Global tropospheric NO2 column distributions: Comparing three-dimensional model calculations with GOME measurements

Tropospheric NO2 columns derived from the data products of the Global Ozone Monitoring Experiment (GOME), deployed on the ESA ERS-2 satellite, have be en compared with model calculations from two global three-dimensional chemi stry transport models, IMAGES and MOZART. The main objectives of the study are an analysis of the tropospheric NO2 data derived from satellite measure ments, an interpretation of it and evaluation of its quality using global m odels, and an estimation the role of NO2 in radiative forcing. The measured and modeled NO2 columns show similar spatial and seasonal patterns, with l arge tropospheric column amounts over industrialized areas and small column amounts over remote areas. The comparison of the absolute values of the me asured and modeled tropospheric column amounts are particularly dependent u pon uncertainties in the derivation of the tropospheric NO2 columns from GO ME and the difficulty of modeling the boundary layer in global models, both of which are discussed below. The measured tropospheric column amounts der ived from GOME data are of the same order as those calculated by the MOZART model over the industrialized areas of the United States and Europe, but a factor of 2-3 larger for Asia. The modeled tropospheric NO2 columns from M OZART as well as the column amounts measured by GOME are in good agreement with NO2 columns derived from observed NO2 mixing ratios in the boundary la yer in eastern North America. The comparison of the models to the GOME data illustrates the degree to which present models reproduce the hot spots see n in the GOME data. The radiative forcing of NO2 has been estimated from th e calculated tropospheric NO2 columns. The local maxima in the radiative fo rcing of tropospheric NO2 for cloud-free conditions over the eastern United States and western Europe represent 0.1-0.15 W m(-2), while values of 0.04 -0.1 W m(2) are estimated on a continental scale in these regions, of the s ame order of magnitude as the forcing of N2O and somewhat smaller than the regional forcing of tropospheric ozone. The globally averaged radiative for cing of tropospheric NO2 is negligible, similar to0.005 W m(-2).