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

Abstract

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 (NOx) 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 GTA, we present an analysis of NO2, VOC and O3 data from federal and provincial governmental monitoring sites in the GTA from 2000–2012. 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.

Trends in atmospheric ammonia at urban, rural, and remote sites across North America

“…Increasing T not only increases soil and vegeta-tion NH 3 emissions but also favors more NH 3 partitioning in the gas phase (Pinder et a., 2012;Sutton et al, 2013); both processes would increase NH 3 mixing ratios. The decreased SO 2 emissions due to the tightened emission control policies since 2008 by the city and provincial governments led to significant declines in SO 2 oxidation products (Hu et al, 2014;Pugliese et al, 2014), which in turn also affected NH 3 -pNH + 4 partitioning, the surface resistance for NH 3 , and increased NH 3 mixing ratios (Yao et al, 2007;Fowler et al, 2015). These hypotheses were supported by the trends in T and pNH + 4 and their correlations with that in NH 3 , as detailed below.…”

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 [] suggests that stagnant conditions (which are increased with climate change [ Leibensperger et al , ]), 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.…”