Biogenic VOCs from Regional Forests Amplify Ozone Pollution in Shanghai Under Extreme Heat Stress Conditions

Compound events of extreme heat and ozone (O₃) pollution pose severe risks to public health in megacities. Using ground-based observations and chemical transport model simulations, we identified distinct synoptic patterns associated with hot-polluted versus cool-clean conditions in Shanghai during the summers of 2022-2025. Hot-polluted days were characterized by higher heat index (HI ≥ 41 °C), elevated O₃ (MDA8 ≥ 160 µg m^- 3), and southwesterly winds at 925 hPa. While anthropogenic NOx levels were comparable, biogenic volatile organic compound (BVOC) emissions from forests south of Shanghai increased significantly under high-temperature conditions. These BVOCs, transported by prevailing southwesterly winds, exacerbated O₃ formation downwind in Shanghai. Our findings highlight that regional biogenic emissions, amplified by extreme heat, are a critical driver of O₃ exceedances, complicating control strategies in the Yangtze River Delta.