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

Abstract

[1] This paper presents a comparison of NO 2 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 ($13 Â 24 km 2 at nadir) NO 2 vertical column density maps. We describe the retrieval, which includes an estimation of the stratospheric and tropospheric fraction of total NO 2 columns, the air mass factor (AMF) correction based on detected tropospheric NO 2 enhancements, and the generation of the gridded data product. We present a validation study of the gridded NO 2 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 À0.38 Â 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.

NO<sub>2</sub> measurements in Hong Kong using LED based long path differential optical absorption spectroscopy

“…February, April and May of 2010. In most of the months, the error bars of OMI data, either within 15 km or within 50 km, do not overlap with the LP-DOAS data, which indicated that there are some unknown systematic errors either in the OMI retrieval (Wenig et al, 2008) or the VCDs to ground level mixing ratios conversion, or both. One possible reason for the lower OMI values over Hong Kong compared to the LP-DOAS measurements is that the OMI a-priori profile used for converting the tropospheric NO 2 VCDs to ground level mixing ratios is averaged over a large area around the Pearl River Delta Region including large fractions of ocean and rural area with lower relative ground level mixing ratio.…”

“…This gridding scheme can also enhance the details in the gridded product by giving smaller OMI pixels higher weights (Wenig et al, 2008). …”

“…Weighting with 1/VCD 2 err yields an average bias toward low VCD values. Following the approach introduced by Wenig et al (2008), we adopt a weighting scheme which only depends on the error caused by the cloud cover C and the OMI pixel size A for the calculation of VCD GRID…”

Dead time effect on the Brewer measurements: correction and estimated uncertainties

“…Effects in the calculation of the total columns of SO 2 and NO 2 are not discussed, because uncertainties from other sources are much higher due to the usually small column amounts (the order of a few DU) of these species (Fioletov et al, 1998;Wenig et al, 2008). Errors in DT are also expected to affect the results of different diagnostic tests in the Brewer, such as the measured intensity of the internal lamps and the determination of the transmittance of the ND filters, which in turn may affect the accuracy of the final products.…”

Ground‐level nitrogen dioxide concentrations inferred from the satellite‐borne Ozone Monitoring Instrument

“…Recent comparisons of tropospheric NO 2 columns from the OMI standard product (Version 1.0.0) with observations reveal a low bias of 14% versus an ensemble of aircraft measurements , of 25% versus Brewer measurements at NASA Goddard [Wenig et al, 2008], and of 15-30% versus a suite of ground-based remote sensing and aircraft measurements . All three manuscripts describe concerns with their data sets that motivate additional validation activities.…”