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Black carbon (BC) aerosols play an important role in modifying the PBL meteorology and severity of haze pollution through its radiative effect (Ding et al. 2016). The radiative effect of BC strongly depends on its mixing state or coating thickness, which is subject to the aging process in the atmosphere (Cheng et al. 2006, 2008). North China Plain (NCP) is one of the areas where both severe haze and high abundance of BC have been reported. By using a single particle soot photometer (SP2) and complimentary measurements, we investigated the aging of BC during several severe haze events in the North China Plain during our winter campaign in 2018. We calculated the size-resolved mixing state and aging rate of BC in the ambient air under different pollution levels. Our results show a fast aging of BC especially during the haze events with high concentrations of air pollutants and condensable vapors. The occurrence of fog episodes in the campaign enables us to analyze the BC-droplet interactions, especially the effect of aerosol size and coating thickness on its activation and removal, which is key for understanding the aerosol-cloud and aerosol fog interactions. Moreover, we find that most coated BC particles can be classified to a "core-shell" type while only a small fraction of BC-containing particles is in an "attached" type. It suggests that the traditional Core-Shell model should still be applicable for modeling the radiative properties of BC in the North China Plain. Based on the Mie model calculation, we determined their absorption enhancement factor of BC due to the aging process and discussed its implication to the formation of severe haze.References:Ding, A. J., Huang, X., Nie, W., Sun, J. N., Kerminen, V. M., Petaja, T., Su, H., Cheng, Y. F., Yang, X. Q., Wang, M. H., Chi, X. G., Wang, J. P., Virkkula, A., Guo, W. D., Yuan, J., Wang, S. Y., Zhang, R. J., Wu, Y. F., Song, Y., Zhu, T., Zilitinkevich, S., Kulmala, M., and Fu, C. B.: Enhanced haze pollution by black carbon in megacities in China, Geophys. Res. Lett., 43, 2873-2879, 10.1002/2016gl067745, 2016.Cheng, Y. F., Eichler, H., Wiedensohler, A., Heintzenberg, J., Zhang, Y. H., Hu, M., Herrmann, H., Zeng, L. M., Liu, S., Gnauk, T., Bruggemann, E., and He, L. Y.: Mixing state of elemental carbon and non-light-absorbing aerosol components derived from in situ particle optical properties at Xinken in Pearl River Delta of China, J. Geophys. Res.-Atmos., 111, 18, 10.1029/2005jd006929, 2006.Cheng, Y. F., Wiedensohler, A., Eichler, H., Su, H., Gnauk, T., Brueggemann, E., Herrmann, H., Heintzenberg, J., Slanina, J., Tuch, T., Hu, M., and Zhang, Y. H.: Aerosol optical properties and related chemical apportionment at Xinken in Pearl River Delta of China, Atmos. Environ., 42, 6351-6372, 10.1016/j.atmosenv.2008.02.034, 2008. [ABSTRACT FROM AUTHOR]

Black carbon (BC) is the most important light-absorbing species in the atmosphere and has a strong positive direct radiative forcing. Recent studies found that BC-containing particles may also modify the structure of planetary boundary layer and thus enhance haze pollution, especially in densely populated areas, for instance, in China. It is however difficult to quantify these effects due to their complex mixing state, hygroscopicity and the high spatial variability of concentration. During an intensive field campaign conducted in the North China Plain in a cold season, the mass size distribution and mixing state of BC was measured with a single particle soot photometer (SP2). During a severe haze event, the BC mass concentration exceeded 15 ug m-3. Stemming from the strong aging process under high relative humidity condition, freshly emitted BC particle was observed to turn into internally mixing within a short time scale. To understand the variation of the hygroscopicity of BC during its aging process, the hygroscopic growth factor of BC-containing particles was measured with a combination of a hygroscopic tandem differential mobility analyzer and a SP2. The particle hygroscopic growth factor was found to be correlated with the volume fraction of BC. During haze events, a fraction of the more-hygroscopic mode particles was observed to contain BC cores; while a minor fraction of the hydrophobic mode particles contained no BC. [ABSTRACT FROM AUTHOR]