Investigating the Spatiotemporal Variability of NO2 and Photochemistry in Urban Areas

The spatial distribution of nitrogen dioxide (NO2) is difficult to measure due to sparse ground-base monitoring and the poor spatial-resolution of space-based sensors. A better understanding of how satellite-derived NO2 columns compare to surface conditions will help in the assessment of regulations for improving air quality and reducing health risks. This dissertation addressed questions on the spatial and temporal variability of NO2 as monitored from the ground, aircraft, and space, as well as how emission reductions influenced the photochemical environment in Houston, Texas. Part one compared satellite (OMI), airborne photometry (GeoTASO), and in situ P-3B aircraft measurements of NO2 columns to those measured by a network of eleven ground-based Pandora spectrometers in Houston, TX during the NASA DISCOVER-AQ Texas campaign in September 2013. Results showed how the spatial resolution of measurements influenced the intercomparison due to the strong spatial variability of NO2 in urban areas. Part two studied the spatial heterogeneity of NO2 during the CalNex 2010 campaign in California by comparing three OMI tropospheric column retrievals (NASA Standard Product, KNMI DOMINO, and BEHR) and a new OMI downscaling technique to in situ aircraft measurements. Near urban environments, the aircraft measurements were not representative of the OMI observations as a result of the spatial heterogeneity of NO2 and the different spatial coverage of these two different observations. When OMI NO2 measurements were downscaled, the aircraft-to-downscale comparisons showed improvement for areas with high NO2 pollution. Finally, in part three, the LaRC photochemical box model was used to evaluate how ozone photochemistry had changed between 2000 and 2014 in Houston, Texas. The model results showed that the decline in the number and severity of ozone events in the Houston Ship Channel was due to significant decreases in highly reactive volatile organic carbons (HRVOCs). Furthermore, on high-ozone days, this chemical system transitioned to a more VOC-sensitive regime resulting in a decrease in the instantaneous ozone production efficiency. These results suggest that further reducing HRVOC emissions is the most efficient way to bring the Houston metropolitan area into compliance with the EPA’s ozone National Ambient Air Quality Standards (NAAQS).