The impacts of precursor reduction and meteorology on ground-level ozone in the Greater Toronto Area

Abstract. Tropospheric ozone (O₃) is a major component of photochemical smog and is a known human health hazard, as well as a damaging factor for vegetation. Its precursor compounds, nitrogen oxides (NO_x) and volatile organic compounds (VOCs), have a variety of anthropogenic and biogenic sources and exhibit non-linear effects on ozone production. As an update to previous studies on ground-level ozone in the Greater Toronto Area (GTA), we present an analysis of NO₂, VOC and O₃ data from federal and provincial governmental monitoring sites in the GTA from 2000 to 2012. We show that, over the study period, summertime 24 h VOC reactivity and NO₂ midday (11:00–15:00) concentrations at all sites decreased significantly; since 2000, all sites experienced a decrease in NO₂ of 28–62% and in measured VOC reactivity of at least 53–71%. Comparing 2002–2003 to 2011–2012, the summed reactivity of OH towards NO₂ and a suite of measured VOCs decreased from 8.6 to 4.6 s⁻¹. Ratios of reactive VOC pairs indicate that the effective OH concentration experienced by primary pollutants in the GTA has increased significantly over the study period. Despite the continuous decrease in precursor levels, ozone concentrations are not following the same pattern at all stations; it was found that the Canada-wide Standard for ozone continues to be exceeded at all monitoring stations. Additionally, while the years 2008–2011 had consistently lower ozone levels than previous years, 2012 experienced one of the highest recorded summertime ozone concentrations and a large number of smog episodes. We demonstrate that these high ozone observations in 2012 may be a result of the number of days with high solar radiation, the number of stagnant periods and the transport of high ozone levels from upwind regions.