Ozone formation in the Houston-Galveston area: A regional chemical transport model study

An episodic simulation (during September 7–11, 1993) is conducted to characterize ozone (O₃) formation and to investigate the dependence of O₃ formation on precursors in the Houston-Galveston (HG) area using a regional chemical transport model. The model reproduces the observed temporal and spatial distributions of O₃ concentrations. Ground level O₃ exhibits an extraordinary diurnal variation in the HG area. Midday O₃ reaches a very high level due to rapid and efficient photochemical production, whereas at night there exists a shallow urban-scale O₃ deficit with near-zero concentrations due to high nighttime NO_x. The simulated net photochemical production rates of O₃ (P(O₃)) in the Houston area are much higher than those in most other U.S. urban cities, reaching 30–40 ppb hr −1 for typical daytime NO_x levels. Furthermore, the NO_x reversal value (the NO_x concentration at which P(O ₃) reaches maximum) is much larger than that in most other U.S. cities. The large abundance and high reactivity of anthropogenic volatile organic compounds (AVOCs), and the coexistence of abundant AVOCs and NO_x in this area as well, are responsible for the high O₃ production rates and the NO_x reversal value. The simulated O₃ production efficiency is typically 3–10 O₃ molecules per NO_x molecule emitted in midday hours in Houston. Heterogeneous conversion of NO ₂ to HONO on the surfaces of soot aerosol accelerates the O₃ production by about one hour in the morning, and leads to an appreciable increase of O₃ level with a midday average of 7 ppb. The model study suggests that the chemistry over most of the Houston metropolitan area is in or close to the transit regime of NO_x-VOC sensitivity, therefore the response of O₃ production to emission reductions of either VOC or NO _x is not very sensitive. Doubling AVOC emissions in the base case does not significantly affect the response of O₃ production to the reduction in AVOC emissions, but greatly impacts the response of O₃ production to the reduction in NO_x emissions. Biogenic VOCs do not contribute significantly to the O₃ pollution in the Houston area, but distant BVOC sources still can affect urban O₃ through long-range transport.