A multi-year evolution of aerosol chemistry impacting visibility and haze formation over an Eastern Asia megacity, Shanghai

A multi-year (2004-2008) study of horizontal visibility and factors controlling its variations was conducted in Shanghai. The seasonal average visibility in spring, summer, autumn, and winter was 7.7 km, 8.4 km, 8.1 km, and 6.5 km, respectively, significantly exceeding the haze criteria of 10 km throughout all four seasons. Although SO2 and NO2 concentrations gradually shifted to lower levels on an annual basis, no distinct improvement of the annual mean visibility was noticed. Via a grouping method, it was found that the water soluble ions and black carbon were the major contributors to haze formation while the role of mineral aerosol was negligible. Of the ions, SO42−, NO3−, NH4+, K+, and C2O42− were most associated with the formation of heavy haze. SO42− and NO3− were both found to have significant decreasing trends with annual decreasing rates of 0.96 and 0.56 μg/m3/yr, respectively. Conversely, NH4+ showed a significant increasing trend with the annual increasing rate of 0.52 μg/m3/yr. Based on a quantitative correlation analysis, the roles of NH4+ and Ca2+ on the acids neutralization were almost equivalent in the earlier years of 2004-2005. While the role of Ca2+ had tremendously dropped to less than 23% in the later years of 2006-2008. Intense control measures on the emissions of construction works and roadside dust were the main driving force for the evident decreasing trend of annual Ca2+ concentrations. This study found that the gradual replacement of CaSO4 and Ca(NO3)2 by (NH4)2SO4 and NH4NO3 in aerosol was the major cause of no improvement of the visibility impairment in Shanghai during recent years.