Ozone air quality standards are exceeded every year in all parts of Ontario. Many traces gases that are important for air quality can be measured with the Fourier transform spectrometer at the University of Toronto Atmospheric Observatory (TAO). Using the TAO dataset along with complementary data from surface networks and satellites, multiple pollution episodes over Toronto have been found each year. The GEOS-Chem chemical transport model can help us determine the geographical sources, emission types, and meteorological conditions that were involved in each pollution episode, and I will present my findings for a few case studies.