Quantitative effects of atmospheric diffusion on surface aerosol extinction in the Pearl River Delta region

Aerosol pollution is closely related to meteorological conditions. In order to accurately evaluate the effectiveness of emission reduction policies, it is very important to separate meteorological effects from emissions in long-term PM changes. In this study, we used surface extinction coefficient (SEC) of aerosol particles as an indicator to estimate the long-term trend of PM pollutant conditions, and proposed a parameter surface ventilation index (SVI) to describe the atmospheric diffusion. A novel method was introduced to quantify the effect of atmospheric diffusion and emissions on SEC from an inter-annual scale, and its applicability and limitations was discussed. The results showed that annual mean SEC exhibited an increasing trend with a rate of 15.6 Mm^-1∙year^-1 from 2000 to 2007 and a decreasing trend with a rate of 14.4 Mm^-1∙year^-1 from 2008 to 2018. The annual frequency distribution of SVI obeyed a lognormal distribution. The SVI value mainly concentrated between 300 and 3600 m^2.s^-1, SEC was negatively correlated with SVI in this range. In most cases, SEC variations were mainly dominated by emissions, and SEC variations at clean sites were more influenced by atmospheric diffusion than that at polluted sites. Emissions remained high during 2003-2007, then gradually decreased during 2008-2014 and finally accelerated decrease during 2015-2018, indicating that control policies have effectively mitigated PM pollution since 2008. In generally, this method can provide valuable information for policy makers to evaluate the emission control measures for air pollution mitigation and prevention.