Daytime Isoprene Nitrates Under Changing NO<sub>x</sub> and O<sub>3</sub>

Mayhew, Alfred W.; Edwards, Peter M.; Hamilton, Jacqueline F.

doi:https://doi.org/10.5194/egusphere-2023-226

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https://doi.org/10.5194/egusphere-2023-226

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21 Feb 2023

| 21 Feb 2023

Daytime Isoprene Nitrates Under Changing NO_x and O₃

Alfred W. Mayhew, Peter M. Edwards, and Jacqueline F. Hamilton

Abstract. Organonitrates are important species in the atmosphere due to their impacts on NO_x, HO_x, and O₃ budgets, and their potential to contribute to secondary organic aerosol (SOA) mass. This work presents a steady-state modelling approach to assess the impacts of changes in NO_x and O₃ concentrations on the organonitrates produced from isoprene oxidation. The diverse formation pathways to isoprene organonitrates dictate the responses of different groups of organonitrates to changes in O₃ and NO_x. For example, organonitrates predominantly formed from the OH-initiated oxidation of isoprene favour formation under lower ozone and moderate NO_x concentrations, whereas organonitrates formed via day-time NO₃ oxidation show the highest formation under high O₃ concentrations with little dependence on NO_x concentrations. Investigating the response of total organonitrates reveals complex and non-linear behaviour with implications that could inform expectations of changes to organonitrate concentrations as efforts are made to reduce NO_x and O₃ concentrations, including a region of NO_x-O₃ space where total organonitrate concentration is relatively insensitive to changes in NO_x and O₃. These conclusions are further contextualised by estimating the volatility of the isoprene organonitrates revealing the potential for high concentrations of low volatility species under high ozone conditions.

Received: 10 Feb 2023 – Discussion started: 21 Feb 2023

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Journal article(s) based on this preprint

31 Jul 2023

Daytime isoprene nitrates under changing NO_x and O₃

Alfred W. Mayhew, Peter M. Edwards, and Jaqueline F. Hamilton

Atmos. Chem. Phys., 23, 8473–8485, https://doi.org/10.5194/acp-23-8473-2023,https://doi.org/10.5194/acp-23-8473-2023, 2023

Short summary

Alfred W. Mayhew et al.

Interactive discussion

Status: closed

RC1: 'Comment on egusphere-2023-226', Anonymous Referee #1, 20 Mar 2023

This is a well-written and interesting theoretical modeling study showing how organic nitrates formed from isoprene oxidation during the daytime in Beijing, China are impacted by changing O₃ and NO_x concentrations. The mechanism used in this work is state of the art and understanding how organic nitrates from isoprene form in the atmosphere especially under the highly polluted environment of Beijing, China is important. The results are quite interesting from a theoretical perspective, but sometimes hard to interpret from an atmospherically relevant perspective. Generally, due to the simplified nature of the modeling here, there are limitations to the conclusions that can be drawn and these limitations and the uncertainty they add to the conclusions should be more clearly specified in the text prior to publication as described more below.
Detailed suggestions:
Line 67: Can you include the exact Vereecken et al. reference here with the date since there are 2 references? Is the mechanism exactly the same as the one referenced in Vereecken? If not, can you provide details on any updates and/or a version number if available?
Line 73: Can you include a reference for the Beijing 2017 campaign?
Line 81: Can you include a reference for why this dilution lifetime of 12 hours was selected?
Line 82, Why was this temperature and relative humidity selected? Are these the averages from the Beijing 2017 campaign at 16:00 local time too?
Section 3.1: Demonstrating that the ranges of various species (kOH, NO, NO2, NO3, OH, HO2, IHN, IPN, ICN, etc.) in the NOx/O3 space modeled matches with the observations is a good first step in building confidence in the model. However, higher confidence in your modeling approach would be to compare the exact observations for each NOx/O3 value at 16:00 during the Beijing 2017 campaign to that from the model. For example, you could use the same color bar but add stars or squares to represent the observations. Or just add plots in the supplement that are side by side using the same color bar, but one for the model and one for the observations. This would be useful to understand how well your model is replicating the contours and variation specifically rather than just confirming it produces results within the range of the observations. It would also show which space on these O₃ and NO_x plots Beijing is typically in. The ranges of O3 and NOx may be exactly as you state, but the full O3 to NOx plots you are representing are not likely to be atmospherically relevant. For example, there is likely more of a curved line of actual space on these plots that reflects real conditions in Beijing, which would be useful for the reader to understand too. All of this would give the reader more confidence that the conclusions from your model on how reductions of NO_x or O₃ will impact SOA and organic nitrates from isoprene oxidation are accurate. Doing something similar for the model results on for the Amazon would also be useful too.
In Figure 7, 8 & beyond, why plot these organic nitrates in molecules cm^-3 instead of mixing ratio like you did for NO, NO2, and NO3 in Figures 5 and 6? It seems easier for the reader to interpret if you put these in mixing ratios as is typically done even if you need to switch from ppb to ppt for the lower yielding organic nitrates. This seems important so that viewers understand your results because IHN has such higher concentrations than the others and this is hidden and easily missed by readers by a very small 1eX written above the color bar for each plot.
Section 3.2 & 3.3: From the authors previous work, Mayhew et al., 2022, the diurnal cycle of these isoprene organic nitrates particularly those from NO3 oxidation have complicated diurnal cycles. Can you comment on whether only showing the steady state at 16:00 impacts the interpretation of your results for atmospherically relevant conditions? How important are IPN, IHN, ICN, the nitrated epoxides (INHE, INPE, INCE) that formed the previous night and then linger into the day and potentially form other oxidation products for the total organic nitrates at 16:00? These organic nitrates formed during the nighttime and their oxidation products are not considered in your current modeling and past work has suggested nighttime formation of organic nitrates to be an important contribution of total organic nitrates (e.g., Kenagy et al., 2020, https://doi.org/10.1029/2020GL087860). How does this assumption to not include the nighttime formation of these organic nitrates impact your conclusions?
Section 3.5 and Figure 11: See comment above, how would not including the contribution of organic nitrates formed during the night impact your results here? Potentially, the text here should be clearer to emphasize that this is not the expected fraction of organic nitrates at 16:00 in the real atmosphere, but instead only the fraction of organic nitrates formed directly from isoprene at 16:00 and does not include organic nitrates formed during the night or organic nitrates formed during the night and further oxidized during the day.
Line 232: You are also not considering the uptake of tertiary organic nitrates and hydrolysis that occurs in the atmosphere and this should also be mentioned here (Vasquez et al., 2020)
Conclusions: See comment above for Section 3.2. Your conclusions here only represent organic nitrates formed during the day from isoprene directly and not organic nitrates formed during nighttime or organic nitrates formed during the nighttime and further oxidized during the daytime. This is important to emphasize as it is different from what will occur in the actual atmosphere. Some description of how this impacts your overall conclusions would be useful so that readers know how to interpret your results.
Lines 289 to 293: If you think about the typical ozone isopleth as a function of NOx and VOC. If VOCs are constant as you are doing in this study (methane + isoprene) then each NOx level is going to produce a certain ozone concentration, so this would produce a curved line across each of these graphs where the conditions are atmospherically relevant. When you talk about some organic nitrates would benefit from reductions in NOx and others with reductions of ozone here and throughout the text, can you further explain what you mean by this under atmospherically relevant conditions? Generally, if you keep NOx the same, and you want to reduce ozone, you would have to reduce VOCs, but this is not what you are simulating here because you keep VOCs constant in all of these graphs (i.e., isoprene + methane). Similarly, if you reduce NOx, you will impact ozone, so you are never going to go straight down the y-axis of one of these graphs in the real atmosphere. Can you explain more how the reader is supposed to interpret these plots from an atmospherically relevant perspective? How is ozone supposed to be reduced (i.e., moving left along the x-axis) when keeping VOCs and NOx constant in the real atmosphere especially as you are also holding all the other levers constant too (e.g., temperature, photolysis)?
For your conclusions on SOA, under atmospherically relevant conditions, the picture may be more complicated than you are implying here. As Pye et al., 2019 (https://doi.org/10.1073/pnas.1810774116) nicely describe as NOx emissions decline, so do oxidants, which ultimately leads to less VOC oxidation and thereby SOA production. How do the results of this study impact your conclusions here?
You are correct in your statements that in Beijing concurrent reductions in VOCs and NOx are likely needed to reduce O3 since reductions in NOx only would likely lead to increases of ozone. On this note, can you provide further context on which regions of your plot in Figure 12 would represent the NOx transition line where below this level of NOx you can expect O3 to start reducing when NOx emissions are reduced? From your current NOx to O3 plots, it is difficult to know where this regime change is expected to occur and this seems important if you are using this plot to state SOA may increase as NOx emissions decline.
Minor items
Figure 1: Typo in the Isoprene carbonyl nitrate should be ICN instead of IHN underneath it.

Citation: https://doi.org/10.5194/egusphere-2023-226-RC1
RC2: 'Comment on egusphere-2023-226', Anonymous Referee #2, 14 Apr 2023

This paper presents some calculations on the concentrations of isoprene oxidation products under a range of NOx and O3 conditions. The paper is detailed and a useful contribution to the literature.
I recommend two additions to give the reader who is interested in the big picture context a deeper understanding of what matters about these results.
First, there are quite a few studies of organonitrates more generally that place isoprene in context including several comprehensive reviews. Orienting the reader to this literature would help them to appreciate the ways in which this study answers an important and unanswered question (to the extent that it does).
Second, the potential for observational tests of the ideas described in this paper including both specific molecular observations of the isoprene daughters described and more general assessments with methods that observe total organo nitrates in he gas or particle phase would be helpful.

Citation: https://doi.org/10.5194/egusphere-2023-226-RC2
AC1: 'Response to Reviewers: egusphere-2023-226', Alfred Mayhew, 09 May 2023

Please find the attached document outlining the changes made to the manuscript titled "Daytime Isoprene Nitrates Under Changing NO_x and O₃" in response to comments made by the anonymous referees.

Citation: https://doi.org/10.5194/egusphere-2023-226-AC1

Interactive discussion

Status: closed

RC1: 'Comment on egusphere-2023-226', Anonymous Referee #1, 20 Mar 2023

This is a well-written and interesting theoretical modeling study showing how organic nitrates formed from isoprene oxidation during the daytime in Beijing, China are impacted by changing O₃ and NO_x concentrations. The mechanism used in this work is state of the art and understanding how organic nitrates from isoprene form in the atmosphere especially under the highly polluted environment of Beijing, China is important. The results are quite interesting from a theoretical perspective, but sometimes hard to interpret from an atmospherically relevant perspective. Generally, due to the simplified nature of the modeling here, there are limitations to the conclusions that can be drawn and these limitations and the uncertainty they add to the conclusions should be more clearly specified in the text prior to publication as described more below.
Detailed suggestions:
Line 67: Can you include the exact Vereecken et al. reference here with the date since there are 2 references? Is the mechanism exactly the same as the one referenced in Vereecken? If not, can you provide details on any updates and/or a version number if available?
Line 73: Can you include a reference for the Beijing 2017 campaign?
Line 81: Can you include a reference for why this dilution lifetime of 12 hours was selected?
Line 82, Why was this temperature and relative humidity selected? Are these the averages from the Beijing 2017 campaign at 16:00 local time too?
Section 3.1: Demonstrating that the ranges of various species (kOH, NO, NO2, NO3, OH, HO2, IHN, IPN, ICN, etc.) in the NOx/O3 space modeled matches with the observations is a good first step in building confidence in the model. However, higher confidence in your modeling approach would be to compare the exact observations for each NOx/O3 value at 16:00 during the Beijing 2017 campaign to that from the model. For example, you could use the same color bar but add stars or squares to represent the observations. Or just add plots in the supplement that are side by side using the same color bar, but one for the model and one for the observations. This would be useful to understand how well your model is replicating the contours and variation specifically rather than just confirming it produces results within the range of the observations. It would also show which space on these O₃ and NO_x plots Beijing is typically in. The ranges of O3 and NOx may be exactly as you state, but the full O3 to NOx plots you are representing are not likely to be atmospherically relevant. For example, there is likely more of a curved line of actual space on these plots that reflects real conditions in Beijing, which would be useful for the reader to understand too. All of this would give the reader more confidence that the conclusions from your model on how reductions of NO_x or O₃ will impact SOA and organic nitrates from isoprene oxidation are accurate. Doing something similar for the model results on for the Amazon would also be useful too.
In Figure 7, 8 & beyond, why plot these organic nitrates in molecules cm^-3 instead of mixing ratio like you did for NO, NO2, and NO3 in Figures 5 and 6? It seems easier for the reader to interpret if you put these in mixing ratios as is typically done even if you need to switch from ppb to ppt for the lower yielding organic nitrates. This seems important so that viewers understand your results because IHN has such higher concentrations than the others and this is hidden and easily missed by readers by a very small 1eX written above the color bar for each plot.
Section 3.2 & 3.3: From the authors previous work, Mayhew et al., 2022, the diurnal cycle of these isoprene organic nitrates particularly those from NO3 oxidation have complicated diurnal cycles. Can you comment on whether only showing the steady state at 16:00 impacts the interpretation of your results for atmospherically relevant conditions? How important are IPN, IHN, ICN, the nitrated epoxides (INHE, INPE, INCE) that formed the previous night and then linger into the day and potentially form other oxidation products for the total organic nitrates at 16:00? These organic nitrates formed during the nighttime and their oxidation products are not considered in your current modeling and past work has suggested nighttime formation of organic nitrates to be an important contribution of total organic nitrates (e.g., Kenagy et al., 2020, https://doi.org/10.1029/2020GL087860). How does this assumption to not include the nighttime formation of these organic nitrates impact your conclusions?
Section 3.5 and Figure 11: See comment above, how would not including the contribution of organic nitrates formed during the night impact your results here? Potentially, the text here should be clearer to emphasize that this is not the expected fraction of organic nitrates at 16:00 in the real atmosphere, but instead only the fraction of organic nitrates formed directly from isoprene at 16:00 and does not include organic nitrates formed during the night or organic nitrates formed during the night and further oxidized during the day.
Line 232: You are also not considering the uptake of tertiary organic nitrates and hydrolysis that occurs in the atmosphere and this should also be mentioned here (Vasquez et al., 2020)
Conclusions: See comment above for Section 3.2. Your conclusions here only represent organic nitrates formed during the day from isoprene directly and not organic nitrates formed during nighttime or organic nitrates formed during the nighttime and further oxidized during the daytime. This is important to emphasize as it is different from what will occur in the actual atmosphere. Some description of how this impacts your overall conclusions would be useful so that readers know how to interpret your results.
Lines 289 to 293: If you think about the typical ozone isopleth as a function of NOx and VOC. If VOCs are constant as you are doing in this study (methane + isoprene) then each NOx level is going to produce a certain ozone concentration, so this would produce a curved line across each of these graphs where the conditions are atmospherically relevant. When you talk about some organic nitrates would benefit from reductions in NOx and others with reductions of ozone here and throughout the text, can you further explain what you mean by this under atmospherically relevant conditions? Generally, if you keep NOx the same, and you want to reduce ozone, you would have to reduce VOCs, but this is not what you are simulating here because you keep VOCs constant in all of these graphs (i.e., isoprene + methane). Similarly, if you reduce NOx, you will impact ozone, so you are never going to go straight down the y-axis of one of these graphs in the real atmosphere. Can you explain more how the reader is supposed to interpret these plots from an atmospherically relevant perspective? How is ozone supposed to be reduced (i.e., moving left along the x-axis) when keeping VOCs and NOx constant in the real atmosphere especially as you are also holding all the other levers constant too (e.g., temperature, photolysis)?
For your conclusions on SOA, under atmospherically relevant conditions, the picture may be more complicated than you are implying here. As Pye et al., 2019 (https://doi.org/10.1073/pnas.1810774116) nicely describe as NOx emissions decline, so do oxidants, which ultimately leads to less VOC oxidation and thereby SOA production. How do the results of this study impact your conclusions here?
You are correct in your statements that in Beijing concurrent reductions in VOCs and NOx are likely needed to reduce O3 since reductions in NOx only would likely lead to increases of ozone. On this note, can you provide further context on which regions of your plot in Figure 12 would represent the NOx transition line where below this level of NOx you can expect O3 to start reducing when NOx emissions are reduced? From your current NOx to O3 plots, it is difficult to know where this regime change is expected to occur and this seems important if you are using this plot to state SOA may increase as NOx emissions decline.
Minor items
Figure 1: Typo in the Isoprene carbonyl nitrate should be ICN instead of IHN underneath it.

Citation: https://doi.org/10.5194/egusphere-2023-226-RC1
RC2: 'Comment on egusphere-2023-226', Anonymous Referee #2, 14 Apr 2023

This paper presents some calculations on the concentrations of isoprene oxidation products under a range of NOx and O3 conditions. The paper is detailed and a useful contribution to the literature.
I recommend two additions to give the reader who is interested in the big picture context a deeper understanding of what matters about these results.
First, there are quite a few studies of organonitrates more generally that place isoprene in context including several comprehensive reviews. Orienting the reader to this literature would help them to appreciate the ways in which this study answers an important and unanswered question (to the extent that it does).
Second, the potential for observational tests of the ideas described in this paper including both specific molecular observations of the isoprene daughters described and more general assessments with methods that observe total organo nitrates in he gas or particle phase would be helpful.

Citation: https://doi.org/10.5194/egusphere-2023-226-RC2
AC1: 'Response to Reviewers: egusphere-2023-226', Alfred Mayhew, 09 May 2023

Please find the attached document outlining the changes made to the manuscript titled "Daytime Isoprene Nitrates Under Changing NO_x and O₃" in response to comments made by the anonymous referees.

Citation: https://doi.org/10.5194/egusphere-2023-226-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Alfred Mayhew on behalf of the Authors (09 May 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (14 May 2023) by Steven Brown

RR by Anonymous Referee #1 (01 Jun 2023)

Suggestions for revision or reasons for rejection

Thanks for considering my suggestions. In particular, thanks for updating your text to more clearly describe the limitations to your conclusions to the real atmosphere from using a simplified modeling approach. This is now much clearer to the reader in all cases and there is less risk of the reader being misled on how to interpret these results. I suggest publication after consideration of the minor suggestions listed below:

Section 3.1: I disagree that comparing the model to observations directly does not add confidence to your model. I think you should include these observations in your paper, but ultimately you can make that call as the authors of the paper. My major concern that the simplified nature of the modeling here limits the conclusions that can be drawn to the real atmosphere does not mean you cannot compare to observations. You can still compare to observations while mentioning that there is expected to be some disagreement due to the simplified nature of the model. However, if there is general agreement, which for most of the species there is except for OH and HO2, then this lends confidence in the model and it shows where there might be limitations for example with HOx.

Section 3.6: The hydrolysis of tertiary organic nitrates are not only going to impact your particle-phase concentrations, but the dominant isomer of the isoprene hydroxy nitrates is a tertiary organic nitrate and it’s loss in the atmosphere in a polluted region like Beijing may be dominantly due to uptake to aerosols and so your organic nitrate gas-phase distribution is also going to be impacted by not including this process.

Conclusions: I agree with the editor that since you cite a number of references in the introduction based on work from the CF3O- CIMS that have led to better understanding of isoprene organic nitrates that this would also be a good tool for validating this work and should be mentioned here too.

Hide

ED: Publish subject to minor revisions (review by editor) (19 Jun 2023) by Steven Brown

AR by Alfred Mayhew on behalf of the Authors (22 Jun 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (26 Jun 2023) by Steven Brown

AR by Alfred Mayhew on behalf of the Authors (26 Jun 2023)

Journal article(s) based on this preprint

31 Jul 2023

Daytime isoprene nitrates under changing NO_x and O₃

Alfred W. Mayhew, Peter M. Edwards, and Jaqueline F. Hamilton

Atmos. Chem. Phys., 23, 8473–8485, https://doi.org/10.5194/acp-23-8473-2023,https://doi.org/10.5194/acp-23-8473-2023, 2023

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Alfred W. Mayhew et al.

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Isoprene nitrates are chemical species commonly found in the atmosphere that are important for their impacts on air quality and climate. This paper investigates modelled changes to day time isoprene nitrate concentrations resulting from changes in NO_x and O₃. The results highlight the complex, non-linear chemistry of this group of species under typical conditions for megacities such as Beijing, with many species showing increased concentrations when NO_xis decreased and/or ozone is increased.


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Daytime Isoprene Nitrates Under Changing NOx and O3

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Daytime Isoprene Nitrates Under Changing NO_x and O₃