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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. D5, PAGES 9315–9324, 1995

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

Daniel J. Jacob

Larry W. Horowitz

J. William Munger

Brian G. Heikes

Russell R. Dickerson

Richard S. Artz

William C. Keene

Abstract

Concentrations of O₃, CO, NO, total reactive nitrogen oxides (NO_y), 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 NO_x-limited to hydrocarbon-limited conditions for O₃ production over the course of September. The transition is diagnosed by large decreases of the H₂O₂/(NO_y-NO_x) and HCHO/NO_y concentration ratios, weakening of the correlation between O₃ and NO_y- NO_x concentrations, and decrease of the slope ΔO₃/Δ(NO_y-NO_x). A high-O₃ 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 O₃ production. Seasonal decline of isoprene emission can greatly accentuate the transition.

Received 1 January 1994; accepted 22 November 1994.

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Citation: Jacob, D. J., L. W. Horowitz, J. W. Munger, B. G. Heikes, R. R. Dickerson, R. S. Artz, and W. C. Keene (1995), Seasonal transition from NO_x- to hydrocarbon-limited conditions for ozone production over the eastern United States in September, J. Geophys. Res., 100(D5), 9315–9324.

Copyright 1995 by the American Geophysical Union.