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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D7, PAGES 12,659–12,666, 1996

Seasonal variation of the ozone production efficiency per unit NO_x at Harvard Forest, Massachusetts

Adam I. Hirsch

Division of Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts

J. William Munger

Division of Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts

Daniel J. Jacob

Division of Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts

Larry W. Horowitz

Division of Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts

Allen H. Goldstein

Division of Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts

Abstract

Weekly values of the net O₃ production efficiency (OPE), defined as the net number Of O₃ molecules produced per molecule Of NO_x (NO+NO₂) consumed, are estimated from a 1990–1994 record of O₃, NO_x, NO_y, CO, and C₂H₂ concentrations at Harvard Forest, Massachusetts. The OPE is inferred from the slope ΔO₃/Δ(NO_y-NO_x) of the linear regression between O₃ and NO_y-NO_x concentrations (NO_y is the sum of NO_x and its oxidation products); and alternatively from the slopes ΔO₃/ΔCO and ΔO₃/ΔC₂H₂ multiplied by regional estimates of the CO/NO_x and C₂H₂/NO_x emission ratios. The mean OPE values inferred from ΔO₃/Δ(NO_y-NO_x) are 3–5 times higher than those inferred from ΔO₃/ΔCO or ΔO₃/ΔC₂H₂; the discrepancy may be due to the effects of HNO₃ and O₃ deposition and also to uncertainties in the CO/NO_x and C₂H₂/NO_x emission ratios. The relative seasonal trends of the OPE derived from ΔO₃/Δ(NO_y-NO_x), ΔO₃/ΔCO, and ΔO₃/ΔC₂H₂ are, however, similar. Thus ΔO₃/Δ(NO_y-NO_x) increases from about 4 mol/mol in May to 8 mol/mol in June–July, and gradually decreases back to 4 mol/mol by early October. The sharp rise of the OPE from May to June is attributed to onset of emission of the biogenic hydrocarbon isoprene. The decline from July to October is attributed to decreases in isoprene emission and in solar radiation. The O3 background at Harvard Forest, defined by the y intercept of the O3 versus NO_y-NO_x regression line, decreases from 40 ppbv in May to 25 ppbv in September, consistent with observations at remote sites in northern midlatitudes. The seasonal trend in the background explains why mean O₃concentrations at Harvard Forest peak in May–June even though the OPE peaks in June–July.

Received 16 September 1995; accepted 30 January 1996.

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Citation: Hirsch, A. I., J. W. Munger, D. J. Jacob, L. W. Horowitz, and A. H. Goldstein (1996), Seasonal variation of the ozone production efficiency per unit NO_x at Harvard Forest, Massachusetts, J. Geophys. Res., 101(D7), 12,659–12,666.

Copyright 1996 by the American Geophysical Union.