Black carbon concentrations and mixing state in the Finnish Arctic

Abstract

Atmospheric aerosol composition was measured using a Single Particle Soot Photometer (SP2) in the Finnish Arctic during winter 2011-2012. The Sammaltunturi measurement site at the Pallas GAW (Global Atmosphere Watch) station receives air masses from different source regions including the Arctic Ocean and continental Europe. SP2 is a unique instrument that can give detailed information about mass distributions and mixing state of refractory black carbon (rBC). As expected, the measurements showed widely varying rBC mass concentrations (0-120 ng m-3), which were related to varying contributions of different source regions and aerosol removal processes. The log-normally distributed rBC core size was relatively constant with an average geometric mass mean diameter of 194 nm. On the average, the number fraction of particles containing rBC was 0.24 and the average rBC core size in these particles was half of the total size (coated to core diameter ratio was 2.0). These numbers mean that the core was larger and had a significantly thicker coating than in typical particles closer to their source regions. Comparison of the measured rBC mass concentration with that of the optically detected equivalent black carbon (eBC) showed a factor of five difference, which could not be fully explained without assuming that a part of the absorbing material is non-refractory. Finally, climate implications of five different rBC mixing state representations were quantified using the Mie approximation and simple direct radiative forcing efficiency calculations. These calculations showed that the observed mixing state (separate non-absorbing and coated rBC particles) means significantly lower warming effect or even a net cooling effect when compared with that of an homogenous aerosol containing the same amounts of rBC and non-absorbing material.