JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101,NO. D1, PAGES 1401-1422, JANUARY 20, 1996 Three-dimensional model studiesof the effect of NOx emissions from aircraft on ozone in the upper troposphere over Europe and the North Atlantic Frode Flatoy and OysteinHov Geophysics Institute, Universityof Bergen, Bergen, Norway Abstract. A mesoscale chemistry transport model is coupled to a numericalweather prediction model(NWP) for Europe and the North Atlantic.It is applied to show that air traffic emissions significantly increase the concentrations of NO• as well as the net chemical formation rate of ozoneover both the Atlantic Oceanand over central Europe. The time periodstudied was July 1-10, 1991, a periodcharacterized by a high-pressure ridgeoverNorth Europe. In convection the verticalgradient of NO.•is strongly reduced because the midtropospheric concentrations increase.In the updraft regionof convective plumes, chemical ozone formation up to 1.8 ppbv/h is calculated in the upper troposphere, and this is about a factor50 higherthan the average for 10.5 ton over continental Europe during the 1 O-day period. The highproduction rates arenot sustained for morethana few hours, and it would be difficult to identify the effect of the process in a local measurement of the ozoneconcentration. The calculations indicate that 10-25 molecules of 03 (0.03- 0.05 ppb/h)are generated on the average per NOx molecule emitted from aircraft(2-3 ppt/h). On convective days overregions with significant surface sources of NO,.,ozone formation in the upper troposphere due to NO• brought thereby convection, outweighs the role of aircraft NOx emissions. The net chemical formation rate of ozone across the model domain depends on the boundary concentrations of NO.,. and 03, but the change in the upper troposphere in thenet chemical formation rateof ozone induced by aircraft NO.,. emissions is quite independent of the concentrations of NO• or 03. 1. Introduction Air traffic takes placein the uppertroposphere and the lower stratosphere and has reached a volumewhere it is likely that the emissions of oxides of nitrogen (NO,.=NO+NO0give rise to a significant increase in the ozoneconcentration in the latitude bandsand heightregions where the air traffic occurs(over North America, Europe, and the North Atlantic flight corridor) (see review by Schumann [1993]). In these regions of the atmosphere, air traffic is a dominant NOr source. Ozone is a radiatively active gas and contributes to the warming of the troposphere, particularly at altitudes above 6 km. The radiative forcing sensitivity of global surface temperature is largestfor changes in ozone around the tropopause where the atmosp- here has a temperature minimum [Lacis et al., 1990]. Ozone absorbs UVB radiation and its photolysisleads to the production of hydroxyl radicals, which in turn, are the single most effective chemical agent in destroying atmospheric trace Copyright 1996by the American Geophysical Union. Paper number 95JD03070. 0148-0227/96/95JD-03070505.00 gases which otherwise would build up and change atmosp- heric composition. An increase in ozone over Europe of about 2%/a throug- hout the troposphere has been found from ozonesonde data since1970. There is no long-term trendover Canada, while there is a small (<1%/a) increaseover the east coast of the United States in the summer. Data from Japanese stations show increases in ozoneonly below 5.5-km altitude(review by Logan [1994]). Global two-dimensional (2D) model calculations have indicated that ozonein the uppertropos- phere may have increased by 4-15% due to aircraft NOx emissions [Isaksen, 1980; Derwent, 1982; Johnson et al., 1992; Crutzen and Briihl, 1990;Becket al., 1992;Fuglest- vedt et al., 1993]. Thesenumbers indicatethat the calculated changein midlatitude,northernhemisphere upper tropos- phericozonedue to aircraft NOx emissions can make up for a large fraction of the observed upper tropospheric ozone increase over Europeand the North Atlantic since1970. But the cause-effect relationship cannot be established with any certainty, andchanges in other processes such asthe frequen- cy of stratosphere-troposphere exchange events can also contribute to the changing ozoneconcentration in the upper troposphere. 1401