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Title: Optical Properties of Moderately-Absorbing Organic and Mixed Organic/Inorganic Particles at Very High Humidities

Relative humidity (RH) affects the water content of an aerosol, altering its ability to scatter and absorb light, which is important for aerosol effects on climate and visibility. This project involves in situ measurement and modeling of aerosol optical properties including absorption, scattering and extinction at three visible wavelengths (467, 530, 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. Extensive instrumentation development and benchmarking with independently measured and modeled values were used to obtain and evaluate these new results. The single scattering albedo value for a dry absorbing polystyrene microsphere benchmark agreed within 0.02 (absolute value) with independently published results at 530 nm. Light absorption bymore » a nigrosin (sample light-absorbing) benchmark increased by a factor of 1.24 +/-0.06 at all wavelengths as RH increased from 38 to 95%. Closure modeling with Mie theory was able to reproduce this increase with the linear volume average (LVA) refractive index mixing rule for this water soluble compound. Absorption by biomass OC aerosol increased by a factor of 2.1 +/- 0.7 and 2.3 +/- 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. Closure modeling with the dynamic effective medium approximation (DEMA) refractive index model was able to capture the increasing absorption trend with RH indicating that the droplets were heterogeneously mixed while containing dispersed insoluble absorbing material within those droplets. Seven other refractive index mixing models including LVA did not adequately describe the measurements for OC. Mixing the biomass OC aerosol with select mass fractions of ammonium sulfate ranging from 25 to 36% and sodium chloride ranging from 21 to 30% resulted in an increase in light scattering and extinction with RH and inorganic mass fraction. However, no detectable difference in light absorption behavior in comparison to pure biomass OC was observed. 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.« less
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Resource Type:
Technical Report
Research Org:
University of Illinois at Urbana-Champaign
Sponsoring Org:
USDOE SC Office of Biological and Environmental Research (SC-23)
Country of Publication:
United States