From the journal Environmental Science: Atmospheres Peer review history

03-Feb-2023

Dear Dr El Haddad:

Manuscript ID: EA-ART-01-2023-000001
TITLE: Assessing the importance of nitric acid and ammonia for particle growth in the polluted boundary layer

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Reviewer 1

The authors have assessed the importance of nitric acid and ammonia for particle growth in the polluted boundary layer. The influences of temperature and the concentrations of precursors on the persistence of supersaturations have been discussed based on chamber studies and model simulations. They proposed that rapid and strong temperature changes and inhomogeneous emissions of ammonium in cities cause rapid growth of particles. The manuscript is well organized and the results are interesting and deeply discussed. It is publishable in Environmental Science: Atmospheres after the following questions have been well addressed.

1. The authors mentioned that particle growth via ammonium nitrate was not sufficient to explain the observed growth rates in 4 out of 10 experiments at -10 °C. And proposed that additional HNO3 from the hydrolysis of N2O5 may contribute to this underestimation. I don’t understand why this mechanism does not act in all the experiments. Why is the role of N2O5 more prominent in these 4 experiments than in others?
2. Although the authors have simply discussed the reason why new particle formation events from nitric acid and ammonia cannot be observed in ambient air in previous work (Marten et al., Environmental Science: Atmospheres, 2022, 2, 491–499), I am still not convinced by their explanations. They pointed out that transient deviations from equilibrium in ambient conditions should have a large impact on new particle formation through the nucleation of nitric acid and ammonia. However, it should be captured at least accidentally based on long-term continuous observations if this process occurs in ambient air. I agree with your assumption that a low time resolution for particle observations might miss the NPF due to a short period of temperature change and inhomogenous emissions of ammonia in ambient air. However, high concentrations of ammonia and nitric acid and low temperature can sustain for several hours in winter, especially in the North China Plain. However, one does not observe NPF events related to nitric acid nucleation based on long-term observations (Liu et al., Nat. Commun. 2022, 13(1): 6887). The possible reasons might be included in the discussion section.
3. As the authors pointed out that the influence of RH on particle formation has not been accounted for in the model. Considering the good performance of the model, does it mean that water molecules should have little contribution to new particle formation?
4. Please add the explanations for the symbols in Eq. 1.

Reviewer 2

In the manuscript, the authors used a kinetic particle microphysics model to explore the importance of nitric acid and ammonia condensation for particle growth in the polluted boundary layer, as a result of temperature change during vertical transport and inhomogeneity of NH3 concentrations in cities (due to traffic). This work is a nice follow-up study of two previous publications: Wang et al. (2020) and Marten et al. (2022). The following comments need to be properly addressed before I can recommend it for publication in ESA.

By the way, the manuscript does not have page numbers and line numbers so it is hard for me to refer to specific locations in the manuscript in my comments.

Main comments:
1. Growth rates due to N2O5.
(1) Section 3.2.2. You mentioned that “In 4 out of 10 experiments at -10 °C, growth via ammonium nitrate formation was not sufficient to explain the observed growth rates. For these experiments, we modeled concentrations of N2O5, and show that this may potentially contribute to growth.” What about the other 6 experiments? If you considered N2O5 condensation for these 6 experiments, would the growth rates be too high?
(2) Is this new? Do the previous measurements reported by Wang et al. (2020) and Marten et al. (2022) also indicate the potential role of N2O5 condensation?
(3) You treated N2O5 as HNO3. Any justification for such treatment? I would expect that N2O5 uptake through heterogeneous reactions shall depend on particle composition and be quite different from kinetic condensation. Any indication of such a process in the atmosphere (i.e., N2O5 uptake at collision rate)? As I understand, N2O5 uptake coefficient through heterogeneous reactions is much less than 1.
(4) Figure 1a. Even with the consideration of N2O5 contribution, the model still substantially underpredicted growth rates in three cases. What are the possible remaining reasons?
(5) Please provide the values of N2O5 concentrations you used for the 4 experiments.

2. Figure 1b. It appears that three points with GR > 200 nm/hr for -10 °C were not included in this plot. Why?
3. One major component of this work is that inhomogeneity of NH3 concentrations in cities (due to traffic) may lead to HNO3-NH3 condensation and thus a longer survival rate of nucleated particles. However, traffics not only emit NH3 but also primary particles. In the exhaust plumes with high NH3 concentrations, concentrations of primary particles and thus coagulation sinks can also be pretty high. If you take into account the high coagulation sinks, will your conclusion change? You can test this using your model.
4. Section 3.3.1 “The nucleation rates used for these models were …”. Please provide specific formulas or parameterizations you used in your calculation of nucleation rates. Do you consider the role of background ionization?
5. Section 3.3.2. “we used hydrocarbon-like organic aerosol (HOA) instead of NH3, since NH3 time-series are averaged out due to the stickiness of the molecule”. I do not understand here. Why NH3 time-series are averaged out due to the stickiness of the molecule? Do you have measurements (references) supporting co-variations of HOA and NH3 emissions from traffic? What is the exact temporal resolution of the time series you discussed here?
6. Section 5.
(1) the first two lines. This is important. Please provide a couple of references to the ambient measurements you mentioned here.
(2) “In the simulations presented in Figure 3c) the inhomogeneities lasted only 2-3 minutes, which would be challenging to capture on an instrument with a 5-minute resolution.” Any indication that in the real atmosphere the inhomogeneities also lasted only 2-3 minutes? I expect that under some conditions (stable boundary layer and low wind speed) the inhomogeneities due to traffic can last more than 2-3 minutes. While SMSP may have a 5-minute resolution, some instruments (such as FMPS, condensation nuclei counters, etc.) can have a much higher resolution. Can the proposed mechanism be directly observed by other instruments?


Minot comments:
1. Abstract, “rapid and strong temperature changes are needed”. Can you be a little bit more specific or quantitative on how rapid and how strong temperature changes are needed?
2. In a number of places, Marten et al. (2022) and Wang et al. (2020) were cited without a reference #.
3. Equation (1): All terms and symbols should be clearly defined.
4. Section 3.3. “The wall and dilution losses of the chamber were replaced with diffusion losses calculated from literature deposition velocities.” Could you provide here the specific values of these losses used in your study?
5. Figure 1a. “thick red border” can’t be seen clearly.
6. Fig 2d. y-axis is N or dN/dlogDp?
7. Section 4.3 “Figure 3a) shows an experiment with fixed production rates of HNO3, NH3, and H2SO4, “. Please provide the values of production rates.
8. Figure 3 caption, first 2 lines. What are the values of injection rates?
9. Figure S2. The figure caption is not very clear. Maybe mark T and T change rates in each panel? Along with dN/dlogDp, can you also provide time series of key parameters/species such as T, NH3, HNO3, saturation concentration of NH3NO3, etc.?
10. Figure S6. Please provide some details on how Kp was calculated.
11. All dN/dlogDp in y-axis of supplement figures need a unit.

Dear Editor, dear reviewers,
We thank you for the comments, which improved significantly the manuscript. Please find below our response to the reviewers’ comments together with the modifications made to the text.

Referee 1
The authors have assessed the importance of nitric acid and ammonia for particle growth in the polluted boundary layer. The influences of temperature and the concentrations of precursors on the persistence of supersaturations have been discussed based on chamber studies and model simulations. They proposed that rapid and strong temperature changes and inhomogeneous emissions of ammonium in cities cause rapid growth of particles. The manuscript is well organized and the results are interesting and deeply discussed. It is publishable in Environmental Science: Atmospheres after the following questions have been well addressed.

1. The authors mentioned that particle growth via ammonium nitrate was not sufficient to explain the observed growth rates in 4 out of 10 experiments at -10 °C. And proposed that additional HNO3 from the hydrolysis of N2O5 may contribute to this underestimation. I don’t understand why this mechanism does not act in all the experiments. Why is the role of N2O5 more prominent in these 4 experiments than in others?

Thank you for this question and comment. We had two methods to generate OH in the chamber. The first was through O3 photolysis and the second through HONO photolysis at low concentration of O3. 4 out of 10 experiments -10 °C were conducted at high O3 concentrations, which formed NO3 and N2O5 after reaction with NO2. The other 6 experiments were performed using HONO. We have modelled the 10 experiments at -10 °C, and could confirm the high concentration of N2O5 during the 4 experiments where O3 was used. For these experiments, we observed growth rates higher than what we could explain based only on HNO3 concentrations.

It was not clear from the paper that N2O5 was modelled for all conditions but that it was negligible for most of the experiments. We have modified this section as follows: A table was added to the ESI (Table S1) and the section below explaining and pointing this out in the main text.

Line 126: "Four experiments, which utilized ozone photolysis to produce OH, had higher ozone concentrations and therefore had higher concentrations of N2O5 via reaction of NO3 radicals with NO2. Six experiments with low O3 concentration and negligible N2O5 utilised HONO photolysis to produce OH, as we intended to avoid NO3 radical reactions with organics present (Table S1)."

2. Although the authors have simply discussed the reason why new particle formation events from nitric acid and ammonia cannot be observed in ambient air in previous work (Marten et al., Environmental Science: Atmospheres, 2022, 2, 491–499), I am still not convinced by their explanations. They pointed out that transient deviations from equilibrium in ambient conditions should have a large impact on new particle formation through the nucleation of nitric acid and ammonia. However, it should be captured at least accidentally based on long-term continuous observations if this process occurs in ambient air. I agree with your assumption that a low time resolution for particle observations might miss the NPF due to a short period of temperature change and inhomogenous emissions of ammonia in ambient air. However, high concentrations of ammonia and nitric acid and low temperature can sustain for several hours in winter, especially in the North China Plain. However, one does not observe NPF events related to nitric acid nucleation based on long-term observations (Liu et al., Nat. Commun. 2022, 13(1): 6887). The possible reasons might be included in the discussion section.

We want to make it clear that although we describe reasons why these events may be overlooked in ambient, we do not think that they will not have been or cannot be measured. We would also like to add that at high condensation sinks and in the absence of inhomogeneities, equilibrium is reached immediately even at high concentrations of ammonia and nitric acid, and activation will not be possible. The papers focus is not to prove that this process is important in ambient, but to provide the conditions when this process would take place and how it would manifest in ambient in terms of aerosol size distributions measured using the typical resolution of the SMPS. Indeed, for this process to happen, we need inhomogeneity in both ammonia and nitric acid concentrations, resulting from either high emissions or large change in temperature. We have added the following to the text to address the reviewer’s comment:

Line 340: "Furthermore, at sustained concentrations of NH4NO3 in absence of inhomogeneities, equilibrium is rapidly reached, and the required supersaturation for rapid growth is no longer present."

Line 351: "To summarise, although it is likely that this mechanism may occur in urban ambient air, it is hard to identify and it needs specific conditions with high heterogeneity coming from either large differences in temperatures or large emissions of ammonia (>10ppb)."

3. As the authors pointed out that the influence of RH on particle formation has not been accounted for in the model. Considering the good performance of the model, does it mean that water molecules should have little contribution to new particle formation?

A sentence was added to the conclusions to clarify that although we believe that water would not have had a large impact on the growth rates in these conditions, in higher RH conditions water may play a role.

Line 361: "Although in the conditions presented in this paper water most likely does not affect ammonium nitrate growth, there are other conditions where water may play a larger role."

In our model we use kinetic nucleation rates typical of urban environments, and so water should not have an effect on the new particle formation rates; or at least its effect is minor compared to the effects of temperature, sulphuric acid and ammonia concentrations. Details on nucleation rate calculation are now given in section 3.3.

Line 149: "The nucleation rates used for these models were temperature dependent and typical of 1 × 107 molecules cm-3 H2SO4 with 3 ppbv NH3 for the “air parcel rising” experiments and no amines.11,24,25 Nucleation rates (JX) were calculated using a model based off of the general dynamic equation that is described in detail in Xiao et al. (2021).2,11 Equation (2) shows a simplified version of the equation, showing that loss rates (losses) are subtracted from the change in concentration of particles of a certain size x (Nx) over time (t). The units of Jx are particles cm-3 s-1.

J_x=(dN_x)/dt-losses (2)

Both the parameterization in Xiao et al. (2021) and the experiments presented in this paper were undertaken in the presence of galactic cosmic rays (i.e. with the clearing field of the CLOUD chamber off). However, we have not considered the effect of varying ionization level during transport."

4. Please add the explanations for the symbols in Eq. 1.
Thank you for pointing out that this was missing A paragraph explaining equation 1 and the symbols can now be found under equation 1.

Line 117: "Where SAB,P is the saturation of ammonium nitrate, dK10 is the kelvin diameter, and dact is the activation diameter. The saturation concentration can be calculated from the mixing ratios of ammonia and nitric acid and the dissociation constant of ammonium nitrate. The Kelvin diameter is the diameter at which SAB,P = 10, and the activation diameter is the minimum diameter at which ammonium nitrate can condense on particles. For more "detailed information see the supporting information of Marten et al. 2022.13

Referee 2

In the manuscript, the authors used a kinetic particle microphysics model to explore the importance of nitric acid and ammonia condensation for particle growth in the polluted boundary layer, as a result of temperature change during vertical transport and inhomogeneity of NH3 concentrations in cities (due to traffic). This work is a nice follow-up study of two previous publications: Wang et al. (2020) and Marten et al. (2022). The following comments need to be properly addressed before I can recommend it for publication in ESA.

By the way, the manuscript does not have page numbers and line numbers so it is hard for me to refer to specific locations in the manuscript in my comments.

Thanks for the comment, we have added line numbers and referenced them in our answers.

Main comments:
1. Growth rates due to N2O5.
(1) Section 3.2.2. You mentioned that “In 4 out of 10 experiments at -10 °C, growth via ammonium nitrate formation was not sufficient to explain the observed growth rates. For these experiments, we modeled concentrations of N2O5, and show that this may potentially contribute to growth.” What about the other 6 experiments? If you considered N2O5 condensation for these 6 experiments, would the growth rates be too high?

As discussed in the response to Reviewer 1, we have added in the methods section a sentence clarifying that N2O5 was modelled for all experiments but was only relevant for 4 out of 10 experiments. Furthermore, we have added a table to the ESI with the calculated values, and a sentence in the results section.

(2) Is this new? Do the previous measurements reported by Wang et al. (2020) and Marten et al. (2022) also indicate the potential role of N2O5 condensation?

Before this investigation, we saw no indication of N2O5. After the discovery, we did re-check previous data. In Marten et al. all experiments and models presented were at 5C where we did not observe the condensation of N2O5. At these conditions, we did not observe a systematically higher CLOUD growth rates than expected based on nitric acid concentrations. In Wang et al. we can confirm that N2O5 did not participate in the nucleation studies since we have API-TOF measurements of the composition of nucleated particles. Some of the experiments presented in Wang et al at -10 are the same as those in this paper, and so if N2O5 is indeed involved, it may have affected these growth rates, but should not have an impact on the conclusions of the paper.

(3) You treated N2O5 as HNO3. Any justification for such treatment? I would expect that N2O5 uptake through heterogeneous reactions shall depend on particle composition and be quite different from kinetic condensation. Any indication of such a process in the atmosphere (i.e., N2O5 uptake at collision rate)? As I understand, N2O5 uptake coefficient through heterogeneous reactions is much less than 1.

(4) Figure 1a. Even with the consideration of N2O5 contribution, the model still substantially underpredicted growth rates in three cases. What are the possible remaining reasons?

To answer questions (3) and (4) about N2O5:
We have added the following sentences to the results section to clarify why we made these choices and what assumptions were made which could result in remaining errors.

Line 214: "The uptake of N2O5 into small ammonium nitrate particles is not well characterized. Uptake rates determined from ambient measurements are approximately a few percent, but strongly increase with the aerosol water content. Without N2O5 measurements, modelling the condensation flux of N2O5 is subject to substantial uncertainties, because (1) the uptake rate coefficient of N2O5 onto small ammonium nitrate particles has not been previously determined, and (2) the wall loss behaviour of N2O5 in the CLOUD chamber is unknown. For our modelled values, we considered a kinetic condensation of the N2O5 as an upper limit and a complete loss of N2O5 onto the walls, which would yield a lowest estimate of N2O5 concentrations, which may explain why the growth rates are still under predicted. Under these assumptions, model to measurement discrepancy decreases from a factor of 10 to factor of 2.5. We concluded, albeit with uncertainty, that N2O5 may be a missing gas contributor to growth, but more experiments would be needed to measure the N2O5 uptake rates as a function of particle size and chemical composition."

(5) Please provide the values of N2O5 concentrations you used for the 4 experiments.

A table was added to the SI with limiting gas concentration (without N2O5), N2O5, and growth rates.

2. Figure 1b. It appears that three points with GR > 200 nm/hr for -10 °C were not included in this plot. Why?

Thank you for pointing out this mistake, it was an accident and the data points have now been added.

3. One major component of this work is that inhomogeneity of NH3 concentrations in cities (due to traffic) may lead to HNO3-NH3 condensation and thus a longer survival rate of nucleated particles. However, traffics not only emit NH3 but also primary particles. In the exhaust plumes with high NH3 concentrations, concentrations of primary particles and thus coagulation sinks can also be pretty high. If you take into account the high coagulation sinks, will your conclusion change? You can test this using your model.

Marten et al 2022 explored the effect of the condensation sink on particle survival, therefore this is not addressed in this paper. The survival of particles decrease with increasing condensation sink, but survival remained high under high concentrations of NH4NO3, even at high condensation sinks (>0.01s-1). We showed that particle survival approaches unity starting from [NH3]x[HNO3] values around 1.2ppb2 or a growth rate from NH4NO3 of 300 nm/h. Under the conditions we have simulated, [NH3]x[HNO3] values 4ppb2 and growth rates are ~800 nm/h, which means that particle survival is relatively independent of the condensation sink.
The following was added to the discussion:

Line 345: "The effect of the larger condensation sink on particle survival was presented in Marten et al. (2022) and revealed that even under condensation sinks over 0.01s-1, [NH3] × [HNO3] concentrations higher than 1.2 ppb2 led to survival rates close to unity. In the experiments simulated in this paper, [NH3] × [HNO3] values were around 4 ppb2, which means that particle survival is relatively independent of the condensation sink."

4. Section 3.3.1 “The nucleation rates used for these models were …”. Please provide specific formulas or parameterizations you used in your calculation of nucleation rates. Do you consider the role of background ionization?

Thank you for the question, the following explanation of modelled nucleation rates was added to section 3.3:

Line 149: "The nucleation rates used for these models were temperature dependent and typical of 1 × 107 molecules cm-3 H2SO4 with 3 ppbv NH3 for the “air parcel rising” experiments and no amines.11,24,25 Nucleation rates (JX) were calculated using a model based off of the general dynamic equation that is described in detail in Xiao et al. (2021).2,11 Equation (2) shows a simplified version of the equation, showing that loss rates (losses) are subtracted from the change in concentration of particles of a certain size x (Nx) over time (t). The units of Jx are particles cm-3 s-1.

J_x=(dN_x)/dt-losses (2)

Both the parameterization in Xiao et al. (2021) and the experiments presented in this paper were undertaken in the presence of galactic cosmic rays (i.e. with the clearing field of the CLOUD chamber off). However, we have not considered the effect of varying ionization level during transport."

5. Section 3.3.2. “we used hydrocarbon-like organic aerosol (HOA) instead of NH3, since NH3 time-series are averaged out due to the stickiness of the molecule”. I do not understand here. Why NH3 time-series are averaged out due to the stickiness of the molecule? Do you have measurements (references) supporting co-variations of HOA and NH3 emissions from traffic? What is the exact temporal resolution of the time series you discussed here?

Thank you for pointing out that this was not clear. All information regarding the NH3 time series are presented in Elser 2018 (https://doi.org/10.1016/j.atmosenv.2017.11.030 ). Ammonia is a sticky gas. During the measurements in Elser et al. we have improved the sampling to be able to perform mobile measurements. This was done by shortening the inlet to 1 m, heating the inlet to 110 C, and increasing the inlet flow to 5 lpm. With this we achieved a relatively quick response of ammonia, which correlated relatively well with HOA. Still, the time-series of ammonia presented some lag and smearing (response time-scales of ~30-60 s), so we used the HOA times series (multiplied by an average NH3/HOA determined our tunnel measurements) to infer the NH3 time series. We edited the text to make clear that this decision was based on the paper Elser et al (2018):

Line 177: "For the highly time-resolved measurements we used hydrocarbon-like organic aerosol (HOA) multiplied by an NH3/HOA factor determined in Elser et al. (2018) instead of NH3.15 Elser et al. showed that NH3 time-series were averaged out with slower response times due to the stickiness of the molecule, and HOA was shown to be the best traffic tracer."

6. Section 5.
(1) the first two lines. This is important. Please provide a couple of references to the ambient measurements you mentioned here.

We added an additional sentence to the beginning of this section to make clear that we are not claiming that this does not or has not happened, but that to the best of our knowledge it has not been reported on, and speculate on why this might be.

Line 329: "In light of these results, it is puzzling that similar size distributions as in Figure 3 have not been frequently observed in ambient measurements. To the best of our knowledge, the rapid particle growth from ammonium nitrate formation has not been previously reported. One of the many reasons for the lack of ambient observations of this phenomena could be the low resolution of particle size distribution measurements"…

(2) “In the simulations presented in Figure 3c) the inhomogeneities lasted only 2-3 minutes, which would be challenging to capture on an instrument with a 5-minute resolution.” Any indication that in the real atmosphere the inhomogeneities also lasted only 2-3 minutes? I expect that under some conditions (stable boundary layer and low wind speed) the inhomogeneities due to traffic can last more than 2-3 minutes. While SMSP may have a 5-minute resolution, some instruments (such as FMPS, condensation nuclei counters, etc.) can have a much higher resolution. Can the proposed mechanism be directly observed by other instruments?

We want to make it clear that although we describe reasons why these events may be overlooked, we do not think that they will not have been measured, and certainly not that they can not be measured. We would also like to add that and in the absence of inhomogeneities, (or longer time inhomogeneities) equilibrium is reached immediately even at high concentrations of ammonia and nitric acid, and activation will no longer be possible. We have added sentences in the first paragraph of the discussions making this clear.

Line 340: "Furthermore, at sustained concentrations of NH4NO3 in absence of inhomogeneities, equilibrium is rapidly reached, and the required supersaturation for rapid growth is no longer present."

Line 351: "To summarise, although it is likely that this mechanism may occur in urban ambient air, it is hard to identify and it needs specific conditions with high heterogeneity coming from either large differences in temperatures or large emissions of ammonia (>10ppb)."

Minor comments:
1. Abstract, “rapid and strong temperature changes are needed ”. Can you be a little bit more specific or quantitative on how rapid and how strong temperature changes are needed?

Line 47: We added: “of 1 °C/min-1” to the abstract

2. In a number of places, Marten et al. (2022) and Wang et al. (2020) were cited without a reference #.

Thank you for pointing this out, it has been amended.

3. Equation (1): All terms and symbols should be clearly defined.

The symbols are now defined and explained below the equation.

Line 117: "Where SAB,P is the saturation of ammonium nitrate, dK10 is the kelvin diameter, and dact is the activation diameter. The saturation concentration can be calculated from the mixing ratios of ammonia and nitric acid and the dissociation constant of ammonium nitrate. The Kelvin diameter is the diameter at which SAB,P = 10, and the activation diameter is the minimum diameter at which ammonium nitrate can condense on particles. For more detailed information see the supporting information of Marten et al. 2022.13"

4. Section 3.3. “The wall and dilution losses of the chamber were replaced with diffusion losses calculated from literature deposition velocities.” Could you provide here the specific values of these losses used in your study?

The following sentences were added to section 3.3

Line 140: "The wall and dilution losses of the chamber were replaced with diffusion losses. Diffusion losses were calculated using equations from Seinfeld and Pandis (2006), with literature deposition velocities for HNO3, NH3, and H2SO4. The height of the boundary layer used was 1km. The production values of HNO3, NH3, and H2SO4 were calculated such that in the absence of a condensation sink, the values were at steady state."

5. Figure 1a. “thick red border” can’t be seen clearly.

Thank you for pointing this out, it seems that the border was not there. We made it more clear by changing the colour to golden and amending the text.

6. Fig 2d. y-axis is N or dN/dlogDp?

Thank you for pointing out that this was unclear, the figure has been rearranged so that it can be seen that 2.d is N(cm-3) and 2.a and c colour scale is dN/dlogdp

7. Section 4.3 “Figure 3a) shows an experiment with fixed production rates of HNO3, NH3, and H2SO4, “. Please provide the values of production rates.
The following sentences were added

Line 294: "The production values of HNO3, NH3, and H2SO4 were calculated such that in the absence of condensation, the concentrations were 1ppb, 0.2ppb, and 1 × 107 molecules cm-3 respectively. The concentrations were chosen as typical background concentrations, resulting in an activation diameter of 200nm for ammonium nitrate growth."

8. Figure 3 caption, first 2 lines. What are the values of injection rates?

The text was changed to “production rates” it is now explained in the text how these were determined.

Line 294: "The production values of HNO3, NH3, and H2SO4 were calculated such that in the absence of condensation, the concentrations were 1ppb, 0.2ppb, and 1 × 107 molecules cm-3 respectively. The concentrations were chosen as typical background concentrations, resulting in an activation diameter of 200nm for ammonium nitrate growth."

9. Figure S2. The figure caption is not very clear. Maybe mark T and T change rates in each panel? Along with dN/dlogDp, can you also provide time series of key parameters/species such as T, NH3, HNO3, saturation concentration of NH3NO3, etc.?

This makes the figure quite messy and deviate from the main aim of the figure which is comparing the dN/dlogDp at different temperature changes of 5°C, 10°C and 15°C. We have listed the conditions in the figure caption so that the reader would be able to relate the figures to them.

10. Figure S6. Please provide some details on how Kp was calculated.

A paragraph and equation were added in the ESI just before figure S6.

"Figure S6 shows the relationship of the dissociation constant, Kp, with temperature. It can be seen that in steady state conditions, where the product of concentrations of gas phase NH3 and HNO3 are equal to the saturation temperature, a change in 10°C from 20°C to 10°C results in more available gas to condense than for 10°C to 0°C. Kp can be calculated by integrating the van’t Hoff equation (Denbigh, 1981). Equation S.1 shows the equation for Kp in units of ppb2 (assuming 1 atm of total pressure) (Mozurkewich, 1993).
lnK_p= 118.87- 24,084/T-6.025 lnT (S.1)"


11. All dN/dlogDp in y-axis of supplement figures need a unit.
Thank you, cm-3 was added

29-Aug-2023

Dear Dr El Haddad:

Manuscript ID: EA-ART-01-2023-000001.R1
TITLE: Assessing the importance of nitric acid and ammonia for particle growth in the polluted boundary layer

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.

I have carefully evaluated your manuscript and the reviewers’ reports, and the reports indicate that revisions are necessary.

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Reviewer 2

The authors have addressed most of the comments and the manuscript has improved. I have some follow-up comments that should be addressed.

1. Growth rates due to N2O5.
It is helpful to clarify that the four experiments with insufficient growth via ammonium nitrate formation are associated with O3 photolysis while others are not. A little surprised that this was not clearly pointed out in the original manuscript.
(1) Thanks for providing Table S1. As stated in Line 134, you “treated the calculated N2O5 concentration as additional HNO3 for these 4 experiments”. Based on Table S1, N2O5 concentrations were much smaller than those of HNO3. How could these smaller concentrations of N2O5 significantly increase the growth rates? For example for Run 1, the addition of 150 ppt N2O5 to 260 ppt HNO3 increased GR from 200 nm/hr (Fig. S1) to ~ 700 nm/hr (Fig. 1) in your model calculation. How could 58% more condensing gas increase the growth rates by 350%? Did I miss something here?

(2) In responding to my original comment 1.4, the authors said: “For our modelled values, we considered a kinetic condensation of the N2O5 as an upper limit and a complete loss of N2O5 onto the walls, which would yield a lowest estimate of N2O5 concentrations, which may explain why the growth rates are still under predicted.” This is important but the answer is just a speculation. The authors should do a quick sensitivity study using smaller values of N2O5 uptake and wall loss rates to see how much this uncertainty may explain the under-prediction. With the model, it should be pretty easy to carry out such a sensitivity study.

2. In replying to comment #2 of Referee 1 and comment #5 of Referee 5 (with regard to “puzzling that similar size distributions as in Figure 3 have not been frequently observed in ambient measurements”, the authors focused on the difficulty in observing these in the atmosphere because of instrument limitation. However, another possible reason is that the experimental conditions may not reflect what occurs in the real atmosphere and the findings of the lab measurements can not be applied to the real atmosphere. This possible reason should also be discussed.

3. Lines 142-143. Please provide the production values of HNO3, NH3, and H2SO4 calculated in your study.

4. Equ. 2. What is the value of x assumed in your study? How did you get dNx/dt? Based on measurements?

5. Line 157. “the effect of varying ionization level during transport”. What does “during transport” mean?

6. “All dN/dlogDp in y-axis of supplement figures need a unit.” “Thank you, cm-3 was added”.
I didn’t find the added unit in Fig. S2-S6, S8.

7. I didn’t find the list of references cited in the supplement.

1. Growth rates due to N2O5.
It is helpful to clarify that the four experiments with insufficient growth via ammonium nitrate formation are associated with O3 photolysis while others are not. A little surprised that this was not clearly pointed out in the original manuscript.

Thank you for this comment; this was an oversight from our side during the preparation of the original manuscript.

(1) Thanks for providing Table S1. As stated in Line 134, you “treated the calculated N2O5 concentration as additional HNO3 for these 4 experiments”. Based on Table S1, N2O5 concentrations were much smaller than those of HNO3. How could these smaller concentrations of N2O5 significantly increase the growth rates? For example for Run 1, the addition of 150 ppt N2O5 to 260 ppt HNO3 increased GR from 200 nm/hr (Fig. S1) to ~ 700 nm/hr (Fig. 1) in your model calculation. How could 58% more condensing gas increase
the growth rates by 350%? Did I miss something here?

There are two explanations for this observation. (1) The relevant quantity for growth is not the total ammonia or nitric acid concentrations, but the supersaturated fraction available for condensation. Therefore, a 60% change in the concentration of nitric acid implies a much larger change in the super saturated fraction. (2) The second factor is the time during which supersaturation is sustained. In the case of the four experiments with significant N2O5 present, there was a continuous production of both N2O5 and HNO3 due to the presence of NO2, O3 and light.

(2) In responding to my original comment 1.4, the authors said: “For our modelled values, we considered a kinetic condensation of the N2O5 as an upper limit and a complete loss of N2O5 onto the walls, which would yield a lowest estimate of N2O5 concentrations, which may explain why the growth rates are still under predicted.” This is important but the answer is just a speculation. The authors should do a quick sensitivity study using smaller values of N2O5 uptake and wall loss rates to see how much this uncertainty may explain the under-prediction. With the model, it should be pretty easy to carry out such a sensitivity study.

Thank you for this comment; we have added the concentrations of N2O5 in the absence of wall losses. They are approximately a factor of 10 larger than concentrations calculated assuming wall losses, which translates into more than a factor 10 higher and an overestimation of growth rates.
Since each experiment starts with low-negligible CS, adjusting the uptake does not change the N2O5 concentration much, but would of course affect the growth rate. Here, we have no data to support or refute a kinetic limit uptake of N2O5 onto nanoparticles and we state in the discussion section that the quantification of the role of N2O5 should be a subject of future research. Our aim was to provide a possible explanation for the higher than expected growth rates during the four experiments when we had O3 photolysis. The focus of the paper is to answer whether the formation of NH4NO3, which is most of the time in equilibrium with the gas phase, can grow nanoparticles under atmospheric conditions. With this we provide the conditions where and when this process is relevant.
Line 136: concentrations where wall loss was not considered are also shown in Table S1.
Line 222: N2O5 concentration calculated without wall loss are also significantly higher and are shown in Table S1.

2. In replying to comment #2 of Referee 1 and comment #5 of Referee 5 (with regard to “puzzling that similar size distributions as in Figure 3 have not been frequently observed in ambient measurements”, the authors focused on the difficulty in observing these in the atmosphere because of instrument limitation. However, another possible reason is that the experimental conditions may not reflect what occurs in the real atmosphere and the findings of the lab measurements can not be applied to the real atmosphere. This possible reason should also be discussed.

The conditions in the chamber are most often not representative of the real atmosphere, with the main difference being that in the real atmosphere, ammonium nitrate is often in equilibrium with its gas-phase components resulting in no particle growth, while during our experiments, supersaturated conditions prevailed, resulting in rapid particle growth. The chamber results have allowed us to verify the chemistry and the particle microphysics behind the formation of ammonium nitrate in the nanoparticle range (results presented in Figure 1). The model is set-up to reflect the real atmosphere, with particles mainly in equilibrium with the gas phase. In the model, we simulate atmospheric scenarios that move these particles out of equilibrium, including a decrease in temperature and point emissions of ammonia. Our main conclusion is that the occurrence of rapid growth by ammonium nitrate formation in the atmosphere requires high heterogeneities either due to large temperature gradients or to significant point sources of ammonia. We have adjusted the text in the discussion to reflect this.
Line 336: Experiments in the CLOUD chamber involved supersaturated conditions of ammonia and nitric acid, resulting in rapid particle growth, allowing the chemistry and particle microphysics of ammonium nitrate growth in the nanoparticle range to be verified. In the real atmosphere ammonium nitrate will mostly be found in equilibrium with its gas-phase counterparts, resulting in no particle growth. The model was therefore designed to simulate ambient atmospheric scenarios that would result in a short perturbation of equilibrium. We observe that in these conditions of high heterogeneities, although short lived, rapid growth due to ammonium nitrate formation can occur.

3. Lines 142-143. Please provide the production values of HNO3, NH3, and H2SO4 calculated in your study.
Since the production rates depend on the concentration used, the equations and example values are shown in the ESI.
Line 144: These calculations and examples of values used are shown in the ESI.
ESI: We calculated suitable production values of HNO3, NH3, and H2SO4 based on the required steady-state concentration in the absence of condensation. The vertical dry deposition fluxes of gas A, FluxA, were calculated using equation S1 with literature deposition velocities, vdep, for HNO3, NH3, and H2SO4, shown in Table S2. The height of the boundary layer, hBL, used was 1km. The production values of HNO3 and NH3 were calculated using equation S2 from the deposition flux, and initial concentration of A, [Ai], considering the deposition flux as the highest loss rate for both gases. The production rate of H2SO4 was calculated considering new particle formation as the highest loss rate of gaseous H2SO4. The initial concentrations and production rates for the experiments in sections “Effect of inhomogeneity of NH3 concentrations in cities” and “Effect of temperature change during vertical transport” are shown in Table S2.

4. Equ. 2. What is the value of x assumed in your study? How did you get dNx/dt? Based on measurements?
We added some more detail on this point in lines 156-161:
For the “air parcel rising” experiments, the nucleation rates used for these models were input rates of 1.7nm particles; they are temperature dependent and typical of 1 × 107 molecules cm-3 H2SO4 with 3 ppbv NH3 and no amines as seen in previous CLOUD experiments and ambient studies, presented in Xiao et al 2021.11,24,25 For the “concentration inhomogeneities” experiments, the nucleation rates were not constrained and kinetic nucleation rates were used, based on clusters of H2SO4 as in Marten et al.

(2022).135. Line 157. “the effect of varying ionization level during transport”. What does “during transport” mean?
Thank you for pointing out this, we added the word “vertical” to clarify.
Line 156:
However, we have not considered the effect of varying ionization level during vertical transport.

6. “All dN/dlogDp in y-axis of supplement figures need a unit.” “Thank you, cm-3 was added”.
I didn’t find the added unit in Fig. S2-S6, S8.
Thank you for pointing out this oversight. cm-3 was added to the figures in the SI.

7. I didn’t find the list of references cited in the supplement.
Thank you for pointing this out, it has been added.

07-Dec-2023

Dear Dr El Haddad:

Manuscript ID: EA-ART-01-2023-000001.R2
TITLE: Assessing the importance of nitric acid and ammonia for particle growth in the polluted boundary layer

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