Microphysical properties of refractory black carbon aerosols for different air masses at a central European background site

Abstract. Uncertainties persist in estimating the radiative forcing of black carbon (BC) due to an incomplete understanding of its microphysical properties. This study investigated the physical properties of refractory black carbon (rBC) at the central European background site Melpitz during summer and winter, using a single-particle soot photometer coupled with a thermodenuder. Different air masses associated with distinct rBC properties were identified in both seasons. 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 observed in the long transportation from the northwest (140 nm), while in winter, the highest concentration (1.23 µg m−3) and largest MMD (216 nm) were both observed in the air mass influenced by the easterly winds. Thickly coated rBC fractions increased during the daytime in summer, indicating that the photochemical processes significantly influence the rBC mixing state. In winter, a higher fraction (27 %) of rBC, with thick coatings in the cold air mass compared to the warm air mass (14 %), suggests the contribution of residential heating emissions to the mixing state. Most rBC retained a low-volatility coating in the thermodenuder samples (63 % mass fraction). In summer, photochemical processes also contribute to coating volatility, showing a higher fraction of rBC particles containing low-volatility coatings during the daytime. In winter, low-volatility coatings showed no significant diurnal variation and were more dependent on ambient temperature. Therefore, rBC coating volatility in winter is more influenced by emission sources, particularly residential heating, rather than atmospheric processes.