Uneven Future PM2.5 and O3 responses to Emission Reductions under Different Warming Levels in East Asia.

Future emission reductions can improve East Asian air quality, but their benefits may not scale with local emission controls because of climate warming and transboundary transport. Using WRF-Chem with a pseudo-global-warming approach, we quantify mid-21st-century PM2.5 and O3 changes over the North China Plain (NCP), South Korea, and Japan under intermediate and net-zero emission pathways and moderate- and high-warming levels. January-April PM2.5 decreases by 18-32% under the intermediate pathway and 36-57% under the net-zero pathway, indicating that emission changes dominate the response. Over the NCP, PM2.5 reductions scale with prescribed emission reductions during low-PM2.5 episodes but exceed them during high-PM2.5 episodes, when secondary inorganic aerosols contribute more strongly. In South Korea and Japan, PM2.5 reductions are smaller than local primary-aerosol emission reductions. Under uniform regional reductions, however, responses more closely match prescribed changes, highlighting source-receptor coupling. High warming enhances ventilation and boundary-layer mixing over the NCP, further reducing PM2.5, but circulation changes over South Korea and Japan partly offset these reductions, leaving open the possibility of enhanced transboundary influence. O3 increases in winter but decreases in summer because of weakened NO titration and suppressed photochemical production, respectively. Strong mitigation lowers summertime daytime-peak O3 even in NOX-rich environments, whereas nighttime O3 remains sensitive to titration. Under identical emissions, warming increases O3 by a few ppb, but this climate penalty does not scale monotonically with warming level, partly due to changes in cloud and radiation. These results emphasize the need for coordinated regional emission reductions amid climate-driven meteorological change.