Ozone Production and Its Sensitivity to NOx and VOCs: Results from the DISCOVER-AQ Field Experiment, Houston 2013

An observation-constrained box model based on the Carbon Bond mechanism, Version 5 (CB05), was used to study photochemical processes along the NASA P-3B flight track and spirals over eight surface sites during the September 2013 Houston, Texas deployment of the NASA DISCOVER-AQ campaign. Data from this campaign provided an opportunity to examine and improve our understanding of atmospheric photochemical oxidation processes related to the formation of secondary air pollutants such as ozone (O₃). O₃ production and its sensitivity to NO_x and VOCs were calculated at different locations and times of day. Ozone production efficiency (OPE), defined as the ratio of the ozone production rate to the NO_x oxidation rate, was calculated using the observations and the simulation results of the box and Community Multiscale Air Quality (CMAQ) models. Correlation of these results with other parameters, such as radical sources and NO_x mixing ratio, was also evaluated. It was generally found that O₃ production tends to be more VOC sensitive in the morning along with high ozone production rates, suggesting that control of VOCs may be an effective way to control O₃ in Houston. In the afternoon, O₃ production was found to be mainly NO_x sensitive with some exceptions. O₃ production near major emissions was mostly VOC sensitive for the entire day, and in other urban areas, O₃ production was either VOC sensitive or in the transition regime. Areas farther from downtown Houston were mostly NO_x sensitive for the entire day. It was also found that the control of NO_x emissions has reduced O₃ concentrations over Houston, but led to larger OPE values. The results from this work strengthen our understanding of O₃ production; they indicate that controlling NO_x emissions will provide air quality benefits over the greater Houston metropolitan area in the long run, but in selected areas, controlling VOC emissions will also be beneficial.