Retrieval of aerosol single scattering albedo at ultraviolet wavelengths at the T1 site during MILAGRO

Surface measurements of direct and diffuse voltages at UV wavelengths were made at the T1 site during the MILAGRO (Megacity Initiative: Local and Global Research Observations) field campaign in March 2006, using a multifilter rotating shadowband radiometer (UV-MFRSR). We used the MFRSR data, together with measurements from a co-located CIMEL Sun photometer at the site operating as part of the AERONET network, to deduce aerosol single scattering albedo (omega) at 368 and 332 nm for four cloud-free days during the study. Our retrievals suggest that T1 aerosols with aerosol extinction optical depth tau(368)> 0.1 that are influenced by Mexico City emissions, blowing dust, and biomass burning, are characterized by low omega(368)=0.73-0.85 and omega(332)=0.70-0.86, with small or no spectral variation of omega between 368 and 332 nm. Our findings are consistent with other published estimates of omega for Mexico City aerosols, including those that suggest that the absorption attributable to these aerosols is enhanced at UV wavelengths relative to visible wavelengths. We also demonstrate, via sensitivity tests, the importance of accurate tau and surface albedo measurements in omega retrievals at UV wavelengths.