Seasonal variation of the size distributions of black carbon in the Arctic

Abstract. Black carbon (BC) aerosol particles strongly absorb solar radiation and heat the atmosphere. BC aerosols also deposit on snow and ice, lowering the surface albedo and accelerate heating of the Arctic. Because these BC radiative effects are size-dependent, an improved understanding of BC size distributions is indispensable for radiative transfer modelling to estimate the aerosol climate effects. We measured BC size distributions at Pallas, northern Finland, for the first time throughout the whole year, to fully capture its seasonal variability connected with the BC atmospheric processing during transport. The shape of the size distribution was very stable, with little seasonal variation. Comparison to previous seasonal observations at Ny-Ålesund in Svalbard, Norway, and Alert in Canada confirmed very similar size distribution shapes at all three sites, suggesting minor spatial variability. Strong temporal variations were observed in the total mass concentration of BC, but not in the shape of the BC core size distributions. The results were additionally used for validation of the state-of-the-art global climate model CAM5-ATRAS monthly BC size at Pallas. Overall, our observational results provide useful constraints for estimating the effects of BC on climate by model simulations, especially in the Arctic, where the measurements were conducted.