Case study of the effects of aerosol chemical composition and hygroscopicity on the scattering coefficient in summer, Xianghe, southeast of Beijing, China

Abstract

The aerosol scattering coefficient is mainly affected by various factors, such as aerosol chemical composition, hygroscopicity and relative humidity. A comprehensive observation of the chemical and physical properties of aerosols was conducted in Xianghe, southeast of Beijing, from June 15 to 26, 2018. Five specific cases were analysed during this observation: (1) rainy (RA); (2) low visibility during the day (LD); (3) high visibility during the day (HD); (4) low visibility at night (LN); and (5) high visibility at night (HN). The mass percentages of a secondary inorganic aerosol (nitrate, sulfate and ammonium) measured with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) were 64.5% and 68.3% during LD and LN, which were higher than those (63.6% and 46.1%) observed during HD and HN, respectively. The mass percentages of secondary organic aerosol (SOA) and primary organic aerosol (POA) to organic aerosol were 76.6% and 23.4%, respectively, during LD. The hygroscopic parameters κ were 0.59 and 0.60 during LD and LN, which were slightly higher than those (0.57 and 0.58) observed during HD and HN, respectively, indicating that the variation of aerosol chemical composition had a limited impact on hygroscopicity. The average proportion, measured with a single particle aerosol mass spectrometer (SPAMS), of biomass burning, dust, heavy metal, soot-like, sea salt, secondary aerosols and other particles were 12.3%, 2.9%, 16.2%, 36.1%, 8.8%, 21.0% and 2.7%. The average proportions of soot-like and secondary aerosols were 42.4% and 19.2% from 0.2 to 1.0 μm. Although the variation and height of the mixing layer were similar between LD and HD, clouds reduced the solar radiation during LD, which caused a higher relative humidity (RH) (67.5%) than that (45.9%) observed during HD. A larger wind speed below 850 hpa during HN caused the mixing layer height (MLH) to reach 1910 m, which resulted in a lower RH (64.8%) than that (68.0%) observed during LN. The light scattering enhancement factors f(RH) during low visibility events were almost greater than those observed during high visibility events, regardless of they were measured during the day (0–9.7%) or night (0–13.1%). The aerosol hygroscopic growth factor (GF) maintained a similar trend with RH, indicating that RH was the main factor affecting the hygroscopic growth of the aerosol, resulting in the decrease in visibility.