The relationship among NOy, O3, N2O, ultra-fine condensation nuclei (CN), and other trace gases in the upper troposphere (UT) and lower stratosphere (LS) observed during SONEX are analyzed with the goal to identify and quantify the sources of NOy in the UT. We use N2O to separate upper tropospheric air from stratospheric influenced air and focus our analysis to the former. The distributions of NOy and O3 show remarkable similarity when they are plotted as a function of N2O. The only difference between NOy and O3 is found in upper tropospheric air where a large number of data points have high values of both NOy and the NOy/O3 ratio. Major sources contributing to these high NOy values are found to be emissions from lightning and surface sources transported to the UT by convection. ¿ 1999 American Geophysical Union