Comparison of optical properties of nitrate and sulfate aerosol and the direct radiative forcing due to nitrate in China

In previous studies, the optical properties of sulfate aerosols were employed to estimate the direct radiative forcing (DRF) due to nitrate aerosols. Ensuing errors have not been rigorously evaluated, which is a major objective of this study. First, we compared the optical properties of nitrate and sulfate aerosols in different spectral regions. Our results show that nitrate is a strongly scattering aerosol and in some spectral regions, its scattering properties are even greater than those of sulfate aerosols. For example, nitrate aerosol single-scattering albedos are about 40% greater than those of sulfates when the wavelength is nearly 2.8 mu m and the relative humidity (RH) is below 40%. We then incorporated the optical parameters of nitrate into a radiative transfer model and estimated the DRF due to nitrate aerosols at the top of the atmosphere (TOA) under both clear- and all-sky conditions and compared them with those of sulfate aerosols. We found that the local forcing due to nitrate aerosols using sulfate optical properties in the simulation can be underestimated by a maximum of 6.2% under all-sky conditions. Using model-simulated nitrate concentrations in China that reproduce observed features reasonably well, we found significant spatial and seasonal changes in DRFs due to nitrate aerosols. DRFs were stronger in winter, spring, and autumn, but much weaker in summer. The annual mean values of the forcings over China were -4.51 W m(-2) and -0.95 W m(-2) under clear-sky and all-sky conditions, respectively. Clouds play an important role in determining the DRF and can greatly reduce the forcing strength and its geographical extent. (c) 2012 Elsevier B.V. All rights reserved.