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 NOx (NO + NO2) consumed, are estimated from a 1990-1994 record of O-3, NOx, NOy, CO, and C2H2 concentrations at Harvard Forest, Massachusetts. The OPE is inferred from the slope Delta O-3/Delta(NOy-NOx) of the linear regression between O-3 and NOy-NOx concentrations (NOy is the sum of NOx and its oxidation products); and alternatively from the slopes Delta O-3/Delta CO and Delta O-3/C2H2 multiplied by regional estimates of the CO/NOx and C2H2/NOx emission ratios. The mean OPE values inferred from Delta O-3/(NOy-NOx) are 3-5 times higher than those inferred from Delta O-3/Delta CO or Delta O-3/Delta C2H2; the discrepancy may be due to the effects of HNO3 and O-3 deposition and also to uncertainties in the CO/NOx and C2H2/NOx emission ratios. The relative seasonal trends of the OPE derived from Delta O-3/(NOy-NOx), Delta O-3/Delta CO, and Delta O-3/C2H2 are, however, similar. Thus Delta O-3/(NOy-NOx) 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 NOy-NOx 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.