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Characterization and source apportionment of aerosol light scattering in a typical polluted city in the Yangtze River Delta, China

Through online observation and offline chem- istry 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 opti- mized the aerosol light scattering estimation method, iden- tified its influencing factors and quantified the contribu- tions 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 in- crease 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 pe- riods. The results indicated that the large growth of nitrate and sulfate was one of the major reasons for the polluted pe- riods. Based on measurements at the three sites, the US Inter- agency Monitoring of Protected Visual Environments (IM- PROVE) algorithm was optimized to evaluate aerosol scat- tering in eastern China. The light absorption capacity of or- ganic 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 accumu- lation 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 scatter- ing (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 visibil- ity in Nanjing, with an estimated collective contribution at 64 %–70 %. The comparison between the clean and polluted period suggested that the increased primary particle emis- sions from vehicles and industry were the major causes of the visibility degradation in urban and industrial regions, re- spectively. In addition, secondary aerosols were a great con- tributor to the reduced visibility.


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