Opposing Effects of PM _2.5 , O ₃ , and CO ₂ Interactions on the East Asian Summer Monsoon Climate before and after the Clean Air Action Plan

Significant changes in PM_2.5, O₃, and CO₂ concentrations occurred across East Asia before and after the implementation of the 2013 Clean Air Action Plan. These shifts led to notable adjustments in the strength of their mutual interactions, resulting in substantial alterations in their combined impact on the East Asian Summer Monsoon (EASM). Using the RegCM-Chem-YIBs regional climate-chemistry-ecosystem model, we analyzed the effects of PM_2.5, O₃, and CO₂ interactions on the EASM climate from 2008 to 2018. Our findings indicate that these interactions reduced surface downward shortwave radiation flux by 20-30 W/m² across East Asia from 2008 to 2012, lowering near-surface temperatures by 0.5-1 K and weakening the EASM. Conversely, surface downward shortwave and longwave radiation fluxes from 2014 to 2018, increased by 2-4 W/m² and 2-8 W/m², respectively, in northern and southeastern China, raising near-surface temperatures by 0.5-1 K, and intensifying the EASM. This study highlights the importance of thorough consideration for the interactions among PM_2.5, O₃, and CO₂ in regional climate projections, particularly over East Asia, where atmospheric pollutant concentrations fluctuate significantly, to precisely elucidate their combined impact on the summer monsoon system.