Impacts of electronically photo-excited NO₂ on air pollution control strategies in the South Coast Air Basin of California

A new path for hydroxyl radical formation via photo-excitation of nitrogen dioxide (NO2) and the reaction of photo-excited NO2 with water is evaluated using the UCI-CIT model for the South Coast Air Basin of California (SoCAB). Two separate studies predict different reaction rates, which differ by nearly an order of magnitude, for the reaction of photo-excited NO2 with water. Impacts of this new chemical mechanism on ozone and particulate matter formation, while utilizing both reaction rates, are quantified by simulating a two-day summer episode. In addition, sensitivity simulations are conducted to evaluate the uncertainty in the rate of reaction of photo-excited NO2 with water reported in the literature. Results indicate that the addition of photo-excited NO2 chemistry increases peak 1-h average ozone concentrations by up to 20.6%. Also, the new chemistry leads to moderate increases in particulate matter concentrations of up to 2.9%. The importance of this new chemistry is then evaluated in the context of pollution control strategies. A series of simulations are conducted to generate isopleths for ozone and particulate matter concentrations, varying baseline nitrogen oxides (NOx) and volatile organic compounds emissions. Results show that including NO2 photo-excitation increases the sensitivity of ozone concentration to changes in NOx emissions. Namely, increasing NOx when NO2 photo-excitation is included, while utilizing the higher reaction rate, leads to an increase in ozone concentration of up to 38.7% higher than a case without photo-excited NO2. Ozone and particulate matter control strategies rely heavily on the variation of NOx and VOC emissions and the addition of the new chemical mechanism increases peak ozone and 24-h average PM concentrations in all locations under all NOx and VOC scaling factors while utilizing both reaction rates. Therefore, three-dimensional air quality models should be modified to include this new OH production mechanism, especially if used to develop emission controls strategies.