Abstract
Abstract. Through online observation and offline chemistry analysis of samples at suburban, urban and industrial sites (NJU, PAES and NUIST, respectively) in Nanjing, a typical polluted city in the Yangtze River Delta, we optimized the aerosol light scattering estimation method, identified its influencing factors and quantified the contributions of emission sources to aerosol scattering. The daily average concentration of PM2.5 during the sampling period (November 2015–March 2017) was 163.1±13.6 µg m−3 for the heavily polluted period, 3.8 and 1.6 times those for the clean (47.9±15.8 µg m−3) and lightly polluted (102.1±16.4 µg m−3) periods, respectively. The largest increase in PM concentration and its major chemical components was found at the size range of 0.56–1.0 µm for the heavily polluted period, and the contributions of nitrate and sulfate were the greatest in the 0.56–1.0 µm fraction (19.4 %–39.7 % and 18.1 %–34.7 %, respectively) for all the three periods. The results indicated that the large growth of nitrate and sulfate was one of the major reasons for the polluted periods. Based on measurements at the three sites, the US Interagency Monitoring of Protected Visual Environments (IMPROVE) algorithm was optimized to evaluate aerosol scattering in eastern China. The light absorption capacity of organic carbon (OC) was estimated to account for over half of the methanol-soluble organic carbon (MSOC) at NJU and PAES, whereas the fraction was lower at NUIST. Based on the Mie theory, we found that the high relative humidity (RH) could largely enhance the light scattering effect of accumulation particles, but it had few effects on the mixing state of particles. The scattering coefficients of particles within the 0.56–1.0 µm range contributed the most to the total scattering (28 %–69 %). The mass scattering efficiency (MSE) of sulfate and nitrate increased with the elevated pollution level, whereas a low MSE of organic matter (OM) was found for the heavily polluted period, probably because a proportion of OM had only a light absorption property. A coupled model of positive matrix factorization (PMF) and the Mie theory was developed and applied for the source apportionment of aerosol light scattering. Coal burning, industry and vehicles were identified as the major sources of the reduced visibility in Nanjing, with an estimated collective contribution at 64 %–70 %. The comparison between the clean and polluted period suggested that the increased primary particle emissions from vehicles and industry were the major causes of the visibility degradation in urban and industrial regions, respectively. In addition, secondary aerosols were a great contributor to the reduced visibility.
Highlights
Atmospheric aerosols play a great role in visibility degradation, radiative balance variation and climate (Liu et al, 2017; Malm and Hand, 2007; Zhang et al, 2017), resulting largely from their light extinction (Seinfeld and Pandis, 2006)
Studies have estimated that the aerosol single-scattering albedo ranges from 0.81 to 0.93 in urban China (Andreae et al, 2008; Cao et al, 2012; Xu et al, 2002, 2012), implying that the deteriorated visibility primarily results from the scattering effect of aerosols
Developed based on the long-term observations in national parks, the US IMPROVE (Interagency Monitoring of Protected Visual Environments) algorithm has been applied to calculate the light extinction of chemical species in aerosols (Watson, 2002)
Summary
Atmospheric aerosols play a great role in visibility degradation, radiative balance variation and climate (Liu et al, 2017; Malm and Hand, 2007; Zhang et al, 2017), resulting largely from their light extinction (Seinfeld and Pandis, 2006). Aerosol light scattering is greatly affected by its chemical composition and hygroscopic growth Developed based on the long-term observations in national parks, the US IMPROVE (Interagency Monitoring of Protected Visual Environments) algorithm has been applied to calculate the light extinction of chemical species in aerosols (Watson, 2002). Through the theoretical calculation, Liu et al (2014) found that smaller particles were in the highly hygroscopic mode, whereas larger particles were in the nearly hydrophobic mode
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