Particulate absorption and its variation with mixing status observed in-situ over Mexico

Light absorbing particles of urban and rural origin were measured in-situ from C-130 and DC-8 aircraft in the troposphere over Mexico during MIRAGE and INTEX-B in March 2006. Thermal analysis of aerosol size distributions and direct measurements of mutiwavelength visible light scattering and absorption enabled evaluation of optical properties of the strongly absorbing soot with and without volatile coating material. As Angstrom exponent, the wavelength dependence of light scattering, increases from -0.2 to 1.5, single scattering albedo of the unheated total particles decreases from ~0.97 to 0.90, indicating transition from pure dust to its mixture with urban and biomass pollution. Greater Angstrom values of 1.5 - 2.1 are, however, associated with increasing total SSA to ~0.95, a trend presumably determined by the sources of the accumulation-mode aerosols. Preliminary attempts to quantify enhanced absorption due to volatile coatings on soot revealed a 0 - 20% reduction in absorption as particles were heated to 300 oC to evaporate those coatings. Variation in the mass scattering/absorption efficiency, submicron SSA and underlying microphysical and chemical properties are also discussed.