Ground-based measurements of ozone and NO₂ slant columns by the SAOZ (Système d'Analyse par Observations Zénithales) and UT-GBS (University of Toronto Ground-Based Spectrometer) instruments during the MANTRA 2004 field campaign are presented herein. During the afternoon of 28 August, a thunderstorm passed over the instruments, which were installed at Vanscoy, Saskatchewan (52° N, 107° W). Enhanced slant columns of ozone and NO₂ were observed by both instruments during the storm, with maximum values of two and 25 times the expected columns, respectively. The enhanced ozone slant columns are primarily due to the longer path traversed by the solar radiation caused by multiple scattering inside the thick cloud layer associated with the thunderstorm. The enhanced NO₂ columns are attributed to NO_x production by lightning. Two different methods are used to separate the NO₂ enhancements into contributions from the longer path length and production by lightning. Combining the observed excess NO₂ with lightning flash data from the Canadian Lightning Detection Network and Environment Canada Doppler radar measurements, the production of NO₂ molecules per lightning flash is determined. Using these two methods, the best estimate of the production rate is found to be (7.45±1.38)×10²⁶ molecules NO₂/flash from the UT-GBS and (6.58±1.21)×10²⁶ molecules NO₂/flash from SAOZ. These results are consistent with previous estimates which range from 10²⁶ molecules NO₂/flash (Noxon, 1976) to (7±3)×10²⁶ molecules NO₂/flash (Franzblau and Popp, 1989).