Ground-level ozone in Montreal, Canada

Analysis of hourly O₃ and NO_x data for nine stations for the period 1984–1990 indicates that urban Montreal forms a net sink for O₃ in a regional regime characterized by the long-range transport of O₃ and its precursors northeastward along the Windsor-Quebec corridor. At central city sites and those adjacent to major transport routes, scavenging of O₃ by NO reduces maximum daily O₃ concentrations by ∼50% on average. This effect is strongest in winter, when increased stability and reduced mixing leads to high NO_x concentrations. The combination of scavenging and channeling of flow along the St Lawrence Valley produces distinct and persistent spatial patterns in the O₃ field. The 82 ppb hourly O₃ standard is exceeded simultaneously at two or more stations in Montreal on about 10 days per year. These episodes tend to be associated with persistent, slow moving anticyclonic systems. The considerable interannual variability in the frequency of high ozone episodes is related to interannual variability in the synoptic meteorological conditions conducive to ozone formation and long-range transport. VOC/NO_x ratios of ∼5 ppbc/ppb at central city locations suggest that local NO_x reduction strategies would likely increase ozone concentrations in urban Montreal. Consequently, effective VOC/NO_x control initiatives need to be regional and international in scope.