Daytime Isoprene Nitrates Under Changing NOx and O3
Abstract. Organonitrates are important species in the atmosphere due to their impacts on NOx, HOx, 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 NOx 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 NOx. For example, organonitrates predominantly formed from the OH-initiated oxidation of isoprene favour formation under lower ozone and moderate NOx concentrations, whereas organonitrates formed via day-time NO3 oxidation show the highest formation under high O3 concentrations with little dependence on NOx 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 NOx and O3 concentrations, including a region of NOx-O3 space where total organonitrate concentration is relatively insensitive to changes in NOx 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.
Alfred W. Mayhew et al.
Status: open (until 04 Apr 2023)
- RC1: 'Comment on egusphere-2023-226', Anonymous Referee #1, 20 Mar 2023 reply
Alfred W. Mayhew et al.
Alfred W. Mayhew et al.
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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 NOx 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.
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 NOx 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 NOx 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.
Figure 1: Typo in the Isoprene carbonyl nitrate should be ICN instead of IHN underneath it.