Seasonal variation and source analyses of aerosol optical properties in Nanjing, China

Abstract

In this study, optical properties of aerosols were investigated from August 2012 to July 2013 in Nanjing, China. A three-wavelength photoacoustic soot spectrometer (PASS-3) was used to measure aerosol scattering and absorption coefficient (Bsca and Babs) with the wavelength dependence (405, 532, and 781 nm). The readings of PASS-3 agreed with those of the conventional instruments. The seasonal and diurnal cycles of optical properties were analyzed using hourly averaged data. Both Bsca and Babs increased in the order of summer (173.4 and 22.9 Mm−1), spring (292.9 and 35.6 Mm−1), fall (380.7 and 44.9 Mm−1) and winter (507.3 and 66.7 Mm−1). The highest aerosol loading occurred in winter. Compared to weak seasonal variation of single scattering albedo (ω532), the highest Ångstrom scattering exponent (Asca, 1.59 ± 0.31) and the lowest Ångstrom absorption exponent (Aabs, 1.11 ± 0.33) appeared in summer. The diurnal variation of Bsca, Babs, Asca, Aabs and ω532 were multimodal and mainly affected by wind speed and relative humidity. The lower mass absorption efficiency (MAE) of elemental carbon (EC) (MAE ~7.2 m2/g) appeared in fall due to biomass burning, while the higher mass scattering efficiency (MSE~3.8 m2/g) in winter was due to aerosol aging. Air masses originating from inland China induced an increase in Bsca and Babs according to backward trajectory cluster analysis. Organic matter, (NH4)2SO4 and EC were the dominant species of light extinction under dry and ambient conditions. Secondary inorganic ions (NH4+, NO3− and SO42−) were significantly correlated with Bsca and Babs. Consistent results were obtained using three types of source analysis methods.