Aerosol chemical composition and light scattering during a winter season in Beijing

Abstract To evaluate PM 2.5 contributions to light scattering under different air pollution levels, PM 2.5 and its major chemical components, PM 10 , size-segregated water-soluble ions, and aerosol scattering coefficient (b sp ) under dry conditions were measured at an urban site in Beijing in January 2013 when heavy pollution events frequently occurred. Measurements were categorized into three pollution levels including heavy-polluted (Air Quality Index (AQI) ≥ 200), light-polluted (200 > AQI ≥ 100) and clean periods (AQI  2.5 mass concentration was 248 μg m −3 during the heavy-polluted period, which was 2.4 and 5.6 times of those during the light-polluted (104 μg m −3 ) and clean (44 μg m −3 ) periods, respectively. The concentrations of SO 4 2− , NO 3 − and NH 4 + increased much more than those of OC and EC during the heavy-polluted period compared with those during the light-polluted and clean periods. Good correlations between PM 2.5 and b sp were found (R 2  > 0.95) during the different pollution levels. The mass scattering efficiency (MSE) of PM 2.5 was 4.9 m 2  g −1 during the heavy-polluted period, which was higher than those during the light-polluted (4.3 m 2  g −1 ) and clean periods (3.6 m 2  g −1 ). To further evaluate the impact of individual chemical components of PM 2.5 on light scattering, a multiple linear regression equation of measured b sp against the mass concentration of (NH 4 ) 2 SO 4 , NH 4 NO 3 , Organic Matter (OM), EC, Fine Soil (FS), Coarse Matter (CM) and Other chemical compounds were performed. (NH 4 ) 2 SO 4 , NH 4 NO 3 and OM were the dominant species contributing to b sp under both dry and ambient conditions. OM contributed more to b sp than the sum of (NH 4 ) 2 SO 4 and NH 4 NO 3 did under the dry condition during all the pollution periods and this was also the case under the ambient condition during the light-polluted and clean periods. However, the total contributions of (NH 4 ) 2 SO 4 and NH 4 NO 3 to b sp under the ambient condition was 55%, much more than the 29% contribution from OM during the heavy-polluted period. High (NH 4 ) 2 SO 4 and NH 4 NO 3 concentrations and their hygroscopicity were the main reasons causing visibility degradation during the heavy-polluted period, and the effect can be enhanced under high RH conditions.