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

Abstract

Abstract. Tropospheric ozone (O 3 ) 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 NO 2 , VOC and O 3 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 NO 2 midday (11:00-15:00) concentrations at all sites decreased significantly; since 2000, all sites experienced a decrease in NO 2 of 28-62 % and in measured VOC reactivity of at least 53-71 %. Comparing 2002Comparing -2003Comparing to 2011Comparing -2012, the summed reactivity of OH towards NO 2 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.

Observed trends in ground-level O<sub>3</sub> in Monterrey, Mexico, during 1993–2014: comparison with Mexico City and Guadalajara

“…This is underlined at OBI with the highest values of O x where the predominant wind direction is NE, consistent with the transport of emissions from the industrial area located to the NE and photochemical processing of air masses (Carrillo-Torres et al, 2017). The daily cycles of O 3 determined within the MMA are consistent with those reported for Los Angeles (VanCuren, 2015) and Toronto (Pugliese et al, 2014). In Toronto, the O 3 maxima were enhanced by the arrival of photochemically processed air masses transported from polluted wind sectors, and decreased during clear air masses.…”

“…In the greater area of Toronto from 2000 to 2012, O 3 levels decreased at urban sites by approximately 0.4 % yr −1 , and at suburban sites by approximately 1.1 % yr −1 , as a consequence of a reduction in the midday averages of NO 2 of 5.8-6.4 % yr −1 and in the VOC reactivity of 9.3 % yr −1 (Pugliese et al, 2014). Emissions estimates suggest an overall national scale decrease during 1980-2008 in US NO x and VOCs emissions of 40 and 47 %, respectively, with city-to-city variability (EPA, 2009;Xing et al, 2013).…”

Ozone distributions over southern Lake Michigan: comparisons between ferry-based observations, shoreline-based DOAS observations and model forecasts

“…Studies have been addressing the role of lake breeze in air quality near the Great Lakes of North America (Levy et al, 2010;Sills et al, 2011;Makar et al, 2010), with a whole campaign, the Border Air Quality and Meteorological Study (BAQS-MET), dedicated to the evaluation of lake breezes. Complexities in the reduction of precursors and continued increases in ozone are of current concern in the Toronto area (Pugliese et al, 2014). Here, we evaluate the Lake Michigan ozone mixing ratios offshore with those onshore, including agreement with ozone forecast models over water and at the shoreline.…”

Toronto area ozone: Long‐term measurements and modeled sources of poor air quality events

“…Given the diversity in sensitivity for each pollution event and the correlation with temperature, it is clear that a continental effort to reduce precursor emissions is required to improve air quality in Toronto in a warming climate. The study by Pugliese et al [2014] suggests that stagnant conditions (which are increased with climate change [Leibensperger et al, 2008]), continue to cause surface O 3 exceedances in the Toronto area. A follow-up study similar to the work presented here, using GEOS-Chem forward and adjoint models with the longer data record (e.g., 2002-2014) would be useful to determine the effects of recent emission controls as well as climate change effects on O 3 over Toronto.…”