Characterization of aerosols from simulated SAGE III measurements applying two retrieval techniques

We investigated the retrieval of aerosol properties and the extinction due to aerosols at the ozone and water vapor channels from simulated measurements at variations of the planned Stratospheric Aerosol and Gas Experiment (SAGE) III aerosol channels. The aerosol quantities surface area, volume, and effective radius are retrieved through the application of two distinct algorithms in the form of the randomized-minimization-search technique (RMST) and the constrained linear inversion (CLI) method. These aerosol quantities are important as inputs in climate, photochemical, and radiative forcing models and are useful in comparing diverse measurements. Ten analytical size distributions fitted to aerosol populations measured in situ are used with a Mie scattering code in conjunction with a Monte Carlo technique to simulate SAGE III measurements. These models consist of variations of prevolcanic and postvolcanic size distributions that exhibit various spectral shapes. Neither the complex components nor the uncertainties of the refractive indices are considered. We developed an objective scheme to estimate the: systematic, random, and total uncertainties of each retrieved quantity that considers the contribution of the particles that lie outside the retrieved size range. Results, based on the 10 selected aerosol models, indicate that in the seven-eight SAGE III channel retrievals, both algorithms obtain estimated total errors in the range 8-50% for the surface area with an average total error (R*) of similar to 25%; for the volume the range is 5-25% with an R* of similar to 12%, and for the effective radius, the range is 6-36% with an R* of 20% though both inversion techniques are applied in different size ranges. The inversion of the six longest channels to study aerosol properties in both the lower stratosphere and the upper troposphere leads to RMST R* values of similar to 32, similar to 15, and similar to 20% and CLI R* values of similar to 48, similar to 22, and similar to 31% for the surface area, volume, and effective radius, respectively. In the seven wavelength retrievals, both algorithms retrieved the extinction coefficients at the unused channel to within their measurement uncertainties except at the 0.385 and 1.550 mu m channels located at the tail ends of the SAGE III aerosol extinction spectrum. The calculated extinction due to aerosols at the water vapor channel at 0.940 mu m and the ozone channel at 0.600 mu m produced R* values of <10 and <15% for both techniques. We have shown that the application of either technique, when properly tailored to the SAGE III system, not only can obtain useful aerosol information in most cases but also can estimate reasonably the extinction due to aerosols at other wavelengths within the SAGE III wavelength range.