Measured profiles of aerosol particle absorption and its influence on clear-sky solar radiative forcing

Using a particle/soot absorption photometer (PSAP), airborne measurements o f aerosol volume absorption coefficients were done in differently polluted air masses over a rural area in Germany under cloudless conditions in Decem ber 1997. Together with measurements of aerosol particle size distribution, particle chemical composition, and meteorological parameters, the PSAP-mea surements were used (1) to estimate vertical profiles of black carbon (BC) volume fraction and (2) to quantify the influence of particle absorption on solar radiative transfer by calculating their solar radiative forcing at t he top of the atmosphere (TOA). Three case studies with different levels of absorption were investigated in detail. The vertical profiles of the volum e absorption coefficient showed maximum values close to the ground (>6 x 10 (-5) m-l in the most absorbing case) and a strong drop-off toward higher al titudes. The estimated BC volume fractions were typically largest near the ground and decreased rapidly with increasing altitude. Radiative transfer c alculations were carried out with and without consideration of the measured absorption of the particles. In the polluted cases and assuming a low surf ace albedo, the negative TOA solar radiative forcing of the aerosol particl es decreased significantly when the measured particle absorption profiles w ere included in the calculations. Assuming a snow-covered ground in these c ases, the aerosol forcing changed from a cooling without aerosol particle a bsorption into a heating if the measured absorption of the particles was co nsidered. An investigation of the forcing dependence on the solar zenith an gle showed an increasing influence of the particle absorption for higher su n elevations.