The Canadian intensive measurement periods of the Eulerian Model Evaluation Field Study (EMEFS) were conducted in the summer of 1988 and the spring of 1990 in central Ontario, Canada. The project involved using instrumented research aircraft to measure vertical profiles of atmospheric aerosols and trace gases over special ground observing sites. Oxidant concentrations were often highest aloft. There were no strong diurnal variations in O3 and H2O2, suggesting that oxidant concentrations reported in this paper, as well as other species, seem to be most influenced by transport mechanisms rather than local chemical transformations. Aerosol and SO2 concentration vertical profiles tended to be similar to those of water vapor mixing ratio, showing a maximum near the surface, which points to a ground level source for these constituents. The inorganic aerosol was dominated by sulphate during the summer measurements, but particulate nitrate was important during the spring. The horizontal transport of these species was calculated for air with back trajectories from the north or south, showing transport to occur primarily from west to east for both cases, with components to the south or north, respectively. The sulphur is transported primarily as SO2 in the spring and about half as SO2 and half as SO4= in the summer. Air from the south is a source of particles, S, H2O, O3, H2O2, and NO2 to this region. Similarly, cloud water concentrations of sulphate and nitrate were higher with southerly trajectories, as were aircraft filter measurements of sulphate, nitrate, ammonium, and nitric acid. ¿ 1998 American Geophysical Union