FY 1995

Comparison of measured and calculated aerosol properties relevant to the direct radiative forcing of tropospheric sulfate aerosol on climate

Quinn, P.K., S.F. Marshall, T.S. Bates, D.S. Covert, and V.N. Kapustin

J. Geophys. Res., 100(D5), 8977–8991, doi: 10.1029/95JD00387 (1995)

The accuracy of the estimated radiative forcing of tropospheric sulfate aerosol depends on the quality and spatial coverage of the aerosol chemical, physical, and optical data that serve as input to global climate models. To augment the available data and to provide a comparison of measured and calculated optical properties, surface measurements were made of the aerosol light scattering and backscattering coefficients, the number size distribution from 0.02 to 9.6 µm, and chemical mass size distributions during two Pacific Ocean field experiments. All measurements were made on an aerosol sample stream dried to 30% relative humidity and are reported as such. The first experiment took place during the Pacific Sulfur/Stratus Investigation at Cheeka Peak, Washington, in April and May of 1991 (PSI 91). The second occurred as part of the Marine Aerosol and Gas Exchange cruise in February and March of 1992 (MAGE 92) which was conducted from 33°N to 12°S along 140°W. The mass size distributions of nonseasalt sulfate and sodium varied widely both spatially and temporally. The shape of the number size distribution remained fairly constant throughout both experiments with an accumulation mode geometric number mean diameter of 0.19 ± 0.03 µm and a geometric mean standard deviation of 1.4 ± 0.06. Measured light scattering and backscattering ranged from 3.7 to 19 × 10⁻⁶ m⁻¹ and 0.64 to 2.8 × 10⁻⁶ m⁻¹, respectively, resulting in an average backscattered fraction of 0.15 with a standard deviation of ±0.009. The light scattering and backscattering coefficients were calculated from a Mie model applied to the measured number size distributions. The mean of the calculated scattering values was 3% higher than the mean of the measured values with a 14% variance about the mean. This variance was within the uncertainty of the calculations indicating that the scattering characteristics of the aerosol were parameterized adequately by the model. The calculated backscattering values were about 40% lower than the measured values, however. The calculated light scattering apportioned to nonseasalt sulfate aerosol was 39 ± 17% of the total calculated scatter. The scattering to mass ratio for sulfate aerosol averaged 5.0 m² g⁻¹ with a standard deviation of ±1.6 m² g⁻¹ and varied with variability in the number size distribution. Further measurements are needed that will allow for the formation of a global-scale database to reveal the extent of the variability in the aerosol chemical, physical, and optical properties relevant to climate forcing.