Age estimation of atmospheric black carbon over Finland from combined aerosol size distribution and radon progeny measurements

Data on aerosol particle size distributions between 0.01 and 10 um were combined with 222Rn and 210Pb particle activities to analyze black carbon characteristics. The activity ratio 210Pb/222Rn was applied to estimate the aerosol and black carbon age. Aerosol with high concentration in the size range of 0.100–0.320 um was related to high black carbon levels with an estimated 2.8 d residence time, and easterly continental winds. Aerosol in the nanometer size range, 0.010–0.032 um, was associated with low black carbon levels, polar winds and 2.0–4.4 d residence time. Aerosol number concentrations varied between 2000-25,000 cm−3, 222Rn activity concentrations between 0.3–13 Bq/m3, radon progeny 210Ph between 110–315 μBq/m3, and black carbon levels between 50–700 ng/ms. Observed mass ratio of black carbon to total aerosol mass in the size range 0.010–10 μm was on average 0.64%, which is substantially smaller than at other remote sites. A near constant ratio of black carbon to the 0.100–0.180 um aerosol fraction is proposed to represent an equilibrium state in an aged aerosol.