Optical properties of moderately absorbing organic and mixed organic/inorganic particles at high humidities

Description

Relative humidity (RH) affects the water content of an aerosol, altering its scattering and absorption of light, which is important for aerosol effects on climate and visibility. This project involves in situ prediction and measurement studies of aerosol optical properties including absorption, scattering, and extinction at three visible wavelengths (467, 530, and 660 nm) for organic carbon (OC) generated by pyrolysis of biomass, ammonium sulfate, and sodium chloride, and their mixtures at controlled RH conditions. Novel components of this project include investigation of: (1) Changes in all three of these optical properties at scanned RH conditions; (2) optical properties at RH values up to 95%, which are usually extrapolated instead of measured; and (3) examination of aerosols generated by the pyrolysis of wood, which is representative of primary atmospheric organic carbon, and its mixture with inorganic aerosol.

Scattering and extinction values were used to determine light absorption by difference and single-scattering albedo values. The single-scattering albedo value for dry absorbing polystyrene microsphere benchmark agreed within 0.02 (absolute value) with independently published results at 530 nm. Light absorption by the nigrosin benchmark increased by a factor of 1.24+/-0.06 at all wavelengths as RH increased from 38 to 95%.

Absorption by biomass OC aerosol increased by a factor of 2.2+/-0.7 and 2.7+/-1.2 between 32 and 95% RH at 467 nm and 530 nm, but there was no detectable absorption at 660 nm. Additionally, the spectral dependence of absorption by OC that was observed with filter measurements was confirmed qualitatively in situ at 467 and 530 nm. Mixing the biomass OC aerosol with select mass fractions of ammonium sulfate ranging from 22 to 48% and sodium chloride ranging from 16 to 39% resulted in an increase in light scattering and extinction with RH and inorganic mass fraction. However, the mixed aerosol showed no detectable difference in light absorption behavior in comparison to pure biomass OC.

The main finding of this research is a measured increase in absorption with increasing RH, which is currently not represented in radiative transfer models even though biomass burning produces most of the primary OC aerosol in the atmosphere.

A new DOE ASR-sponsored project will study the effect of chemical aging with ultraviolet light and ozone on the optical and cloud-nucleating properties of the biomass OC aerosol at controlled RH conditions.