TRANSPORT OF (222)RADON TO THE REMOTE TROPOSPHERE USING THE MODEL OF ATMOSPHERIC TRANSPORT AND CHEMISTRY AND ASSIMILATED WINDS FROM ECMWF AND THE NATIONAL-CENTER-FOR-ENVIRONMENTAL-PREDICTION NCAR

The Model of Atmospheric Transport and Chemistry (MATCH) is used to si mulate the transport of Rn-222 using both European Centre for Medium-R ange Weather Forecasts (ECMWF) winds and National Center for Environme ntal Prediction/National Center for Atmospheric Research (hereafter re ferred to as NCEP) reanalysis winds. These winds have the advantage of being based on observed winds but have the disadvantage that the subg rid-scale transport processes are not routinely archived. MATCH derive s subgrid-scale mixing rates for the boundary layer using a nonlocal s cheme and for moist convective mixing using one of two parameterizatio ns (Tiedtke [1989] or Pan and Wu [1997]). This paper describes the abi lity of the model to recreate mixing rates of Rn-222 using the forecas t center winds. Radon 222 is a species with a continental crust source and a simple sink involving radioactive decay with an e-folding times cale of 5.5 days. This atmospheric constituent is therefore a good tra cer for testing the vertical transport in the chemical transport model , as well as the horizontal transport from continental regions to remo te oceanic regions. The various simulations of Rn-222 are compared wit h observations as well as with each other, allowing an estimate of the uncertainty in transport due to uncertainties in the winds and subgri d-scale processes. The calculated vertical profiles over the western U nited States are somewhat similar to observed, and the upper troposphe ric concentrations compare reasonably well in their spatial distributi on with data collected during Tropospheric Ozone II (TROPOZ II), altho ugh the model values tend to be higher than observed values, especiall y in the upper troposphere. The model successfully simulates specific observed pollution events at Cape Grim. It has more difficulty at site s farther from continental source regions, although the model captures the seasonal structure of the pollution events at these sites (Macqua rie Island, Amsterdam Island, Kerguelen Island, and Crozet Island). In clusion of a moist convective mixing scheme in MATCH increases Rn-222 concentrations in the upper troposphere by 50% compared to not having moist convective mixing, while surface concentrations do not appear to be very sensitive to moist convection. In addition, differences betwe en the upper tropospheric concentrations of radon predicted using the ECMWF and NCEP winds can be 30% for large areas of the globe, due to e ither differences in the forecast center winds themselves or the moist convective mixing schemes used in conjunction with them. This has imp lications for model simulations of radiatively and chemically importan t trace species in the atmosphere.