Atmospheric Chemistry Program Seminar

Revisiting dry deposition of trace gases and particles in the atmosphere

Delphine K. Farmer
Associate Professor, Department of Chemistry, Colorado State University

"Dry deposition is a key process that removes trace gases and particles from the atmosphere, and thus one factor that controls the atmospheric lifetime of pollutants and short-lived climate forcers. In fact, dry deposition is the single largest component of uncertainty in our understanding of aerosol effects on climate. Despite its importance, dry deposition of trace gases and particles is poorly constrained by observations due to the instrumental challenge in measuring surface-atmosphere exchange. Instruments must be adequately fast, sensitive and selective to measure low concentrations on the rapid (<1 s) timescale of turbulence. We have developed several measurement techniques that use the eddy covariance approach to flux measurements over terrestrial surfaces incorporating both spectroscopy and mass spectrometry. This talk will be divided into two parts – the first considering the mechanisms that control forest-atmosphere exchange of acidic organic molecules, and the second revisiting our understanding of size-resolved particle deposition in the atmosphere. We contrast these observations with previous measurements in the literature, and with commonly used resistance models, highlighting several model-measurement discrepancies. To further investigate the mechanisms of particle deposition, we use black carbon deposition as an inert tracer for particle wet and dry deposition. We show that wet deposition dominates in an agricultural environment in Oklahoma, and provide observational constraints on black carbon lifetime in this region. Our suite of observations inform a revised model parameterization, which we incorporate in a global chemical transport model to investigate how dry deposition can impact particle loading and the radiative balance of the atmosphere. Together, these new measurements highlight the importance of observational constraints in developing, validating, and revising models of fundamental chemical and physical processes in the atmosphere – and in reducing uncertainties in our understanding of climate."