the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Aug 2026
Impact of urban-industrial air pollution on chemically reactive greenhouse gases is highly sensitive to organic nitrate formation
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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-4590', Anonymous Referee #1, 18 Sep 2026
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RC2: 'Comment on egusphere-2026-4590', Anonymous Referee #2, 19 Oct 2026
This manuscript presents a well-conceived exploration of the biases in representing the impacts of emissions from highly polluted regions with the simple chemistry schemes used in global models. These are attributed to underestimation of organic nitrate formation associated with lower VOC reactivity, and the study demonstrates the consequences of this for assessment of the climate impacts of anthropogenic emissions. Some of the results are as expected, reflecting the omission of fast chemistry in a CTM, while others are new and provide fresh insight, for example highlighting the existence of chemical compensation points where coarse model resolution ceases to overestimate local ozone production. The study makes a useful contribution to the literature in clearly quantifying and explaining these effects using a new and original approach.
General Comments
I would have liked to have seen stronger evidence for the role of organic nitrates as the key driver of the differences. This could have been demonstrated with sensitivity studies in the CTM, e.g., through addition of an accelerated organic nitrate formation pathway or through emission of organic nitrate directly, following Holmes et al., 2014 but adjusted for polluted rather than marine environments. This latter approach was adopted for wildfires by Lin et al., 2024, https://doi.org/10.1029/2024GL109369. This would allow a more critical test of the effects of organic nitrates.
It would also have been good to provide a simple exploration of dilution in the polluted BL in HMS (alluded to in the conclusions) to demonstrate that this effect doesn't substantially bias the HMS results and that the overall findings still hold. While I recognize that this may be beyond the scope of the present study, an estimate of its likely impact would be valuable.
If the VOC in the CTM are less reactive than in the HMS, more will be transported out of the region to support additional (slower) ozone production in the global atmosphere. What role does this play in the O3 and OH budget differences?
Specific commentsIt would help the reader to note in the introduction why the focus of the study is on South Korea. This only becomes evident in section 2 when KORUS is first mentioned.
Section 2.4: the differences in ozone lifetime are interesting, but the purpose of this section isn't clear. How is the reader meant to interpret the differences between the CTM and HMS? A sentence or two of introduction and explanation is needed here.
Line 154-155: "The CTM PO3 is only 71% of the HMS because the lifetime is longer..." The lifetime is longer because loss is less, and this is because production is less, so there is some circularity here. Please rephrase the sentence.
Line 168: Why do the termination reactions differ between models? A sentence or two of explanation would be useful here.
Section 3.1 focuses on differences in OPE, but doesn't explain why the differences in the OH response between models is so large. Inclusion of more reactive VOC in the CTM may remove more OH unless there is stronger OH recycling, and it is not clear that organic nitrate chemistry provides this. A statement of explanation is needed here.
Table 1: the emissions for methyl vinyl ketone and methacrolein are both zero, which doesn't match the total.
Fig 1: for consistency with other panels, modelled NOx should be plotted over DC8 NOx.
Citation: https://doi.org/10.5194/egusphere-2026-4590-RC2
Data sets
Methane loss (L-CH4) and tropospheric ozone (O3) burden in response to a 10% KORUS-AQ emission perturbation in the UCI chemistry-transport model Calum 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
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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.