Evaluation of aerosol wet deposition in the NASA GEOS Chemistry-Climate Model using 210Pb

Wet deposition is the dominant sink for a whole suite of aerosols in the troposphere but model parameterizations of this process are highly uncertain, substantially contributing to large uncertainties in the simulated loadings and radiative forcing of aerosols. Here we evaluate and constrain aerosol wet deposition in the NASA Goddard Earth Observing System (GEOS) Chemistry-Climate Model (GEOS CCM) by simulating aerosol tracer lead-210 (²¹⁰Pb). Lead-210 (radioactive half-life 22.3 years) is the decay daughter of radon-222 (²²²Rn, half-life 3.8 days), a radioactive gas emitted from continental crust. Lead-210 attaches immediately after production to ambient submicron aerosols and is subsequently subject to dry and wet deposition. Because of its relatively well-known sources and chemically inert nature with wet deposition as the principal sink, ²¹⁰Pb is a useful tracer for testing wet deposition processes in a global model. Our simulation of moist transport and removal as part of the GOCART aerosol module includes scavenging in convective updrafts, midlevel entrainment and detrainment, and first-order rainout and washout from large-scale precipitation. We calculate the global budget and mean lifetime (against deposition, 6.7 days) of tropospheric ²¹⁰Pb. We compare GEOS free-running simulations with climatological records of ²¹⁰Pb concentrations and deposition fluxes at surface sites worldwide, and with the upper troposphere and/or lower stratosphere (UT/LS) ²¹⁰Pb climatology constructed from ∼25 years of aircraft and balloon data. We conduct GEOS "replay" simulations and test the wet deposition parameterization against ²¹⁰Pb profile measurements during eleven NASA airborne campaigns. The use of more physically based parameterizations of wet scavenging in the model will also be discussed.