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

Weekly values of the net O 3 production efficiency (OPE), defined as the net number of O 3 molecules produced per molecule of NO x (NO + NO 2 ) consumed, are estimated from a 1990–1994 record of O 3 , NO x , NO y , CO, and C 2 H 2 concentrations at Harvard Forest, Massachusetts. The OPE is inferred from the slope ΔO 3 /Δ(NO y − NO x ) of the linear regression between O 3 and NO y ‐NO x concentrations (NO y is the sum of NO x and its oxidation products); and alternatively from the slopes ΔO 3 /ΔCO and ΔO 3 /ΔC 2 H 2 multiplied by regional estimates of the CO/NO x and C 2 H 2 /NO x emission ratios. The mean OPE values inferred from ΔO 3 /Δ(NO y − NO x ) are 3–5 times higher than those inferred from ΔO 3 /ΔCO or ΔO 3 /ΔC 2 H 2 ; the discrepancy may be due to the effects of HNO 3 and O 3 deposition and also to uncertainties in the CO/NO x and C 2 H 2 /NO x emission ratios. The relative seasonal trends of the OPE derived from ΔO 3 /Δ(NO y − NO x ), ΔO 3 /ΔCO, and ΔO 3 /ΔC 2 H 2 are, however, similar. Thus ΔO 3 /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 O 3 background at Harvard Forest, defined by the y intercept of the O 3 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 O 3 concentrations at Harvard Forest peak in May–June even though the OPE peaks in June–July.