On June 28, 1989, a severe thunderstorm over North Dakota developed into a squall line and then into a mesoscale convective complex (MCC) with overshooting tops as high as ~14 km and a cirrus anvil that covered more than 300,000 km2. In this paper we describe the trace gas concentrations prior to, in, and around the storm; paper 2 presents numerical simulations. Observations of O3 and &thgr;eq unaffected by upstream convection for at least 3 days prior to the flights placed the undisturbed tropopause between 10.7 and 11 km. The anvil outflow, sampled at altitudes of 10.8 to 12.2 km, extended well into what used to be the stratosphere. Air inside the anvil was characterized by notably low concentrations of O3 and high CO relative to the out-of-cloud environment. Elevated concentrations of NO and NOy, due to lightning and upward transport, were observed in the anvil. A tongue of air with tropospheric characteristics lay above stratospheric air, showing that extensive stratosphere-troposphere exchange had occurred. The effects of this mechanism on atmospheric budgets of trace species depend on the fate of the air that enters the anvil and on the frequency of MCCs. Assuming that the symmetry was cylindrical and that the material transported during the observations at the east edge of the anvil was representative of the entire cirrus anvil cloud, we estimate a minimum flux of 2¿1010 g of O3 into the troposphere and a maximum flux of 3--7¿1013 g of H2O into the stratosphere. This is a greater flux of water than the stratospheric water budget can support, and thus most of this water must return to the troposphere; the ice crystals were of sufficient size to have substantial settling velocity. If, however, even a small fraction of the mass of such anvils remains in the stratosphere, then convective transport of reactive tropospheric trace species such as NOy, CO, and NMHC may dominate the chemistry of the lower stratosphere in this midlatitude region. More detailed estimates of the fluxes, taking into account the rear anvil as well, are presented in the companion paper. ¿ American Geophysical Union 1996