Origin and variability of upper tropospheric nitrogen oxides and ozone at northern mid-latitudes

Year-long measurements of NO _x and ozone performed during the NOXAR project are compared to results from the ECHAM4.L39(DLR)/CHEM (E39/C) and GISS coupled chemistry-climate models. The measurements were taken on flights between Europe and the eastern United States and between Europe and the Far East in the latitude range 40-65°N. Our comparison concentrates on the upper troposphere and reveals strong longitudinal variations in seasonal mean NO _x of more than 200 pptv, which both models are able to reproduce qualitatively. Vertical profiles show maximum NO _x values 2-3 km below the tropopause ("E-shape") with a strong seasonal cycle. E39/C simulates a maximum located at the tropopause and with a reasonable seasonal cycle. The GISS model reproduces the seasonal cycle but not the profile's shape due to its coarser vertical resolution. A comparison of NO _x frequency distributions reveals that both models are capable of reproducing the observed variability, except that E39/C shows no very high NO _x mixing ratios. Both models show that lightning and surface NO _x emissions contribute the most to the seasonal cycle of NO _x at tropopause altitudes. The impact of lightning in the upper troposphere does not vary strongly with altitude, whereas the impact of surface emissions decreases with altitude. Among all sources, lightning contributes the most to the variability of NO _x in the upper troposphere in northern mid-latitudes during summer.