LAUSR

Significance Dry deposition is a key sink of atmospheric particles, which impact human and ecosystem health, and the radiative balance of the planet. However, the deposition parameterizations used in climate and air-quality models are poorly constrained by observations. Dry deposition of submicron particles is the largest uncertainty in aerosol indirect radiative forcing. Our particle flux observations indicate that dry deposition velocities are an order of magnitude lower than models suggest. Our updated, observation-driven parameterizations should reduce uncertainty in modeled dry deposition. The scheme increases modeled accumulation mode aerosol number concentrations, and enhances the combined natural and anthropogenic aerosol indirect effect by −0.63 W m−2, similar in magnitude to the total aerosol indirect forcing in the Intergovernmental Panel on Climate Change report. Wet and dry deposition remove aerosols from the atmosphere, and these processes control aerosol lifetime and thus impact climate and air quality. Dry deposition is a significant source of aerosol uncertainty in global chemical transport and climate models. Dry deposition parameterizations in most global models were developed when few particle deposition measurements were available. However, new measurement techniques have enabled more size-resolved particle flux observations. We combined literature measurements with data that we collected over a grassland in Oklahoma and a pine forest in Colorado to develop a dry deposition parameterization. We find that relative to observations, previous parameterizations overestimated deposition of the accumulation and Aitken mode particles, and underestimated in the coarse mode. These systematic differences in observed and modeled accumulation mode particle deposition velocities are as large as an order of magnitude over terrestrial ecosystems. As accumulation mode particles form most of the cloud condensation nuclei (CCN) that influence the indirect radiative effect, this model-measurement discrepancy in dry deposition alters modeled CCN and radiative forcing. We present a revised observationally driven parameterization for regional and global aerosol models. Using this revised dry deposition scheme in the Goddard Earth Observing System (GEOS)-Chem chemical transport model, we find that global surface accumulation-mode number concentrations increase by 62% and enhance the global combined anthropogenic and natural aerosol indirect effect by −0.63 W m−2. Our observationally constrained approach should reduce the uncertainty of particle dry deposition in global chemical transport models.