A Case Study of Aerosol Optical Properties and Radiative Effects Computed From Airborne Measurements During the ACE-Asia Campaign

A range of aerosol optical properties can be calculated from sunphotometer measurements, using retrieval of aerosol size distributions from optical thickness spectra as an intermediate step, along with the information on aerosol refractive index. We retrieved the aerosol size distributions from aerosol extinction and layer optical thickness spectra derived from NASA Ames Airborne Tracking Sunphotometer (AATS-14) measurements, during the ACE-Asia campaign. In layers dominated by small particles, the shapes of the retrieved size distributions in the radius range of optically efficient particles are generally in agreement with those measured in situ aboard the same aircraft on which the sunphotometer was flown, in accord with close wavelength dependencies of the corresponding optical thicknesses. Focusing on a vertical profile with three distinct layers (marine boundary, pollution and dust), we combined the retrieved size distributions with a size-resolved refractive index model based on aerosol chemical composition to calculate aerosol optical properties. The aerosol model resulted in aerosol single scattering albedos of 0.78-0.81 and 0.93-0.96 at mid-visible, in the pollution and dust layers respectively. In the case of the dust layer, the modeled values are consistent with those obtained from the in situ measurements during the campaign, whereas in the pollution layer our model suggests significantly more absorbing aerosol than implied by the measurements. We carried out a comparison between the vertical profile of the modeled lidar ratio and that derived from co-located airborne sunphotometer and shipborne lidar measurements. The aerosol model resulted in lidar ratios between 34 and 44 sr in the pollution layer, larger than those obtained from the measurements, with relative differences from 7 to 46%. The differences can be explained by non-uniqueness of the result of the size distribution retrieval and lack of information on vertical variability of particle refractive index. In order to obtain a more accurate estimate of aerosol optical properties for radiative transfer calculations, we constrained the aerosol model by lidar/sunphotometer measurements and single scattering albedos implied by the in situ measurements in pollution and dust dominated cases during the campaign. We demonstrate that, if the extinction profile is known, then information on the vertical structure of aerosol single scattering albedo and the asymmetry parameter is not significant for estimating aerosol radiative forcing at TOA and the surface, while it is of importance for estimating vertical profiles of radiative forcing and heating rates.