Effects of the southwest vortex on the vertical structure of ozone in the Sichuan Basin in China

To investigate the mechanism by which the southwest vortex (SWV) affected the vertical distribution of O₃ in the lower troposphere over the Sichuan Basin (SCB), the SWV cases from April to September of each year from 2015 to 2018, the concentrations of O₃ in the same periods, meteorological factors (such as temperature, humidity, and wind), and the convective stability index were analysed. The results revealed 78 dry SWVs, 74 weak precipitation SWVs, and 37 heavy precipitation SWVs over the SCB. The dry SWV increased the mean O₃ column concentration (MOCC) over the SCB (+1.75 %), and the precipitation SWVs reduced the MOCC, with the weak precipitation SWVs (−7.44 %) having a greater reduction than the heavy precipitation SWVs (−5.42 %). Furthermore, the O₃ concentrations at various altitude layers were impacted to varied degrees, with the strongest scavenging effect occurring at the 850 hPa layer. The scavenging effect was especially significant in areas with high O₃ concentrations, such as the Chengdu Plain. The transit of precipitation SWVs resulted in the cooling and humidification of all layers, which could have suppressed O₃ photochemical reactions and reduced the O₃ concentration, whereas the dry layer during the transit of dry SWVs increased the O₃ concentration at the bottom of the basin. In terms of the different dispersion situations, Chongqing was dominated by horizontal winds during the transit of SWVs. However, the wind speed in the central and western SCBs was greater and the vertical component of the wind was stronger, which was favourable for O₃ dispersion in Chengdu.