Validation of the NO2 Tropospheric Column Product from Ozone Mapping Instrument by In-Situ Aircraft Data in Urban Environments

Validation of the Earth Observing System Aura's sensor Ozone Mapping Instrument (OMI) is especially crucial in urban regions, where numerous sources of pollutants are present at concentrations higher than rural settings. Using data from the Student Airborne Research Program (June 2012) and the CalNex (May-June 2010), in-situ profiles of Nitrogen Dioxide (NO₂) were measured and converted to tropospheric NO₂ columns to compare to the satellite sensor. Cloud free skies and a direct satellite overpass on the chosen flight days contributed to the ideal conditions for the validation of OMI's NO₂ tropospheric column product. The NO₂ density columns were determined by integrating the aircraft NO₂ vertical profiles using linear interpolations and the assumption of a constant free tropospheric background. Initial results show a significant low bias of the OMI tropospheric column densities in comparison to in-situ measurements in these highly polluted areas. A fraction of the bias is likely the result of the combination of sub-pixel variability (e.g. NO₂ hotspots) and the poor spatial resolution of OMI (13x24km² at nadir). Little validation of this product has been conducted in significantly polluted urban regions. However, during the INTEX-B campaign in 2006, two spirals were performed in NASA's DC-8 over Mexico City. These measurements showed a high bias from the satellite, and this is likely due to the elevation of the city (7,940 ft) and the changing sensitivity of the instrument at different altitudes. Additional research needs to be done to investigate why these biases are occurring in urban regions and if the satellite algorithms can be modified to better represent the polluted environments.