Measurement report: Size-resolved mass concentration of equivalent black carbon-containing particles larger than 700 nm and their role in radiation

Abstract

Abstract. Black carbon (BC) mass size distribution (BCMSD) is crucial in both the environment and the climate system due to BC's intense size-dependent absorption of solar radiation. BC-containing particles larger than 700 nm (BC>700) could contribute to more than half of bulk BC mass concentration. Unfortunately, previous methods concentrated on BC-containing particles of less than 700 nm because of technical limitations. The contribution of BC to absorption and the radiative effect might be underestimated without consideration of BC>700. In this study, equivalent BCMSD (eBCMSD) from 150 nm up to 1.5 µm was measured at high time resolution of 1 h for the first time by an aerodynamic aerosol classifier in tandem with an aethalometer in two field campaigns over eastern China, namely Changzhou located in the Yangtze River Delta and Beijing located in the North China Plain. The results revealed that the pattern of eBCMSD in Changzhou (Beijing) was mostly bimodal (unimodal) peaking at 240 and 1249 nm (427 nm). The peak diameter of eBCMSD in Changzhou did not shift significantly with increasing pollution (240 to 289 nm). In contrast, the peak diameter of eBCMSD in Beijing shifted towards larger sizes, from 347 to 527 nm, with increasing pollution, indicating that the aging process at the urban site was different from that at the regional background site. eBCMSD in both Changzhou and Beijing had a significant diurnal cycle with a smaller (larger) value of eBCMSD during daytime (nighttime). Equivalent BC>700 (eBC>700) was ubiquitous and varied significantly with different locations and pollution levels. The campaign-averaged contribution of eBC>700 to bulk eBC mass concentration (meBC,bulk), bulk absorption coefficient (σab,bulk) and estimated direct radiative forcing of eBC (DRFeBC) in Changzhou and Beijing were 27.8 % (20.9 %–36.5 %) and 24.1 % (17.5 %–34.2 %), 19.6 % (15.8 %–24.6 %) and 25.9 % (19.6 %–33.7 %) and 20.5 % (18.4 %–22.2 %) and 21.0 % (16.3 %–26.1 %), respectively. meBC,bulk, σab,bulk and DRFeBC of eBC>700 in Changzhou (Beijing) increased by a factor of 3.6 (5.1) from 0.11 (0.07) to 0.40 (0.36) µg m−3, by a factor of 3.2 (5.5) from 0.54 (0.63) to 1.75 (3.45) Mm−1 and by a factor of 2.4 (4.7) from 0.1 (0.1) to 0.24 (0.47) W m−2, respectively, with the aggravation of pollution. The contribution of eBC>700 to meBC,bulk and σab,bulk had a significant diurnal cycle with a higher (lower) fraction during daytime (nighttime) in both Changzhou and Beijing. A case study indicated that the contribution of eBC>700 to meBC,bulk, σab,bulk and DRFeBC could reach up to 50 %, 50 % and 40 %, respectively. It was highly recommended to consider the whole size range of BC-containing particles in model estimation of the BC radiative effect.