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Long term changes in nitrogen oxides and volatile organic compounds inToronto and the challenges facing local ozone control

Jeffrey A. Geddes a, Jennifer G. Murphy a,*, Daniel K. Wang b

a University of Toronto, Department of Chemistry, 80 St George St, Toronto, ON, Canada M5S 3H6b 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

Article history:Received 7 October 2008Received in revised form13 March 2009Accepted 16 March 2009

Keywords:Ground level ozoneAir pollutionNitrogen oxidesVolatile organic compoundsToronto Canada

a b s t r a c t

Ground level ozone represents a significant air quality concern in Toronto, Canada, where the national65 ppb 8-h standard is repeatedly exceeded during the summer. Here we present an analysis of nitrogendioxide (NO2), ozone (O3), and volatile organic compound (VOC) data from federal and provincialgovernmental monitoring sites from 2000 to 2007. We show that summertime VOC reactivity and ambientconcentrations of NO2 have decreased over this period of time by up to 40% across Toronto and thesurrounding region. This has not resulted in significant summertime ozone reductions, and in some urbanareas, it appears to be increasing. We discuss the competing effects of decreased ozone titration leading toan 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 summerozone correlates strongly with maximum daily temperatures, and we explore the effect of atmospherictransport from the southwest which has a significant influence on early morning levels before localproduction begins. A mathematical model of instantaneous ozone production is presented which suggeststhat, given the observed decreases in NOx and VOC reactivity, we would not expect a significant change inlocal ozone production under photochemically relevant conditions. These results are discussed in thecontext of Toronto’s recent commitment to cutting local smog-causing pollutants by 20% by 2012.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Tropospheric ozone is a major component of photochemicalsmog, and is a significant human health hazard (Mudway and Kelly,2000; Sillman, 2003), but as a secondary pollutant its atmosphericlevels are often difficult to control. Its precursor compounds,nitrogen oxides (NOx) and volatile organic compounds (VOC), havea wide variety of sources and can exhibit a non-linear effect on localozone production, while its accumulation is strongly influenced bymeteorological processes. For example, recent studies of long termmonitoring data in Japan and Taiwan report steadily increasingozone levels despite significant reductions in precursor concen-trations and attribute this to increasing background levels in airtransported from neighbouring regions (Chou et al., 2006; Itanoet al., 2007). Therefore, it is vital to distinguish the impacts of localproduction, meteorology, and background levels to understandannual trends and variability in ozone.

Ground level ozone was recently estimated to cost the provinceof Ontario several billions of dollars in economic losses due to

human health impacts and an additional two hundred milliondollars in agricultural crop damages each year (MOE, 2005; OMA,2005). As a result, there have been major provincial and municipalefforts including both regulatory initiatives and incentive programsto 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-causingpollutants by 20% from 2004 levels by 2012 (TEO, 2007). It istherefore worthwhile to assess how past changes in precursoremissions have impacted ozone levels in this region.

In the troposphere during the day, interconversion of NO andNO2 occurs with O3 on the time scale of minutes by the followingreactions:

NO2Dhv/NODO (R1a)

ODO2DM/O3DM (R1b)

NODO3/NO2DO2 (R2)

This rapid chemistry constitutes a null cycle in which there is noproduction or loss of either NOx or O3. Net production of ozone is

* Corresponding author.E-mail address: [email protected] (J.G. Murphy).

Contents lists available at ScienceDirect

Atmospheric Environment

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1352-2310/$ – see front matter � 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.atmosenv.2009.03.053

Atmospheric Environment 43 (2009) 3407–3415


initiated when VOC are oxidized by OH, producing peroxy radicalsthat react with NO to form NO2.

OHDVOCDO2/H2ODRO2 (R3)

RO2DNO/RODNO2 (R4)

This NO2 can rapidly photolyze during the daytime to regenerateNO and form O3 via reactions (R1a) and (R1b). We therefore definethe 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 whenozone is formed by reaction (R2) followed by reaction (R1). Anotherway 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 fromvehicles (Ban-Weiss et al., 2008) but this may change in the futurewith the introduction of new particulate filters on heavy-dutydiesel vehicles. (Shorter et al., 2005).

The relationship between the ozone production rate and ozoneprecursors is non-linear and depends on the relative proportions ofNOx and VOC. At low atmospheric concentrations of NOx, the mainloss of peroxy radicals is by the following (where R can be H or anyorganic group):

RO2DR0O2/ROOR0DO2 (R5)

Under these conditions, ozone production increases linearlywith increases in NOx, and is essentially independent of VOC.However, at high concentrations of NOx, NO2 can compete withVOC for the OH radical by the following reaction:

NO2DOHDM/HNO3DM (R6)

This reaction removes OH and suppresses the production ofperoxy radicals via reaction (R3). Therefore, under these conditionsozone production is inversely proportional to NOx levels and moresensitive to VOC reactivity. At high NOx levels, reaction (R6) is themain sink of OH and NOx in the atmosphere, leading to a plateau inthe rate of this reaction with increasing NOx. This means that NO2

controls its own lifetime, and that concentrations of NO2 cannot beexpected to scale linearly with NOx emissions.

In this paper we document significant decreases in ozoneprecursor levels within the Greater Toronto Area (city of Torontoand the surrounding region) from 2000 to 2007, and show thatdespite reductions of up to 40% in NO2 levels and VOC reactivity, Ox

levels have only shown a slight response, and average ozone levelsin Toronto have remained constant. We demonstrate the strong linkbetween O3 and local meteorological conditions by showing thatmaximum ozone is strongly dependent on high summer temper-atures and the degree of regional transport.

An analytical solution for instantaneous ozone production ispresented, which suggests that Toronto has been in a VOC-limitedproduction regime where we would not expect a significant decreasein 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 toreducing locally-generated smog-causing pollutants.

2. Methods

2.1. Study region and monitoring stations

Toronto, Canada (43�40’N, 79�23’W) is located in southernOntario on the northwest shore of Lake Ontario. The Greater Tor-onto Area (GTA) has a population of 5.5 million within 7000 km2,

and lies along the ‘‘Windsor-Quebec’’ corridor, the most populatedand industrialized region of Canada. During the summer, the GTA isoften affected by warm southerly and southwesterly air flow fromthis corridor which may also have passed over American cities (e.g.Cleveland, Detroit, Pittsburgh). Local meteorology is also ofteninfluenced by regular land-lake breezes. According to a 2004estimate, transportation accounted for roughly 63% of NOx emis-sions in Toronto, with diesel trucks contributing disproportionately(36%); the remainder of emissions are from natural gas and elec-tricity (ICF, 2007). In Ontario, transportation is a large source of VOCemissions (33%), with industrial emissions and general solvent usecontributing another 33% cumulatively; the last third of emissionsare from other miscellaneous sources (EC, 2008).

NOx, O3, and VOC data are available from the federal andprovincial cooperative National Air Pollution Surveillance (NAPS)network. Hourly NO2 and O3 data for eight sites in the GreaterToronto Area are publicly available from 2000 forward andupdated annually at http://www.airqualityontario.ca. NOx and O3

measurements are made by automated continuous chemilumi-nescent and UV-absorption analyzers respectively which satisfyrequirements of the US EPA as equivalent or reference methods.Sampling heights varied from 4 m to 12 m. Recent detailed inter-comparisons assessing the performance of molybdenum converterson chemiluminescent analyzers show that sometimes over 50% ofwhat is reported as NO2 during photochemically active times of theday 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 whereNO2 will be a high fraction of total nitrogen oxides, the error islikely significantly less than 50%, and that this bias has a minimalimpact on the observed trends. VOC data throughout the sameperiod was obtained from Environment Canada for four stations inthe area (Downtown, West 1, Junction, and Brampton), where 24-hsamples are collected once every 6 days by evacuated canisterfollowed by analysis by gas chromatography with flame ionizationdetection (for C2 hydrocarbons) or mass spectrometric detection(for C3–C12 hydrocarbons) as described by Wang et al. (2005). Lossof hydrocarbons to processes occurring on the internal canistersurfaces was evaluated by analyzing canister samples within a fewhours for terpenes, then again after storage for several weeks, andno decrease in terpene concentrations was observed. However, nocomparisons to on-line GC measurements have been made to date.Carbonyl compounds were automatically sampled using 2,4-dini-trophenylhydrazine coated silica Sep-Pak cartridges for 24 h, thenseparated and identified using HPLC and UV DAD detection at365 nm (EPA, 1999). Hourly meteorological data at Toronto’s PearsonInternational Airport, Toronto’s major weather reporting stationwhich has been in operation continuously during the period ofinterest, was obtained from 2000 to 2007 from the National ClimateData and Information Archive operated by Environment Canada.

The MOE and EC site locations in this study represent a variety ofurban and suburban environments across the Greater Toronto Area.The location and name of each site is shown in Fig. 1. Some are inclose proximity to major intersections or major highways, andsome are located close to large recreational parks or in residentialneighbourhoods (see Fig. 1).

2.2. Data

Daily 8-h maximum ozone and Ox were calculated for each site.Diurnal cycles were examined to determine the most photochem-ically relevant hours during the day for ozone production. This wasidentified to be between 11:00 and 15:00, so hourly data for NO2

was averaged during this 4-h-period for each day (hereafterreferred to as ‘‘daily NO2 midday average’’). Annual summer

J.A. Geddes et al. / Atmospheric Environment 43 (2009) 3407–34153408


averages were calculated, where summer is defined as May toSeptember, being the most photochemically relevant time of year.Annual averages also remove seasonal variability in the time seriesmaking it easier to identify trends. VOC reactivity was calculated asthe 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 andanthropogenic reactivity were distinguished by separating the sumof isoprene, cymene, pinenes, limonene, and camphene (consid-ered ‘‘biogenic’’ (Seinfeld and Pandis, 2006)) from the rest of themeasured VOC (considered ‘‘anthropogenic’’, consisting of 40different VOCs). Annual trends were calculated by linear regression,and their significance is measured by p-values calculated froma standard T-test where we are testing the null hypothesis that theslope of the regression line is equal to zero. Therefore the p-valuesreported represent the probability of observing a trend of thatextreme in a T-distribution of the same degrees of freedom.

Maximum daily temperatures were calculated during eachsummer, and net wind vectors were calculated for a 12-h periodprior to each afternoon (from midnight to 12:00 pm). This was doneby summing the magnitude of each x- and y-component of thewind at every hour, then calculating the resultant vector by trigo-nometry. The result is a single value that estimates the strength ofthe wind and net direction of air transport for each day throughoutthe immediately preceding night and early morning.

3. Results and discussion

3.1. Precursor levels

Annual summer averages of daily midday NO2 are shown inFig. 2a and b. It is clear that an overall decrease is presentthroughout the whole study region. The most urban station(Downtown Toronto) and the station closest to a significanthighway (West 2) report the highest NO2 in the region, reflectingthe important transportation source of NOx and indicating thatthese sites likely represent some of the highest NO2 levels foundthroughout the region. Stations furthest from the main roads andurban centre (e.g. Newmarket and Oakville) report the lowestlevels. Downtown Toronto, West 2, and Oshawa all have the

steepest slopes, between �1.2 and �1.4 ppb$year�1 (p < 0.01),while all other sites have strong decreasing trends of �0.4to �1.0 ppb year�1 (with p < 0.01 except for Newmarket). Overall,the North, East and Downtown Toronto summer averages in 2007were over 35% lower than 2000 levels (the highest difference is inEast Toronto, at 40%). At the West 2 station, 2007 levels werealready 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 changeless than 30% since the year it began reporting (13% since 2003).

We observe the steepest decrease in NO2 at stations closest tomajor transportation arteries and intersections, even though thenumber of vehicles registered in Ontario increased by over 700,000between 2000 and 2007 (Statistics Canada, 2000, 2007). This isevidence that the reductions are driven strongly by improvementsin vehicle technology. Decreasing NO2 in areas where its abundanceis high will lead to increases in local OH concentrations (because ofReaction (R6)). This changes the lifetime of NO2 so that it is oxidizedmore quickly near its source, meaning that a reduction in emissionscan have a proportionally even larger reduction in ambientconcentrations. Whatever combination of these factors, the factthat NO2 levels across Toronto and surrounding areas havedecreased between 13% and 40% reflects considerable achieve-ments in NOx emission controls and improvements in technologyover the past decade.

From the available data, the situation appears to be similar forvolatile organic compounds across Toronto. The annual summeraverages in anthropogenic VOC reactivity, shown in Fig. 3, showa steadily decreasing trend since 2000. VOC reactivity at theBrampton and Downtown sites in 2007 was already 40% lower thanreactivity in 2001 and 2002 respectively. VOC reactivity attributedto biogenic compounds shows no apparent trend.

While the magnitude of biogenic reactivity seems to be quitespatially consistent (around 0.4–0.7 s�1 at all locations), themagnitude of anthropogenic VOC reactivity does not (frombetween 2 and 3 s�1 in Brampton and West 1 to around 3–5 s�1 atthe Downtown and Junction stations). Consequently, the relativeimportance of biogenic emissions is not evenly distributed. At theWest 1 and Brampton site, for example, biogenic VOC reactivity in2007 represented almost 25% of all recorded reactivity, whereas atthe Downtown station the proportion is closer to 18%. At the

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 toa highway. Superscript ‘‘b’’ denotes stations which are located in suburbs.

J.A. Geddes et al. / Atmospheric Environment 43 (2009) 3407–3415 3409


Junction station, the most recent data in 2005 showed the biogenicfraction to be about 13%. Emission controls are more easily imple-mented for anthropogenic VOC, thus controlling total reactivity isnot an equal task across the Toronto region, and progress inreducing anthropogenic emissions will not result in a uniformdecrease in overall VOC reactivity. Additionally, as anthropogenicVOC reactivity continues to decrease, the biogenic fraction will rise,making it increasingly difficult to control VOC reactivity during thephotochemical season most relevant to ozone production. More-over, photochemical conditions favouring ozone production such asincreasing temperature and light can also favour higher biogenicemission rates (for example, see Kesselmeier and Staudt, 1999 andHarley et al., 1999). Indeed, the correlation between daily biogenicVOC reactivity and maximum temperature at each station rangesfrom 0.02 to 0.05 s�1/�C (R2 ¼ 0.10–0.42).

This analysis of VOC reactivity is limited, since the governmentaldata used here do not include oxygenated compounds at allstations. Formaldehyde, acetaldehyde, acrolein, acetone, and pro-pionaldehyde were measured at the Junction station in 2000, 2001,2004 and 2005, and the average sum of their reactivity during the

summer is shown in Fig. 3. While the trend here seems to beincreasing, this is difficult to say with the limited availability ofdata. However, we are able to estimate the contribution ofoxygenated VOC to overall reactivity at the Junction station. Thisvalue is w1 s�1 and is therefore an important contribution that isnot being accurately captured at other Toronto stations. In addition,our analysis using simple VOC reactivity only considers the initialOH reaction, and subsequent reactions (which may be especiallyimportant for longer chained VOC) are not always represented.However, these data show that efforts to reduce anthropogenic VOCemissions are in general producing significant results throughoutToronto, but also suggest several challenges facing pollutantreductions like the increasing fraction of, and the effect oftemperature on, biogenic emissions.

3.2. Ozone and Ox

Fig. 4a and b show annual summer daily 8-h maximum O3 forToronto and surrounding sites respectively, and Fig. 5a and b showannual summer daily 8-h maximum Ox. Linear regression showsthat the trends in ozone are not statistically different from zero, andat some of the more urban sites it may be increasing. The slopes inOx are all decreasing, but with varying levels of significance (fromp < 0.05 to p > 0.50). The most significant decrease is seen at theBrampton site, where the slope is �0.87 ppb year�1, however thistrend should be taken cautiously, since it is probably in part due tothe fact that data from 2000 is not available (which for all othersites was around 2005 levels, and would weaken the significance).Nevertheless, there seems to be some evidence that ozoneproduction may be decreasing slowly in the area. Annual summerdaily 8-h maximum Ox averages show less variability between sites,which illustrates the importance of using Ox (¼ NO2 þ O3) toremove the titration effect.

The titration reaction (R2) explains many of the differencesbetween the trends in O3 and Ox and between urban andsurrounding sites. For example, Newmarket, located around 40 kmnorth of Toronto, has consistently higher maximum O3 averages butlower maximum Ox averages than Downtown Toronto. Becausethere is significantly more NO at the Downtown station to react

Fig. 2. Annual summer midday NO2 concentrations (ppb) in (a) Toronto and (b)surrounding areas. Slopes in ppb year�1. Error bars represent 1 standard deviation ofthe mean.

Fig. 3. Annual summer VOC reactivity in the GTA (s�1). Slopes in s�1 year�1. Error barsrepresent 1 standard deviation of the mean. Closed squares represent anthropogenicVOC, open squares represent biogenic VOC, and closed circles represent oxygenatedVOC (Junction Station only).



with O3 via reaction (R2), the ozone is temporarily titrated there.Thus there appear to be competing effects responsible for the lackof a trend in ozone; mainly, the fact that decreasing NOx and VOCreactivity levels could be slowly driving down the production ofozone (as seen by the weakly decreasing trend in Ox levels), whileat the same time having less NO to titrate ozone causes levels toincrease in proximity to NO sources. Unravelling these competingchemical influences is complicated by the variability in ozoneresulting from meteorological conditions, which is explored in thefollowing section.

3.3. Meteorological considerations

Numerous studies report high correlations between ozonelevels and temperature for sites all over the United States (Vuko-vich, 1994; Sillman and Samson, 1995; Vukovich and Sherwell,2003; Abdul-Wahab et al., 2005), and several methods have beendeveloped to derive ‘‘meteorologically adjusted’’ trends. These

include probabilistic models, moving average filters, and clusteranalysis (Cox and Chu, 1993; Yang and Miller, 2002; Wise andComrie, 2005; Walsh et al., 2008). The eight-year time seriesavailable in the present study is probably too short to be exploredaccurately by most of these methods, but recognizing and under-standing the effect of temperature on ozone production is obvi-ously crucial in explaining year-to-year variability in O3 and Ox

levels throughout the GTA. We illustrate this in Fig. 6, where thenumber of ozone exceedances in North Toronto is plotted alongwith the number of days where the maximum temperaturereached above 30 �C. This figure shows that a great deal of theannual variability in high ozone levels can be explained by vari-ability in the number of summer days with high temperatures. Infact, summer Ox levels at this station correlate with maximum dailytemperatures with a slope of 1.7 ppb/�C (R2 ¼ 0.33). Mechanisti-cally, increased temperatures could lead to enhanced localproduction of O3 by enhancing HOx production, increasing localbiogenic VOC emissions (or volatilization of VOC), suppressing the

Fig. 4. Annual summer average maximum 8-h O3 concentrations (ppb) for (a) Torontoand (b) surrounding sites. Slopes in ppb year�1. Error bars represent 1 standarddeviation of the mean.

Fig. 5. Annual summer average maximum 8-h Ox concentrations (ppb) for (a) Torontoand (b) surrounding sites. Slopes in ppb year�1. Error bars represent 1 standarddeviation of the mean.



net formation of peroxyacetylnitrate, or may simply correlate withhigh solar radiation (Jacob et al., 1993; Sillman, 2003). For example,a correlation between biogenic VOC reactivity and maximum dailytemperature can be seen for at the Toronto sites, and might becomestronger if hourly instead of 24-h average VOC concentrations wereavailable.

Because high temperatures in Toronto are often associated withsoutherly flow, it is also possible that the correlation betweenozone and high temperatures is driven by transport of ozone andprecursor compounds from the polluted upwind region. Since we

were interested in separating and comparing the effects ofincreased local production and increased transport on warm days,we used the net wind vectors for each day from midnight to noon ofthat day as described in the Methods section. Fig. 7 shows a polargraph where the radius represents the summer net wind vectorsfrom 2000 to 2007 for each day, and the points are coloured byaverage Toronto daily maximum 8-h Ox levels. In general, when airis being transported from the southwest, Ox reaches higher levelsthan on days when air is being transported from the north andnorthwest.

Each summer, we defined days when the wind speed anddirection resulted in a net movement of >120 km from the southand southwest (135�–270�) as days influenced by ‘‘west to south-east flow (W–SE)’’ (n ¼ 197); days when the net wind speedand 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 than120 km as ‘‘stagnant’’ (or influenced most by local conditions,n ¼ 596). The data in the W–SE group represent days most likelyinfluenced by polluted air masses from surrounding urban areas inCanada and the US, while data in the W–NE group are generallyinfluenced by transport from remote regions.

Hourly averages of Ox, O3, and NO2 for these separate conditionsat the North Toronto site are shown in Fig. 8. North Toronto wasselected since it is representative of a location influenced directlyby GTA emissions (no matter what direction air might be trans-porting), but not as biased toward high NO2 as stations such asDowntown or West 2. We see that on days that were influenced byW–SE flow there are significantly higher morning Ox levels than on

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.

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).



days with stagnant conditions or influenced by W–NE flow. Stag-nant days show a sustained Ox increase of around 3 ppb h�1 from8 am to 3 pm, at which point levels start to decrease. Meanwhile,days with W–SE flow show similar production rates, but only untilabout noon at which point levels start to level out. These resultssuggest that stagnant conditions allow ozone to accumulate fora longer period through the day, probably because precursors arenot swept out of the city. This includes occasions when the land-lake breeze causes precursors to be swept out across the lake in theearly morning, but swept back into the city later in the day,resulting in very little net transport of air. The diurnal profile of oddoxygen on days with strong W–SE flow more closely follows theinstantaneous rate of ozone production, which likely peaks nearnoon because ozone and its precursors are being advected awayfrom the city and not left to accumulate. The evidence suggests thatunder strong W–SE flow, airmasses over Toronto start out withmore Ox, but that daytime increases are less significant. Interest-ingly, the maximum 8-h average in Ox is quite similar for bothstagnant and strong W–SE flow days but on stagnant days theimpact of titration is more significant, which keeps the ozonelower.

Previous studies using back trajectory clusters during summermonths from 1989 to 1995 and from 1994 to 2003 similarly foundthat the highest ozone levels are associated with trajectories fromthe south and southwest, passing over the Detroit-Windsor area(Brankov et al., 2003; Johnson et al., 2007). Along with the presentevidence, these results are highly relevant to the Canada-wideStandard for Ozone which makes a provision for ‘‘transboundary-influenced communities’’ that fail to meet the standard even aftertheir ‘‘best efforts’’ to reduce precursor emissions (CCME, 2000).

3.4. Computational model

While variability in meteorological conditions drives much ofthe interannual variability in ozone over Toronto, it is useful toassess the extent to which changes in precursor concentrationshave impacted local ozone productions rates. In this section, weapply a model that was developed by Murphy et al. (2006) tocalculate the instantaneous production rate of ozone as a functionof NO2 levels and VOC reactivity. By situating the surface observa-tions of NO2 and VOC from Toronto during the last 8 years on thisplot, we can estimate how the local production regime may or maynot be changing and suggest control measures that would mostefficiently result in improved air quality.

We refer readers to the above paper for a full derivation, but byassuming conditions of rapid ozone production where chainpropagation is more significant than chain termination and that theproduction of each RO2 ultimately results in an HO2 radical, wearrive at the following equation for instantaneous ozoneproduction:

PðO3Þ ¼ k4½HO2 þ RO2�½NO� ¼ 2k3½VOC�½OH�

where [OH] is estimated by:

With an initial O3 concentration of 40 ppb, relative humidity of 65%,and an average formaldehyde concentration of 4 ppb, the estimatedmidday summertime P(HOx) for Toronto was calculated as

2.3 ppb h�1. To solve the above equations, we estimate a to be 0.04(Cleary et al., 2005), [NO2]/[NO] ¼ 3, k6 ¼ 1.1 �10�11 s cm3molec�1

(Sander et al., 2003), k4eff ¼ 8 � 10�12 s cm3molec�1 (Tyndall et al.,2001), and k5 ¼ 5.0 � 10�12 s cm3molec�1 (Tyndall et al., 2001;Sander et al., 2003). Finally, we are able to plot P(O3) as a function ofVOC reactivity (¼Sk1i[VOC]i) and NO2. The result is shown in Fig. 9.

This contour plot clearly shows two separate ozone productionregimes; one in which decreasing NO2 levels decreases ozoneproduction (NOx-limited), and one in which decreasing NO2 levelsincreases ozone production (VOC-limited). Since the data show bothprecursors have been decreasing linearly, and at roughly consistentrates over most of the Greater Toronto Area we can estimate a vectorof precursor changes from 2000 to 2007 on the contour plot to

indicate how instantaneous local ozone production might bechanging year-to-year. We use a wide arrow on the plot to indicatethe general chemical space over which the GTA region atmosphere

½OH� ¼�ðk6½NO2� þ ak3½VOC�Þ þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðk6½NO2� þ ak3½VOC�Þ2þ24PðHOxÞk5eff

�k3½VOC�k4eff ½NO�

�2r

12k5eff

�k3½VOC�k4eff ½NO�

�2

Fig. 8. Hourly summer averages at North Toronto for Ox, O3, and NO2. Lines with circlesrepresent net wind conditions <120 km (local); lines with squares represent net windconditions >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).



has transitioned between 2000 and 2007. According to this simplemodel we do not expect instantaneous summertime ozoneproduction to have changed very much in the last 8 years. Theprecursor changes produce a vector (red arrow) that does not crosscontour lines in Fig. 9. It is interesting to note that if either the VOCreactivity or NO2 concentrations had changed in isolation, therewould have been more dramatic changes in ozone production. Forexample a 40% reduction in NO2 with constant VOC reactivity at 4 s�1

leads to model predictions of a doubling in ozone production rates,while a 40% reduction in VOC reactivity with constant NOx reducesozone production rates by approximately half.

While this model is quite simple and relies on several assump-tions, we note that the ozone production rates that it predicts(w4 ppb h�1) are consistent with the rates of Ox increase(w3 ppb h�1) observed in the GTA (Fig. 8). The model is useful as anaid to approximate the chemical regime of ozone production inToronto based on the non-linear relationship with NOx and VOClevels, and provides a sound scientific explanation for why the past8 years of precursor reductions have not resulted in statisticallysignificant reductions in O3 and Ox. Because the model calculationswere for a single value of P(HOx), the impact of possible changes toP(HOx), which may be driven by changes in O3 or formaldehydelevels, are not represented by the model realization in Fig. 9. Thereis no strong evidence for changes in P(HOx) between 2000 and2007, and this would have a greater impact on the magnitude of O3

production rates than on their NOx-dependence. We can say witha great deal of certainty that present levels place Toronto in a VOC-limited production regime and thus VOC emission reductions willbe most effective at decreasing local ozone production duringphotochemically relevant periods.

4. Conclusion

Significant reductions in NOx and VOC have been observed inthe city of Toronto and surrounding areas. At most sites, summer-time NO2 concentrations and VOC reactivity have decreased by

between 30 and 40% since 2000, likely in response to improvingvehicle technology, regulatory initiatives and incentive programsby the province and municipality to control emissions. Throughoutthe same period summer ozone levels have shown no significantchanges, with many sites showing average conditions in 2007 weresimilar to those in 2000. We interpret this as a response tocompeting effects; a nominal reduction in ozone production asdemonstrated by slightly decreasing Ox levels, counteracted by thereduced titration effect on a short time scale. We have also shownevidence that air transported from the west and south on some ofthose warm days is contributing to higher 8-h ozone maxima,mainly through higher early morning concentrations before localproduction begins. For example, we have shown that days influ-enced by strong W–SE transport reach similar 8-h maximum ozoneconcentrations as stagnant days on which ozone starts out lowerbut accumulation extends later into the day.

These results lend insight into ozone production in the GreaterToronto Area, but also point at several future challenges. Forexample, an increasing fraction of biogenic VOC reactivity meansthat it will become increasingly difficult to control total VOC reac-tivity, which our analysis indicates would be the most effectivestrategy for significant reductions in a VOC-limited ozone produc-tion regime. Moreover, biogenic VOC reactivity in Toronto hasshown a correlation with maximum daily temperature, and we cantherefore expect some efforts may be compounded by increasingtemperatures in a warming climate. The loss of O3 due to reactionswith biogenic VOCs (thus counteracting increases in O3 productionfrom reactions with OH) was estimated to be at least an order ofmagnitude lower than O3 production over the conditions presentedhere. In a VOC-limited production regime, the collection of VOCdata with higher time resolution and good spatial coverage acrossthe city and surrounding areas ought to be a priority. The VOC dataused in this analysis is based on 24-h averages, and we are thereforelacking specific information about VOC reactivity during photo-chemically relevant times of the day. Moreover, we have noted thelack of oxygenated VOC data which comprises a significant fraction

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 toconditions in Toronto. Contour lines represent changes in ozone production of 1 ppb h�1. The diagonal shaded arrow represents how general conditions across the Toronto area havechanged 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 ozoneproduction than changes in both VOC reactivity and NO2.



of VOC reactivity in the area. Also, there continue to be strongincentives to reduce NOx emissions, due to the role NOx plays in theproduction of fine particulate matter (PM2.5).

These conclusions should have important implications on boththe recent commitment to a 20% reduction in ozone precursoremissions in Toronto since 2004, and on the Canada-wide Standardfor Ozone which provides leniency for transboundary-influencedcommunities that cannot meet the standard despite their bestefforts to reduce emissions.

Acknowledgements

The authors are grateful for support from Environment Canadaand the Natural Sciences and Engineering Research Council ofCanada.

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