Modelling of Ozone Formation at a Rural Site in Southern Ontario

A one-dimensional time-dependent photochemical diffusion model has been used to simulate the chemical and vertical transport processes occurring at a rural location within Southern Ontario. The model output has been compared with the extensive measurements of several atmospheric trace gases which were made at a rural site in Southern Ontario as part of SONTOS (Southern Ontario Oxidant Study) during the summer of 1992. In particular, the measurements include various VOCs, odd nitrogen components and ROx which have been explicitly included in the model. For the case examined, observations at the site are typical of unpolluted background conditions and the model is able to reproduce the observed variation of many of the key trace gas species observed. Though the site is remote from large population centers there appear to be important local emissions of NOx and non-methane hydrocarbons (NMHC). Modelling results suggest that for the day studied approximately 10 ppb of ozone was photochemically produced, and that the production of ozone was limited by the availability of NOx (NO + NO2). At the observed levels isoprene does not play a major role in the production of ozone. A comparison of observations and model calculations for peroxy radical and HNO3 concentrations indicate a disagreement between the model and measurements.