COMPARISON OF MEASURED AND CALCULATED AEROSOL PROPERTIES RELEVANT TO THE DIRECT RADIATIVE FORCING OF TROPOSPHERIC SULFATE AEROSOL ON CLIMATE

The accuracy of the estimated radiative forcing of tropospheric sulfat e aerosol depends on the quality and spatial coverage of the aerosol c hemical, physical, and optical data that serve as input to global clim ate models, To augment the available data and to provide a comparison of measured and calculated optical properties, surface measurements we re made of the aerosol light scattering and backscattering coefficient s, the number size distribution from 0.02 to 9.6 mu m, and chemical ma ss size distributions during two Pacific Ocean field experiments. All measurements were made on an aerosol sample stream dried to 30% relati ve humidity and are reported as such. The first experiment took place during the Pacific Sulfur/Stratus Investigation at Cheeka Peak, Washin gton, in April and May of 1991 (PSI 91), The second occurred as part o f the Marine Aerosol and Gas Exchange cruise in February and March of 1992 (MAGE 92) which was conducted from 33 degrees N to 12 degrees S a long 140 degrees W, The mass size distributions of nonseasalt sulfate and sodium varied widely both spatially and temporally, The shape of t he number size distribution remained fairly constant throughout both e xperiments with an accumulation mode geometric number mean diameter of 0.19 +/- 0.03 mu m and a geometric mean standard deviation of 1.4 +/- 0.06. Measured light scattering and backscattering ranged from 3.7 to 19 x 10(-6) m(-1) and 0.64 to 2.8 x 10(-6) m respectively, resulting in an average backscattered fraction of 0.15 with a standard deviation of +/- 0.009. The light scattering and backscattering coefficients we re calculated from a Mie model applied to the measured number size dis tributions, The mean of the calculated scattering values was 3% higher than the mean of the measured values with a 14% variance about the me an. This variance was within the uncertainty of the calculations indic ating that the scattering characteristics of the aerosol were paramete rized adequately by the model. The calculated backscattering values we re about 40% lower than the measured values, however. The calculated l ight scattering apportioned to nonseasalt sulfate aerosol was 39 +/- 1 7% of the total calculated scatter. The scattering to mass ratio for s ulfate aerosol averaged 5.0 m(2) g(-1) with a standard deviation of +/ - 1.6 m(2) g(-1) and varied with variability in the number size distri bution, Further measurements are needed that will allow for the format ion of a global-scale database to reveal the extent of the variability in the aerosol chemical, physical, and optical properties relevant to climate forcing.