Comment on acp-2022-105

Abstract

Black carbon (BC) particles exert a significant influence on the earth's climate system. However, large uncertainties remain when estimating the radiative forcing by BC because the corresponding microphysical properties have not been well addressed. Knowleadge of the BC mixing states of different aging degree can help better characterise the corresponding environmental and climate effects. In this study, the BC size distributions were studied based on three different field campaigns at an urban site, a suburban site, and a background site in China using a single particle soot photometer (SP2) in tandem with a differential mobility diameter. Measurements from the SP2 indicate that the BC particles were composed of either fresh or aged aerosols. The mean number fractions of the fresh BC aerosols were 51 %, 67 %, and 21 % for the urban, suburban, and background sites, respectively. The corresponding mobility diameters of these aged (fresh) BC-containing aerosols were 294 nm (193 nm), 244 nm (161 (nm), and 257 nm (162 nm). The measured aged (fresh) BC core number median diameters were 115 nm (114 nm), 107 nm (95 nm), and 127 nm (111 nm) for urban, suburban, and background sites, respectively. The corresponding aged (fresh) core mass median diameters were 187 nm (154 nm), 182 nm (146 nm), and 238 nm (163 nm) respectively. The mean diameter of the aged BC-containing aerosols was larger than that of the fresh BC-containing aerosols, while the mean BC core diameter of the aged BC-containing aerosols was smaller than that of the fresh BC-containing aerosols. About 10 % of the BC-containing aerosols with the BC core were attached to the other non-BC components, which were mainly generated by coagulation between the BC and non-BC components. The measurement results in our study can help better understand the BC size distributions and mixing status in the different atmospheres in China and can be further used in modeling studies to help constrain the uncertainties of the BC radiative effects.