Seasonal dependence of factors of year-to-year variations in South China AOD and Hong Kong air quality

This study investigates the year-to-year variations in aerosol optical depth (AOD) over South China and plausible connections with Hong Kong air quality changes. The inter-annual variability of AOD over South China is found to display a pronounced seasonal dependence, with the largest standard deviation in October and March, which is about two times of that in January. Hong Kong Air Pollution Index (API) also shows the largest variability in October and March. Analysis shows that the precipitation and circulation pattern associated with inter-annual variations of aerosol over South China is very different depending on the season. In October, precipitation decrease and anomalous lower-level wind convergence contribute to an increase in aerosol. In July, anomalous lower-level wind convergence contributes to an increase in both aerosol and precipitation. In March, the aerosol change has no obvious statistical relationship with local precipitation change, and the transport by anomalous winds appears to be a main factor for aerosol variations. The link between year-to-year changes in Hong Kong air quality and South China aerosol displays a strong seasonality. While the air quality change in Hong Kong is associated with regional aerosol, precipitation, and wind changes over South China in October and July, the air quality change in March appears to be related to local factors. The link of Hong Kong air quality to regional precipitation changes over South China tends to be opposite in October and July. This study points out the necessity of distinguishing the individual seasons when addressing the year-to-year changes in aerosol and air quality. The seasonal dependence of the aerosol-precipitation relationship may include the effects of the mean state, the meteorological condition, and the role of atmospheric circulation.