Important Effect of Secondary Inorganic Salt Extinction on Visibility Impairment in the Northern Suburb of Nanjing

Abstract

Observed data regarding the visibility and aerosol chemical composition from May 2013 to May 2014 were used to analyze the variation of visibility, the relationship between aerosol chemical composition and visibility variations, and their contributions to atmospheric light extinction. An important effect of secondary inorganic salt extinction on the visibility impairment was determined. The present study suggests that the average visibility during the observation period was (6.78±3.68) km, and there was obvious seasonal variation in the visibility. Fine particles with size less than 2.1 μm have a great influence on visibility, with the main chemical components of SO42-, NO3-, NH4+, and OC. The secondary inorganic ions make significant contributions to visibility degradation. The mean light extinction coefficient of Nanjing was (527.2±295.2) Mm-1, which was calculated by using the revised IMPROVE equation. Regarding the chemical composition of PM2.1, the most contributive species to the light extinction coefficient were ammonium sulfate, ammonium nitrate, and organic species, which accounted for 80.6%. Although the light extinction contribution of organic matter was as high as 43.51% on a clear day (VR>10 km), with the decrease of visibility, the extinction contribution of organic matter decreased, but the contribution of secondary inorganic salt increased. The contribution of extinction was 58.96% for heavy haze days with low visibility (VR<5 km). This proves that the secondary inorganic salt extinction plays a significant role in visibility impairment.