SEASONAL TRANSITION FROM NOX- TO HYDROCARBON-LIMITED CONDITIONS FOR OZONE PRODUCTION OVER THE EASTERN UNITED-STATES IN SEPTEMBER

Concentrations of 0(3), CO, NO, total reactive nitrogen oxides (NOy), H2O2, and HCHO were measured from September 4 to October 1, 1990, at a mountain ridge site in Shenandoah National Park, Virginia. The data show evidence for a transition from NOx-limited to hydrocarbon-limited conditions for 0(3) production over the course of September. The transition is diagnosed by large decreases of the H2O2/(NOy-NOx) and HCHO/NOy concentration ratios, weakening of the correlation between 0(3) and NOy- NOx concentrations, and decrease of the slope Delta O-3/Delta(NOy-NOx). A high-0(3) episode occurring in late September was associated with only 0.34 ppbv H2O2, indicative of hydrocarbon-limited conditions. A seasonal transition in photochemical regime over the eastern United States in September would be expected from theory; the production rate of odd hydrogen radicals decreases by a factor of 2 over the course of the month, due to decreasing UV radiation and humidity, allowing HNO3 production to become the dominant sink for odd hydrogen in the boundary layer and resulting in hydrocarbon-limited conditions for 0(3) production. Seasonal decline of isoprene emission can greatly accentuate the transition.