Heatwave-driven amplification of ozone formation offsets emission control benefits in VOC-limited megacities.

Amid an ongoing warming climate, long-term increases in surface ozone (O3) concentrations have been reported across East Asia, including South Korea, despite sustained efforts to reduce precursor emissions. In this study, we use an extreme heat episode as a physically plausible analog of near-future warming conditions to examine how climate-driven temperature increases may offset the air quality benefits of emission reductions in two major metropolitan areas in South Korea: Seoul and Busan. Specifically, we selected the unprecedented heatwave during the summer of 2018, which recorded the highest seasonal mean temperature over the past decade and represents meteorological conditions analogous to those expected under continued climate warming. Heatwave-specific biogenic volatile organic compound (BVOC) emissions were contrasted with those during non-heatwave conditions using surface observations, satellite-based formaldehyde (HCHO) retrievals from TROPOMI, and WRF-Chem simulations. The results show that temperature was positively correlated with both HCHO and O3 in Seoul (R = 0.59 and 0.69, respectively) and Busan (R = 0.60 and 0.57, respectively), consistent with enhanced temperature-driven BVOC emissions and subsequent oxidation reflected by HCHO. These findings indicate that rising temperatures intensify photochemical reactions and alter O3 production sensitivity in major megacities through the amplification of BVOC emissions, particularly under VOC-limited ozone formation regimes. Our results suggest that continued climate warming, represented here by historically observed extreme heat conditions, may impose a climate change penalty on urban ozone, implying that substantially greater emission reductions will be required to attain equivalent air quality benefits in major East Asian cities under future warming.