Abstract
We compare tropospheric NO 2 column measurements from the Ozone Monitoring Instrument (OMI) aboard the EOS Aura satellite with coincident in situ aircraft measurements on vertical spirals over the southern United States, Mexico, and the Gulf of Mexico during the INTEX-B campaign in March 2006. Good correlation with no significant bias ( r 2 = 0.67 , slope = 0.99 ± 0.17 , n = 12 ) is found for the ensemble of comparisons when the aircraft could spiral sufficiently low to sample most of the NO 2 column. Urban spirals where large extrapolations were needed below the aircraft floor (1000 ft) showed poorer agreement. We use the OMI observations together with a global chemical transport model (GEOS-Chem) to estimate emissions of nitrogen oxides over the eastern United States and Mexico in March 2006. Comparison to EPA's National Emissions Inventory 1999 (NEI99) calls for a decrease in power plant emissions and an increase in on-road vehicle emissions relative to that inventory. The rise in vehicular emissions is offsetting the reduction in power plant and industry emissions. These findings are consistent with independent assessments. Our OMI-derived emission estimates for Mexico are higher by a factor of 2.0 ± 0.5 than bottom-up emissions, similar to a comparison between the recently released Mexican NEI99 inventory and the bottom-up showing that the Mexican NEI99 inventory is 1.6–1.8 × higher.
Highlights
Nitrogen oxides ðNOx 1⁄4 NO þ NO2Þ are shortlived (3–24 h at the surface) species that catalyze ozone production and contribute to aerosol formation in the troposphere
We find a small increase of 3.2% in total NOx emissions between March 1999 (NEI99, 0.452 Tg N) and March 2006, indicating that the increase in vehicular emissions is offsetting the reduction in power plant and industry emissions
The first phase of the Intercontinental Chemical Transport Experiment—Phase B (INTEX-B) campaign during March 2006 provided extensive in situ observations of NO2 concentrations throughout the troposphere over the southern United States, the Gulf of Mexico, and the Mexican mainland including Mexico City
Summary
Nitrogen oxides ðNOx 1⁄4 NO þ NO2Þ are shortlived (3–24 h at the surface) species that catalyze ozone production and contribute to aerosol formation in the troposphere. Since 1995, satellite retrievals of tropospheric NO2 columns extend the existing ground-based and aircraft observational database with ever more spatial detail and coverage These satellite observations offer the possibility to improve our understanding of NOx sources and chemistry by testing or improving emission inventories through inverse (top-down) modelling techniques Most evaluation of GOME and SCIAMACHY NO2 retrievals has relied on serendipitous overlap with other measurements (e.g. ground-based Schaub et al, 2006 or aircraft-based Martin et al, 2004) These comparisons relied heavily on assumptions on the spatial and temporal representativity of the validation data for satellite scenes with sizes of 60 Â 30 km (SCIAMACHY) and 320 Â 40 km (GOME). In this paper we constrain NOx emissions over the United States, and over Mexico, where emission inventories for Mexican NOx sources are still highly uncertain (Kuhns et al, 2005)
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