the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Sep 2026
The Dominance and Future Variations of Background Ozone in China under Different Climate and Emission Scenarios
Abstract. Background ozone, defined as ozone present in the absence of domestic anthropogenic emissions, has become increasingly dominant of the total surface ozone in China as the domestic emissions decline. Using the GEOS-Chem model driven by meteorological fields from GCAP2.0, this study investigates the changes in China background ozone (CBO) from the present-day (PD) to the middle of the 21st century (MC) under the SSP1-1.9 and SSP5-8.5 scenarios. CBO consistently dominates the total surface ozone in China (CTO), accounting for about 90 % of the annual mean ozone in both PD and MC conditions. Under SSP1-1.9, CTO declines by 4.7 ppbv from PD to MC, with CBO contributing 3.7 ppbv (79 %) of this reduction. The decrease in CBO is largely due to a sharp decline in transboundary transport of foreign pollution ozone (FPO, −4.7 ppbv) resulting from global emission reductions, partially offset by a climate-driven increase in natural background ozone (NBO, +1.0 ppbv). Under SSP5-8.5, the annual mean CTO increases by 7.3 ppbv, of which 6.5 ppbv is attributed to the rise in CBO. The increase is primarily driven by the enhancement of NBO, associated with intensified photochemical production under warmer conditions. We also show that in a low-emission future with controlled warming, the reduction in CBO strengthens climate mitigation and creates a positive climate-ozone feedback, along with health co-benefits. These contrasting trajectories reveal that background ozone is a dynamic component shaped by global cooperation and climate change, a reality that must be considered when setting air quality targets.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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Status: open (until 19 Oct 2026)
- RC1: 'Comment on egusphere-2026-1839', Anonymous Referee #2, 28 Sep 2026 reply
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RC2: 'Comment on egusphere-2026-1839', Anonymous Referee #1, 28 Sep 2026
reply
This study examines how background ozone over China evolves from the present day to the mid 21st century under SSP1-1.9 and SSP5-8.5 scenarios. The authors use GEOS-Chem driven by GCAP2.0 meteorology to quantify China total surface ozone and then decompose it into natural background ozone, foreign source, and domestic source components. They find that background ozone dominates annual mean surface ozone in China in both periods, on the order of 90 percent. Under SSP1-1.9, total ozone decreases and most of that change comes from reduced background ozone due to weaker foreign influence, partly offset by a climate driven rise in natural background. Under SSP5-8.5, total ozone increases, mainly because warming enhances natural background through stronger photochemical production.
I find the research question timely and policy-relevant. The manuscript is generally well organized, and the attribution framework is useful. I would support publication after minor revisions to address following points.
Firstly, as found in prior literature and stated by the authors themselves, GEOS-Chem tends to overestimate surface ozone in China. Even if that does not strongly affect the relative attribution across components, it does matter for the absolute ppbv changes reported and for how readers interpret policy relevance. Have the authors considered bias correcting the simulated ozone first, or at least discussing how sensitive the reported absolute changes are to the model bias?
Secondly, the excessive use of acronyms throughout the manuscript slows down reading. I often had to stop to recall what each term meant, especially with CTO, CBO, CPO, NBO, FPO, PNB, plus PD and MC. Since the decomposition is conceptually straightforward, it may read much more cleanly if the authors rely more on plain language in the main text. As I understand it, the authors are basically decomposing China total ozone into three components: natural background, domestic contribution, and foreign contribution. It would be helpful to reduce the acronyms used unless necessary.
Thirdly, the manuscript concludes that the foreign anthropogenic influence weakens under SSP1-1.9. It would help to be more specific on the mechanism. Is this driven mainly by lower ozone and precursor concentrations in the transboundary inflow, or by weakening of the flow strength itself, or both? A simple diagnostic using boundary inflow concentrations versus transport flux would make this claim more concrete.
Lastly, for the increase in natural background ozone under warming, the paper mentions several contributing processes (lines 448-450), including biogenic VOC emissions, soil NOx, lightning NOx, and stratosphere to troposphere exchange. The qualitative discussion is useful, but it would be stronger if the authors could quantify the relative contributions, even approximately, using available GEOS-Chem diagnostics or sensitivity simulations. At minimum, it would help to identify which one or two processes dominate the natural background ozone increase in SSP5 8.5, and whether the ranking varies by season or region.
Citation: https://doi.org/10.5194/egusphere-2026-1839-RC2
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General comment
This manuscript entitled “The Dominance and Future Variations of Background Ozone in China under Different Climate and Emission Scenarios” investigated the background ozone concentration over China under several emission scenarios. This topic can contribute to understanding the background ozone concentration at the present status and in the future; thus, an important topic could be considered for possible publication in the journal Atmospheric Chemistry and Physics. However, the manuscript also needs to be revised to improve the presentation quality. In particular, the description of the modeling details should be added for a clear explanation of the modeling design. Please address the following comments.
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