Mixing state of refractory black carbon in fog and haze at rural sites in winter on the North China Plain

Abstract. The variability of the mixing state of refractory black carbon aerosol (rBC) and the corresponding complicated light absorption capacity impose great uncertainty for its climate forcing assessment. In this study, field observations using a single particle soot photometer (SP2) were conducted to investigate the mixing state of rBC under different meteorological conditions at a rural site on the North China Plain. The results showed that the hourly mass concentration of rBC during the observation periods was 2.6 ± 1.5 µg m−3 on average with a moderate increase (3.1 ± 0.9) during fog episodes. The mass–equivalent size distribution of rBC exhibited an approximately lognormal distribution with a mass median diameter (MMD) of 213 nm. We found that the count median diameter (CMD) of rBC particles during snowfall episodes was obviously larger than that before–snowfall, indicating that smaller rBC–containing particles were much more effectively removed by snowfall. The droplet collision and Wegener–Bergeron–Findeisen (WBF) processes are possible explanations. Based on the Mie–scattering theory simulation, the relative and absolute coating thicknesses of rBC–containing particles were estimated to be 1.6 and ~52 nm for the rBC core with a mass–equivalent diameter (Dc) of 170–190 nm, respectively, indicating that the most of rBC–containing particles were thinly coated. Furthermore, moderate light absorption enhancement (Eabs = 1.3) and relatively low absorption cross–section (MAC = 5.5 m2/g) at 880 nm were observed at the GC site in winter compared with other typical rural sites. The relationships between the microphysical properties of rBC and meteorological conditions were also studied. We found that the coatings of rBC–containing particles increase only when both ambient RH and secondary components increase at the same time, with the thickest coating during fog events and the thinnest on clean days. And −4 ~ 0 °C may be the most suitable temperature range for coating formation of rBC in this study. The sulfate formation from aqueous–phases reactions may have a limited contribution to Eabs under high RH conditions (RH > 80 %). The enhancement of Eabs of rBC–containing particles was strongly related to an increase in the mass fraction of nitrate instead of organics at appropriate temperature conditions.