Significant uncertainties remain with aerosol light extinction properties, their relative humidity (RH) dependence, and the resulting impacts on atmospheric radiative transfer. The water uptake properties of laboratory generated biomass smoke were probed with two instruments: a newly developed and validated humidified aerosol single scattering albedometer (H-CAPS PMssa) and a controlled humidity nephelometry system. The H-CAPS PMssa simultaneously measures in-situ aerosol light extinction and scattering using a cavity-attenuated phase shift-single scattering albedo PM monitor (CAPS-PMSSA, Aerodyne, Inc.). The CAPS-PMSSA allows humidity-dependent retrieval of aerosol light absorption by difference and provides a direct quantification of aerosol single scattering albedo (SSA). Laboratory combustion experiments used a controlled temperature tube furnace as a closed, controlled combustion environment. Results show firstly the presence of inorganic species, particularly prevalent with halophytic plant species from saline soil regions, is necessary for strong hygroscopicity. Smoldering combustion at T < 600C diminished the hygroscopic response, while flaming combustion at T > 700 C enhanced it. Online in-situ measurements of aerosol composition using aerosol mass spectrometry showed a high ratio of chloride plus sulfate ions to organic carbon characterized the more hygroscopic smoke samples. The results will be discussed in the context of the importance of aerosol water uptake to atmospheric visibility, aerosol chemical evolution, and the direct and indirect effects of aerosols on climate.
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