The influence of terrestrial transport on visibility and aerosol properties over the coastal East China Sea.

Abstract

Air pollutants from East Asia continent can affect the physio-chemical and optical properties of marine aerosols under seasonal winds. We investigated the change of visibility and haze frequency from 1974 to 2017 over the coastal East China Sea (ECS), and reconstructed the light extinction coefficients according to the chemical compositions of PM2.5 samples collected at Huaniao Island in the ECS. The annual average visibility significantly decreased from over 25\u202fkm in the early 1970s to <18\u202fkm in recent 4\u202fyears. The occurrence of daily maximum haze frequency was approximately 3-h later with respect to land sites, which could be explained by the diffusion of air pollutants from nearby cities (haze peak around rush hour) to the coastal ECS as well as the formation of secondary aerosols enhanced by photochemical reactions around noon at the condition of affluent gaseous precursors. Meanwhile, anthropogenic chloride transported from the land could increase the concentration of Cl- in marine aerosol, which may weaken the Cl- depletion phenomenon over coastal ECS and even induced considerable Cl- enrichment during the severe haze event in Jan. 2013. The largest contributor to the light extinction was (NH4)2SO4 followed by NH4NO3 and OM in almost all seasons. Especially in winter and spring, (NH4)2SO4 accounted for 45% and 52% of total light extinction, respectively. The estimated bext was lower than the monitored values, suggesting that the contribution of some aerosol components (e.g. NH4Cl and large mode components) might be underestimated. Further study on the combination of observation and estimation of specific aerosol contribution to the visibility impairment are needed.