Laboratory Water Uptake Studies of Biomass Smoke and Assessment of BC Parameterization in DOE Climate Model

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Description

Aerosol interactions with water control the evolution of optical properties and their cloud condensation nuclei activity that affect the Earth’s radiative budget. Water uptake increases--light scattering by aerosol growth, absorption by black carbon by lensing, cloud reflectivity, and lifetime and aerosol scavenging--with competing effects on climate forcing. Parameterizations must capture process details including composition of fresh emissions and the evolution of mixing state as they age to assess these effects in climate model. We report laboratory studies of fresh biomass burning smoke from Southwest US fuels at LANL. We measured a suite of aerosol properties with a focus on aerosol light extinction and their hygroscopic response. These properties were measured with a photoacoustic extinctiometer, a humidity-controlled nephelometry system, and a cavity-assisted phase shift instrument. A range of gas-phase species and particle sizing provided complementary detail. The fuels combusted included a range of coniferous, deciduous, and grass species including several invasive species like cheat grass and salt cedar. Whereas optical properties were strongly dictated by the flaming versus smoldering nature of the burn, we find that aerosol hygroscopicity is intimately linked to fuel type and its chemical composition--particularly its ion content. Hygroscopicity ranged from nearly hydrophobic to similar to that of pure deliquescent salts. Our results are used to guide development of a predictive framework of water uptake and optical properties aerosols from the fuel elemental composition, fire combustion phase, and eventual aging in the atmosphere. We also assessed the treatment of aerosols by the global CAMChem model. In particular, sensitivity of BC in MAM4 where it is transferred from the primary Aitken mode to the accumulation mode after accumulating 8 monolayers of material on aging is evaluated by varying this threshold from from 1 to 12 monolayers. We compare the timescales of this BC conversion in CAMChem that range from hours to several days to those in a more detailed BC mixing-state model PartMC MOZAIC. We examine the effects of parameters on global vertical distributions of BC to assess their importance and develop a strategy to constrain them with field data.