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Observational Constraints on the Mechanisms that Control Size- and Chemistry- Resolved Aerosol Fluxes over a Colorado Forest

The ultimate goal of the project is improving parameterizations of aerosol dry deposition, and integrating them into chemical transport models through ongoing collaborations with other researchers. Dry deposition is a known sink for atmospheric aerosols, but the mechanisms of this process, and thus the impact of deposition on aerosol lifetimes and chemistry, are, however, so poorly understood that accumulation mode dry deposition is the single largest contributor to uncertainty in cloud condensation nuclei. Studies have typically focused on developing parameterizations for size-resolved particle number flux, but recent work has suggested that this approach may be limited, as the chemical components of aerosols deposit at different rates. We hypothesize that these chemically resolved fluxes are controlled by the volatility of the aerosol, and are influenced by thermal gradients within forest canopies. Rapid in-canopy formation of oxidized volatile organic compounds leads to aerosol formation, which causes an upward flux component that, for some chemical species, out-competes deposition. In order to understand the processes controlling aerosol fate over forests, one needs to understand particle fluxes with both size and chemistry resolution, fluxes of semi-volatile oxidized species in the gas phase, aerosol volatility and thermal gradients throughout the forest canopy, and wet deposition. In particular, we will determine: (1) Seasonal variation in accumulation mode aerosol dry deposition rates over a temperate forest, and the potential importance of this sink on aerosol lifetime, (2) The role of volatility in controlling chemistry-resolved particle fluxes, and (3) The impact of fluxes of semi-volatile oxidized organic species in controlling aerosol fluxes.

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