Enhanced surface ozone during the heat wave of 2013 in Yangtze River Delta region, China

In order to explore the relationship between heat wave and elevated ground-level ozone in Yangtze River Delta (YRD) region, we have investigated observational air temperature and ozone during summer of 2013, also applied with regional chemistry-climate model (RegCM-CHEM4). Observations indicate that YRD experienced severe heat waves with maximum temperature up to 41.1 °C, 6.1 °C higher than the definition of heat wave in China, and along with maximum ozone reaching 160.5 ppb, exceeding the national air quality standard (secondary level) as 74.7 ppb. Moreover, ozone was found to increase at a rate of 4-5 ppb K− 1 within the temperature range of 28-38 °C, but decreased by a rate of −1.3-−1.7 ppb K− 1 under extremely high temperature (Fig. 1). According to observations, a typical heat wave case (HW: 24/7-31/7) and a non-heat wave case (NHW: 5/6-12/6) were selected to reveal the mechanism between heavy ozone and heat waves. Numerical simulation discovers that, among chemical reaction, dry deposition, vertical turbulence and horizontal advection, chemical reaction plays the most important role in ozone formation when high temperature can result in 12 ppb ozone enhancement compared to NHW days. Chemical reaction can be influenced by several factors. High temperature environment usually controlled by anti-cyclone, and combined with sink airflow, leading to a more stagnant condition. During HW, less water vapor in YRD from south contributed to less cloud cover, which favored a strong solar radiation environment. As photochemical reaction strongly depends on the availability of solar radiation, ozone significantly increased during heat waves in YRD. High temperature also slightly promote the effect of dry deposition velocity, vertical turbulence and horizontal advection, which beneficial to ozone remove, but the magnitude is much smaller than chemical effect. Our research suggests that the chemical reaction will potentially lead to substantial elevated ozone in a warmer climate, which should be taken into account in future ozone related issues.192