Atmospheric Black Carbon Concentrations in the Finnish Arctic over Five Decades: Comparisons between measurements and model

Abstract:

Black carbon particles (BC) strongly absorb solar radiation and may be the second largest contributor to global warming after greenhouses gases. However, the magnitude of the climate forcing by BC is quite uncertain, with a global average value estimated up to + 1.1Wm². There is limited long term BC field data that can be used to validate global models to estimate the radiative forcing. Here we report (1) the measurements of BC concentrations, [BC], on filters collected weekly over 47 years, from 1964 to 2010 at Kevo, Finland (69^o 45' N, 27^o 02' E), (2) the se-asonal and long term trends, (3) the relative impact of potential source regions of BC and (4) compare the [BC] with the values calculated by the OsloCTM3model that simulates BC transport and removal rates and emission inventories as input.

BC Concentrations were determined in weekly aerosol samples collected on Whatman 42 or Glass fiber filters using optical and/or thermal optical methods. The data so obtained provides the longest record of BC measurements. The highest annual mean concentrations were observed in 1965-1966, 670ng m^-3 (local wood burning may have contributed at that time). Concentrations decreased sharply, averaging ~300 ng m^-3 during 1967-80, ~250 ng m^-3 for 1981- 90, ~140 for 1991-2000, and ~100 ng m^-3 for 2001-10. From 1970 to 2010 the concentration decreased by 78% or 1.8% per yr. The highest concentrations were observed in winter, and the lowest in summer. Model calculations using the OsloCTM3 yield BC values ~4 times lower than those observed between 1998 and 2008, however the time-trend is reproduced quite faithfully. The model underestimates BC for the earlier periods as well. There are limitations in global models to reproduce Arctic surface concentration, but the comparison of model and observations might also indicate that the emissions in source regions are underestimated. It can be used to improve historical emission inventories for the regions affecting the surface concentration in the European Arctic. Revised emission inventories will impact the radiative forcing trend calculations, including the snow albedo effect.