Springtime warming and biomass burning causing ozone episodes in South and Southwest China

A detailed analysis of springtime ozone outbreaks in South/Southwest China is presented in this paper, providing an insight into a regional photochemical and climate problem. A major ozone episode in 2013 was the first ever in April and the worst in Hong Kong up to 2018, measuring a peak ozone concentration of 293 mu gm(-3). This multi-day, ozone pollution was evidenced by similar conditions in the Pearl River Delta (PRD), and an even more severe episode in Kunming (Yunnan) in Southwest China. Concurrently, widespread air temperature composite anomalies of up to about +4 degrees K were observed in the region, particularly during 6Z (14:00 local time). The global annual geopotential height anomaly implied increased atmospheric stability and inhibited dispersion-consistent with global warming impacts for the region. Backward trajectories, satellite observations, and transport model simulations characterized the biomass burning sources. Results indicated that activities in Indochina, South and Southwest China, and Africa were the main sources in South China while those in Burma dominated Southwest China. The close succession of outbreaks from west to east (Kunming, Guangzhou, and Hong Kong) suggests an eastward transport of ozone and precursors.