the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Large Simulated Future Changes in the Nitrate Radical Under the CMIP6 SSP Scenarios: Implications for Oxidation Chemistry
Scott Archer-Nicholls
Rachel Allen
Nathan Luke Abraham
Paul Thomas Griffiths
Alexander Thomas Archibald
Abstract. The nitrate radical (NO3) plays an important role in the chemistry of the lower troposphere, acting as the principle oxidant during the night. Previous model simulations suggest that the levels of NO3 have increased dramatically since the pre-industrial. Here, we show projections of the evolution of the NO3 radical from 1850–2100 using the UKESM1 Earth System model under the CMIP6 SSP scenarios. Our model results highlight diverse trajectories for NO3, with some scenarios and regions undergoing rapid growth of NO3 to unprecedented levels over the course of the 21st Century, and others seeing sharp declines. The local increases in NO3 are driven not only by local changes in emissions of nitrogen oxides but have an important climate component, with NO3 being favoured in warmer future climates. The changes in NO3 lead to changes in the oxidation of important secondary organic aerosol precursors, with potential impacts to particulate matter pollution regionally and globally. This work highlights the potential for substantial future growth in NO3 and the need to better understand the formation of SOA from NO3 to accurately predict future air quality and climate implications.
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Scott Archer-Nicholls et al.
Status: closed (peer review stopped)
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RC1: 'Comment on acp-2022-706', Anonymous Referee #1, 25 Nov 2022
General Comments
Archer-Nicholls et al. report model results on historical trends and future projections in nitrate radical (NO3) abundance in the lower atmosphere at global scale. There is a focus on regional hot spots, especially South Asia, where both the historical trends and future projections show large differences. There is also a focus on the relevance of these changes for oxidation of biogenic volatile organic compounds (BVOC), which in turn are relevant to the efficiency with which these species produce secondary organic aerosol (SOA). The future projections are based on a set of emissions scenarios from the recent literature. Figure 2 shows the core result of the analysis of global maps of O3, NO2 and NO3differences between the present day, preindustrial, and a series of future projections. Presuming that the O3 and NO2differences are correct, the NO3 differences can be largely, although not fully, understood in terms of the changes in NOx emissions and their effects on O3 distributions.
The paper is of interest to ACP and publishable with minor revisions, as outlined below. The most important comment, listed first below, is that the scope of the paper is somewhat limited compared to what it could be. The paper stops at mixing ratios and oxidation rates, without really predicting more about the associated changes in fates of BVOC oxidation products.
Major Comments
While the paper is of value in assessing trends in NO3 mixing ratio and BVOC oxidation rates, it stops short of assessing other important quantities such as organic nitrogen and SOA mass. For example, mass yields tend to be oxidant specific, and that effect is not captured here. Previous papers that have examined the mass yield dependences for SOA or organic nitrogen should be cited and compared to this model where possible. Relevant references are listed below.
- von Kuhlmann et al., Sensitivities in global scale modeling of isoprene. Atmos. Chem. Phys., 2004. 4: p. 1-17.
- Horowitz, L.W., et al., Observational constraints on the chemistry of isoprene nitrates over the eastern United States. J. Geophys. Res., 2007. 112(D12): p. D12S08.
- Hoyle, C.R., et al., Anthropogenic influence on SOA and the resulting radiative forcing. Atmos. Chem. Phys., 2009. 9(8): p. 2715-2728.
- Brown, S.S., et al, Nocturnal isoprene oxidation over the Northeast United States in summer and its impact on reactive nitrogen partitioning and secondary organic aerosol. Atmos. Chem. Phys., 2009. 9: p. 3027-3042.
- Pye, H.O.T., et al., Global modeling of organic aerosol: the importance of reactive nitrogen (NOx and NO3). Atmos. Chem. Phys., 2010. 10(22): p. 11261-11276.
- Hoyle, C.R., et al, A review of the anthropogenic influence on biogenic secondary organic aerosol. Atmos. Chem. Phys., 2011. 11: p. 321-343.
7.Schwantes, R.H., et al. , Comprehensive isoprene and terpene chemistry improves simulated surface ozone in the southeastern U.S. Atmos. Chem. Phys. Discuss., 2019. 2019: p. 1-52.
Specific Comments
Line 9: The nitrate radical is not always, or perhaps even in an integrated or average way, the principal oxidant during the night. This is more typically ozone. Figures within the paper show the importance of ozone compared to nitrate radical. Suggest rephrasing as either “principal oxidant together with ozone”, or as “principal oxidant in areas with substantial NOx pollution.”
Line 24: Omit the word “rapidly”. Reaction 1 is quite slow.
Line 28: See comment above from the abstract – need to qualify NO3 as most important nighttime oxidant since it always acts together with O3 and in locations without NOx emissions is an unimportant oxidant. A small but important caveat. See for example Edwards et al. Nature Geosci, 2017. 10(7): p. 490-495.
Line 75: Worth noting here also that the rate constant for reaction (1) has among the strongest temperature dependence of any major atmospheric bimolecular reaction, so the source reaction is also sensitive to temperature increases. This effect is certainly more modest than the N2O5 equilibrium but worth noting.
Line 100: Agree with the caveats stated here that the simplification of a large, fixed uptake coefficient for N2O5 will affect the model predictions of various processes, including BVOC oxidation. It would be useful to see a sensitivity test with a smaller uptake coefficient (e.g., 0.01 rather than 0.1 since the former is likely the more appropriate order of magnitude for the troposphere) for predictions regarding major process chemistry using the specific model in this paper rather than the reference to Jones et al. The authors may wish to comment on the feasibility of inclusion of such a test, or at least qualitatively predict the outcome, if they elect not to do so. See McDuffie et al. Journal of Geophysical Research: Atmospheres, 2018. 123(8): p. 4345-4372. for a discussion of the complexity in the N2O5 uptake coefficient and its range of variability.
Line 134: Doubling of monoterpenes to account for isoprene is not clear. It was stated above that isoprene is treated separately as its own species?
Line 139, 141: The expressions for kNO3 appear incomplete or else the units are other than expected for a bimolecular rate constant. The prefactor should be a much smaller number if these units are in cm3 s-1.
Line 159-161: There appear to be other features in the comparison of figure 1 for model measurement disagreement. Most obvious is boundary layer height, and presumably vertical mixing throughout the model. The NO2 gradient near the surface is very strong in the model but not as strong in the observations. This is reflected in the ozone simulation as well. The large NO3at higher altitude relative to the model is also certainly a consequence of the NO2 at higher altitude, again something that could be attributed to vertical mixing that is too small (vertical gradients that are too large) in the model.
Line 167-174 and Table 1: A useful comparison of model to observations for NO3. The authors state that these are all from surface observations. Related to the preceding comment, the vertical distribution is likely the most difficult aspect for a coarse resolution model, and even observations with small differences in elevation above surface might differ considerably in how accurately they are simulated. The authors may wish to add this caveat to the discussion.
Figure 3: Useful here would be to also plot absolute temperature across the top axis to provide the reader an easy reference to the temperature changes that are actually inferred by the models. Similarly, rather than a natural logarithm, a base 10 log on the y axis would make the translation of the equilibrium ratios easier to understand at glance rather than having to invert an exponential function.
Line 260-262: The choice of presentation using rates is somewhat misleading since it is an average rate over a diel cycle and a month. An integral (i.e., a total mass within a given time period) would be a more appropriate quantity in figures 4 and 5. The figures themselves would presumably not change, but the mass would place the figures in better context for emissions inventories of BVOC, which are typically in mass units rather than rates.
Line 297: The caveat about diel boundary layer variability is almost certainly not limited to East Asia, as implied.
Technical corrections
Line 99: Its rather than it’s
Line 122: -pinene is missing either an alpha or a beta, likely.
Figure 1c: NO3 is given in ppbv when pptv is almost certainly what was intended.
Line 295: No comma after the word include
Citation: https://doi.org/10.5194/acp-2022-706-RC1 -
RC2: 'Comment on acp-2022-706', Anonymous Referee #2, 11 Jan 2023
The study discusses the evolution of NO3 radicals from 1850-2100 based on model simulations by UKEMS1 Earth System Model under different climatic scenarios. Special attention is paid to South Asia where NO3 is expected to increase to unprecedented levels. In general, the study is well established and well written. A few comments are listed as follows for the authors to consider.
- Abstract: It is expected that some quantitative results be mentioned in the abstract, instead of using vague descriptions like “dramatic increase”, “rapid growth” and “sharp decline”.
- Fig. 1, (1) The legend and lines in Fig.1 overlap which needs to be modified later. (2)The upper-limit of the x-axis may be larger, in order to allow the maxima concentration (the upper limit of the error bar) be included in the figure (The same suggestion for Fig.S4).
- Fig. 3: The x-axis could be changed into fractions (i.e. 1/298).
- Fig. 5: The y-axis could be changed into log scale in order to make the zonal distribution clearer. (Same suggestion for Fig. S5)
- Discussion: The discussion part is too short. It is suggested that discussion could be incorporated into results.
- An uncertainty analysis of the simulations (or at least an uncertainty analysis of the kinetics parameters) is required in the article or supplement. What is the most important cause of model uncertainty? And how will it influence the model results?
- The model results demonstrate that NO3 levels may double by the end of 21st Its further implications could be discussed in depth in the conclusion part. Does it mean more BVOCs oxidized by NO3 and more SOA or particulate nitrate production in the future and so what?
Citation: https://doi.org/10.5194/acp-2022-706-RC2
Status: closed (peer review stopped)
-
RC1: 'Comment on acp-2022-706', Anonymous Referee #1, 25 Nov 2022
General Comments
Archer-Nicholls et al. report model results on historical trends and future projections in nitrate radical (NO3) abundance in the lower atmosphere at global scale. There is a focus on regional hot spots, especially South Asia, where both the historical trends and future projections show large differences. There is also a focus on the relevance of these changes for oxidation of biogenic volatile organic compounds (BVOC), which in turn are relevant to the efficiency with which these species produce secondary organic aerosol (SOA). The future projections are based on a set of emissions scenarios from the recent literature. Figure 2 shows the core result of the analysis of global maps of O3, NO2 and NO3differences between the present day, preindustrial, and a series of future projections. Presuming that the O3 and NO2differences are correct, the NO3 differences can be largely, although not fully, understood in terms of the changes in NOx emissions and their effects on O3 distributions.
The paper is of interest to ACP and publishable with minor revisions, as outlined below. The most important comment, listed first below, is that the scope of the paper is somewhat limited compared to what it could be. The paper stops at mixing ratios and oxidation rates, without really predicting more about the associated changes in fates of BVOC oxidation products.
Major Comments
While the paper is of value in assessing trends in NO3 mixing ratio and BVOC oxidation rates, it stops short of assessing other important quantities such as organic nitrogen and SOA mass. For example, mass yields tend to be oxidant specific, and that effect is not captured here. Previous papers that have examined the mass yield dependences for SOA or organic nitrogen should be cited and compared to this model where possible. Relevant references are listed below.
- von Kuhlmann et al., Sensitivities in global scale modeling of isoprene. Atmos. Chem. Phys., 2004. 4: p. 1-17.
- Horowitz, L.W., et al., Observational constraints on the chemistry of isoprene nitrates over the eastern United States. J. Geophys. Res., 2007. 112(D12): p. D12S08.
- Hoyle, C.R., et al., Anthropogenic influence on SOA and the resulting radiative forcing. Atmos. Chem. Phys., 2009. 9(8): p. 2715-2728.
- Brown, S.S., et al, Nocturnal isoprene oxidation over the Northeast United States in summer and its impact on reactive nitrogen partitioning and secondary organic aerosol. Atmos. Chem. Phys., 2009. 9: p. 3027-3042.
- Pye, H.O.T., et al., Global modeling of organic aerosol: the importance of reactive nitrogen (NOx and NO3). Atmos. Chem. Phys., 2010. 10(22): p. 11261-11276.
- Hoyle, C.R., et al, A review of the anthropogenic influence on biogenic secondary organic aerosol. Atmos. Chem. Phys., 2011. 11: p. 321-343.
7.Schwantes, R.H., et al. , Comprehensive isoprene and terpene chemistry improves simulated surface ozone in the southeastern U.S. Atmos. Chem. Phys. Discuss., 2019. 2019: p. 1-52.
Specific Comments
Line 9: The nitrate radical is not always, or perhaps even in an integrated or average way, the principal oxidant during the night. This is more typically ozone. Figures within the paper show the importance of ozone compared to nitrate radical. Suggest rephrasing as either “principal oxidant together with ozone”, or as “principal oxidant in areas with substantial NOx pollution.”
Line 24: Omit the word “rapidly”. Reaction 1 is quite slow.
Line 28: See comment above from the abstract – need to qualify NO3 as most important nighttime oxidant since it always acts together with O3 and in locations without NOx emissions is an unimportant oxidant. A small but important caveat. See for example Edwards et al. Nature Geosci, 2017. 10(7): p. 490-495.
Line 75: Worth noting here also that the rate constant for reaction (1) has among the strongest temperature dependence of any major atmospheric bimolecular reaction, so the source reaction is also sensitive to temperature increases. This effect is certainly more modest than the N2O5 equilibrium but worth noting.
Line 100: Agree with the caveats stated here that the simplification of a large, fixed uptake coefficient for N2O5 will affect the model predictions of various processes, including BVOC oxidation. It would be useful to see a sensitivity test with a smaller uptake coefficient (e.g., 0.01 rather than 0.1 since the former is likely the more appropriate order of magnitude for the troposphere) for predictions regarding major process chemistry using the specific model in this paper rather than the reference to Jones et al. The authors may wish to comment on the feasibility of inclusion of such a test, or at least qualitatively predict the outcome, if they elect not to do so. See McDuffie et al. Journal of Geophysical Research: Atmospheres, 2018. 123(8): p. 4345-4372. for a discussion of the complexity in the N2O5 uptake coefficient and its range of variability.
Line 134: Doubling of monoterpenes to account for isoprene is not clear. It was stated above that isoprene is treated separately as its own species?
Line 139, 141: The expressions for kNO3 appear incomplete or else the units are other than expected for a bimolecular rate constant. The prefactor should be a much smaller number if these units are in cm3 s-1.
Line 159-161: There appear to be other features in the comparison of figure 1 for model measurement disagreement. Most obvious is boundary layer height, and presumably vertical mixing throughout the model. The NO2 gradient near the surface is very strong in the model but not as strong in the observations. This is reflected in the ozone simulation as well. The large NO3at higher altitude relative to the model is also certainly a consequence of the NO2 at higher altitude, again something that could be attributed to vertical mixing that is too small (vertical gradients that are too large) in the model.
Line 167-174 and Table 1: A useful comparison of model to observations for NO3. The authors state that these are all from surface observations. Related to the preceding comment, the vertical distribution is likely the most difficult aspect for a coarse resolution model, and even observations with small differences in elevation above surface might differ considerably in how accurately they are simulated. The authors may wish to add this caveat to the discussion.
Figure 3: Useful here would be to also plot absolute temperature across the top axis to provide the reader an easy reference to the temperature changes that are actually inferred by the models. Similarly, rather than a natural logarithm, a base 10 log on the y axis would make the translation of the equilibrium ratios easier to understand at glance rather than having to invert an exponential function.
Line 260-262: The choice of presentation using rates is somewhat misleading since it is an average rate over a diel cycle and a month. An integral (i.e., a total mass within a given time period) would be a more appropriate quantity in figures 4 and 5. The figures themselves would presumably not change, but the mass would place the figures in better context for emissions inventories of BVOC, which are typically in mass units rather than rates.
Line 297: The caveat about diel boundary layer variability is almost certainly not limited to East Asia, as implied.
Technical corrections
Line 99: Its rather than it’s
Line 122: -pinene is missing either an alpha or a beta, likely.
Figure 1c: NO3 is given in ppbv when pptv is almost certainly what was intended.
Line 295: No comma after the word include
Citation: https://doi.org/10.5194/acp-2022-706-RC1 -
RC2: 'Comment on acp-2022-706', Anonymous Referee #2, 11 Jan 2023
The study discusses the evolution of NO3 radicals from 1850-2100 based on model simulations by UKEMS1 Earth System Model under different climatic scenarios. Special attention is paid to South Asia where NO3 is expected to increase to unprecedented levels. In general, the study is well established and well written. A few comments are listed as follows for the authors to consider.
- Abstract: It is expected that some quantitative results be mentioned in the abstract, instead of using vague descriptions like “dramatic increase”, “rapid growth” and “sharp decline”.
- Fig. 1, (1) The legend and lines in Fig.1 overlap which needs to be modified later. (2)The upper-limit of the x-axis may be larger, in order to allow the maxima concentration (the upper limit of the error bar) be included in the figure (The same suggestion for Fig.S4).
- Fig. 3: The x-axis could be changed into fractions (i.e. 1/298).
- Fig. 5: The y-axis could be changed into log scale in order to make the zonal distribution clearer. (Same suggestion for Fig. S5)
- Discussion: The discussion part is too short. It is suggested that discussion could be incorporated into results.
- An uncertainty analysis of the simulations (or at least an uncertainty analysis of the kinetics parameters) is required in the article or supplement. What is the most important cause of model uncertainty? And how will it influence the model results?
- The model results demonstrate that NO3 levels may double by the end of 21st Its further implications could be discussed in depth in the conclusion part. Does it mean more BVOCs oxidized by NO3 and more SOA or particulate nitrate production in the future and so what?
Citation: https://doi.org/10.5194/acp-2022-706-RC2
Scott Archer-Nicholls et al.
Scott Archer-Nicholls et al.
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