Validation of OMI tropospheric NO2 column densities using direct-Sun mode Brewer measurements at NASA Goddard Space Flight Center

[1] This paper presents a comparison of NO2 data measured with the Ozone Monitoring Instrument (OMI) on board the EOS-AURA satellite with ground-based direct-Sun Brewer measurement data. Since its deployment in July 2004, OMI has provided more than 2 years of daily high-resolution (similar to 13 x 24 km 2 at nadir) NO2 vertical column density maps. We describe the retrieval, which includes an estimation of the stratospheric and tropospheric fraction of total NO2 columns, the air mass factor (AMF) correction based on detected tropospheric NO2 enhancements, and the generation of the gridded data product. We present a validation study of the gridded NO2 data set using data from a Brewer MK3 double monochromator in direct-Sun mode located at NASA Goddard Space Flight Center in Greenbelt, Maryland, USA. Monthly averages of coinciding measurements correlate well (r = 0.9) but OMI data are about 25% lower than the Brewer measurement data ( slope 0.75, intercept -0.38 x 10(15) molecules/cm(2)). We present a detailed uncertainty analysis for both ground and satellite data and discuss the possible reasons for the observed differences.