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

Using a particle/soot absorption photometer (PSAP), airborne measurements of aerosol volume absorption coefficients were done in differently polluted air masses over a rural area in Germany under cloudless conditions in December 1997. Together with measurements of aerosol particle size distribution, particle chemical composition, and meteorological parameters, the PSAP-measurements 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 the top of the atmosphere (TOA). Three case studies with different levels of absorption were investigated in detail. The vertical profiles of the volume 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 altitudes. The estimated BC volume fractions were typically largest near the ground and decreased rapidly with increasing altitude. Radiative transfer calculations were carried out with and without consideration of the measured absorption of the particles. In the polluted cases and assuming a low surface albedo, the negative TOA solar radiative forcing of the aerosol particles decreased significantly when the measured particle absorption profiles were included in the calculations. Assuming a snow-covered ground in these cases, the aerosol forcing changed from a cooling without aerosol particle absorption into a heating if the measured absorption of the particles was considered. An investigation of the forcing dependence on the solar zenith angle showed an increasing influence of the particle absorption for higher sun elevations.