Effects of Chemical Composition of PM2.5 on Visibility in a Semi-rural City ofSichuan Basin

The rate of visibility deterioration in Ya'an city in the Sichuan Basin has been accelerating since the 2000s. Issues related to the air quality as well as meteorological conditions are reported in this study. Fine particulate matters (PM2.5) were collected in Ya'an from June 2013 till June 2014. The chemical compositions of the samples were determined. The annual average visual range (VR), PM2.5 concentrations, and ambient light extinction coefficient (b(ext)) were 11.9 +/- 9.2 km, 64.1 +/- 41.6 mu g m(-3), and 452 +/- 314 Mm(-1), respectively. The highest concentration of PM2.5, the highest b(ext), and the lowest VR were all seen in winter, followed by spring, autumn, and summer. Organic matter (OM), ammonium sulfate [(NH4)(2)SO4], and ammonium nitrate [NH4NO3] were the major constituents, accounting for 32.8%, 28.3%, and 12.1%, respectively, of the total PM2.5 mass. The revised Interagency Monitoring of Protected Visual Environments (IMPROVE) equation was applied to estimate ambient b(ext). On an annual basis, (NH4)(2)SO4 was the most significant contributor (43.1%), followed by OM (27.1%) and NH4NO3 (22.4%), which, in total, accounted for 92.6% of the ambient b(ext). Rayleigh, elemental carbon, fine soil, nitrogen dioxide, and chloride salt accounted for a minor fraction (7.4%). Up to similar to 40.8% of the ambient b(ext) was ascribed to relative humidity (RH), of which 26.4% and 14.0% were attributed to the hygroscopic growth of (NH4)(2)SO4 and NH4NO3, respectively. More efforts are needed to reduce the average daily PM2.5 concentration of < 59 mu g m(-3) to avoid the occurrence of haze under a high average RH of 78.3 +/- 10%, which significantly impacts visibility through various physicochemical processes. Emissions of precursor gases, such as sulfur dioxide, nitrogen oxides, ammonia, and volatile organic compounds, should be reduced to improve the air quality and visibility in Ya'an