Ground level ozone represents a significant air quality concern in Toronto, Canada, where the national
65 ppb 8-h standard is repeatedly exceeded during the summer. Here we present an analysis of nitrogen
dioxide (NO2), ozone (O3), and volatile organic compound (VOC) data from federal and provincial
governmental monitoring sites from 2000 to 2007. We show that summertime VOC reactivity and ambient
concentrations of NO2 have decreased over this period of time by up to 40% across Toronto and the
surrounding region. This has not resulted in significant summertime ozone reductions, and in some urban
areas, it appears to be increasing. We discuss the competing effects of decreased ozone titration leading to
an increase in O3, and decreased local ozone production, both caused by significant decreases in NOx
concentrations. In addition, by using local meteorological data, we show that annual variability in summer
ozone correlates strongly with maximum daily temperatures, and we explore the effect of atmospheric
transport from the southwest which has a significant influence on early morning levels before local
production begins. A mathematical model of instantaneous ozone production is presented which suggests
that, given the observed decreases in NOx and VOC reactivity, we would not expect a significant change in
local ozone production under photochemically relevant conditions. These results are discussed in the
context of Toronto’s recent commitment to cutting local smog-causing pollutants by 20% by 2012.