Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).
Impact of urban-industrial air pollution on chemically reactive greenhouse gases is highly sensitive to organic nitrate formation
Calum Patrick Wilsonand Michael John Prather
Abstract. We compute net tropospheric ozone production (P-O3) and methane oxidation (L-CH4) caused by 45 days of South Korean emissions using a chemistry-transport model (CTM) and a hybrid modelling system (HMS). The CTM has global 1° x 1° resolution; the HMS has detailed chemical mechanisms and a refined regional grid (0.1° x 0.1°), but simplified, staged transport. The emissions cause +22.1 Gmol P-O3 and +2.0 Gmol L-CH4 in the CTM vs. +31.2 Gmol P-O3 and +4.3 Gmol L-CH4 in the HMS. Differences are attributed to lower volatile organic compound (VOC) reactivity in the CTM, thus decreased organic nitrate export to the free troposphere. Time-integrated O3 mass perturbations (𝛿O3) are similar in both models and constitute a globally-averaged +0.03 DU summertime O3 enhancement. Due to contrasting L-CH4, the net steady-state effective radiative forcing sustained by the emissions is +0.49 mW m-2 in the CTM and -0.25 mW m-2 in the HMS.
Received: 30 Jul 2026 – Discussion started: 21 Aug 2026
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Review of “Impact of urban-industrial air pollution on chemically reactive greenhouse gases is highly sensitive to organic nitrate formation”, by Calum P. Wilson and Michael J. Prather
General comments
This is a very nice and concisely presented study that illustrates the important role played by higher hydrocarbon chemistry, via organic nitrate production, on the relationship between emissions and impacts on ozone and methane. The study highlights that the relatively basic chemistry in typical tropospheric chemistry models (exemplified by the UCI CTM here) produces significantly different impacts on O3 and CH4 compared to a model with much more extensive hydrocarbon chemistry.
My main comments about the study are to do with slightly better documenting the differences in chemistry (are processes like halogen and heterogeneous chemistry also very different between the models; or do you think they are relatively insignificant?). I also wondered if it might be relatively easy to produce a version of the HMS with exactly the same chemistry as the CTM, in order to better separate the effects of chemistry and modelling system differences.
If a bit more discussion on these points and a few specific clarifications as listed below are added, this will make a really excellent addition to the literature.
Specific comments
L9 I am unsure of the “+” prefixes are necessary.
L29 “nearly complete set of chemical reactions” – can you elaborate a bit on how complete? (see also later comment)
Table 1 Is there a typo in the MVKMACR numbers? It looks like 0.0+0.0=0.5?
L90-95 The focus of the discussion here is on gas-phase organic chemistry, which is clearly very comprehensive in the HMS. But what about other, potentially important areas of chemistry, like halogens and heterogeneous chemistry? Are these also different in the two modelling approaches?
L144 It may be worth adding a sentence to introduce the inert tracer in Figure 3, saying that this illustrates dispersion.
Figure 3 It might be useful to add coastlines? The text (line 143) refers to the global atmosphere, but Figure 3 only shows the Northern Hemisphere. Maybe briefly explain that over the timescales considered, essentially all of the chemistry occurs in the NH.
L154 So I guess a “steady-state +0.03 DU global ozone enhancement” is equivalent to the 327 Gmol days in Table 2. It may be worth a sentence clarifying this equivalence.
L155 It may be worth clarifying that a longer O3 perturbation lifetime means that the O3 chemistry (both production and loss) is slower.
L159 Add ‘the’ before PL.
L182 I don’t think intermediate is a verb. Suggest “…which is intermediate between…”
L182-3 Clarify when you say “high” and “low” you mean in the model compared to observed.
Could you downgrade the chemistry in the HMS to be exactly the same as the CTM chemistry? Then you’d be able to explicitly separate chemistry and model system differences?
Table 2: Could you clarify the merged PL/DP boxes for P-O3 and OPE?
L203 BL -> BL-RL. The caption should clarify the HMS 3-day and 1-day results.
L228 “Surprising” is perhaps going a bit far – but it certainly is interesting that the chemical effects outweigh the resolution effects.
The Conclusion section could perhaps say slightly more about the implications for net climate forcings – the ERFs derived from the CTM and HMS are very different, and suggest that the net impact on climate (via O3 and especially CH4) of pollution may be poorly characterized by the current generation of global chemistry models. That feels like quite an important conclusion.
Methane loss (L-CH4) and tropospheric ozone (O3) burden in response to a 10% KORUS-AQ emission perturbation in the UCI chemistry-transport modelCalum Patrick Wilson https://doi.org/10.5061/dryad.djh9w0wgm
Model code and software
Modelling system for computing the tropospheric O3 and CH4 perturbations from South Korean Emissions (KORUS-AQ period)Calum Patrick Wilson https://doi.org/10.5061/dryad.f4qrfj78x
Calum Patrick Wilsonand Michael John Prather
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We model the change in greenhouse gas (ozone and methane) levels in the atmosphere caused by South Korean air pollution. We compare results obtained using a global chemistry-transport model with simplified chemistry, and a specialised hybrid model with detailed chemistry but simplified transport. Air pollution in the chemistry-transport model produces less ozone, and destroys much less methane, than the hybrid model. This leads to net global warming in the former model and cooling in the latter.
We model the change in greenhouse gas (ozone and methane) levels in the atmosphere caused by...
Review of “Impact of urban-industrial air pollution on chemically reactive greenhouse gases is highly sensitive to organic nitrate formation”, by Calum P. Wilson and Michael J. Prather
General comments
This is a very nice and concisely presented study that illustrates the important role played by higher hydrocarbon chemistry, via organic nitrate production, on the relationship between emissions and impacts on ozone and methane. The study highlights that the relatively basic chemistry in typical tropospheric chemistry models (exemplified by the UCI CTM here) produces significantly different impacts on O3 and CH4 compared to a model with much more extensive hydrocarbon chemistry.
My main comments about the study are to do with slightly better documenting the differences in chemistry (are processes like halogen and heterogeneous chemistry also very different between the models; or do you think they are relatively insignificant?). I also wondered if it might be relatively easy to produce a version of the HMS with exactly the same chemistry as the CTM, in order to better separate the effects of chemistry and modelling system differences.
If a bit more discussion on these points and a few specific clarifications as listed below are added, this will make a really excellent addition to the literature.
Specific comments
L9 I am unsure of the “+” prefixes are necessary.
L29 “nearly complete set of chemical reactions” – can you elaborate a bit on how complete? (see also later comment)
Table 1 Is there a typo in the MVKMACR numbers? It looks like 0.0+0.0=0.5?
L90-95 The focus of the discussion here is on gas-phase organic chemistry, which is clearly very comprehensive in the HMS. But what about other, potentially important areas of chemistry, like halogens and heterogeneous chemistry? Are these also different in the two modelling approaches?
L144 It may be worth adding a sentence to introduce the inert tracer in Figure 3, saying that this illustrates dispersion.
Figure 3 It might be useful to add coastlines? The text (line 143) refers to the global atmosphere, but Figure 3 only shows the Northern Hemisphere. Maybe briefly explain that over the timescales considered, essentially all of the chemistry occurs in the NH.
L154 So I guess a “steady-state +0.03 DU global ozone enhancement” is equivalent to the 327 Gmol days in Table 2. It may be worth a sentence clarifying this equivalence.
L155 It may be worth clarifying that a longer O3 perturbation lifetime means that the O3 chemistry (both production and loss) is slower.
L159 Add ‘the’ before PL.
L182 I don’t think intermediate is a verb. Suggest “…which is intermediate between…”
L182-3 Clarify when you say “high” and “low” you mean in the model compared to observed.
Could you downgrade the chemistry in the HMS to be exactly the same as the CTM chemistry? Then you’d be able to explicitly separate chemistry and model system differences?
Table 2: Could you clarify the merged PL/DP boxes for P-O3 and OPE?
L203 BL -> BL-RL. The caption should clarify the HMS 3-day and 1-day results.
L228 “Surprising” is perhaps going a bit far – but it certainly is interesting that the chemical effects outweigh the resolution effects.
The Conclusion section could perhaps say slightly more about the implications for net climate forcings – the ERFs derived from the CTM and HMS are very different, and suggest that the net impact on climate (via O3 and especially CH4) of pollution may be poorly characterized by the current generation of global chemistry models. That feels like quite an important conclusion.