Oxidized nitrogen and ozone production efficiencies in the springtime freetroposphere over the Alps

The Free Tropospheric Experiment (FREETEX'98) was conducted at the Jungfrau joch Observatory in the Swiss Alps (3580 m above sea level) during the well -documented spring maximum in ozone. In spring the Jungfraujoch frequently lies in the free troposphere but can also be influenced by air from the pla netary boundary layer. Measurements of NOx, NOy, peroxyacetylnitrate (PAN), HCHO, O-3, CO, nonmethane hydrocarbons, peroxy radicals, j((OD)-D-1),j(NO2 ) and a variety of other tropospheric constituents crucial to ozone photoch emical cycles were made over a 1-month period. Two independent measurements of NOx NOy, and PAN showed good agreement. Average free tropospheric dayti me NO levels were about 50 pptv, sufficient to sustain photochemical ozone formation. Although high mixing ratios were encountered, PAN decomposition did not contribute to NOx production during FREETEX'98. Ozone production ef ficiencies (EN) derived from observed Delta O-3/(NOz) ratios in Free tropos pheric air were 20-30 molecules of O-3 produced per NOx molecule oxidized a nd agreed well with a photochemical model. A much lower ozone production ef ficiency of 4 was determined in a photochemically aged air mass arriving fr om southern Europe, in line with other. measurements and calculations in re gimes containing high levels of oxidized nitrogen. Model simulations indica ted that by sequestering NOx and HO2, low-temperature formation of peroxyni tric acid (PNA) decreased ozone production by 20% and instantaneous ozone p roduction efficiencies by 40%, whereas PAN formation had little effect. The model reproduced well the observed sharp transformation from ozone product ion to ozone destruction (defined as Delta O-3/Delta(NOz) = 0) at 20-25 ppt v NO. The observed and calculated strong dependence of E-N on NOx concentra tion in the low-NO, regime highlights the difficulty in assigning a single O-3 production efficiency value to remote regions, where most of the CO and CH4 in the atmosphere are oxidized.