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

摘要

Tropospheric ozone (O) is a major component of photochemical smog andis a known human health hazard, as well as a damaging factor for vegetation.Its precursor compounds, nitrogen oxides (NO) and volatile organiccompounds (VOCs), have a variety of anthropogenic and biogenic sources andexhibit non-linear effects on ozone production. As an update to previousstudies on ground-level ozone in the Greater Toronto Area (GTA), we presentan analysis of NO, VOC and O data from federal and provincialgovernmental monitoring sites in the GTA from 2000 to 2012. We show that, overthe study period, summertime 24 h VOC reactivity and NO midday(11:00–15:00) concentrations at all sites decreased significantly; since2000, all sites experienced a decrease in NO of 28–62% and inmeasured VOC reactivity of at least 53–71%. Comparing 2002–2003 to2011–2012, the summed reactivity of OH towards NO and a suite ofmeasured VOCs decreased from 8.6 to 4.6 s. Ratios of reactiveVOC pairs indicate that the effective OH concentration experienced byprimary pollutants in the GTA has increased significantly over the studyperiod. Despite the continuous decrease in precursor levels, ozoneconcentrations are not following the same pattern at all stations; it wasfound that the Canada-wide Standard for ozone continues to be exceeded atall monitoring stations. Additionally, while the years 2008–2011 hadconsistently lower ozone levels than previous years, 2012 experienced one ofthe highest recorded summertime ozone concentrations and a large number ofsmog episodes. We demonstrate that these high ozone observations in 2012 maybe a result of the number of days with high solar radiation, the number ofstagnant periods and the transport of high ozone levels from upwind regions.