Meteorology and topographic influences on nocturnal ozone increase during the summertime over Shaoguan, China

Abstract

Ozone (O3) is a typical photochemical product, generally with a maximum value in the late afternoon and a minimum value at night. This study aimed to investigate the variation characteristics of nocturnal O3 in a subtropical urban city (Shaoguan, Guangdong Province, China). The measurement campaigns were conducted from 23 July to July 28, 2020. O3 lidar, aerosol lidar, and wind profile lidar were utilised to detect the vertical distributions of the O3 concentrations, aerosol extinction coefficient, and wind velocity. The topographic features, pollutant data (PM10, PM2.5, O3, SO2, NO2, and CO), and meteorological parameters (temperature, humidity, wind speed, and wind direction) were also analysed. The results showed that the concentration of nocturnal O3 over Shaoguan peaked at midnight. The horizontal wind was dominated by southwest and south winds, and the vertical wind was dominated by downdrafts during the fixed-point observations in Shaoguan. Horizontal transport might have a high impact on the increase in the nocturnal O3 because of high pollutant emissions in upwind cities. This was further justified by the 24-h back trajectories of the air mass generated by the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. Vertical transport of O3 in the lower or middle troposphere might be induced by the subtropical high and nocturnal mountain-valley breezes. Thus, the vertical mixing of O3 affects the O3 concentration at night.