November 2007
Validation of OMI tropospheric NO² observations during INTEX-B and application to constrain NO_x emissions over the eastern United States and Mexico

We compare tropospheric NO2 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 (r² = 0.67, slope=0.99 ± 0.17, n = 21) is found for the ensemble of comparisons when the aircraft could spiral sufficiently low to sample most of the NO2 column. Urban spirals where large extrapolations were needed below the aircraft floor (1000 feet) 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. These findings are consistent with independent assessments. Our OMI-derived emission estimates for Mexico are higher by a factor of 2.0 ± 0.3 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 x higher.

The figures above show the monthly mean (March 2006) tropospheric NO2 columns observed by OMI at 13:30 hrs on days that the instrument took a measurement under cloud-free conditions. The circles indicate the location of aircraft profiles over sea (white) and land (black). The inset panel shows the comparison of coincident OMI tropospheric NO2 columns with those determined from LIF in situ measurements from the DC-8 aircraft. Vertical bars indicate OMI retrieval uncertainty, and horizontal bars illustrate the estimated uncertainty of the DC-8 LIF measurements. The dashed line represents the reduced major-axis regression through all data with a slope of 1.40±0.21 (r²=0.79). The solid line represents the reduced major axis (RMA) regression for situations when less than 30% of the DC-8 column has been extrapolated (diamonds) with a slope of 0.99±0.17 (r²=0.67).

For more information see Boersma et al. [2007].