Heterogeneous reactions have the potential to significantly impact urban ozone formation and total reactivity of the atmosphere. This dissertation used comparisons between photochemical modeling predictions and field measurements to examine heterogeneous N₂O₅ chemistry in Southeast Texas. Heterogeneous reactions of N₂O₅ can lead to two different products: nitric acid (HNO₃) and nitryl chloride (ClNO₂). The formation of HNO₃ results in a loss of reactive nitrogen from the atmosphere. In contrast ClNO₂ photolysis forms Cl radicals and NO₂, both of which promote ozone formation in the troposphere. Preliminary modeling identified key uncertainties and the need to perform more refined modeling which included updated PM emissions estimates, an updated gas-phase N₂O₅ hydrolysis reaction rate constant, updated reactive uptake coefficients, and the inclusion of ClNO₂ as a product of heterogeneous N₂O₅ uptake. Refined modeling which incorporated all of these improvements was carried out and was the first comprehensive modeling of this chemistry performed for an urban air pollution episode. Comparisons of aerosol surface area concentrations, N₂O₅ concentrations, HNO₃ concentrations, and ClNO₂ concentrations with ambient data showed that model predictions were reasonable. The exceptions to this were 1) over-predictions of aerosol surface area concentration peaks at altitudes above 1500 meters and 2) over-prediction of N₂O₅ concentrations in the Houston Ship Channel. Further analysis is needed to identify the reasons for these over-predictions. Other key findings from this modeling include the model prediction of inland chlorine concentrations high enough to form ClNO₂ and the prediction that a large portion of atmospheric chlorine is cycled through ClNO₂, therefore making the inclusion of ClNO₂ into photochemical models essential for properly simulating chlorine chemistry. In addition, modeling suggested that the chemistry leads to significant increases of NO[subscript x] at night, but decreases in daytime NO[subscript x] concentrations and that the overall effect was to decrease ozone concentrations. Further investigation into the effect of ClNO₂ as a chlorine source showed that likely ozone increases in the Houston area caused by the presence of this compound are on the order of several ppb. Further analyses showed that vertical dispersion and local atmospheric composition moderated the effect of nitryl chloride on ozone mixing ratios. / text

Ambient observations have indicated that ozone formation in the Houston area is frequently faster and more efficient, with respect to NOx consumed, than other urban areas in the country. It is believed that these unique characteristics of ozone formation in the Houston area are associated with the plumes of reactive hydrocarbons, emanating from the industrial Houston Ship Channel area. Thus, accurate quantification of industrial emissions, particularly of reactive hydrocarbons, is critical to effectively address the rapid ozone formation and the consequent high levels of ozone in the area. Industrial emissions of hydrocarbons have significant temporal variability as evidenced by various measurements, but they have been assumed to be continuous at constant levels for air quality regulation and photochemical modeling studies. This thesis examines the effect of emission variability from industrial sources on ozone formation in the HoustonGalveston area. Both discrete emission events and variability in continuous emissions are examined; new air quality modeling tools have been developed to perform these analyses. Also, this thesis evaluates the impact of emission variability on the effectiveness of emission control strategies in the Houston-Galveston area. Overall, the results indicate that industrial emission variability plays a substantial role in ozone formation and that controlling emission variability can be effective in ozone reduction. These results suggest that a quantitative treatment of emission variability should be included in the development of air quality plans for regions with extensive industrial activity, such as Houston.