Constraints From Airborne 210Pb Observations on Aerosol Scavenging and Lifetime in a Global Chemical Transport Model

Cloud (in-cloud and below-cloud) scavenging is the dominant loss process for a whole suite of tropospheric aerosols and largely determines their burden, distribution, and lifetime. Its parameterization in current global models has been identified by the IPCC AR5 as a major source contributing to uncertainties in model estimated aerosol lifetime and radiative forcing. The atmospheric radionuclide, ²¹⁰Pb (radioactive half-life of 22.3 years), is produced by radioactive decay of land-emitted gaseous ²²²Rn. It attaches to ambient submicron aerosols and its fate becomes that of those aerosols. It has long been recognized as an excellent tracer for studying cloud scavenging of aerosols. In a previous study using the GEOS-Chem chemical transport model, the global lifetime of tropospheric ²¹⁰Pb aerosol was estimated to be ~9 days. Detailed parameterizations of scavenging processes (e.g., scavenging in ice and mixed-phase cloud) have been recently implemented in the model, and new GEOS products (MERRA) containing more information about cloud and precipitation are now used as the driving meteorology. However, the model simulated tropospheric ²¹⁰Pb lifetime and spatial distribution have not been examined in terms of its sensitivity to the updated scavenging parameterization and meteorology. In this study, we use ²¹⁰Pb profiles measured during the NASA aircraft campaigns in the past two decades to constrain cloud scavenging parameterization of aerosols in GEOS-Chem. The airborne measurements cover four continents and two oceans, allowing us to obtain an estimated global tropospheric ²¹⁰Pb aerosol lifetime constrained by observations from different regions and altitudes. We assess the performance and impacts of the scavenging scheme in various climate (e.g., tropics, Polar Regions, and upper troposphere) and conduct sensitivity tests on parameterizations of in-cloud and below-cloud processes. Initial results indicate that the global tropospheric ²¹⁰Pb aerosol lifetime can vary from 5.3 to 8.8 days depending on whether or not in-cloud scavenging by snow, in-cloud impaction scavenging, and temperature-dependent cloud water content are included in the model. We find that inclusion of scavenging in mixed-phase and ice clouds significantly improves simulated ²¹⁰Pb in the upper troposphere.