A Case Study of Ozone Production in a Rural Area of Central Ontario

An O3 episode observed at Dorset, a rural site in central Ontario, during a stagnant high pressure period of the intensive Eulerian Model Evaluation Field Study (EMEFS), in the summer of 1988, is simulated using a photochemical box model with a two-layer treatment. In the model analysis, natural hydrocarbon chemistry is simulated based on an isoprene-only scenario. Sensitivity tests indicate that local isoprene emissions are an important contributor to local O3 production, relative to anthropogenic hydrocarbons (AHCs), during the event.

The model calculated isoprene contribution to the local O3 production, defined as the ratio of the O3 amount formed in the absence of AHCs to that in the presence of AHCs, is characterized by a strong NOx dependence. A minimum value (50%) of the contribution was found at a NOx level of 6 ppbv for the representative hydrocarbon composition during the episode. At this NOx level, O3 production was strongly influenced by the presence of AHCs. At significantly higher or lower NOx levels, isoprene is more important than AHCs in the local O3 production.