Estimating aerosol optical extinction across eastern China in winter during 2014–2019 using the random forest approach

Abstract

Air quality has been continuously improved in recent years across eastern China, yet severe haze pollution episodes still frequently occur in winter. Optical extinction of aerosols can provide a proxy for assessing haze pollution levels, especially for submicron particles (i.e., PM1, aerodynamic diameter less than 1.0 μm) because they are close to the wavelength of visible solar radiation and are very efficient in reducing atmospheric visibility. In past decades, many studies had indicated that optical extinction showed an opposite trend to fine aerosol particles (i.e., PM2.5, aerodynamic diameter less than 2.5 μm). However, little work has examined the changing trend of optical extinction since the implementation of the toughest-ever clean air policy in 2013. Has the trend of optical extinction been consistent with PM2.5 in recent years? Here a random forest model was developed to predict optical extinction across eastern China in winter for the period of 2014–2019. The model captured the observed spatiotemporal variations of optical extinction (cross-validation R2 = 0.72 and RMSE = 0.12), taking advantage of the extensive network datasets available including air quality data, meteorological data, land cover data, and other predictors. The wintertime optical extinction was predicted to be the highest in North China, suggesting that the region still suffers from the adverse impacts of haze pollution. From 2014 to 2018, we estimated that optical extinction exhibited a shrinking trend across eastern China, at a decline rate of 3.6% each year, and the spatial coverage affected by haze pollution was also cut down. Moreover, grid-based statistics indicated that the decreasing trends of optical extinction were slower than those of PM2.5 mass concentrations, which could attribute to the enhancement of the ratio of the secondary aerosols to PM2.5 mass concentrations in recent years. This study provides the latest investigation into the trend of aerosol optical properties and highlights the importance of reducing secondary aerosol formation to control haze pollution across eastern China.