Vertical evolution of black and brown carbon during pollution events over North China Plain

Abstract

The vertical distribution of carbonaceous aerosol impacts climate change, air quality and human health, but there is a lack of in-situ vertical observations of black (BC) and brown carbon (BrC). Thus, the characteristic of vertical profiles of BC concentration, particle number concentration (PNC), O₃ concentration and optical absorption of BC and BrC were observed in a suburban site over North China Plain, where heavy pollution of PM_2.5 and O₃ always occurred in winter and summer, respectively. In winter, during a heavy pollution episode, the BC and PNC was near uniformly distributed within mixing layer (ML) (15.2 ± 6.7 μg m^-3 and 678 ± 227 p cm^-3, respectively) and decreased with altitude above the ML. The BC heating rate reached about 0.13 K h^-1 during the heaviest pollution day. In summer, the BC concentration (2.9 ± 1.3 μg m^-3) in ML during the middle O₃ pollution events was higher than that (1.7 ± 0.6 μg m^-3) during the light O₃ pollution. The light absorption coefficients of BC at 880 nm and BrC at 375 nm measured in the early morning were lower than that in the daytime, and the contribution of BrC to total light absorption of carbonaceous aerosols was in the range of 27-47%. In addition, BC was effectively transported to high altitude than BrC in the daytime. The light absorption of secondary BrC in the daytime was higher 10-20% than that in the early morning. Simultaneously, the contribution of secondary BrC to the total BrC light absorption at 375 nm was range from 32% to 68% within 1000 m.

Keywords: Black carbon; Brown carbon; Ozone; Secondary BrC; Vertical profile.