Insights into extinction evolution during extreme low visibility events: Case study of Shanghai, China.

Apportionment of ambient extinction coefficient is essential for quantifying the causes of visibility degradation. Previous studies focused on either seasonal or episode-average extinction coefficients. The extinction evolution during different types of low visibility events was still unclear and seldom investigated. In this study, hourly-resolution apportionment of ambient extinction coefficient, including dry extinction coefficient and hygroscopic portion, during three low visibility events (i.e., dust storm, autumn and winter haze) and one clear episode was retrieved through online measurement in Shanghai, China. PM soil and coarse particles contributed 90% of PM mass and 62% of total extinction coefficient throughout the dust storm event. Secondary inorganic aerosol and organic matter dominated the autumn and winter haze events, accounting for 52% and 31% of PM mass, 35% and 27% of extinction coefficient, respectively. Hygroscopic enhancement by inorganic particles contributed another 22-27% of extinction coefficient during the two haze events. However, higher relative humidity elevated the extinction percentage of inorganic aerosol and hygroscopic enhancement during the autumn haze, and the percentage of organic matter decreased correspondingly. In contrast, the extinction of each contributor increased proportionally and the percentages could keep at a stable level during the winter haze. Furthermore, the mass extinction efficiency of major PM chemical components was found to increase with the accumulation of mass loading. These findings indicated the importance of reducing the mass level of organic matter and secondary inorganic aerosol during the autumn or winter haze events. The control of precursors of sulfur and nitrogen oxides seemed more effective for visibility improvement during the autumn events with higher relative humidity.