out there and would have found the Helmig et al., references for example. Clearly the authors cannot cite every study, but there are key summary papers that could be included: for example, Andreae et al., 2015 and Brown et al., 2013.
Reply: We appreciate the reviewer for the valuable comments and suggestions. we are very sorry for bringing you such a confusion for the citation of references. We agree with the reviewer’s opinion that there are many studies worldwide reporting vertical measurements of atmospheric trace gases, many of which also have important guidance and implications in the design and analysis of our studies. As suggested by the reviewer, we have added some important international studies in the reference lists in this paragraph to remove this confusion. [P: 3; L: 49-55]
“Vertical measurements of atmospheric trace gases in the lower troposphere have been made by a variety of approaches, e.g., sounding and tethered balloon 11-15, unmanned aerial vehicle (UAV) 16-18, tower 19-24, remote sensing techniques 25-27, and aircraft 28-32. Advantages and disadvantages of these approaches have been discussed in the literature 33, 34. In summary, towers could provide an economic and flexible way to regularly perform vertical gradient measurements of trace gases, particularly in urban and suburban environments.”
Lines 55-56. Minor grammatical point, “targeting” should be “targeted”.
Reply: We appreciate the reviewer for pointing out this mistake and we have corrected it throughout the manuscript. [P: 3; L: 56-58]
“Direct deployment of instruments at multiple altitudes of a tower (in situ observations) was the most popular way to make regular observations of targeted species 7, 21, 35.”
Line 62. Leave out the word “were”.
Reply: We appreciate the reviewer for pointing out this mistake and we have corrected it in the manuscript. [P: 3; L: 63-66]
“Instead of in situ observations, air samples at multiple heights of a tower could be drawn by long tubes and sequentially analyzed using only one set of instruments deployed at a certain level (most frequently on ground) 36-39.”
Line 69. “inertia property” should be “inert properties”. I assume the author mean chemical inertness, so maybe say “chemically inert properties”.
Reply: We appreciate the reviewer for the valuable suggestions and this sentence has been rephrased as suggested by the reviewer. [P: 4; L: 69-71]
“Tubes made of perfluoroalkoxy alkane (PFA), polytetrafluoroethylene (PTFE), and fluorinated ethylene propylene are widely recommended for analyzing organic and inorganic species due to their excellent chemically inert properties 15, 40-43.”
Lines 81-82. The statement that these effects have not been investigated so far is not correct, see General Comment above.
Reply: We appreciate the reviewer for the valuable comments and suggestions. We have rephrased related statements in the manuscript. [P: 4; L: 84-89]
“Assessment of the long tubing (<200 m) in measuring ambient concentrations of ozone, NOx, and a small set of non-methane hydrocarbons (including seven alkanes and two aromatics) have been also performed in the literature 14, 15, 43. However, interactions between much longer tubes (e.g., several hundreds of meters) and more trace gases (particularly for various organic trace gases) should be also systematically investigated.”
Line 103-104. The typical chemiluminescence method for NOx uses a catalyst and actually responds to more than just NO and NO2, so we need more details here.
Reply: We appreciate the reviewer for the valuable suggestions and we have provided names and models of the instruments used in this study. We agree with the reviewer’s opinion that measurements of the chemiluminescence method for NOx may be impacted by other more oxidized NOy species. Responses of the chemiluminescence method to these NOy species are quite complicated, but we don’t think they have significant impacts on the results of the tubing assessment. Thus, they were not discussed in the manuscript. [P: 5; L: 117-119]
“NOx was measured using the chemiluminescence (42i, Thermo Fisher Scientific Inc., USA) and UV absorption method (2B Tech, USA)”
References:
Dickerson, et al., On the use of data from commercial NOx analyzers for air pollution studies, Atmos Environ, 214, 116873, https://doi.org/10.1016/j.atmosenv.2019.116873, 2019.
Line 116. The “delay time” has to be at least 155 sec because that is the residence time in the tubing, so you really mean something else here. See the General Comments.
Reply: We appreciate the reviewer for the valuable comments. The residence time and delay time are two distinct definitions. The residence time is a characteristic of the air flow in a tube and is defined as the time required for an air sample to traverse a tubing. However, the delay time is a characteristic of the trace gas in a tubing, which is defined as the amount of time required for the instruments to measure 90% of the change in its concentration before entering the tubing. Definitions of residence time [P: 5; L: 98-100] and delay time [P: 6-7; L: 139-160] have been provided in the manuscript.
Line 118. “of’ should be “on”.
Reply: We appreciate the reviewer for pointing out this mistake and we have corrected it in the manuscript. [P: 6; L: 133-135]
“As validated in previous works, tubing delays of most organic compounds are independent on concentration and humidity 48, 49, but dependent of C* and tubing size 40, 41, 44.”
Lines 122-123. This explanation of how tubing delays were quantified is not sufficient, as this topic is not well known. So, the equations need to be given here and the authors also need to explain what they mean by the terms “normalized tubing delay” and “normalized concentration”
Reply: We appreciate the reviewer for the valuable comments and suggestions. We have provided more explanations on the definition of tubing delays and the double exponential fitting equation has been given in the manuscript. [P: 6-7; L: 139-160]
“In this study, tubing delays of organic compounds were defined and computed using the same methods in previous works 40, 41, 44. Briefly, the tubing delay of a trace gas is defined as the amount of time required to reach 90% of the change in its concentration before entering the tubing 40, 41, 44. It should be noted that the tubing delay is distinct from the residence time as discussed in section 2.1. For a tubing with fixed length and inside flow rate, all the chemical species have the same residence time but have different tubing delays. Tubing delays of organic compounds were calculated according to the depassivation curves of their ion signals measured at the outlet end of the tubing (Figures 1 and S8-S10). Ion signals of organic compounds were firstly normalized to those measured at the start time of the step-function change at the outlet end of the tubing. Then, the normalized depassivation curves were fitted using the double exponential method, as formulated in Eq. (1), to reduce impacts of the noise in measured ion signals. The tubing delay of an organic compound is determined as the time when its double exponential fitting line declined to 0.1, as shown in Figure 1(b). As shown in Figure 1(c), the inner parts of the instruments and auxiliary tubes (these tubes have ODs of 1/4'' and 1/8'' and were used to connect instruments and long tubes) used in laboratory tests can also cause delay times for the measurements of organic compounds and should be excluded in the analysis. However, the measured delay times caused by the instruments and auxiliary tubes were much lower than those caused by long tubes, as shown in Figure S11. Therefore, the delay times of targeted organic compounds caused by the instruments and auxiliary tubes were directly subtracted from those measured for long tubes.
〖y=y〗_0+A_1 exp⁡{(-(x-x_0))/τ_1 } +A_2 exp{(-(x-x_0))/τ_2 } (1)
”
As for “normalized concentration”, concentrations of the organic compounds in their depassivation curves were normalized to those measured at the start time of the step-function change. The definition of normalized concentration has been provided in the manuscript. [P: 6; L: 145-147] The tubing delay of each organic compound was normalized to its largest value when measured at different tubing lengths and flow rates. Specific definitions of normalized tubing delays have been provided in the caption of Figure 3. [P: 27; L: 768-774] In addition, we also revised the labels of the Y axis in panels (b-d) in Figure 2 to explain normalized tubing delays of organic compounds.

Figure 2. Tubing delays of various organic compounds as a function of (a) C*, (b) residence time, (c) tubing length, and (d) flow rate; In panel (a), HA is the abbreviation for higher alkanes. In panels (b-c), tubing delays of organic compounds were normalized to those measured for the 400 m long tubing at the flow rate of 6 SLPM. In panel (d), tubing delays were normalized to those measured for the 400 m long tubing at the flow rate of 13 SLPM. In panels (b-d), circles are color coded by C*.
Line 127. “of “ should be “on”
Reply: We appreciate the reviewer for the valuable comments. This sentence has been removed because the detailed description of the field campaign has been moved to the main manuscript from the SI.
Line 130. “amount” should be “mount” and “tubing” should be “tubes”
Reply: We appreciate the reviewer for pointing out these mistakes and they have been corrected in the manuscript. [P: 7; L: 167-169]
“Five heights, namely 40, 70, 120, 220, and 335 m, on the SMT (Figure S13) were selected to mount inlets of the tubes (Table S3).”
Lines 139, 141, 152. “tubing” should be “tube”
Reply: We appreciate the reviewer for pointing out these mistakes and they have been corrected throughout the manuscript.
Line 163. The first thing that should be shown in this section is a plot of signal versus time for a number of compounds – in other words plots of what the basic data look like on time scales that we can see the effects of the diffusion/dispersion, and the chemical absorption/desorption.
Reply: We appreciate the reviewer for the valuable comments and suggestions. we have provided a new figure (Figure 1 in the revised manuscript) to show the normalized passivation and depassivation curves of four selected organic compounds with different tubing delays for the 400 m long tubing at the flow rate of 13 SLPM. In addition, we also discussed impacts of molecular diffusion/dispersion on measured tubing delays of the organic compounds. [P: 10-11; L: 250-266]

Figure 1. (a) Passivation and (b) depassivation curves of four selected ion signals measured by PTR-ToF-MS for the 400 m long tubing at a flow rate of 13 SLPM. Ion signals were normalized to those measured at the start time (t=0 s) of the step-function change; Grey solid lines in panel (b) are double exponential fitting lines. (c) Depassivation curves of m/z69 for the 400 m long tubing at different flow rates.

Line 165. I’m not completely sure how to interpret the plots in Figure1 because I don’t know what “normalized tubing delay” is.
Reply: We appreciate the reviewer for the valuable comments. The tubing delay of each organic compound was normalized to its largest value when measured at different tubing lengths and flow rates. To make this point clearer, we have provided definitions of normalized tubing delay in the caption of Figure 2. [P: 27; L: 768-774] In addition, we also revised the labels of the Y axis in panels (b-d) in Figure 2 to explain normalized tubing delays of organic compounds.
“Figure 2. Tubing delays of various organic compounds as a function of (a) C*, (b) residence time, (c) tubing length, and (d) flow rate; In panel (a), HA is the abbreviation for higher alkanes. In panels (b-c), tubing delays of organic compounds were normalized to those measured for the 400 m long tubing at the flow rate of 6 SLPM. In panel (d), tubing delays were normalized to those measured for the 400 m long tubing at the flow rate of 13 SLPM. In panels (b-d), circles are color coded by C*.”
Line 167-168. How were the delay times of the PTR and auxiliary tubing “excluded”? Show us how that was done.
Reply: We appreciate the reviewer for the valuable comments and suggestions. In our study, the delay times of various organic compounds caused by inner parts of the PTR-ToF-MS and auxiliary tubes were measured and calculated using the same method as that used for the assessment of long tubes. The delay times of various organic compounds caused by PTR and auxiliary tubes were much lower than those measured for long tubes, as shown in Figures 1(c) and S11. In subsequent analysis, the delay times of targeted organic compounds caused by PTR and auxiliary tubes were directly subtracted from those measured for long tubes. We have provided more introductions on this point in the revised manuscript. [P: 7; L: 153-160]
“As shown in Figure 1(c), the inner parts of the instruments and auxiliary tubes (these tubes have ODs of 1/4'' and 1/8'' and were used to connect instruments and long tubes) used in laboratory tests can also cause delay times for the measurements of organic compounds and should be excluded in the analysis. However, the measured delay times caused by the instruments and auxiliary tubes were much lower than those caused by long tubes, as shown in Figure S11. Therefore, the delay times of targeted organic compounds caused by the instruments and auxiliary tubes were directly subtracted from those measured for long tubes.”
Line 174. I assume this 200 s is after subtracting the residence time in the tubing?
Reply: We appreciate the reviewer for the valuable comments. The reviewer’s opinion is right. The 200 s is estimated after subtracting the residence time in the tubing.
Line 182-183. What do you mean by “is closely associated with the residence time of the air stream”? Certainly, the diffusion/dispersion effect is associated with that, but the chemical effects are associated with the surface area of the tubing.
Reply: We appreciate the reviewer for the valuable comments and suggestions. The residence time of the air stream in a long tubing is mainly determined by the tubing length and flow rate, which are two of the important factors for determining tubing delays of organic compounds. The change in both tubing length and flow rate can be reflected by the change in the residence time of the air stream, and this is what we want to express in this sentence. In our study, the long tubes were continuously drawn at large flow rates (12-15 SLPM) to reduce the residence time of air samples in tubing. In this condition, molecular diffusion/dispersion may play insignificant roles in affecting measured concentrations of targeted trace gas after passing through in a long tube.
Lines 187-190. I don’t understand the reason for excluding these data, and what is meant by “stochastic errors”.
Reply: We appreciate the reviewer for the valuable comments. In the revised manuscript, all the species were used in the calculation and thus these sentences have been removed from the manuscript.
Line 191-192. More rapidly than what?
Reply: We appreciate the reviewer for the valuable comments and this sentence has been rephrased. [P:11; L: 290-291]
“For the 400 m long tubes, tubing delays of organic compounds increase rapidly with the decrease in flow rate.”
Line 195. More rapidly than what?
Reply: We appreciate the reviewer for the valuable comments and this sentence has been rephrased. [P:12; L: 296-298]
“For a fixed flow rate of 13 SLPM, tubing delays of organic compounds also increase rapidly with the increase in tubing length.”
Line 200. What does viscosity have to do with this effect? I think the author mean highly absorptive compounds.
Reply: We appreciate the reviewer for the valuable comments. The viscous species here refers to the organic compounds with small C* values and we have rephrased this sentence in the manuscript. [P:12; L: 301-304]
“Previous work 44 mainly focused on tubing delays of the organic compounds that have small C* values in tubes with short lengths (<3 m), small ODs (≤1/4'') and flow rates (≤3 SLPM, laminar flow).”
Lines 201-204. The conditions used by the author are close to the transition between laminar and turbulent flow (this needs to be discussed as noted above), so turbulent flow might be responsible for some of the effects observed here.
Reply: We appreciate the reviewer for the valuable comments and suggestions. we agree with the reviewer’s opinion and provided related discussions in the manuscript. [P:12; L: 306-309]
“Reynolds numbers of sample gas streams in the tubing (OD: 1/2'') used in this study are generally larger (~900–2700) than those used in the literature (≤1000) 40, 41, 44. Therefore, the air sample streams in long tubes in this study are close to the transition between laminar and turbulent flow, which may be the most likely reason for the non-linear responses of tubing delays of organic compounds to changes in tubing length and flow rate.”
Line 211-212. I think the author mean smaller than the time between concentration steps.
Reply: We appreciate the reviewer for pointing out this mistake and we have revised in the manuscript. [P:12; L: 316-318]
“For the organic compounds that have tubing delays smaller than the time between concentration steps, their ion signals measured at both ends of the tubing agreed well within 20%, as shown in Figure 3.”
Line 226-228. This statement is misleading, if there is NO and O3 in the sample line, NO will be lost due to titration, all the ambient data show this. Putting in NO alone isn’t a very useful test.
Reply: We appreciate the reviewer for the valuable comments. In this section, these results were obtained from laboratory tests with only NO in the gas. This experiment was performed to check if there are any interactions between NO and tubing walls. In following sections, the impacts of chemical reactions on measurement uncertainties of various trace gases were discussed.
Lines 246-252. All these statements about NO, NOx and O3 depend on the relative amounts of NO and O3. The only reason NO at night doesn’t look so far off is that there is so much of it relative to O3, another site or another time where NO is lower relative to O3 would show more serious problems. In fact, that is what the authors see with their system.
Reply: We appreciate the reviewer for the valuable comments. In nighttime, due to the absence of sunlight, chemical species have the same reaction environments both inside the tubing and in the ambient air. The reaction between ozone and NO may reach a dynamical equilibrium in ambient air. In this condition, the concentrations of NO and ozone were not expected to be significantly changed after traversing the 400 m tubing.
Line 266. I think “estimated” is a more appropriate word than “determined”.
Reply: We appreciate the reviewer for the valuable suggestion. The word “determined” has been replaced with the word “determined” in the manuscript. [P: 14; L: 367]
Line 272. What is “k”?
Reply: We appreciate the reviewer for the valuable comments. The variable k is the slope of linear fitting line. We have added definitions of k and b before their first usage in the manuscript. [P: 6; L: 129-131]
“In this study, scatter plots and linear fittings (y=kx+b; k is the slope and b is the intercept) were mainly used to perform the comparison analysis. The coefficient of determination, denoted by R2, was used to evaluate the goodness of fit.”
Line 276. “excepted” should be “expected”.
Reply: We appreciate the reviewer for pointing out this mistake and it has been corrected in the manuscript. [P: 14; L: 376]
Line 279, 283. “tubing” should be “tubes”
Reply: We appreciate the reviewer pointing out these mistakes and they have been corrected throughout the manuscript.
Line 289, 308. “due to that the” should be “due to the fact that”.
Reply: We appreciate the reviewer for the suggestions and they have been revised in the manuscript. [P: 16; L:416-425]
Line 298, 302. What is “k”?
Reply: We appreciate the reviewer for the valuable comments. The variable k is the slope of linear fitting line. We have added definitions of k and b before their first usage in the manuscript. [P: 6; L: 129-131]
Line 303. I disagree, there is clearly loss of NO at the lower ranges of NO.
Reply: We appreciate the reviewer for the valuable comments. The clearly loss of NO at the lower ranges of NO concentrations mainly occurred in daytime with the presence of high ozone concentrations, which has been highlighted in the manuscript. [P: 14; L: 376-380]
“As expected, concentrations of O3 and NO measured using the two tubes agreed well in nighttime. By contrast, concentrations of O3 and NO measured using the 400 m long tubing in daylight (LT 07:00-13:00) decreased on average by 18% and 53%, respectively, in comparison to those made using the 5 m long tubing (Figure S17).”
Line 353. I don’t know what “Except for tall towers” means. Do they mean “in addition to tall towers”?
Reply: We appreciate the reviewer for the valuable comments and we have rephrased this sentence in the manuscript. [P:17; L: 466-468]
“In addition to tall towers or high-rise buildings, PFA Teflon tubes can also be used to establish vertical observation systems of atmospheric trace gases based on mobile platforms, such as tethered balloon 15, 43 and UAV.”
Line 356. “targeting” should be “targeted”.
Reply: We appreciate the reviewer for pointing out this mistake and it has been revised in the manuscript. [P:17; L: 469]
I believe the journal policy requires that the data need to be made available.
Reply: We appreciate the reviewer for the concerns. The reviewer can contact the corresponding author for data after the publication of the paper.
Supporting Information
Page 5, What are the variables and their units in this equation? The equation should have a slight correction for pressure.
Reply: We appreciate the reviewer for the valuable comments and suggestions. All the variables and their units have been provided in the revised SI. [P:5-6 in SI] The flow rate of the air sample stream, denoted by Q, in a tubing is measured by MFCs using the standard liter per minute (SLPM) as unit, which has considered the correction for pressure.
Figure S1, there needs to be an expanded time scale plot so we can actually see what the signal versus time looks like. My guess is that the 5 second timescale used for the PTR will not capture the rise of the signal very well.
Reply: We appreciate the reviewer for the valuable comments. The data shown in Figure S1 has time resolutions of 1 s and was mainly used to show the residence time of the air sample stream in the 400 m tubing. We have provided a new figure (Figure 1) in the main manuscript to show changes in PTR-ToF-MS signals as a function time.
What is the difference between measured and calculated residence time due to, is it temperature and pressure differences (you haven’t specified the units of Q). Could the difference be due to the residence times in the auxiliary tubing and associated parts of the system?
Reply: We appreciate the reviewer for the valuable comments. The difference between measured and calculated residence time could be mainly attributed to two reasons: 1) the residence times caused by auxiliary tubes and associated parts of the system; 2) measurement errors for the tubing length and the flow rate. [P: 5 in SI]
“Therefore, the measured residence time of the sample gas in a 400 m-long tubing (OD: 1/2'') at a flow rate of 13 SLPM is 155 s, which is slightly larger than that (131 s) calculated using Eq. (S1). The difference between the measured and calculated residence time may be mainly attributed to measurement errors for the tubing length and the flow rate, as well as the residence times caused by auxiliary tubes and associated parts of the instrument.”
We need to see the units in Equation S3.
Reply: We appreciate the reviewer for the valuable suggestions. The units of all the variables in Equation S3 have been provided in the revised SI. [P:6 in SI]
“where Pi is the pressure of sample gas at tubing inlet (hPa), Po is the pressure of sample gas at tubing outlet, Q is the flow rate of sample gas stream in tubing (cm-3 s-1), d is the inner diameter of the tubing (cm), η is the viscosity of sample gas in tubing (Pa s-1), and L is tubing length (cm).”
Page S7. What is the time resolution of the PTR based on? Is this the averaging time? Could you change it to get better resolution on some of the faster profiles?
Reply: We appreciate the reviewer for the valuable suggestions. In laboratory tests, measurements of the PTR-ToF-MS were set at a time resolution of 1 s. In field campaigns, measurements of the PTR-ToF-MS are associated with a time resolution of 5 s.
Equation S4 is an exponential (actually double exponential), not hyperbolic fit. Show us some fits. In the main paper you say delay times were based on time to reach 90% of the change, are you now says that they are based on tau 1 and tau 2? This is not clear.
Reply: We appreciate the reviewer for the valuable comments and suggestions. Equation S4 is a double exponential function and we have corrected this mistake in the manuscript. Before the calculation of tubing delays, ion signals of targeted organic compounds were firstly normalized to those measured at the start time of the step-function change. The double exponential method was then used to fit the depassivation curves of the species. The tubing delay of an organic compound is estimated as the time when its double exponential fitting line declined to 0.1. The estimated tubing delays are not determined by the fitting parameters, namely tau 1 and tau 2. We have provided more detailed explanations in the manuscript. [P: 6-7; L: 139-153]
“In this study, tubing delays of organic compounds were defined and computed using the same methods in previous works 40, 41, 44. Briefly, the tubing delay of a trace gas is defined as the amount of time required to reach 90% of the change in its concentration before entering the tubing 40, 41, 44. It should be noted that the tubing delay is distinct from the residence time as discussed in section 2.1. For a tubing with fixed length and inside flow rate, all the chemical species have the same residence time but have different tubing delays. Tubing delays of organic compounds were calculated according to the depassivation curves of their ion signals measured at the outlet end of the tubing (Figures 1 and S8-S10). Ion signals of organic compounds were firstly normalized to those measured at the start time of the step-function change at the outlet end of the tubing. Then, the normalized depassivation curves were fitted using the double exponential method, as formulated in Eq. (1), to reduce impacts of the noise in measured ion signals. The tubing delay of an organic compound is determined as the time when its double exponential fitting line declined to 0.1, as shown in Figure 1(b).”
Figure S9. Show us some actual fits to Equation S4!
Reply: We appreciate the reviewer for the valuable suggestions. We have provided a new figure (Figure 1) in the manuscript and Figure 1(b) shows the actual fits to the double exponential equation.

Figure 1(b) in the revised manuscript.
Box Model section. Does the model have O3 + alkenes producing OH radicals?
Reply: We appreciate the reviewer for the valuable comments. The model has included the chemical mechanisms that O3 reacts with alkenes to produce OH radicals (see the website for more details: http://mcm.york.ac.uk/home.htt). However, the reactions between ozone and alkenes are smaller than reactions with NO3 and only produce a small fraction of OH radicals when passing through the tubing.
Figure S11 shows that there is something seriously wrong with the box model. OH radical concentrations of 5 x10¹⁰ molec/cm3 are a factor of 10⁴ too high!
Reply: We appreciate the reviewer for pointing out this mistake. The units of the radicals were wrongly converted when making the plot. We have corrected this mistake in the revised SI by replotting the figure (Figure S14).

Figure S14. Time series of modelled NO3 and OH radical concentrations in light and dark environments when the initial mixing ratio of ozone was set as 100 ppb.
Figure S12. Your box model shows you that the NO measurements are completely compromised by using the long inlet. This needs to be explained in the main text.
Reply: We appreciate the reviewer for the valuable comments and suggestions. The changes in NO concentrations derived from model simulations have been discussed in the manuscript. [P: 13; L: 346-351]
“According to box model results (Figure 5), concentrations of hydroxyl radical (OH) decrease by 55% while concentrations of nitrate radical (NO3) increase by more than a factor of 3 in the following 180 s (Figure S11) after setting solar radiation to zero (entering the tubing) when ambient ozone mixing ratio was 100 ppb. NO will be completely consumed in less than 100 s, accompanied by an increase of 4.2% in NO2 and a decrease of 1.8% in ozone.”
Figure S17. Your data show that you cannot measure NO properly with long inlets.
Reply: We appreciate the reviewer for the valuable comments. Figure S18 shows that the NO measurements made by long tubes and tower platforms significantly deviated in daytime but agreed well in nighttime, which are consistent with the conclusions drawn in this study. We have revised Figure S18 to clearly identify the data in daytime and nighttime periods.

Figure S18. Time series of NO mixing ratios measured at the four altitudes on SMT from January 12 to 16, 2021. The grey areas indicate the nighttime period (LT 19:00-05:00).
In summation, this paper needs major revisions if it is to be acceptable for publication.
Reply: We appreciate the reviewer for the valuable comments and suggestions to improve the quality of our manuscript.
References
Andreae, M.O., et al., The Amazon Tall Tower Observatory (ATTO): overview of pilot measurements on ecosystem ecology, meteorology, trace gases, and aerosols, Atmos Chem. Phys., 15, 10723-10776, 2015, doi: 10.5194/acp-15-10723-2015.
Helmig, D., Johnson, B., Oltmans, S.J., Neff, W., Eisele, F., Davis, D.D., Elevated boundary-layer ozone at South Pole. Atmospheric Environment, 42, 2788-2803, 2008 doi:10.1016/j.atmosenv.2006.12.032.
Helmig, D., Johnson, B., Warshawsky, M., Morse, T., Neff, W., Eisele, F., Davis, D.D., Nitric oxide in the boundary-layer at South Pole during the Antarctic Tropospheric Chemistry Investigation (ANTCI). Atmospheric Environment, 42, 2817-2830, 2008 doi:10.1016/j.atmosenv.2007.03.061.
Helmig, D., Thompson, C.R., Evans, J., Boylan, P., Hueber, J., and J.-H. Park, Highly elevated atmospheric levels of volatile organic compounds in the Uintah Basin, Utah, Enivron. Sci. Technol., 48, 4707-4715, 2014, doi.10.1021/es405046r
Johnson, B.J., Helmig, D., and Oltmans, S.J., Evaluation of ozone measurements from a tethered balloon-sampling platform at South Pole Station in December 2003, Atmos. Environ, 42, 2780-2787, 2008,
Karion, A., Sweeney, C., Tans, P., and Newberger, T.: AirCore: An innovative atmospheric sampling system, J. Atmos. Ocean. Technol., 27, 1839-1853, 2010.
Steven S. Brown, et al., Nitrogen, Aerosol Composition and Halogens on a Tall Tower (NACHTT): Overview of a Wintertime Air Chemistry Field Study in the Front Range Urban Corridor of Colorado, J. Geophys. Res., 118, 10.1002/jgrd.50537, 2013.
Reply: We appreciate the reviewer for the recommendations.

12-Dec-2022

Dear Dr Yuan:

Manuscript ID: EA-ART-08-2022-000110.R1
TITLE: Emerging investigator series: Assessment of Long Tubing in Measuring Atmospheric Trace Gases: Applications on Tall Towers

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

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Reviewer 1

The authors sufficiently addressed the comments therefore I recommend to accept the manuscript.

Reviewer 2

See enclosed file

This text has been copied from the PDF response to reviewers and does not include any figures, images or special characters.

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Response to Reviewer #1
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1. The authors sufficiently addressed the comments therefore I recommend to accept the manuscript.
Reply: We appreciate the reviewer for the valuable comments and suggestions.


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Response to Reviewer #2
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1. The paper has been much improved over the original version, however there are still some issues that need to be attended to.
The authors seem not to understand that the diffusion/dispersion analysis described by Karion et al applies to flow through a tube, no matter the specific reason for that flow. In addition, the authors have miscalculated this effect, so let’s go through it here:
Deff = D + a2V2/48D
Where D is the bimolecular diffusion coefficient, a is the radius of the tube, and V is the average flow velocity. The effective diffusion distance (Xrms) is then;
Xrms = (2Defft)1/2
Where t is the residence time in the tube (155 sec at 13 SLPM).
Using the author’s own data, assuming D = 0.100 cm2/sec for a relatively large organic molecule, and paying attention to units:
V = 400 m/155 sec = 2.58 m/sec = 258 cm/sec
Deff = 0.100 + ((0.475)2×(258)2)/(48×0.100) ≈3130 cm2/sec
Xrms = (2×3130×155)1/2 = 984 cm
At an average velocity of 258 cm/sec, that equals an effective time 984/258 = 3.8 sec
This is essentially what the authors’ Figure 1c shows (ignoring the subtilties of what an rms time means relative to an exponential decay time). The authors don’t really explain Figure 1c, so I’m not sure if m/z 69 is unit mass resolution, ie. isoprene + furan, or one of those compounds individually. Either way, neither of those compounds would be expected to be significantly retained based on their C*. So, Figure 1c shows what time constant can be expected for an unretained compound. This analysis needs to be presented up front at the beginning of the results section since it is fundamental to understanding how the system works.
Reply: We appreciate the reviewer for the valuable comments and suggestions. In the last version of the manuscript, we only assess the effects of molecular diffusion without considering the impacts of molecular dispersion (namely Taylor dispersion). As suggested by the reviewer, we have reassessed the influence time of molecular diffusion and dispersion, denoted by tm here, on measured concentrations of trace gases after traversing the tubing. We can take isoprene and chlorobenzene as examples, the two organic compounds have significantly different saturation concentrations and measured tubing delays after traversing the 400 m tubing (as summarized in Tables S3 and S4). However, the estimated tm for isoprene and chlorobenzene has minor differences and all increase with the increase of flow rate. This is different from the observed results that the measured tubing delays of organic compounds decrease with the increase in flow rate, which is well illustrated in Figure 1(c). In addition, the estimated tm for isoprene is comparable to and even larger than its measured tubing delays at large flow rates. However, the estimated tm for chlorobenzene is much smaller than its measured tubing delay. According to these results, we believe that gas-surface interactions play predominant roles in determining the tubing delays of organic compounds, especially for the species with smaller C* values. The roles of molecular diffusion and dispersion become more significant than those of gas-surface interactions for the species with larger C* values. We have rephrased related sentences and provided more discussions in the revised manuscript to make this point clearer. [P: 10-11; L: 255-276] and [P: 9-10 in SI]
In Fig. 1(c), the ion signal of m/z 69 only refers to isoprene and we have revised the label of the Y axis. The raw ion signals of PTR-ToF-MS were processed using high-resolution peak fittings. Therefore, the ion signals of isoprene and furan can be well separated.
“As shown in Figure 1, concentrations of organic compounds have significant but differentiated delay times after traversing the 400 m long tubing at the flow rate of 13 SLPM. In addition to gas-surface interactions, the longitudinal mixing of trace gases, caused by molecular diffusion and dispersion 45, can also have effects on their concentrations after traversing a long tubing. According to the method (see details in SI) used in the literature 45, the influence times of molecular diffusion and dispersion, denoted by tm, on measurements of organic compounds after traversing a tubing can be estimated. Taking isoprene (C*= 2.0×109 μg m-3) and chlorobenzene (C*= 6.6×107 μg m-3) as examples, the two organic compounds have very significant differences in both saturation concentrations and measured tubing delays, as summarized in Tables S3 and S4. However, the estimated tm for isoprene and chlorobenzene has minor differences and all increase with the increase in flow rate. This is totally different from the observed results that the measured tubing delays of organic compounds rapidly decrease with the increase in flow rate, as shown in Figure 1(c). In addition, the estimated tm for chlorobenzene (Table S4) is much smaller than its measured tubing delay. By contrast, the estimated tm for isoprene (Table S3) is comparable to and even larger than its measured tubing delays at large flow rates. These results indicate that gas-surface interactions play predominant roles in determining the tubing delays of organic compounds, particularly for the species with smaller C* values associated with tubing delay much larger than several seconds. The effects of molecular diffusion and dispersion become more important for the species with larger C* values associated with tubing delay in the range of several seconds.”
I also have some specific comments:
2. Somewhere in the abstract it should be noted that the effective time constants imposed by the sampling system limits how the data can be interpreted.
Reply: We appreciate the reviewer for the valuable comments and suggestions. In real applications, the measurements of a trace gas made through a long tubing cannot be used to interpret its temporal variability at time scales lower than its tubing delay. We have highlighted this point in the abstract of the revised manuscript. [P: 2; L: 33-37]
“Measurements of various trace gases made by the vertical observation system can well characterize their concentrations and vertical distributions. However, the measurements of a trace gas made by the vertical observation system at each altitude cannot be used to interpret its temporal variability at time scales lower than its tubing delay.”
3. Line 109. Figure S1 doesn’t show what this sentence says it does.
Reply: We appreciate the reviewer for pointing out this mistake. Figure S1 is used to illustrate what the residence time is in this study and we have revised this sentence in the manuscript. [P: 5; L: 101-103]
“The residence time of sample gas (Figure S1) in a section of tube, which is defined as the ratio of the tubing length to the inside flow velocity, is inversely proportional to flow rate.”
4. Line 210. I assume the platinum catalyst is immediately in front of the PTRMS in the flow system? This should be made clear.
Reply: We appreciate the reviewer for the suggestion. The platinum catalyst is immediately in front of the inlet of the PTR-ToF-MS and we have provided related descriptions in the manuscript. [P: 8; L: 200-201]
“The platinum catalyst is immediately in front of the inlet of the PTR-ToF-MS and has no impacts on measurements of the other instruments.”
5. Lines 275-284. This is all wrong, see above calculation.
Reply: We appreciate the reviewer for the valuable comments. In the last version of the manuscript, we only assess the effects of molecular diffusion without considering the impacts of molecular Taylor dispersion. As suggested by the reviewer, we have reassessed and provided discussions on the effects of molecular diffusion and dispersion on measured concentrations of trace gases after traversing the tubing in the revised manuscript. [P: 10-11; L: 255-276]
6. Lines 325-327. This is not clear, are you saying that because the relationships in Figure 2 are not linear that this is different from previous studies. Your relationships are close to linear, and might be impacted by the fact that your highest flow rates extend into the turbulent flow regime. So, your data are not all that different (especially considering their variability) from previous results.
Reply: We appreciate the reviewer for the valuable comments. In our study, the tubing delays of organic compounds exhibit slight non-linear responses to changes in both tubing length and flow rate. We believe that this result is consistent with the discussions on the effects of molecular diffusion and dispersion on tubing delays of trace gases. Previous works mainly focused on investigating tubing delays of the organic compounds with large C* values in short tubes and at low flow rates. In this condition, the effective dispersion distances of the organic compounds in tubing are very small and can be neglected. Tubing delays of the organic compounds were mainly determined by gas-surface interactions. In our study, the large increase in tubing length and flow rate will non-linearly enhance the impacts from molecular diffusion and dispersion on measured concentrations of trace gases after traversing the tubing. In addition, we agree with the reviewer’s opinion that the air sample streams in long tubes in this study are close to the transition between laminar and turbulent flow, which may also be one of the reasons for the non-linear responses of tubing delays of organic compounds to changes in tubing length and flow rate. We have provided more discussions in the manuscript to make this point clearer. [P: 12; L: 310-320]
“Previous work 44 mainly focused on tubing delays of the organic compounds that have small C* values in tubes with short lengths (<3 m), small ODs (≤1/4'') and flow rates (≤3 SLPM, laminar flow). Reynolds numbers of sample gas streams in the tubing (OD: 1/2'') used in this study are generally larger (~900–2700) than those used in the literature (≤1000) 40, 41, 44. Therefore, the air sample streams in long tubes in this study are close to the transition between laminar and turbulent flow. In addition, the increase in tubing length and flow rate will non-linearly enhance the impacts from molecular diffusion and dispersion on measured concentrations of trace gases after traversing the tubing. These are the two most likely reasons for the non-linear responses of tubing delays of organic compounds to changes in tubing length and flow rate.”
7. Lines 410-412. Figure 5 does not show this, it only shows Ox and NOx.
Reply: We appreciate the reviewer for the valuable comments. Figure 5 shows the comparison of Ox and NOx measurements made by the 5 m and 400 m tubes at ground level. The comparison of NO and O3 measurements made by the 5 m and 400 m tubes was shown in Figure S17.
8. Lines 501. This is the wrong figure number.
Reply: We appreciate the reviewer for pointing out this mistake and we have rechecked the citation of figure numbers in the manuscript.
9. Figure 5a. What are the grey vertical lines? The caption says the figure is for ground-level data, but there appears to be distinct signal in O3 and NO2 that goes up and down, especially in the daytime period at the end of the timeline. Could it be that you have the wrong figure?
Reply: We appreciate the reviewer for the valuable comments. The figure is correct and the grey vertical lines in Figure 5(a) indicates the tubing length. The concentrations of O3 and NO2 were made alternatively through the 5 m and the 400 m tubes. Therefore, the distinct signal in O3 and NO2 that goes up and down mainly occurred in daytime and are caused by the difference in their concentrations made through the 5 m and 400 m tubes (Figure S17).
10. Figure 6a. What is the cause of the points that are low in observations through the tubing, but high in the in situ observations?
Reply: We appreciate the reviewer for the valuable comments. We have rechecked the raw measurements of the instruments and experimental records. The low NOx concentrations measured through the tubing were mainly interfered by instrumental calibration using zero air. These data points have been removed from the comparison analysis and we also revised Figure 6(a) in the manuscript. [P: 32]
11. Figure 6f. I disagree that there is good agreement in these vertical profiles. There is more than 10ppbv difference in Ox at 120m.
Reply: We appreciate the reviewer for the valuable comments. We have rechecked the raw measurements of the instruments and experimental records, and there are no problems with the observations. As introduced in the method section, the vertical observation system sequentially measures concentrations of trace gases at each altitude. The effective data samples obtained by the vertical observation system were much fewer than those of the tower platforms in one hour. In this condition, the vertical observation system may miss some high or low concentrations of trace gases at each altitude if ambient concentrations of trace gases quickly changed. As shown in Figures S18 and S19, the concentrations of NO and O3 strongly varied during the nighttime between January 13 to 15, 2021. This may be the reason for the difference in Ox concentrations at 120 m.

19-Feb-2023

Dear Dr Yuan:

Manuscript ID: EA-ART-08-2022-000110.R2
TITLE: Emerging investigator series: Assessment of Long Tubing in Measuring Atmospheric Trace Gases: Applications on Tall Towers

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. I am pleased to accept your manuscript for publication in its current form. I have copied any final comments from the reviewer(s) below.

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Reviewer 2

The paper is now acceptable for publication.

Reviewer 1

The authors addressed the comments of the second review round. This round improved quality of figures and eliminated some minor mistakes.

The only reservation I have is the newly included calculations for effective time t_m, which are reported in Tables S3 and S4. Following the calculations provided in the section “Tubing delays of organic compounds”, I get that t_m decreases with increasing flow. The authors report the opposite. Could authors please double check their calculations are correct and if there were any mistakes. It would help to report values for t (residence times) that were used, which are probably different than the calculated values.