Direct radiative feedback due to biogenic secondary organic aerosol estimated from boreal forest site observations

We used more than five years of continuous aerosol measurements to estimate the direct radiative feedback parameter associated with the formation of biogenic secondary organic aerosol (BSOA) at a remote continental site at the edge of the boreal forest zone in Northern Finland. Our upper-limit estimate for this feedback parameter during the summer period (ambient temperatures above 10 degrees C) was -97 +/- 66 mWm(-2) K-1 (mean +/- STD) when using measurements of the aerosol optical depth (f(AOD)) and -63 +/- 40 mWm(-2) K-1 when using measurements of the 'dry' aerosol scattering coefficient at the ground level (f(sigma)). Here STD represents the variability in f caused by the observed variability in the quantities used to derive the value of f. Compared with our measurement site, the magnitude of the direct radiative feedback associated with BSOA is expected to be larger in warmer continental regions with more abundant biogenic emissions, and even larger in regions where biogenic emissions are mixed with anthropogenic pollution.