Nitrogen oxides and ozone in the tropopause region of the Northern Hemisphere: Measurements from commercial aircraft in 1995/1996 and 1997

Measurements of nitrogen oxides (NO and NO2) and ozone (O-3) were performed from a Swissair B-747 passenger aircraft in two extended time periods (May 1995 to May 1996, August to November 1997) in the framework of the Swiss N OXAR and the European POLINAT 2 project. The measurements were obtained on a total of 623 flights between Europe and destinations in the United States and the Far East. NO2 measurements were obtained only after December 1995 and were less precise than the NO measurements. Therefore daytime NO2 value s were derived from measured NO and O-3 concentrations assuming photostatio nary equilibrium. The completed NOx data set (measured NOx measured NO2 dur ing night, and calculated NO2 during day) includes a complete annual cycle and is the most extensive and representative data set currently available f or the upper troposphere (UT) and the lower stratosphere (LS) covering a si gnificant proportion of the northern hemisphere between 15 degreesN and 65' degreesN. NOx concentrations in midlatitudes (30 degrees -60 degreesN) show ed a marked seasonal variation both in the UT and the LS with a maximum in summer (median/mean values of 159/264 pptv in UT, 199/237 pptv in LS) and a minimum in winter (51/99 pptv in UT, 67/91 pptv in LS). Mean NOx concentra tions were generally much higher than the respective median values, in part icular in the UT, which reflects the important contribution from comparativ ely few very high concentrations observed in large-scale convection/lightni ng and small-scale aircraft plumes. Seasonal mean NOx concentrations in the UT were up to 3-4 times higher over continental regions than over the Nort h Atlantic during summer. Lightning production of NO and convective vertica l transport from the polluted boundary layer thus appear to have dominated the upper tropospheric NOx budget over these continental regions, particula rly during summer. Ozone concentrations at aircraft cruising levels typical ly varied by an order of magnitude due to the strong vertical gradient in t he LS. Seasonal mean values were dominated by large-scale dynamical process es controlling the altitude of the tropopause and the O-3 abundance in the LS. O-3 in the UT in midlatitudes showed a broad maximum between June and A ugust, typical of observations in the free troposphere.