This article appeared in a journal published by Elsevier. The attached
copy is furnished to the author for internal non-commercial research
and education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling or
licensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of the
article (e.g. in Word or Tex form) to their personal website or
institutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies are
encouraged to visit:
http://www.elsevier.com/copyright
Author's personal copy
Atmospheric Environment 43 (2009) 3407–3415
Contents lists available at ScienceDirect
Atmospheric Environment
journal homepage: www.elsevier.com/locate/atmosenv
Long term changes in nitrogen oxides and volatile organic compounds in
Toronto and the challenges facing local ozone control
Jeffrey A. Geddes a, Jennifer G. Murphy a, *, Daniel K. Wang b
a
b
University of Toronto, Department of Chemistry, 80 St George St, Toronto, ON, Canada M5S 3H6
Environmental Technology Centre, Environment Canada, 335 River Road South, Ottawa, ON, Canada K1A 0H3
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 October 2008
Received in revised form
13 March 2009
Accepted 16 March 2009
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.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Ground level ozone
Air pollution
Nitrogen oxides
Volatile organic compounds
Toronto Canada
1. Introduction
Tropospheric ozone is a major component of photochemical
smog, and is a significant human health hazard (Mudway and Kelly,
2000; Sillman, 2003), but as a secondary pollutant its atmospheric
levels are often difficult to control. Its precursor compounds,
nitrogen oxides (NOx) and volatile organic compounds (VOC), have
a wide variety of sources and can exhibit a non-linear effect on local
ozone production, while its accumulation is strongly influenced by
meteorological processes. For example, recent studies of long term
monitoring data in Japan and Taiwan report steadily increasing
ozone levels despite significant reductions in precursor concentrations and attribute this to increasing background levels in air
transported from neighbouring regions (Chou et al., 2006; Itano
et al., 2007). Therefore, it is vital to distinguish the impacts of local
production, meteorology, and background levels to understand
annual trends and variability in ozone.
Ground level ozone was recently estimated to cost the province
of Ontario several billions of dollars in economic losses due to
* Corresponding author.
E-mail address: [email protected] (J.G. Murphy).
1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.atmosenv.2009.03.053
human health impacts and an additional two hundred million
dollars in agricultural crop damages each year (MOE, 2005; OMA,
2005). As a result, there have been major provincial and municipal
efforts including both regulatory initiatives and incentive programs
to control emissions of ozone precursor compounds. In 2000,
Canada adopted a new nation-wide 8-h average of 65 ppb (CCME,
2000), called the Canada-wide Standard (CWS) for ozone. In 2007,
Toronto committed to reducing locally-generated smog-causing
pollutants by 20% from 2004 levels by 2012 (TEO, 2007). It is
therefore worthwhile to assess how past changes in precursor
emissions have impacted ozone levels in this region.
In the troposphere during the day, interconversion of NO and
NO2 occurs with O3 on the time scale of minutes by the following
reactions:
NO2 Dhv/NODO
(R1a)
ODO2 DM/O3 DM
(R1b)
NODO3 /NO2 DO2
(R2)
This rapid chemistry constitutes a null cycle in which there is no
production or loss of either NOx or O3. Net production of ozone is
Author's personal copy
3408
J.A. Geddes et al. / Atmospheric Environment 43 (2009) 3407–3415
initiated when VOC are oxidized by OH, producing peroxy radicals
that react with NO to form NO2.
OHDVOCDO2 /H2 ODRO2
(R3)
RO2 DNO/RODNO2
(R4)
This NO2 can rapidly photolyze during the daytime to regenerate
NO and form O3 via reactions (R1a) and (R1b). We therefore define
the total oxidant, Ox, as the sum of NO2 and O3 ([Ox] ¼ [NO2] þ [O3]).
In this way, Ox increases when ozone is formed by reactions (R3)
and (R4) followed by reaction (R1), whereas it is conserved when
ozone is formed by reaction (R2) followed by reaction (R1). Another
way in which Ox can increase is through the direct emission of NO2
from combustion sources. Recent studies indicate that NO2
represents a small fraction (1–5%) of total NOx emissions from
vehicles (Ban-Weiss et al., 2008) but this may change in the future
with the introduction of new particulate filters on heavy-duty
diesel vehicles. (Shorter et al., 2005).
The relationship between the ozone production rate and ozone
precursors is non-linear and depends on the relative proportions of
NOx and VOC. At low atmospheric concentrations of NOx, the main
loss of peroxy radicals is by the following (where R can be H or any
organic group):
RO2 DR0 O2 /ROOR0 DO2
(R5)
Under these conditions, ozone production increases linearly
with increases in NOx, and is essentially independent of VOC.
However, at high concentrations of NOx, NO2 can compete with
VOC for the OH radical by the following reaction:
NO2 DOHDM/HNO3 DM
(R6)
This reaction removes OH and suppresses the production of
peroxy radicals via reaction (R3). Therefore, under these conditions
ozone production is inversely proportional to NOx levels and more
sensitive to VOC reactivity. At high NOx levels, reaction (R6) is the
main sink of OH and NOx in the atmosphere, leading to a plateau in
the rate of this reaction with increasing NOx. This means that NO2
controls its own lifetime, and that concentrations of NO2 cannot be
expected to scale linearly with NOx emissions.
In this paper we document significant decreases in ozone
precursor levels within the Greater Toronto Area (city of Toronto
and the surrounding region) from 2000 to 2007, and show that
despite reductions of up to 40% in NO2 levels and VOC reactivity, Ox
levels have only shown a slight response, and average ozone levels
in Toronto have remained constant. We demonstrate the strong link
between O3 and local meteorological conditions by showing that
maximum ozone is strongly dependent on high summer temperatures and the degree of regional transport.
An analytical solution for instantaneous ozone production is
presented, which suggests that Toronto has been in a VOC-limited
production regime where we would not expect a significant decrease
in O3 production given the observed decreases in precursor levels.
These results lend important insight into local air quality and policy,
especially in the context of the city of Toronto’s recent commitment to
reducing locally-generated smog-causing pollutants.
and lies along the ‘‘Windsor-Quebec’’ corridor, the most populated
and industrialized region of Canada. During the summer, the GTA is
often affected by warm southerly and southwesterly air flow from
this corridor which may also have passed over American cities (e.g.
Cleveland, Detroit, Pittsburgh). Local meteorology is also often
influenced by regular land-lake breezes. According to a 2004
estimate, transportation accounted for roughly 63% of NOx emissions in Toronto, with diesel trucks contributing disproportionately
(36%); the remainder of emissions are from natural gas and electricity (ICF, 2007). In Ontario, transportation is a large source of VOC
emissions (33%), with industrial emissions and general solvent use
contributing another 33% cumulatively; the last third of emissions
are from other miscellaneous sources (EC, 2008).
NOx, O3, and VOC data are available from the federal and
provincial cooperative National Air Pollution Surveillance (NAPS)
network. Hourly NO2 and O3 data for eight sites in the Greater
Toronto Area are publicly available from 2000 forward and
updated annually at http://www.airqualityontario.ca. NOx and O3
measurements are made by automated continuous chemiluminescent and UV-absorption analyzers respectively which satisfy
requirements of the US EPA as equivalent or reference methods.
Sampling heights varied from 4 m to 12 m. Recent detailed intercomparisons assessing the performance of molybdenum converters
on chemiluminescent analyzers show that sometimes over 50% of
what is reported as NO2 during photochemically active times of the
day may actually be interferences from higher nitrogen oxides
(Dunlea et al., 2007; Steinbacher et al., 2007). We estimate that,
since most of the stations in this study are urban locations where
NO2 will be a high fraction of total nitrogen oxides, the error is
likely significantly less than 50%, and that this bias has a minimal
impact on the observed trends. VOC data throughout the same
period was obtained from Environment Canada for four stations in
the area (Downtown, West 1, Junction, and Brampton), where 24-h
samples are collected once every 6 days by evacuated canister
followed by analysis by gas chromatography with flame ionization
detection (for C2 hydrocarbons) or mass spectrometric detection
(for C3–C12 hydrocarbons) as described by Wang et al. (2005). Loss
of hydrocarbons to processes occurring on the internal canister
surfaces was evaluated by analyzing canister samples within a few
hours for terpenes, then again after storage for several weeks, and
no decrease in terpene concentrations was observed. However, no
comparisons to on-line GC measurements have been made to date.
Carbonyl compounds were automatically sampled using 2,4-dinitrophenylhydrazine coated silica Sep-Pak cartridges for 24 h, then
separated and identified using HPLC and UV DAD detection at
365 nm (EPA, 1999). Hourly meteorological data at Toronto’s Pearson
International Airport, Toronto’s major weather reporting station
which has been in operation continuously during the period of
interest, was obtained from 2000 to 2007 from the National Climate
Data and Information Archive operated by Environment Canada.
The MOE and EC site locations in this study represent a variety of
urban and suburban environments across the Greater Toronto Area.
The location and name of each site is shown in Fig. 1. Some are in
close proximity to major intersections or major highways, and
some are located close to large recreational parks or in residential
neighbourhoods (see Fig. 1).
2.2. Data
2. Methods
2.1. Study region and monitoring stations
Toronto, Canada (43 40’N, 79 23’W) is located in southern
Ontario on the northwest shore of Lake Ontario. The Greater Toronto Area (GTA) has a population of 5.5 million within 7000 km2,
Daily 8-h maximum ozone and Ox were calculated for each site.
Diurnal cycles were examined to determine the most photochemically relevant hours during the day for ozone production. This was
identified to be between 11:00 and 15:00, so hourly data for NO2
was averaged during this 4-h-period for each day (hereafter
referred to as ‘‘daily NO2 midday average’’). Annual summer
Author's personal copy
J.A. Geddes et al. / Atmospheric Environment 43 (2009) 3407–3415
3409
Fig. 1. Monitoring stations within Toronto and the surrounding area used in this analysis. Superscript ‘‘a’’ denotes stations which are urban or located in close proximity to
a highway. Superscript ‘‘b’’ denotes stations which are located in suburbs.
averages were calculated, where summer is defined as May to
September, being the most photochemically relevant time of year.
Annual averages also remove seasonal variability in the time series
making it easier to identify trends. VOC reactivity was calculated as
the product of VOC concentration and the rate constant against OH
(found in Atkinson (1997) and Seinfeld and Pandis (2006)).
Summer annual averages of the sum of all VOC reactivity
(Ski[VOCi]) were calculated. The contributions of biogenic and
anthropogenic reactivity were distinguished by separating the sum
of isoprene, cymene, pinenes, limonene, and camphene (considered ‘‘biogenic’’ (Seinfeld and Pandis, 2006)) from the rest of the
measured VOC (considered ‘‘anthropogenic’’, consisting of 40
different VOCs). Annual trends were calculated by linear regression,
and their significance is measured by p-values calculated from
a standard T-test where we are testing the null hypothesis that the
slope of the regression line is equal to zero. Therefore the p-values
reported represent the probability of observing a trend of that
extreme in a T-distribution of the same degrees of freedom.
Maximum daily temperatures were calculated during each
summer, and net wind vectors were calculated for a 12-h period
prior to each afternoon (from midnight to 12:00 pm). This was done
by summing the magnitude of each x- and y-component of the
wind at every hour, then calculating the resultant vector by trigonometry. The result is a single value that estimates the strength of
the wind and net direction of air transport for each day throughout
the immediately preceding night and early morning.
3. Results and discussion
3.1. Precursor levels
Annual summer averages of daily midday NO2 are shown in
Fig. 2a and b. It is clear that an overall decrease is present
throughout the whole study region. The most urban station
(Downtown Toronto) and the station closest to a significant
highway (West 2) report the highest NO2 in the region, reflecting
the important transportation source of NOx and indicating that
these sites likely represent some of the highest NO2 levels found
throughout the region. Stations furthest from the main roads and
urban centre (e.g. Newmarket and Oakville) report the lowest
levels. Downtown Toronto, West 2, and Oshawa all have the
steepest slopes, between 1.2 and 1.4 ppb$year1 (p < 0.01),
while all other sites have strong decreasing trends of 0.4
to 1.0 ppb year1 (with p < 0.01 except for Newmarket). Overall,
the North, East and Downtown Toronto summer averages in 2007
were over 35% lower than 2000 levels (the highest difference is in
East Toronto, at 40%). At the West 2 station, 2007 levels were
already almost 35% lower than when it began reporting in 2003.
Similar magnitudes of change are found at the surrounding sites
(w30–38%), and Oakville is the only station which reported a change
less than 30% since the year it began reporting (13% since 2003).
We observe the steepest decrease in NO2 at stations closest to
major transportation arteries and intersections, even though the
number of vehicles registered in Ontario increased by over 700,000
between 2000 and 2007 (Statistics Canada, 2000, 2007). This is
evidence that the reductions are driven strongly by improvements
in vehicle technology. Decreasing NO2 in areas where its abundance
is high will lead to increases in local OH concentrations (because of
Reaction (R6)). This changes the lifetime of NO2 so that it is oxidized
more quickly near its source, meaning that a reduction in emissions
can have a proportionally even larger reduction in ambient
concentrations. Whatever combination of these factors, the fact
that NO2 levels across Toronto and surrounding areas have
decreased between 13% and 40% reflects considerable achievements in NOx emission controls and improvements in technology
over the past decade.
From the available data, the situation appears to be similar for
volatile organic compounds across Toronto. The annual summer
averages in anthropogenic VOC reactivity, shown in Fig. 3, show
a steadily decreasing trend since 2000. VOC reactivity at the
Brampton and Downtown sites in 2007 was already 40% lower than
reactivity in 2001 and 2002 respectively. VOC reactivity attributed
to biogenic compounds shows no apparent trend.
While the magnitude of biogenic reactivity seems to be quite
spatially consistent (around 0.4–0.7 s1 at all locations), the
magnitude of anthropogenic VOC reactivity does not (from
between 2 and 3 s1 in Brampton and West 1 to around 3–5 s1 at
the Downtown and Junction stations). Consequently, the relative
importance of biogenic emissions is not evenly distributed. At the
West 1 and Brampton site, for example, biogenic VOC reactivity in
2007 represented almost 25% of all recorded reactivity, whereas at
the Downtown station the proportion is closer to 18%. At the
Author's personal copy
3410
J.A. Geddes et al. / Atmospheric Environment 43 (2009) 3407–3415
Fig. 3. Annual summer VOC reactivity in the GTA (s1). Slopes in s1 year1. Error bars
represent 1 standard deviation of the mean. Closed squares represent anthropogenic
VOC, open squares represent biogenic VOC, and closed circles represent oxygenated
VOC (Junction Station only).
summer is shown in Fig. 3. While the trend here seems to be
increasing, this is difficult to say with the limited availability of
data. However, we are able to estimate the contribution of
oxygenated VOC to overall reactivity at the Junction station. This
value is w1 s1 and is therefore an important contribution that is
not being accurately captured at other Toronto stations. In addition,
our analysis using simple VOC reactivity only considers the initial
OH reaction, and subsequent reactions (which may be especially
important for longer chained VOC) are not always represented.
However, these data show that efforts to reduce anthropogenic VOC
emissions are in general producing significant results throughout
Toronto, but also suggest several challenges facing pollutant
reductions like the increasing fraction of, and the effect of
temperature on, biogenic emissions.
Fig. 2. Annual summer midday NO2 concentrations (ppb) in (a) Toronto and (b)
surrounding areas. Slopes in ppb year1. Error bars represent 1 standard deviation of
the mean.
Junction station, the most recent data in 2005 showed the biogenic
fraction to be about 13%. Emission controls are more easily implemented for anthropogenic VOC, thus controlling total reactivity is
not an equal task across the Toronto region, and progress in
reducing anthropogenic emissions will not result in a uniform
decrease in overall VOC reactivity. Additionally, as anthropogenic
VOC reactivity continues to decrease, the biogenic fraction will rise,
making it increasingly difficult to control VOC reactivity during the
photochemical season most relevant to ozone production. Moreover, photochemical conditions favouring ozone production such as
increasing temperature and light can also favour higher biogenic
emission rates (for example, see Kesselmeier and Staudt, 1999 and
Harley et al., 1999). Indeed, the correlation between daily biogenic
VOC reactivity and maximum temperature at each station ranges
from 0.02 to 0.05 s1/ C (R2 ¼ 0.10–0.42).
This analysis of VOC reactivity is limited, since the governmental
data used here do not include oxygenated compounds at all
stations. Formaldehyde, acetaldehyde, acrolein, acetone, and propionaldehyde were measured at the Junction station in 2000, 2001,
2004 and 2005, and the average sum of their reactivity during the
3.2. Ozone and Ox
Fig. 4a and b show annual summer daily 8-h maximum O3 for
Toronto and surrounding sites respectively, and Fig. 5a and b show
annual summer daily 8-h maximum Ox. Linear regression shows
that the trends in ozone are not statistically different from zero, and
at some of the more urban sites it may be increasing. The slopes in
Ox are all decreasing, but with varying levels of significance (from
p < 0.05 to p > 0.50). The most significant decrease is seen at the
Brampton site, where the slope is 0.87 ppb year1, however this
trend should be taken cautiously, since it is probably in part due to
the fact that data from 2000 is not available (which for all other
sites was around 2005 levels, and would weaken the significance).
Nevertheless, there seems to be some evidence that ozone
production may be decreasing slowly in the area. Annual summer
daily 8-h maximum Ox averages show less variability between sites,
which illustrates the importance of using Ox (¼ NO2 þ O3) to
remove the titration effect.
The titration reaction (R2) explains many of the differences
between the trends in O3 and Ox and between urban and
surrounding sites. For example, Newmarket, located around 40 km
north of Toronto, has consistently higher maximum O3 averages but
lower maximum Ox averages than Downtown Toronto. Because
there is significantly more NO at the Downtown station to react
Author's personal copy
J.A. Geddes et al. / Atmospheric Environment 43 (2009) 3407–3415
Fig. 4. Annual summer average maximum 8-h O3 concentrations (ppb) for (a) Toronto
and (b) surrounding sites. Slopes in ppb year1. Error bars represent 1 standard
deviation of the mean.
with O3 via reaction (R2), the ozone is temporarily titrated there.
Thus there appear to be competing effects responsible for the lack
of a trend in ozone; mainly, the fact that decreasing NOx and VOC
reactivity levels could be slowly driving down the production of
ozone (as seen by the weakly decreasing trend in Ox levels), while
at the same time having less NO to titrate ozone causes levels to
increase in proximity to NO sources. Unravelling these competing
chemical influences is complicated by the variability in ozone
resulting from meteorological conditions, which is explored in the
following section.
3.3. Meteorological considerations
Numerous studies report high correlations between ozone
levels and temperature for sites all over the United States (Vukovich, 1994; Sillman and Samson, 1995; Vukovich and Sherwell,
2003; Abdul-Wahab et al., 2005), and several methods have been
developed to derive ‘‘meteorologically adjusted’’ trends. These
3411
Fig. 5. Annual summer average maximum 8-h Ox concentrations (ppb) for (a) Toronto
and (b) surrounding sites. Slopes in ppb year1. Error bars represent 1 standard
deviation of the mean.
include probabilistic models, moving average filters, and cluster
analysis (Cox and Chu, 1993; Yang and Miller, 2002; Wise and
Comrie, 2005; Walsh et al., 2008). The eight-year time series
available in the present study is probably too short to be explored
accurately by most of these methods, but recognizing and understanding the effect of temperature on ozone production is obviously crucial in explaining year-to-year variability in O3 and Ox
levels throughout the GTA. We illustrate this in Fig. 6, where the
number of ozone exceedances in North Toronto is plotted along
with the number of days where the maximum temperature
reached above 30 C. This figure shows that a great deal of the
annual variability in high ozone levels can be explained by variability in the number of summer days with high temperatures. In
fact, summer Ox levels at this station correlate with maximum daily
temperatures with a slope of 1.7 ppb/ C (R2 ¼ 0.33). Mechanistically, increased temperatures could lead to enhanced local
production of O3 by enhancing HOx production, increasing local
biogenic VOC emissions (or volatilization of VOC), suppressing the
Author's personal copy
3412
J.A. Geddes et al. / Atmospheric Environment 43 (2009) 3407–3415
Fig. 6. Number of 65-ppb ozone exceedances at North Toronto (line with triangles)
and number of days above 30 C (line with squares) each summer from 2000 to 2007.
net formation of peroxyacetylnitrate, or may simply correlate with
high solar radiation (Jacob et al., 1993; Sillman, 2003). For example,
a correlation between biogenic VOC reactivity and maximum daily
temperature can be seen for at the Toronto sites, and might become
stronger if hourly instead of 24-h average VOC concentrations were
available.
Because high temperatures in Toronto are often associated with
southerly flow, it is also possible that the correlation between
ozone and high temperatures is driven by transport of ozone and
precursor compounds from the polluted upwind region. Since we
were interested in separating and comparing the effects of
increased local production and increased transport on warm days,
we used the net wind vectors for each day from midnight to noon of
that day as described in the Methods section. Fig. 7 shows a polar
graph where the radius represents the summer net wind vectors
from 2000 to 2007 for each day, and the points are coloured by
average Toronto daily maximum 8-h Ox levels. In general, when air
is being transported from the southwest, Ox reaches higher levels
than on days when air is being transported from the north and
northwest.
Each summer, we defined days when the wind speed and
direction resulted in a net movement of >120 km from the south
and southwest (135 –270 ) as days influenced by ‘‘west to southeast flow (W–SE)’’ (n ¼ 197); days when the net wind speed
and direction resulted >120 km net movement from the north
(270 –45 ) as days influenced by ‘‘west to northeast flow (W–NE)’’
(n ¼ 431); and days when the net wind movement was less than
120 km as ‘‘stagnant’’ (or influenced most by local conditions,
n ¼ 596). The data in the W–SE group represent days most likely
influenced by polluted air masses from surrounding urban areas in
Canada and the US, while data in the W–NE group are generally
influenced by transport from remote regions.
Hourly averages of Ox, O3, and NO2 for these separate conditions
at the North Toronto site are shown in Fig. 8. North Toronto was
selected since it is representative of a location influenced directly
by GTA emissions (no matter what direction air might be transporting), but not as biased toward high NO2 as stations such as
Downtown or West 2. We see that on days that were influenced by
W–SE flow there are significantly higher morning Ox levels than on
Fig. 7. Daily maximum 8-h Ox levels as a function of the net 12-h wind vector for the immediately preceding morning and night (radius in kilometres).
Author's personal copy
J.A. Geddes et al. / Atmospheric Environment 43 (2009) 3407–3415
3413
days with stagnant conditions or influenced by W–NE flow. Stagnant days show a sustained Ox increase of around 3 ppb h1 from
8 am to 3 pm, at which point levels start to decrease. Meanwhile,
days with W–SE flow show similar production rates, but only until
about noon at which point levels start to level out. These results
suggest that stagnant conditions allow ozone to accumulate for
a longer period through the day, probably because precursors are
not swept out of the city. This includes occasions when the landlake breeze causes precursors to be swept out across the lake in the
early morning, but swept back into the city later in the day,
resulting in very little net transport of air. The diurnal profile of odd
oxygen on days with strong W–SE flow more closely follows the
instantaneous rate of ozone production, which likely peaks near
noon because ozone and its precursors are being advected away
from the city and not left to accumulate. The evidence suggests that
under strong W–SE flow, airmasses over Toronto start out with
more Ox, but that daytime increases are less significant. Interestingly, the maximum 8-h average in Ox is quite similar for both
stagnant and strong W–SE flow days but on stagnant days the
impact of titration is more significant, which keeps the ozone
lower.
Previous studies using back trajectory clusters during summer
months from 1989 to 1995 and from 1994 to 2003 similarly found
that the highest ozone levels are associated with trajectories from
the south and southwest, passing over the Detroit-Windsor area
(Brankov et al., 2003; Johnson et al., 2007). Along with the present
evidence, these results are highly relevant to the Canada-wide
Standard for Ozone which makes a provision for ‘‘transboundaryinfluenced communities’’ that fail to meet the standard even after
their ‘‘best efforts’’ to reduce precursor emissions (CCME, 2000).
3.4. Computational model
While variability in meteorological conditions drives much of
the interannual variability in ozone over Toronto, it is useful to
assess the extent to which changes in precursor concentrations
have impacted local ozone productions rates. In this section, we
apply a model that was developed by Murphy et al. (2006) to
calculate the instantaneous production rate of ozone as a function
of NO2 levels and VOC reactivity. By situating the surface observations of NO2 and VOC from Toronto during the last 8 years on this
plot, we can estimate how the local production regime may or may
not be changing and suggest control measures that would most
efficiently result in improved air quality.
We refer readers to the above paper for a full derivation, but by
assuming conditions of rapid ozone production where chain
propagation is more significant than chain termination and that the
production of each RO2 ultimately results in an HO2 radical, we
arrive at the following equation for instantaneous ozone
production:
PðO3 Þ ¼ k4 ½HO2 þ RO2 ½NO ¼ 2k3 ½VOC½OH
where [OH] is estimated by:
Fig. 8. Hourly summer averages at North Toronto for Ox, O3, and NO2. Lines with circles
represent net wind conditions <120 km (local); lines with squares represent net wind
conditions >120 km and between 135 and 270 (influenced by strong W–SW flow);
lines with triangles represent net wind conditions >120 km and between 270 and 45
(influenced by strong W–NE flow).
2.3 ppb h1. To solve the above equations, we estimate a to be 0.04
(Cleary et al., 2005), [NO2]/[NO] ¼ 3, k6 ¼ 1.1 1011 s cm3molec1
(Sander et al., 2003), k4eff ¼ 8 1012 s cm3molec1 (Tyndall et al.,
2001), and k5 ¼ 5.0 1012 s cm3molec1 (Tyndall et al., 2001;
Sander et al., 2003). Finally, we are able to plot P(O3) as a function of
VOC reactivity (¼Sk1i[VOC]i) and NO2. The result is shown in Fig. 9.
This contour plot clearly shows two separate ozone production
regimes; one in which decreasing NO2 levels decreases ozone
production (NOx-limited), and one in which decreasing NO2 levels
increases ozone production (VOC-limited). Since the data show both
precursors have been decreasing linearly, and at roughly consistent
rates over most of the Greater Toronto Area we can estimate a vector
of precursor changes from 2000 to 2007 on the contour plot to
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
ðk6 ½NO2 þ ak3 ½VOCÞ þ ðk6 ½NO2 þ ak3 ½VOCÞ2 þ24PðHOxÞk5eff kk3 ½VOC
½NO
4eff
½OH ¼
k3 ½VOC 2
12k5eff k ½NO
4eff
With an initial O3 concentration of 40 ppb, relative humidity of 65%,
and an average formaldehyde concentration of 4 ppb, the estimated
midday summertime P(HOx) for Toronto was calculated as
indicate how instantaneous local ozone production might be
changing year-to-year. We use a wide arrow on the plot to indicate
the general chemical space over which the GTA region atmosphere
Author's personal copy
3414
J.A. Geddes et al. / Atmospheric Environment 43 (2009) 3407–3415
Fig. 9. Contour plot of instantaneous O3 production as a function of VOC reactivity and NO2 concentrations based on the simple model presented by Murphy et al. (2006) applied to
conditions in Toronto. Contour lines represent changes in ozone production of 1 ppb h1. The diagonal shaded arrow represents how general conditions across the Toronto area have
changed between 2000 and 2007 according to the data presented here. The vertical shaded arrow illustrates how changes in VOC reactivity have a stronger influence on ozone
production than changes in both VOC reactivity and NO2.
has transitioned between 2000 and 2007. According to this simple
model we do not expect instantaneous summertime ozone
production to have changed very much in the last 8 years. The
precursor changes produce a vector (red arrow) that does not cross
contour lines in Fig. 9. It is interesting to note that if either the VOC
reactivity or NO2 concentrations had changed in isolation, there
would have been more dramatic changes in ozone production. For
example a 40% reduction in NO2 with constant VOC reactivity at 4 s1
leads to model predictions of a doubling in ozone production rates,
while a 40% reduction in VOC reactivity with constant NOx reduces
ozone production rates by approximately half.
While this model is quite simple and relies on several assumptions, we note that the ozone production rates that it predicts
(w4 ppb h1) are consistent with the rates of Ox increase
(w3 ppb h1) observed in the GTA (Fig. 8). The model is useful as an
aid to approximate the chemical regime of ozone production in
Toronto based on the non-linear relationship with NOx and VOC
levels, and provides a sound scientific explanation for why the past
8 years of precursor reductions have not resulted in statistically
significant reductions in O3 and Ox. Because the model calculations
were for a single value of P(HOx), the impact of possible changes to
P(HOx), which may be driven by changes in O3 or formaldehyde
levels, are not represented by the model realization in Fig. 9. There
is no strong evidence for changes in P(HOx) between 2000 and
2007, and this would have a greater impact on the magnitude of O3
production rates than on their NOx-dependence. We can say with
a great deal of certainty that present levels place Toronto in a VOClimited production regime and thus VOC emission reductions will
be most effective at decreasing local ozone production during
photochemically relevant periods.
4. Conclusion
Significant reductions in NOx and VOC have been observed in
the city of Toronto and surrounding areas. At most sites, summertime NO2 concentrations and VOC reactivity have decreased by
between 30 and 40% since 2000, likely in response to improving
vehicle technology, regulatory initiatives and incentive programs
by the province and municipality to control emissions. Throughout
the same period summer ozone levels have shown no significant
changes, with many sites showing average conditions in 2007 were
similar to those in 2000. We interpret this as a response to
competing effects; a nominal reduction in ozone production as
demonstrated by slightly decreasing Ox levels, counteracted by the
reduced titration effect on a short time scale. We have also shown
evidence that air transported from the west and south on some of
those warm days is contributing to higher 8-h ozone maxima,
mainly through higher early morning concentrations before local
production begins. For example, we have shown that days influenced by strong W–SE transport reach similar 8-h maximum ozone
concentrations as stagnant days on which ozone starts out lower
but accumulation extends later into the day.
These results lend insight into ozone production in the Greater
Toronto Area, but also point at several future challenges. For
example, an increasing fraction of biogenic VOC reactivity means
that it will become increasingly difficult to control total VOC reactivity, which our analysis indicates would be the most effective
strategy for significant reductions in a VOC-limited ozone production regime. Moreover, biogenic VOC reactivity in Toronto has
shown a correlation with maximum daily temperature, and we can
therefore expect some efforts may be compounded by increasing
temperatures in a warming climate. The loss of O3 due to reactions
with biogenic VOCs (thus counteracting increases in O3 production
from reactions with OH) was estimated to be at least an order of
magnitude lower than O3 production over the conditions presented
here. In a VOC-limited production regime, the collection of VOC
data with higher time resolution and good spatial coverage across
the city and surrounding areas ought to be a priority. The VOC data
used in this analysis is based on 24-h averages, and we are therefore
lacking specific information about VOC reactivity during photochemically relevant times of the day. Moreover, we have noted the
lack of oxygenated VOC data which comprises a significant fraction
Author's personal copy
J.A. Geddes et al. / Atmospheric Environment 43 (2009) 3407–3415
of VOC reactivity in the area. Also, there continue to be strong
incentives to reduce NOx emissions, due to the role NOx plays in the
production of fine particulate matter (PM2.5).
These conclusions should have important implications on both
the recent commitment to a 20% reduction in ozone precursor
emissions in Toronto since 2004, and on the Canada-wide Standard
for Ozone which provides leniency for transboundary-influenced
communities that cannot meet the standard despite their best
efforts to reduce emissions.
Acknowledgements
The authors are grateful for support from Environment Canada
and the Natural Sciences and Engineering Research Council of
Canada.
References
Abdul-Wahab, S.A., Bakheit, C.S., Al-Alawi, S.M., 2005. Principal component and
multiple regression analysis in modelling of ground-level ozone and factors
affecting its concentrations. Environmental Modelling & Software 20, 1263–
1271.
Atkinson, R., 1997. Gas-phase tropospheric chemistry of volatile organic compounds .1.
Alkanes and alkenes. Journal of Physical and Chemical Reference Data 26,
215–290.
Ban-Weiss, G.A., Mclaughlin, J.P., Harley, R.A., Kean, A.J., Grosjean, E., Grosjean, D.,
2008. Carbonyl and nitrogen dioxide emissions from gasoline- and dieselpowered motor vehicles. Environmental Science & Technology 42, 3944–3950.
Brankov, E., Henry, R.F., Civerolo, K.L., Hao, W., Rao, S.T., Misra, P.K., Bloxam, R.,
Reid, N., 2003. Assessing the effects of transboundary ozone pollution between
Ontario, Canada and New York, USA. Environmental Pollution 123, 403–411.
CCME, 2000. Canada-Wide Standards for Particulate Matter (PM) and Ozone.
Canadian Council of Ministers of the Environment. http://www.ccme.ca/assets/
pdf/pmozone_standard_e.pdf (last accessed October 2008).
Chou, C.C.K., Liu, S.C., Lin, C.Y., Shiu, C.J., Chang, K.H., 2006. The trend of surface
ozone in Taipei, Taiwan, and its causes: implications for ozone control strategies. Atmospheric Environment 40, 3898–3908.
Cleary, P.A., Murphy, J.G., Wooldrige, P.J., Day, D.A., Millet, D.B., Goldstein, A.H.,
Cohen, R.C., 2005. Observations of total alkyl nitrates within the Sacramento
urban plume. Atmospheric Chemistry and Physics Discussions 5, 4801–4843.
Cox, W.M., Chu, S.H., 1993. Meteorologically adjusted ozone trends in urban areas –
a probabilistic approach. Atmospheric Environment Part B-Urban Atmosphere
27, 425–434.
Dunlea, E.J., Herndon, S.C., Nelson, D.D., Volkamer, R.M., San Martini, F.,
Sheehy, P.M., Zahniser, M.S., Shorter, J.H., Wormhoudt, J.C., Lamb, B.K.,
Allwine, E.J., Gaffney, J.S., Marley, N.A., Grutter, M., Marquez, C., Blanco, S.,
Cardenas, B., Retama, A., Villegas, C.R.R., Kolb, C.E., Molina, L.T., Molina, M.J.,
2007. Evaluation of nitrogen dioxide chemiluminescence monitors in a polluted
urban environment. Atmospheric Chemistry and Physics 7, 2691–2704.
EC, 2008. Criteria Air Contaminants Emission Summaries: 2005 Air Pollutant
Emissions for Ontario. Environment Canada. http://www.ec.gc.ca/pdb/cac/
Emissions1990-2015/2005/2005_ON_e.cfm (last accessed October 2008).
EPA, 1999. TO-11 Compendium of Methods for the Determination of Toxic Organic
Compounds in Ambient Air: Determination of Formaldehyde in Ambient Air
Using Adsorbent Cartridge Followed by High Performance Liquid Chromatography (HPLC). US Environmental Protection Agency. http://www.epa.gov/
ttnamti1/files/ambient/airtox/to-11ar.pdf> (last accessed January 2009).
Harley, P.C., Monson, R.K., Lerdau, M.T., 1999. Ecological and evolutionary aspects of
isoprene emission from plants. Oecologia 118, 109–123.
Itano, Y., Bandow, H., Takenaka, N., Saitoh, Y., Asayama, A., Fukuyama, J., 2007.
Impact of NOx reduction on long-term ozone trends in an urban atmosphere.
Science of the Total Environment 379, 46–55.
ICF, 2007. Greenhouse Gases and Air Pollutants in the City of Toronto: Towards
a Harmonized Strategy for Reducing Emissions. ICF International in collaboration
with Toronto Atmospheric Fund and Toronto Environment Office. http://www.
toronto.ca/teo/pdf/ghg-aq-inventory-june2007.pdf (last accessed October 2008).
3415
Jacob, D.J., Logan, J.A., Gardner, G.M., Yevich, R.M., Spivakovsky, C.M., Wofsy, S.C.,
Sillman, S., Prather, M.J., 1993. Factors regulating ozone over the United-States
and its export to the Global atmosphere. Journal of Geophysical ResearchAtmospheres 98, 14817–14826.
Johnson, D., Mignacca, D., Herod, D., Jutzi, D., Miller, H., 2007. Characterization and
identification of trends in average ambient ozone and fine particulate matter
levels through trajectory cluster analysis in eastern Canada. Journal of the Air &
Waste Management Association 57, 907–918.
Kesselmeier, J., Staudt, M., 1999. Biogenic volatile organic compounds (VOC): an
overview on emission, physiology and ecology. J. Atmos. Chem. 33, 23–88.
MOE, 2005. Transboundary Air Pollution in Ontario. Ontario Ministry of the Environment. http://www.ene.gov.on.ca/envision/techdocs/5158e.pdf (last accessed
October 2008).
Mudway, I.S., Kelly, F.J., 2000. Ozone and the lung: a sensitive issue. Molecular
Aspects of Medicine 21, 1–48.
Murphy, J.G., Day, D.A., Cleary, P.A., Wooldrige, P.J., Millet, D.B., Goldstein, A.H.,
Cohen, R.C., 2006. The weekend effect within and downwind of Sacramento:
Part 2. Observational evidence for chemical and dynamical contributions.
Atmospheric Chemistry and Physics Discussions 6, 11971–12019.
OMA, 2005. The Illness Costs of Air Pollution: 2005–2026 Health & Economic
Damage Estimates. Ontario Medical Association. http://www.oma.org/Health/
smog/report/ICAP2005_Report.pdf (last accessed October 2008).
Sander, S.P., Kurylo, J.M., Orkin, V.L., Golden, D.M., Huie, R.E., Finlayson-Pitts, B.J.,
Kolb, C.E., Molina, M.J., Friedle, R.R., Ravishankara, A.R., Moortgat, G.K., 2003.
NASA/JPL Data Evaluation.
Seinfeld, J.H., Pandis, S.N., 2006. Atmospheric Chemistry and Physics: From Air
Pollution to Climate Change. John Wiley & Sons Inc., New Jersey.
Shorter, J.H., Herndon, S., Zahniser, M.S., Nelson, D.D., Wormhoudt, J.,
Demerjian, K.L., Kolb, C.E., 2005. Real-time measurements of nitrogen oxide
emissions from in-use New York City transit buses using a chase vehicle.
Environmental Science & Technology 39, 7991–8000.
Sillman, S., 2003. Tropospheric ozone and photochemical smog. In: Holland, H.D.,
Turekian, K.K. (Eds.), Treatise on Geochemistry. Elsevier-Pergammon, Oxford,
pp. 407–432.
Sillman, S., Samson, F.J., 1995. Impact of temperature on oxidant Photochemistry in
urban, polluted rural and remote environments. Journal of Geophysical
Research-Atmospheres 100, 11497–11508.
Statistics Canada, 2000. Canadian Vehicle Survey: Annual. http://www.statcan.ca/
english/freepub/53-223-XIE/53-223-XIE2000000.pdf (last accessed October 2008).
Statistics Canada, 2007. Canadian Vehicle Survey: Annual. http://www.statcan.ca/
english/freepub/53-223-XIE/53-223-XIE2007000.pdf (last accessed October 2008).
Steinbacher, M., Zellweger, C., Schwarzenbach, B., Bugmann, S., Buchmann, B.,
Ordonez, C., Prevot, A.S.H., Hueglin, C., 2007. Nitrogen oxide measurements at
rural sites in Switzerland: bias of conventional measurement techniques.
Journal of Geophysical Research-Atmospheres 112, D11307. doi:10.1029/
2006JD007971.
Toronto Environment Office (TEO) with The City of Toronto Energy Efficiency Office,
2007. Toronto Environment Office (TEO) with The City of Toronto Energy Efficiency Office, 2007. Change Is in the Air: Climate Change, Clean Air, and
Sustainable Energy Action Plan. http://www.toronto.ca/changeisintheair/pdf/
clean_air_action_plan.pdf (last accessed October 2008).
Tyndall, G.S., Cox, R.A., Granier, C., Lesclaux, R., Moortgat, G.K., Pilling, M.J.,
Ravishankara, A.R., Wallington, T.J., 2001. Atmospheric chemistry of small
organic peroxy radicals. Journal of Geophysical Research-Atmospheres 106,
12157–12182.
Vukovich, F.M., 1994. Boundary-Layer ozone variations in the eastern United-States
and their association with meteorological variations – long-term variations.
Journal of Geophysical Research-Atmospheres 99, 16839–16850.
Vukovich, F.M., Sherwell, J., 2003. An examination of the relationship between
certain meteorological parameters and surface ozone variations in the
Baltimore–Washington corridor. Atmospheric Environment 37, 971–981.
Walsh, K.J., Milligan, M., Woodman, M., Sherwell, J., 2008. Data mining to characterize ozone behavior in Baltimore and Washington, DC. Atmospheric
Environment 42, 4280–4292.
Wang, D., Fuentes, J.D., Travers, D., Dann, T., Connolly, T., 2005. Non-methane
hydrocarbons and carbonyls in the Lower Fraser Valley during PACIFIC 2001.
Atmospheric Environment 39, 5261–5272.
Wise, E.K., Comrie, A.C., 2005. Meteorologically adjusted urban air quality trends in
the Southwestern United States. Atmospheric Environment 39, 2969–2980.
Yang, J., Miller, D.R., 2002. Trends and variability of ground-level O3 in Connecticut
over the period 1981–1997. Journal of the Air & Waste Management Association
52, 1354–1361.