Summertime ozone sensitivity to temperature in China: observational evidence and mechanistic attribution in urban and rural areas in the Yangtze River Delta region

Abstract. As climate change increases frequency and intensity of hot extremes, understanding how elevated temperatures influence surface ozone (O3) across different chemical regimes is critical. We combine observations from summer 2022 in the Yangtze River Delta (YRD) with targeted United Kingdom Chemistry and Aerosols (UKCA) Box experiments to quantify and attribute the temperature sensitivity of ground-level O3 in a VOC-limited urban megacity (Shanghai) and a predominantly NOx-limited rural region (Lishui). Observed daily-maximum O3 increases with temperature at both sites, with a stronger response in Shanghai. Two-dimensional T-RH distributions show that O3 rises with temperature and decreases with relative humidity (RH), implying that part of the temperature response is mediated by drying. Factorial box-model experiments separate the roles of temperature-driven shifts in chemical partitioning, diurnal thermal structure, temperature-dependent isoprene emissions, and humidity. In Shanghai, the increase in O3 between 30 °C and 40 °C is dominated by BVOC-driven chemistry, with temperature-amplified isoprene emissions explaining most of the observed response. In Lishui, O3 sensitivity is governed primarily by radical-NOx chemistry and thermal PAN-NOx cycling, with a much smaller role for BVOCs. Fixed RH scenarios substantially reduce the apparent contribution of “thermal chemistry”, highlighting humidity-dependent Ox loss and HOx-NOx cycling as key modifiers of the climate penalty. These regime-specific O3-temperature mechanisms, including BVOC-driven in the urban core, radical-NOx-PAN-driven in the rural area, and significant humidity modulation, provide a process basis for heat-resilient emission controls and demonstrate the value of observation-constrained box models as a bridge to regional chemistry-climate simulations.