Optical Properties of Dispersed Aerosols in the Near Ultraviolet (355 nm): Measurement Approach and Initial Data

Aerosols play an important role in climate change. The direct climate effect of aerosols is related to the optical properties of aerosols. In order to study these properties, an aerosol albedometer combining cavity ring-down spectroscopy (CRDS) with integrating sphere nephelometry was developed at a near ultraviolet (UV) wavelength. Extinction and scattering coefficients of dispersed particulate matter were measured simultaneously at λ=355nm. Several samples have been analyzed, including ammonium sulfate, secondary organic aerosols (SOA) resulting from the ozonolysis of α-pinene and toluene, re-dispersed soil dust samples, biomass burning aerosols, and ambient aerosols. Particle size and number density were controlled during measurements of ammonium sulfate and α-pinene (SOA) and a linear relationship between particle number concentration and extinction and scattering coefficients was found. Extinction cross sections for these two types of size-selected (Dp = 300 nm) samples were within the range of 1.65 - 2.60 × 10^-9 cm². Inversions of Lorentz-Mie theory allowed extraction of refractive indices for these materials. The near UV refractive index of ammonium sulfate was 1.56 ± 0.12 while the refractive index of pinene SOA was 1.45 ± 0.02. Minimal light absorption was observed for these aerosols due to scattering cross sections indistinguishable from extinction. Single scatter albedo (SSA) values of soil dusts and biomass burning aerosols were in the range of 0.74 and 0.84, showing significant absorption. Results for ambient aerosols were also considered and compared to values obtained with a particle-soot absorption photometer (PSAP) which measured absorption at visible wavelengths. Results for optical absorption made with the aerosol albedometer at 355 nm and PSAP at visible range correlated well. Absorption angstrom exponents in the range of 0.6 - 1.6 were observed.