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In this study we present the seasonal variability of ozone productionefficiencies (EN), defined as the net number of ozone molecules producedper molecule of nitrogen oxides (nitrogen oxide (NO) + nitrogen dioxide(NO2)=NOx) oxidized to NOz (total reactive nitrogen(NOy)–NOx) determined from field measurements of a seven-yearperiod (1998–2004) at the Swiss high-alpine research station Jungfraujoch(JFJ), 3580 m a.s.l. This dataset is a unique long-term data series ofnitrogen levels in the free troposphere over Central Europe and hence itoffers an excellent opportunity to perform such an analysis and providefurther evidence to the photochemical origin of the ozone spring maximum atlocations of the northern hemisphere distant from nearby pollution sources.Experimentally derived daily EN values have been selected for 571 daysout of the 2557 days from 1998 to 2004, from which an average ozoneproduction efficiency of 18.8±1.3 molecules of O3 produced per moleculeof NOx oxidized was calculated. This value indicates the great potentialand importance of photochemical ozone production in the free troposphere.The monthly means of experimentally derived daily EN values show aseasonal variation with lower values from May to August, which can beprobably attributed to more efficient vertical transport of polluted airmasses from the atmospheric boundary layer up to JFJ. In agreement,theoretically derived monthly EN values show similar seasonal variation.The ratio NOy/CO, a parameter to assess the aging process that hasoccurred in an air parcel, was used as a criterion to disaggregate the 571selected days between undisturbed and disturbed free tropospheric (FT). Themonthly means of experimentally derived EN values for the undisturbed FTconditions show a distinct seasonal cycle with higher values in the coldseason from November to April. The EN values for undisturbed FTconditions are particularly higher than the respective monthly EN valuesfor disturbed FT conditions from February to October. It should be notedthat the monthly EN values of March (EN=35.8) and April(EN=34.9) are among the highest values throughout the year forundisturbed FT conditions at JFJ. These results highlight the key andpossibly the dominant role for photochemistry in the observed build-up oftropospheric ozone in the winter-spring transition period

Seasonal variability of measured ozone production efficiencies in the lower free troposphere of Central Europe