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Laepple, T, Schultz, MG, Lamarque, JF, Madronich, S, Shetter, RE, Lefer, BL, Atlas, E (2005). Improved albedo formulation for chemistry transport models based on satellite observations and assimilated snow data and its impact on tropospheric photochemistry. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 110(D11), D11308.

[ 1] Present parameterizations of the UV surface albedo in global chemistry transport models are generally based on a crude land cover classification and do not account for interannual variations of the snow-covered surface or the large variability in the albedo of snow-covered surfaces. We developed an improved scheme based on 2 years of Moderate-Resolution Imaging Spectroradiometer ( MODIS) albedo data, a fine-resolution MODIS land cover map, Global Ozone Monitoring Experiment (GOME) albedo data, and daily assimilated snow cover maps from the European Centre for Medium-Range Weather Forecasts or the National Centers for Environmental Prediction. The new parameterization improves the calculation of photolysis frequencies in particular in the subarctic region as shown by a comparison of the calculated ratio of upwelling and downwelling actinic fluxes with spectral measurements from the Tropospheric Ozone Production About Spring Equinox (TOPSE) campaign ( January - May 2000). The impact of surface albedo changes on tropospheric photochemistry has been investigated using the global MOZART-2 chemistry transport model. Compared with the original model version, the surface albedo changes alter the tropospheric oxidizing capacity ( OH concentrations) between - 20 and + 200% locally and + 5% in the global annual mean. About half of this change results from a new value adapted for the ocean UV albedo. Locally, NOx concentrations were found to decrease by up to 40% and were most pronounced where the snow boundary crosses the high-emission regions in Europe, North America, and Asia. The interannual variability of snow and sea ice cover can lead to changes in the global tropospheric OH-concentration of 0.5%, which is of similar magnitude compared with the impacts of varying water vapor, transport, ozone column, and emissions as discussed in previous studies.



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