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

Abstract. Black carbon (BC) mass size distribution (BCMSD) is crucial in both environment and climate system due to BC’s intense size-dependent absorption of solar radiation. BC-containing particles of size larger than 700 nm (BC>700) could contribute to larger than half of bulk BC mass concentration. Unfortunately, previous methods concentrated on BC-containing particles less than 700 nm because of technical limitation. The contribution of BC to absorption and radiative effect would 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 level of eBCMSD in both Changzhou and Beijing increased with increasing pollution. 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 size from 347 to 527 nm with increasing pollution, indicating the aging process in urban site was different from that in regional background site. eBCMSD in both Changzhou and Beijing had significant diurnal cycle with lower (higher) level 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) as well as 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) %, as well as 20.5 (18.4 ~ 22.2) % and 21.0 (16.3 ~ 26.1) %, respectively. meBC,bulk, σab,bulk as well as DRFeBC of eBC>700 in Changzhou (Beijing) varied by 3.6 (5.1) times from 0.11 (0.07) to 0.40 (0.36) μg m−3, 3.2 (5.5) times from 0.54 (0.63) to 1.75 (3.45) Mm−1 as well as 2.4 (4.7) times 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 significant diurnal cycle with 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 whole size range of BC-containing particles in the model estimation of BC radiative effect.