EPSRC : Jordan Stewart : EP/R513386/1 Atmospheric oxidants are responsible for the degradation and ultimate removal of the vast majority of emissions into our atmosphere. A complete understanding of this chemistry is therefore essential if we are to develop effective and timely policies that address the pressing societal problems of air quality and climate change. Chlorine atoms are the least understood of these oxidants, and are currently not included in models that routinely forecast air quality and climate. However, work over the last decade suggests chlorine could be more important than previously thought. A major unanswered question is the source of chlorine in mid-continental regions, where models predict there to be very little. By contributing to an existing international measurement program looking at air quality in global megacities, this project will make measurements of key chlorine compounds in a mid-continental megacity (Toronto). This will provide vital data on the sources of chlorine in this important environment.