Evaluate transport processes in MERRA driven chemical transport models using updated 222Rn emission inventories and global observations

Abstract:

Convective and synoptic processes play a major role in determining the transport and distribution of trace gases and aerosols in the troposphere. The representation of these processes in global models (at ~100-1000 km horizontal resolution) is challenging, because convection is a sub-grid process and needs to be parameterized, while synoptic processes are close to the grid scale. Depending on the parameterization schemes used in climate models, the role of convection in transporting trace gases and aerosols may vary from model to model. ²²²Rn is a chemically inert and radioactive gas constantly emitted from soil and has a half-life (3.8 days) comparable to synoptic timescale, which makes it an effective tracer for convective and synoptic transport. In this study, we evaluate the convective and synoptic transport in two chemical transport models (GMI and GEOS-Chem), both driven by the NASA’s MERRA reanalysis. Considering the uncertainties in ²²²Rn emissions, we incorporate two more recent scenarios with regionally varying ²²²Rn emissions into GEOS-Chem/MERRA and compare the simulation results with those using the relatively uniform ²²²Rn emissions in the standard model. We evaluate the global distribution and seasonality of ²²²Rn concentrations simulated by the two models against an extended collection of ²²²Rn observations from 1970s to 2010s. The intercomparison will improve our understanding of the spatial variability in global ²²²Rn emissions, including the suspected excessive ²²²Rn emissions in East Asia, and provide useful feedbacks on ²²²Rn emission models. We will assess ²²²Rn vertical distributions at different latitudes in the models using observations at surface sites and in the upper troposphere and lower stratosphere. Results will be compared with previous models driven by other meteorological fields (e.g., fvGCM and GEOS4). Since the decay of ²²²Rn is the source of ²¹⁰Pb, a useful radionuclide tracer attached to submicron aerosols, improved understanding of emissions and transport of ²²²Rn will provide insights into the transport, distribution, and wet deposition of ²¹⁰Pb aerosols.