Abstract
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 (bext) were 11.9 ± 9.2 km, 64.1 ± 41.6 µg m–3, and 452 ± 314 Mm–1, respectively. The highest concentration of PM2.5, the highest bext, and the lowest VR were all seen in winter, followed by spring, autumn, and summer. Organic matter (OM), ammonium sulfate [(NH4)2SO4], 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 bext. On an annual basis, (NH4)2SO4 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 bext. Rayleigh, elemental carbon, fine soil, nitrogen dioxide, and chloride salt accounted for a minor fraction (7.4%). Up to ~40.8% of the ambient bext was ascribed to relative humidity (RH), of which 26.4% and 14.0% were attributed to the hygroscopic growth of (NH4)2SO4 and NH4NO3, respectively. More efforts are needed to reduce the average daily PM2.5 concentration of < 59 µg m–3 to avoid the occurrence of haze under a high average RH of 78.3 ± 10%, which significantly impacts visibility through various physico-chemical 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.
Highlights
Visibility, a fundamental metric of air quality, has been deteriorating globally from 1973 to 2007, especially occurred+ These authors contributed to this work.Visibility impairment is caused by the light scattering and absorption by atmospheric particles and gases
Up to ~40.8% of the ambient bext was ascribed to relative humidity (RH), of which 26.4% and 14.0% were attributed to the hygroscopic growth of (NH4)2SO4 and NH4NO3, respectively
The visual range (VR) in Ya’an was higher than those reported in Chinese megacities, including Beijing, Guangzhou, Shanghai, Chengdu, Xi’an, Fig. 5
Summary
Visibility, a fundamental metric of air quality, has been deteriorating globally from 1973 to 2007, especially occurred+ These authors contributed to this work.Visibility impairment is caused by the light scattering and absorption by atmospheric particles and gases. Light scattering is controlled by multiple factors, including concentration, size distribution, and chemical composition of ambient aerosols as well as meteorological conditions (Watson, 2002). Light extinction related to gaseous components in atmosphere are relatively well understood and estimated at various atmospheric conditions. Particle light extinction is more complex than that affected by gaseous components. The amount of light scattered by an ensemble of particles can be accurately estimated using Mie theory when its size distribution and index of refraction are known. Meteorological factors, including relative humidity (RH), wind, precipitation, temperature, have impacts on the concentration and optical property of PM2.5 as well, thereby contributing to the visibility degradation (Malm et al, 2003; Zhao et al, 2011; Chen et al, 2014)
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