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

 
Poster PDF

Authors

Mark J. Rood — University of Illinois at Urbana-Champaign
Tami Bond — University of Illinois, Urbana

Category

Aerosol Properties

Description

Optical properties at varying RH conditions for primary OC aerosols generated by pyrolysis of biomass. Extinction σep and scattering σsp for three wavelengths are shown in the top two (a), (b), and lower left graph (c); absorption σap by difference is shown in the bottom graph (d).
This project involves in situ closure studies between predictions and measurements of aerosol optical properties including absorption, scattering, and extinction at three visible wavelengths, for organic carbon (OC) biomass combustion aerosols, inorganic aerosols, and their mixtures at controlled relative humidity (RH) conditions. Novel components of this project include investigation of: (1) changes in all three of these optical properties at continuously scanned RH conditions; (2) optical properties at RH values up to 95%, which are usually extrapolated instead of confirmed; and (3) examination of aerosols generated by the pyrolysis of wood, which is representative of primary atmospheric organic carbon and its mixture with inorganic aerosols. This year’s activities focused on the measurement of wood pyrolysis generated OC aerosols. Light absorbing benchmarks (polystyrene microspheres (PSMs) and nigrosin) were additionally evaluated to verify the operation of the instrumentation. The newly developed equipment measures scattering and extinction to infer light absorption and single-scattering albedo values. The measured single-scattering albedo for PSMs agreed within 0.02 with previously reported results at 530 nm wavelength. Increasing RH values from 39% to 95% enhanced the light absorption of the nigrosin benchmark by a factor of 1.25. Such an enhancement is higher than expected, but modeling results suggest that it could be caused by incomplete dissolution of nigrosin in the aqueous droplets. Results for the biomass OC aerosols indicate an increase in light absorption by a factor of two between 32% and 95% RH. Additionally, the spectral dependence of absorption by OC that was observed previously with filter measurements was confirmed in situ. Work in the immediate future will study the effect of mixing the pyrolysis-generated OC aerosols with inorganic compounds on the optical properties of the resulting aerosols at controlled RH conditions.