Seasonal characteristics in physical properties of refractory black carbon aerosols at a central European background site Melpitz

Black carbon (BC) is the most absorbing atmospheric aerosol and, therefore, influences the Earth’s climate system. Uncertainties in BC climate forcing estimates can be attributed to a limited understanding of its size distribution, mixing state, morphology, spatiotemporal distribution, and optical properties, all of which require more representative and long-term measurements (Bond et al., 2013; Liu et al., 2020). To investigate the long-term variation of BC physical properties, continuous measurements were conducted at the central European rural background site Melpitz (Germany) from August 2021 to February 2022. Mass concentrations, size distributions, and mixing state of BC were measured by a Single Particle Soot Photometer (SP2). A thermodenuder (300⁰C) was connected upstream of the SP2 to remove the volatile coating of BC. In addition, the light absorption coefficients were measured using a multi-angle absorption photometer (MAAP).

Different air masses associated with distinct refractory black carbon (rBC) properties were identified in summer (August) and winter (December). In summer, rBC exhibited a similar mass concentration (~0.16 μg m^-3) among different air masses, with the smallest mass median diameter (MMD) of rBC overserved in the long transportation from the northwest (140nm), while in winter, the highest concentration (1.23 μg m^-3) and largest MMD (216 nm) were both observed in easterly air masses. Thickly coated rBC fractions increased during the daytime in summer, indicating that photochemical processes significantly influence the rBC mixing state. In winter, a higher fraction (27%) of thickly coated rBC in the cold air mass compared to the warm air masses (14%) suggests the contribution of residential heating emissions to the mixing state. Most rBC particles retained a low-volatile coating when passing the thermodenuder with a mass fraction of 58%. In summer, photochemical processes also contribute to the volatility of coating, showing a higher fraction of rBC particles containing low-volatile coatings during the daytime. In winter, low-volatile coatings showed no significant diurnal variation and were more dependent on ambient temperature. Therefore, the volatility of rBC coatings in winter is more influenced by emission sources, particularly residential heating, rather than atmospheric processes. The optical properties of rBC showed seasonal variations as well, which were caused by changes in size distribution and mixing state.

 

Bond, T. C., et al. (2013). "Bounding the role of black carbon in the climate system: A scientific assessment." Journal of Geophysical Research: Atmospheres 118(11): 5380-5552.

Liu, D., et al. (2020). "Lifecycle of light-absorbing carbonaceous aerosols in the atmosphere." npj Climate and Atmospheric Science 3(1).