From the journal Environmental Science: Atmospheres Peer review history

17-Aug-2022

Dear Dr China:

Manuscript ID: EA-ART-07-2022-000097
TITLE: An Automated Size and Time-resolved Aerosol Collector Platform Integrated with Environmental Sensors to Study Vertical Profile of Aerosol

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************

Reviewer 1

The work described a vertical gradient of size-resolved aerosol composition by deploying an automated Size and Time-resolved Aerosol Collector platform integrated with sensors on the Atmospheric Radiation Measurement’s tethered balloon system. The sampler was tested at the one sampling site. Also, the authors carefully evaluate their size and compositions of individual particles in different altitude. Certainly, the sampler is very useful for the potential readers. I believe that work is deserved to be published after one minor revision.

It has been found that atmospheric particles have complex and diverse chemical compositions and multi-components in nature, including but not limited to arbonaceous particles (e.g., black carbon and organic carbon particles), sea spray aerosols (e.g., sea salt), bioaerosols (e.g., fungi,bacteria, viruses, and pollen), and dust, and their size distributions are also highly variable (Cheng et al.,2022; Fröhlich-Nowoisky et al., 2016; Laskin et al., 2012; Lata et al., 2021; Philip et al., 2014; Riemer et al., 2019; Tomlin et al., 2021). There references should be reviewing paper instead of some specific work, such as (Laskin et al., 2012; Riemer et al., 2019;
(2017), Atmospheric Aerosol Chemistry: Spectroscopic and Microscopic Advances, Anal. Chem., 89(1), 430-452.; (2016), A review of single aerosol particle studies in the atmosphere of East Asia: morphology, mixing state, source, and heterogeneous reactions, J. Clean. Prod., 112, Part 2, 1330-1349; (2010), Nature and Climate Effects of Individual Tropospheric Aerosol Particles, Annu. Rev. Earth Pl. Sc., 38(1), 17-43

For Figure 1, could the authors can have some simply flow chart which can show how aerosol movement.

Reviewer 2

Cheng et al. developed an automated size and time-resolved aerosol collector (STAC) platform for investigating the vertical characteristics of atmospheric aerosol. The platform is battery-powered and lightweight, and can be deployed on unmanned aerial systems such as tethered balloons. The performance of STAC was characterized in several ARM campaigns. The experimental particle size range with a 50% cut-off was from as small as 0.07 microns to 2.3 microns. Results show that the STAC system is suitable for collecting ambient particles with different size ranges and meteorological data at different altitudes. The authors also demonstrated that the STAC system can be used to probe the physicochemical properties of atmospheric aerosols such as phase state, chemical composition, and optical properties. Overall, the manuscript is well written. The developed aerosol sampling platform is valuable to the community. I recommend the manuscript be published in Environmental Science: Atmospheres, only after my following comments are fully addressed.

1. Abstract: In the abstract, the authors mentioned that a micro-aethalometer was used to measure black carbon mass concentration. However, I did not see any aethalometer data discussed in the main manuscript.

2. Density of the particles assumed for Equation 1:

Density of the particles varied significantly with their chemical composition. It seems that organic compounds made up a major fraction of particles measured in this study (Figure 4). Usually, organic compounds have a much lower density than other particle components (such as inorganic salts). Using a fixed density of 1.6 g cm-3 in Equation 1 may lead to large uncertainty. Why not provide a range of ambient particle density and discuss how applying different density value will affect the analysis and results of this study?

3. Table 1: Is there a reason why the number of nozzles is different for different stages?

4. dp,50,exp: How was “dp,50,exp” obtained? Please provide an example.

5. The sampling flow rate is critical to maintain correct size cuts. In this study, the flow rate was kept at 3 LPM and the STAC performance was characterized under this flow rate condition. Was the sampling flow rate monitored during the flight? How stable was the flow?

6. Section 2.4: The authors mentioned that “each impactor sample was collected for 30 min”. The sample collection duration needed depends largely on particle concentration. The authors demonstrated that the collected samples can be used for offline analysis of phase state, chemical composition, optical properties, and hygroscopic and ice nucleation properties. Can the authors comment on the sampling time duration needed to perform each of the abovementioned analyses, given a typical ambient particle concentration (for example, 10 ug m-3)?

7. Section 3.2.1: How accurate was the OPC size measurement? Have the authors compared the performance of the OPC with other standard instruments such as SMPS + APS?

8. Section 3.2.2: The text says, “The size mode for Stages A, B, C, D, and E are ~0.90, ~0.45, ~0.36, and ~0.11 μm, respectively”. However, the information of the size mode for Stage A is missing.

Similarly, “The ATD particles collected on Stage A to D have size ranges of ~0.45-7.1 μm, ~0.23-2.5 μm, ~0.18-1.8 μm with size modes at ~1.4, 0.71, 0.45, and 0.45 μm, respectively”. Information of the size range for Stage D is missing.

9. Figure 3: Please add two lines in each sub-figure to show 50% cut-off size and 100% cut-off size so that the reader can make comparison and understand the results better.

10. The authors mentioned that “we have observed particles larger than the 100% collection efficiency cut-off size of the previous stage, which might bounce from higher to lower stages”. What was the fraction of the particles collected on a lower stage from the higher stage due to particle bounce? Please clarify.

11. CCSEM-EDX: Size distribution of the particles collected on each stage was measured by CCSEM-EDX. As the authors mentioned, the CCSEM-EDX provides the projected area equivalent diameter, while many other analytical methods give the aerodynamic diameter. Is there a way to convert area equivalent diameter to a more widely-used aerodynamic diameter?

12. Section 3.3 Page 11 lines 1-3: The authors demonstrated that one of the explanations that the observed size ranges exceed the expected collection size ranges of Stages C and D is that the collected particles might be in a liquid state. Have the authors tried to add a “mini” diffusion dryer before the sampling line to examine the possibility of this explanation?

13. Section 3.3 Page 11 lines 5-7: The observed size modes of the particles varied with altitude in this study. The authors attributed this observation to different atmospheric processing along the vertical column. However, samples were collected at each altitude for 30 min, and each sample was collected at a different time. In an extreme case, could it be just the evolution of the entire vertical column over time? Any suggestion to eliminate this possibility?

14. Figure S3: I suggest the authors move Figure S3 to the main text. Figure S3 is very different from Figure 4 in regard to chemical composition. This figure is important and demonstrates the robustness of the STAC sampling.

15. Conclusion: Can the authors briefly discuss the limitation of the STAC system? Any suggestions for future improvement?

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

We want to thank the reviewers for their comments. Addressing those comments has improved the quality of the manuscript. Below, we list each reviewer's comment (regular font), followed by our response (indented, bold font), followed by corresponding changes in the revised manuscript (indented, blue font). RL and RSL represent the line number in the revised main manuscript and SI, respectively.
REVIEWER REPORT(S):
Referee: 1
Comments to the Author
The work described a vertical gradient of size-resolved aerosol composition by deploying an automated size and Time-resolved Aerosol Collector platform integrated with sensors on the Atmospheric Radiation Measurement's tethered balloon system. The sampler was tested at the one sampling site. Also, the authors carefully evaluate their size and compositions of individual particles in different altitude. Certainly, the sampler is very useful for the potential readers. I believe that the work is deserved to be published after one minor revision.
We appreciate the positive feedback from the reviewer. Below are our responses to each comment:
It has been found that atmospheric particles have complex and diverse chemical compositions and multi-components in nature, including but not limited to carbonaceous particles (e.g., black carbon and organic carbon particles), sea spray aerosols (e.g., sea salt), bioaerosols (e.g., fungi, bacteria, viruses, and pollen), and dust, and their size distributions are also highly variable (Cheng et al.,2022; Fröhlich-Nowoisky et al., 2016; Laskin et al., 2012; Lata et al., 2021; Philip et al., 2014; Riemer et al., 2019; Tomlin et al., 2021). There references should be reviewing paper instead of some specific work, such as (Laskin et al., 2012; Riemer et al., 2019; (2017), Atmospheric Aerosol Chemistry: Spectroscopic and Microscopic Advances, Anal. Chem., 89(1), 430-452.; (2016), A review of single aerosol particle studies in the atmosphere of East Asia: morphology, mixing state, source, and heterogeneous reactions, J. Clean. Prod., 112, Part 2, 1330-1349; (2010), Nature and Climate Effects of Individual Tropospheric Aerosol Particles, Annu. Rev. Earth Pl. Sc., 38(1), 17-43
We appreciate the reviewer's suggestion. We have revised this part as below by only citing review papers.
It has been found that atmospheric particles have complex and diverse chemical compositions and multi-components in nature, including but not limited to carbonaceous particles (e.g., black carbon and organic carbon particles), sea spray aerosols (e.g., sea salt), bioaerosols (e.g., fungi, bacteria, viruses, and pollen), and dust, and their size distributions are also highly variable (Ault and Axson, 2017; Laskin et al., 2015; Li et al., 2016b; Riemer et al., 2019).
For Figure 1, could the authors can have some simply flow chart which can show how aerosol movement.
Thanks for your suggestion which we believe will helpreaders to better understand the STAC platform . We have revised the manuscript as below:

Figure 1. Schematic showing the major components of Size- and Time-resolved Aerosol Collector (STAC) platform and 4-stage STAC impactor. Numbers indicate (1) Sample inlet, (2) STAC box, (3) RH sensor, (4) MetOne OPC, (5) 4-stage STAC impactor, (6) battery, (7) inlet aluminum manifold, (8) outlet aluminum manifold, (9) printed circuit board (PCB) control plate, (10) electrical 3-way valves, (11) sample pressure sensor, (12) power switch, (13) pump, (14) Temperature and altitude sensors, (15) PC, (16) impactor nozzle plate, (17) top plate, and (18) impaction plate. A micro-aethalometer (microAeth, Model MA200, AethLabs) is attached (not shown here) to the outside bottom of the STAC box. Yellow arrows indicate the direction of aerosol flow.






 
Referee: 2
Comments to the Author
Cheng et al. developed an automated size and time-resolved aerosol collector (STAC) platform for investigating the vertical characteristics of atmospheric aerosol. The platform is battery-powered and lightweight, and can be deployed on unmanned aerial systems such as tethered balloons. The performance of STAC was characterized in several ARM campaigns. The experimental particle size range with a 50% cut-off was from as small as 0.07 microns to 2.3 microns. Results show that the STAC system is suitable for collecting ambient particles with different size ranges and meteorological data at different altitudes. The authors also demonstrated that the STAC system can be used to probe the physicochemical properties of atmospheric aerosols such as phase state, chemical composition, and optical properties. Overall, the manuscript is well written. The developed aerosol sampling platform is valuable to the community. I recommend the manuscript be published in Environmental Science: Atmospheres, only after my following comments are fully addressed.
We appreciate the positive feedback from the reviewer. The reviewer raised some critical points, which we believe were addressed in the revised version and strengthened the article. Below are our responses to each comment:
1. Abstract: In the abstract, the authors mentioned that a micro-aethalometer was used to measure black carbon mass concentration. However, I did not see any aethalometer data discussed in the main manuscript.
Thanks for pointing that out. Micro-Aethalometer data has been shown in Fig. S1 to demonstrate that we have light-absorption measurements for STAC flights. As we mentioned in section 2.4 ("In this study, we focus only on the vertical distribution of aerosol composition using single particle analysis via CCSEM-EDX and scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS)"), we decided not to discuss micro-Aethalometer data in detail. To make this clear, we add the following sentence in Sect. 3.3:
Representative raw data from the OPC and microAeth are shown in Fig. S2, indicating the variation of particle concentration and their light absorption properties at different altitudes.
2. Density of the particles assumed for Equation 1: Density of the particles varied significantly with their chemical composition. It seems that organic compounds made up a major fraction of particles measured in this study (Figure 4). Usually, organic compounds have a much lower density than other particle components (such as inorganic salts). Using a fixed density of 1.6 g cm-3 in Equation 1 may lead to large uncertainty. Why not provide a range of ambient particle density and discuss how applying different density value will affect the analysis and results of this study?
We appreciate the reviewer's constructive comment. We agree with the reviewer that different particle densities can affect the theoretical 50% cut-off size calculation. Several studies have shown that the density of ambient particles can be around 1.6 g cm-3 (e.g., Hand and Kreidenweis, 2002; Hu et al., 2012; Li et al., 2018; McMurry et al., 2002; Morawska et al., 1999; Pitz et al., 2003), which validates our choice of mean density. However, as the reviewer mentioned, there are several studies reported that ambient particle density might vary between 1 and 2.5 g cm-3 depending on the particle types and size (e.g., Cross et al., 2007b; Hu et al., 2012; Morawska et al., 1999; Pitz et al., 2003; Pokorná et al., 2022; Zhou et al., 2022; Zieger et al., 2017). To account for this variation in ambient particle density, we add the sentence below in section 2.3 and include dp,50,theo calculated by using ρp equal to 1 and 2.5 g cm-3 as uncertainties:
Based on measured parameters (Px, U, and D) and assumed particle density (ρp = 1.6 g cm-3), we used equations 1 and 2 to calculate each Stage's theoretical 50% cut-off size (dp,50,theo) and listed it in Table 1. However, several studies have reported that particle density might vary between 1 and 2.5 g cm-3 (e.g., Cross et al., 2007b; Hu et al., 2012; Morawska et al., 1999; Pitz et al., 2003; Pokorná et al., 2022; Zhou et al., 2022; Zieger et al., 2017). To account for the ambient particle density variation, we also include dp,50,theo calculated by using ρp equal to 1 and 2.5 g cm-3 as uncertainties in Table 1.
3. Table 1: Is there a reason why the number of nozzles is different for different stages?
Thanks for asking this question. We have a different number of nozzles to maintain similar pressure drop across each Stage, which is determined based on Marple and Willeke, 1975.
4. dp,50,exp: How was "dp,50,exp" obtained? Please provide an example.
Please refer to the new Fig. S1 as shown below, which shows the average particle size distribution normalized by the size distribution mode after STAC impactor for before and after STAC impactor stage D. Shaded areas represent measurement uncertainties. The dp50,exp¬ was determined as the diameter where collection efficiency (calculated by equation 3) equals 50%.

Figure S1. Representative normalized average particle size distribution normalized before (solid green line) and after (solid blue line) STAC impactor stage D and size-resolved collection efficient (red dash line). Size distributions were normalized by the size distribution mode after the STAC impactor. Shaded areas represent measurement uncertainties. The dp50,exp was determined as the diameter where collection efficiency (calculated by equation 3) equals 50%.
5. The sampling flow rate is critical to maintain correct size cuts. In this study, the flow rate was kept at 3 LPM and the STAC performance was characterized under this flow rate condition. Was the sampling flow rate monitored during the flight? How stable was the flow?
We appreciate the reviewer's valid point that variation in the ambient condition might affect the pump performance. The STAC platfrom has an inline pressure sensor to monitor the pressure in the sampling line as an indicator of the performance of valves and pump. The pressure fluctuations are within 5%. Therefore, we would consider the variation in the flow rate is negligible. To acknowledge this, we add the following sentence in Section 2.1:
The changes in flow rate are negligible since pressure fluctuations are within 5% during the flights.
6. Section 2.4: The authors mentioned that "each impactor sample was collected for 30 min". The sample collection duration needed depends largely on particle concentration. The authors demonstrated that the collected samples can be used for offline analysis of phase state, chemical composition, optical properties, and hygroscopic and ice nucleation properties. Can the authors comment on the sampling time duration needed to perform each of the abovementioned analyses, given a typical ambient particle concentration (for example, 10 ug m-3)?
We collected samples for 10- 30 mins during the campaign, depending on the particle concentrations observed on the ground. The particle concentration in the ambient can vary several orders of magnitude (101 to 105 cm-3). Considering the lowest particle concentration (10 cm-3) and collecting particles for 30 mins with a 3 LPM flow rate, we can collect approximately 9105 particles for a STAC impactor, which is sufficient for various offline analyses. Typically, similar particle concentrations are needed for chemical composition, optical properties, and ice nucleation measurements. For RH-dependent phase state measurements and hygroscopicity measurements, lower particle loadings are needed as a particle will grow after water uptake. We select an area away from the center impaction point for these experiments, where particle loadings are lower. We added the following sentence in the revised manuscript.
Under this sampling condition, around 105 particles per STAC impactor are deposited. This particle loading is suitable for multi-modal offline analyses. Typically, similar particle concentrations are needed for chemical composition, optical properties, and ice nucleation measurements. Lower particle loadings are needed for relative humidity-dependent phase state and hygroscopicity measurements as a particle will grow and coalesce after water uptake. We select an area away from the center impaction point for these experiments, where particle loadings are lower.
7. Section 3.2.1: How accurate was the OPC size measurement? Have the authors compared the performance of the OPC with other standard instruments such as SMPS + APS?
The OPC we used to calculate the collection efficiency of particles larger than 1 μm was a brand-new instrument that the manufacturer had just calibrated before receiving it. The uncertainty for each size range is less than 10%, which has already been accounted into the uncertainty of our results. To make this point clear to readers, we modified the manuscript as below:
To calculate the collection efficiency of particles larger than 1 μm, we used an optical particle counter (OPC, Model 804, Met One Instruments, Inc.) to measure the concentration of the particles before and after Stage A in size ranges of 0.7-1, 1.0-2.5, 2.5-5.0, and 5.0-10.0 μm. The OPC was calibrated before the experiment, and the uncertainty of each size range was within 10%. This uncertainty has been accounted into the measurement uncertainties.
8. Section 3.2.2: The text says, "The size mode for Stages A, B, C, D, and E are ~0.90, ~0.45, ~0.36, and ~0.11 μm, respectively". However, the information of the size mode for Stage A is missing.

Similarly, "The ATD particles collected on Stage A to D have size ranges of ~0.45-7.1 μm, ~0.23-2.5 μm, ~0.18-1.8 μm with size modes at ~1.4, 0.71, 0.45, and 0.45 μm, respectively". Information of the size range for Stage D is missing.
We appreciate the reviewer for pointing this out. We have revised this part as below:
As shown in Fig.3 (a-e), SRFA particles collected on Stages A, B, C, D, and E have size ranges of ~0.36-5.7 μm, ~0.23-2.8 μm, ~0.14-1.4 μm, ~0.071-1.8 μm, and ~0.045-0.71 μm, respectively. The size mode for Stages A, B, C, D, and E are ~1.4 μm, ~0.90, ~0.43, ~0.38, and ~0.11 μm, respectively. The ATD particles collected on Stage A to D have size ranges of ~0.45-7.1 μm, ~0.23-2.5 μm, ~0.18-1.8 μm, and ~0.11-1.8 μm, with size modes at ~1.4, 0.75, 0.45, and 0.40 μm, respectively (Fig. 3 (f-i)), agrees well with the experiment estimated collection efficiency.
9. Figure 3: Please add two lines in each sub-figure to show 50% cut-off size and 100% cut-off size so that the reader can make comparison and understand the results better.
We agree with the reviewer that adding two lines for 50% and 100% cut-off size will help readers compare and understand the results better. However, as we mentioned in our reply to comment 11, we cannot convert the projected area equivalent diameter to aerodynamic diameter, and our cut-off size is based on aerodynamic diameter. Therefore, adding lines for cut-off aerodynamic size might not lead to the equivalent cut-off size of the projected area equivalent diameter, which could mislead the readers. Thus, we decide not to add lines of cut-off sizes.
10. The authors mentioned that "we have observed particles larger than the 100% collection efficiency cut-off size of the previous stage, which might bounce from higher to lower stages". What was the fraction of the particles collected on a lower stage from the higher Stage due to particle bounce? Please clarify.
Thanks for asking these questions. There were ~14.1% of ATD particles on Stage D and ~7.5% and ~3.8% SRFA particles on Stage D and E, respectively, larger than the 100% collection efficiency cut-off size of the previous Stage. One reason might be that large particles bounced from larger stages to smaller stages, and we are not able to quantify the fraction of particles bounced from a higher stage to a lower stage since that depends on the particle properties and impaction plate surface, which need lots of calibration (see Mitchell et al., 2009). Moreover, those particles with an area equivalent diameter larger than the 100% collection efficiency cut-off size might be an aerodynamic diameter smaller than the 100% collection efficiency cut-off size. Thus, it will not be easy to distinguish between these two effects. We made the following modification to the sentence:
We have observed ~14.1% of ATD particles on Stage D and ~7.5% and ~3.8% SRFA particles on Stage D and E, respectively, larger than the 100% collection efficiency cut-off size of the previous Stage, which might bounce from higher to lower stages (Cheng and Yeh, 1979). The fraction of these bounced particles depends on the particles' properties, such as viscosity and density, and environmental conditions, such as relative humidty and temperature (Mitchell et al., 2009).
11. CCSEM-EDX: Size distribution of the particles collected on each Stage was measured by CCSEM-EDX. As the authors mentioned, the CCSEM-EDX provides the projected area equivalent diameter, while many other analytical methods give the aerodynamic diameter. Is there a way to convert area equivalent diameter to a more widely-used aerodynamic diameter?
Thanks for making this valid comment. It is possible to convert the projected area equivalent diameter to aerodynamic diameters, but this conversion has several limitations and introduces large uncertainties. For example, one should know the density of particles and assume the spherical shape of particles. Further, different phase states (e.g., solid, semi-solid, and liquid particles also introduce uncertainties in this conversion. In our study, as we investigate irregular shape dust particles and also atmospheric particles, we report particle size as projected area equivalent diameter.
12. Section 3.3 Page 11 lines 1-3: The authors demonstrated that one of the explanations that the observed size ranges exceed the expected collection size ranges of Stages C and D is that the collected particles might be in a liquid state. Have the authors tried to add a "mini" diffusion dryer before the sampling line to examine the possibility of this explanation?
We did not add any dryer system since we wanted to collect particles in the same condition as in the ambient. Furthermore, we plan to investigate the phase state of these particles in future studies, so it is crucial to collect particles in ambient conditions.
13. Section 3.3 Page 11 lines 5-7: The observed size modes of the particles varied with altitude in this study. The authors attributed this observation to different atmospheric processing along the vertical column. However, samples were collected at each altitude for 30 min, and each sample was collected at a different time. In an extreme case, could it be just the evolution of the entire vertical column over time? Any suggestion to eliminate this possibility?
We thank the reviewer for pointing this out. As the reviewer suggested, it is possible to observe the evolution of the entire vertical column over time under certain conditions. In this case, we would see very similar particle concentrations at all altitudes and observe specific meteorological conditions. However, we did not observe any of these, suggesting that it is indeed atmospheric processing rather than the evolution of the entire vertical column. We agree with the reviewer that a shorter sampling time would avoid this issue.

14. Figure S3: I suggest the authors move Figure S3 to the main text. Figure S3 is very different from Figure 4 in regard to chemical composition. This figure is important and demonstrates the robustness of the STAC sampling.
We agree with the reviewer. Fig. S3 has been moved to the main manuscript and labeled as Figure 6.
15. Conclusion: Can the authors briefly discuss the limitation of the STAC system? Any suggestions for future improvement?
Thanks for these constructive comments. We discussed integrating other environmental sensors in next-generation STAC. In the revised version, we discussed the limitations of the system and added the following sentences in our conclusion:
The current limitation of the STAC platform includes the lack of information about sample loading on the substrate. Future studies will include data from controlled laboratory experiments with a range of particle concentrations monitored by OPC with different sampling durations. The STAC software can be modified to convert the system into an adaptive and triggered sampling system to automatically switch the valve and move to the next sampling impactor once particle loading reaches a certain number using the integrated OPC particle concentrations data.

 
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12-Sep-2022

Dear Dr China:

Manuscript ID: EA-ART-07-2022-000097.R1
TITLE: An Automated Size and Time-resolved Aerosol Collector Platform Integrated with Environmental Sensors to Study Vertical Profile of Aerosol

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 1

i wish the sampler can widely be used in the science community. Then the paper can be accepted in the future. I am satisifed with the revision and fully accept this paper published.

Reviewer 2

The authors have sufficiently addressed all my comments, especially for the concerns regarding the density of the particles used to perform 50% cut-off size calculation, particle sampling duration for different analytical purposes, and the limitation of the SATC system. I don’t have additional comments and would like to recommend the publication of this manuscript in Environmental Science: Atmospheres.