SEASONAL-VARIATION OF THE OZONE PRODUCTION EFFICIENCY PER UNIT NOX ATHARVARD-FOREST, MASSACHUSETTS

Weekly values of the net O-3 production efficiency (OPE), defined as t he 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 regress ion 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/D elta 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 De lta 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 th e 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 t he 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 fr om about 4 mol/mol in May to 8 mol/mol in June-July, and gradually dec reases back to 4 mol/mol by early October. The sharp rise of the OPE f rom May to June is attributed to onset of emission of the biogenic hyd rocarbon isoprene. The decline from July to October is attributed to d ecreases in isoprene emission and in solar radiation. The O-3 backgrou nd 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 Sept ember, consistent with observations at remote sites in northern midlat itudes. The seasonal trend in the background explains why mean O-3 con centrations at Harvard Forest peak in May-June even though the OPE pea ks in June-July.