Nonstationary Summer Ozone-Temperature Climate Penalty over East Asia: Decadal Trends and Regional Variability

The sensitivity of surface ozone (O₃) to temperature, often termed the climate penalty factor (CPF), quantifies the increase in O₃ per unit temperature rise. While previous studies have characterized CPF under present-day conditions, its temporal evolution over multi-decadal timescales remains poorly understood. This study investigates long-term changes in summer (JJA) CPF across East Asia from 1980 to 2024 using GEOS-Chem simulations driven by MERRA-2 reanalysis. Anthropogenic emissions are held constant to isolate meteorology-driven changes, whereas biogenic emissions are allowed to respond to meteorological conditions. Using a regression-based decomposition approach, we separate the contributions of direct temperature effects from indirect effects mediated by co-varying meteorological conditions. Preliminary results reveal that CPF has increased in most East Asian regions over the past four decades, with distinct spatial patterns. Northern regions exhibit CPF changes primarily driven by direct temperature effects, while southern coastal regions show dominant contributions from indirect effects. These findings suggest that the mechanisms underlying O₃-temperature sensitivity differ regionally and have evolved over time. Our results demonstrate the nonstationary nature of CPF and its regional heterogeneity, with implications for projecting future air quality and designing region-specific control strategies in a warming climate.