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.