Seasonal transition from NOx‐ to hydrocarbon‐limited conditions for ozone production over the eastern United States in September

Concentrations of O 3 , CO, NO, total reactive nitrogen oxides (NO y ), H 2 O 2 , 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 NO x ‐limited to hydrocarbon‐limited conditions for O 3 production over the course of September. The transition is diagnosed by large decreases of the H 2 O 2 /(NO y ‐NO x ) and HCHO/NO y concentration ratios, weakening of the correlation between O 3 and NO y ‐ NO x concentrations, and decrease of the slope ΔO 3 /Δ(NO y ‐NO x ). A high‐O 3 episode occurring in late September was associated with only 0.34 ppbv H 2 O 2 , 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 HNO 3 production to become the dominant sink for odd hydrogen in the boundary layer and resulting in hydrocarbon‐limited conditions for O 3 production. Seasonal decline of isoprene emission can greatly accentuate the transition.