This study investigates temporal variations and long-term (1996–2015) trends
of ground-level O3 (ozone) and its precursors, NOx (nitrogen
oxides),
and volatile organic compounds in Windsor, Ontario, Canada. During the
20-year study period, NOx, non-methane hydrocarbon concentrations, and ozone
formation potential decreased significantly by 58 %, 61 %, and 73 %,
respectively, while O3 concentrations increased by 33 % (20.3 ppb in
1996 vs. 27 ppb in 2015). Our analysis revealed that the increased annual
O3 concentrations in Windsor were due to (1) decreased O3
titration (by 50 % between 1996 and 2015) owing to declining nitric
oxide concentrations, which is suggested by a slightly decreasing trend of
annual mean total O3 concentrations after the titration effect is
removed, (2) reduced local photochemical production of O3 because of
dwindling precursor emissions, and (3) an increased background O3 level
that has a greater impact on the low-to-median concentrations. The net effect of
those factors is decreasing peak O3 levels during the smog season from
May to September but an overall increasing trend of annual means. These
results indicate that the emission control measures are effective in
reducing peak ozone concentrations. However, challenges in lowering annual
O3 levels call for long-term collaborative efforts in the region and
around the globe.
Introduction
Ozone (O3) at the ground level is a main component of smog. Exposure to
high O3 concentrations causes wheezing and shortness of breath,
resulting in absence from schools and hospital admissions (USEPA, 2018).
People with respiratory diseases, children, and elders are at higher risk
from O3 exposure. Recent studies suggest that long-term exposure to
high O3 levels is associated with permanent lung damage and deaths
from respiratory causes (USEPA, 2018). High O3 concentrations also
result in reduced crop yields by inhibiting breathing ability of plants,
slowing down the photosynthesis rates, and making plants more susceptible to
diseases (IDNR, 2018).
As a secondary air pollutant, ground-level O3 is formed by
photochemical reactions between nitrogen oxides (NOx) and volatile organic
compounds (VOCs) in the presence of sunlight. Non-methane hydrocarbons
(NMHCs) are more reactive than methane and other VOCs in forming ozone (NAS,
1999); therefore, NMHCs are used to represent O3 precursors (e.g., Jun
et al., 2007; Akimoto et al., 2015). Because the reactivity of each NMHC is
different, Carter (1994) and other researchers used O3 formation
potential (OFP) to quantify contributions of individual NMHCs or a group of
NMHCs (Jia et al., 2016). Similarly, a study in Hong Kong investigated
associations between O3 and its precursors, i.e., NOx and 21
NMHCs,
during 2005–2014 (Wang et al., 2017). O3 concentrations in Hong Kong
increased (0.56 ppb yr-1, p<0.01), while NOx decreased
(-0.71 ppb yr-1, p<0.01). The study further showed that there were no
significant changes in NMHCs (-0.03 ppb yr-1, p>0.1) during the
10-year study period. Nevertheless, the calculated daytime average
contribution to O3 concentrations by aromatics decreased (-0.23 ppb yr-1,
p<0.05), while that by alkenes increased (0.14 ppb yr-1,
p<0.05) and that by alkanes and biogenic VOCs did not change
significantly (-0.04 ppb yr-1, 0.24 ppb yr-1, respectively,
p>0.05; Wang et al., 2017).
In Ontario, Canada, emissions of NOx and VOCs decreased by 52 % (from 651
to 311 kt) and 54 % (from 789 to 363 kt),
respectively,
during 1996–2015 (ECCC, 2018a). However, the Ontario-wide O3 composite mean
increased by 22 %, from 22.4 ppb in 1996 (MOE, 2006) to 27.4 ppb in 2015
(MOECC, 2017). Previous studies showed that changes in O3
concentrations were attributed to background O3 and changes in
photochemical O3 production caused by the decrease in NOx and VOC
concentrations (e.g., Shin et al., 2012). Because NO (nitric oxide) reacts
with O3 to form NO2 (nitrogen dioxide) and O2, also known as
NO titration, decreased NO concentrations may lead to increases in O3
concentrations due to weakened titration effect (Sicard et al., 2011; Akimoto
et al., 2015). To remove the impact of the NO titration on ambient O3
concentrations, total ozone (TO) was previously employed in trend
analysis. For example, Akimoto et al. (2015) used TO in their ambient ozone
study in four areas in Japan where O3 concentrations were high (i.e.,
Tokyo, Nagoya, Osaka, and Fukuoka). During the 20-year study period, NO
concentrations decreased from 16 ppb in 1990 to 6 ppb in 2010. The
increasing rates of annual TO (0–0.22 ppb yr-1) were much smaller than those
of O3 (0.22–0.37 ppb yr-1) in the four areas during 1990–2010. The
authors concluded that the decrease in the NO titration effect was one of
the causes for the increased O3 concentrations in Japan.
Recently, continuous O3 observations (2 years or longer) from more than
9600 stationary platforms around the world were assembled to assess a suite
of metrics relevant to its impact on human health, vegetation, and climate
under the International Global Atmospheric Chemistry (IGAC) Tropospheric
Ozone Assessment Report (TOAR) project (Schultz et al., 2017; IGAC, 2018).
Using 2010–2014 means from over 3300 vegetation sites, the highest ozone
levels were found in midlatitudes of the Northern Hemisphere, including
the southern USA; the Mediterranean Basin; northern India; northern, northwestern, and
eastern China; the Republic of Korea, and Japan (Mills et al., 2018). In a
study of over 2000 monitoring sites worldwide, negative (i.e. decreasing)
trends in peak O3 concentrations (i.e. fourth-highest daily maximum 8 h
average) were observed at most North American sites and at some European
sites, with very few sites exhibiting positive trends (Fleming et al., 2018).
Similar studies reported that O3 levels (monthly mean of the daytime
average and monthly mean of the daily maximum 8 h average) continued to
decrease significantly over eastern North America and Europe, while Asia
experienced increasing O3 concentrations through the end of 2014 (Chang
et al., 2017; Gaudel et al., 2018). In eastern North America, summertime
daytime averages and daily maximum 8 h concentrations declined at a
slower rate at urban sites than at rural sites during 2000–2014 (Chang et
al., 2017). Those studies showed that, over North America and Europe,
decreasing peak O3 levels are attributable to reduction in precursor
emissions, and a relatively slower decreasing rate at urban locations
suggests weakened O3 titration. In Asia, growing precursor emissions
led to increasing ozone concentrations.
Built on our understanding of spatial variations (Fleming et al., 2018;
Mills et al., 2018), this study evaluated temporal variations and trends of
ground-level O3 and its precursors (NOx and VOC) in Windsor, an urban
location in Southern Ontario, Canada, during the 20-year study period of
1996–2015. The main objective was to identify the driving force of long-term
trends of O3 concentrations in Windsor during the past 20 years as
well as seasonal and diurnal variations. Findings of this study will shed
light on the effectiveness of emission control policies and help develop
feasible approaches for reducing O3 concentrations in this region.
MethodologySelection of station in Windsor
There are two air quality monitoring stations in Windsor, Windsor Downtown
and Windsor West, which are 3.5 km apart (Fig. 1). Both stations monitor
O3 and a number of common air pollutions (e.g., NO, NO2, NOx,
SO2, and PM2.5; MECP, 2018). The Windsor Downtown station was
selected in this study due to (1) fewer invalid or missing O3 values
(1824 vs. 2660 during 1996–2015) and (2) a longer record of NO, NO2,
and NOx data available (1996–2015) compared to the Windsor West station
(2001–2015); 24 h VOC samples were collected once every 6 d
at the Windsor West station (ECCC, 2016).
Air quality monitoring stations in Windsor, Ontario, Canada.
Data sources
Hourly O3, NO, NO2, and NOx concentrations in Windsor
(1996–2015) were obtained from the Ontario Ministry of the Environment,
Conservation and Parks (MECP); 24 h VOC data at the Windsor West
station during 1996–2015 were downloaded from National Air Pollution
Surveillance (NAPS) website (ECCC, 2018b).
Data processingO3, NO, NO2, NOx, and VOC concentrations and
ratios
Numbers of data flags “-999” (i.e., invalid data), blank cells, and “0”
data points in hourly O3, NO, NO2, and NOx concentrations
were counted by year. Then data flags -999 were replaced with blank
cells to maintain consecutive date and time for individual pollutants. If the
total percentage of data flags and blank cells is greater than 40 % (3504 h yr-1)
in a year, data in that year are considered to be invalid and excluded
from further analysis. This is the case for hourly NO, NO2, and
NOx concentrations in 2003. Results of data screening can be found in
Zhang (2016) and in the Supplement (Table S1).
There are 176 VOCs reported in the NAPS dataset, of which 118 were
used in this study. Missing samples were identified by comparing the
sampling schedule with the dates of available samples in each year. Blank
and 0 cells were counted for individual compounds in each year. A
compound is excluded from analysis if the total number of blank and 0
cells is greater than 70 % during the study period of 1996–2015. Blank and
0 cells were also counted for each sample. Samples with less than 60 % of
compounds that registered valid readings were removed. To reduce the undue
influence of a few unusual events with extremely high concentrations,
outliers were identified and removed.
Sixteen NMHCs were excluded from analysis because less than 30 % of
samples had valid readings. Thus, 102 compounds were retained for further
analysis. Out of 877 samples, 14 were excluded. The remaining 863 samples each
had at least 60 % compounds with valid readings (range of 64 %–100 %,
mean of 88 %, and median of 91 %), and they were used to calculate total NMHCs
and OFPs. Daily NOx-to-NMHC ratios (referred to as NOx-to-VOC ratios
hereafter) were calculated for the dates when VOC data are available. Hourly
NO2/NOx ratios were calculated as well.
Total O3 concentrations
Following Akimoto et al. (2015), TO in Windsor was calculated with Eq. (1),
[TO]=[O3]+[NO2]-0.1×[NOx]1=[O3]+[DO3],
where [DO3] ([NO2]-0.1 × [NOx])
represents loss of O3 due
to in situ NO titration; [O3], [NO2], and [NOx] are hourly
concentrations; and the constant 0.1 is the fraction of NO2 in primary
NOx emissions (Itano et al., 2007). In this study, the NO2
fraction was determined from the slopes of regression of [Ox] (=
[O3] + [NO2]) vs. [NOx] in Windsor in each year during the
morning NO and NO2 peak hours (from 05:00 to 08:00; all times are referred to in
local time – LT) as described in
Kurtenbach et al. (2012). The 20-year average fraction was 0.1, consistent
with that in the previous O3 study in Japan (Itano et al., 2007).
NMHC concentrations and ozone formation potential
OFPs for individual VOCs were calculated using Eq. (), as
described in Yan et al. (2017),
OFPi=Conci×MIRi,
where Conci (µg m-3) is the ambient concentration of the ith
NMHC, and MIRi is the corresponding maximum incremental reactivity
coefficient in the unit of grams of ozone formed per gram of VOC added in the
system (Carter, 1999). OFPs for individual samples in each year were
calculated.
Temporal variation and trend
The analysis of variance (ANOVA) was used to determine whether there were
statistical differences in O3 and TO concentrations between weekdays
and weekends. Linear regression was employed to examine long-term
(1996–2015) trends of (1) annual means and means in the smog (May–September)
and non-smog season (October–April) for O3 and TO; (2) the annual mean for
NO, NO2, NOx, OFP, DO3, NMHC concentrations, and the ratio of
NO/NOx; and (3) various annual percentile levels (5th, 25th,
50th, 75th, and 95th) of hourly O3 and TO.
Hourly O3, TO, and DO3 concentrations do not follow a normal
distribution. Thus, the Mann–Kendall test, a non-parametric trend detection
method (Gilbert, 1987), was used to detect long-term trends in each month of
a year and at each hour in a day. Sen's method (Sen, 1968) was used to
estimate the slope of seasonal and diurnal trends when the trend is
significant at the 95 % level. Long-term trends of O3 and TO in
Windsor were compared to quantify the impact of the NO titration on O3
concentrations.
All analyses outlined in Sect. 2.3–2.4 were carried out in Minitab
version 16 (Minitab Inc., State College, Pennsylvania, USA) and MATLAB
release 2017a (The MathWorks, Inc., Natick, Massachusetts, USA).
Results and discussionGeneral statistics
As shown in Table 1, the 20-year mean O3 concentration was 24 ppb in
Windsor. Higher O3 levels were observed in the smog season than the
non-smog season, reflecting photochemical production under sunny and warm
conditions. TO concentrations were higher than O3 concentrations in all
seasons and at all concentration percentile levels because TO includes the
fraction of O3 lost through the NO titration. TO concentrations showed
lower variability (i.e., lower coefficient of variation) than O3
concentrations, which is expected because O3 reacts with NO while TO is
not affected by the NO titration (Akimoto et al., 2015).
General statistics of O3 and TO concentrations in Windsor
during 1996–2015 (SD and CV stand for standard deviation and coefficient of
variation, respectively).
Diurnal variation of O3 concentrations in Windsor during 1996–2015 is
shown in Fig. 2a and Table S2. There was a gradual increase in O3
concentrations from 06:00 to 14:00 and a gradual
decrease from 15:00 to 06:00 next day. A similar trend was observed
for TO. The diurnal variations for O3 and TO indicate that O3
photochemical production was enhanced by increased solar radiation and
temperature (So and Wang, 2003). DO3 followed an opposite trend than
O3, i.e. lower at noon to afternoon (11.2 ppb from 11:00–15:00) than that at other hours of the day (16.2 ppb), suggesting that
relative loss due to the titration effect was reduced when O3
concentrations were high.
O3 concentrations were higher during the smog season than in the
non-smog season, especially around noon, due to photochemical production
(Fig. 2b and c). DO3 levels were lower throughout the day in the smog
season, suggesting that relative loss due to the titration effect was reduced
when O3 concentrations were high. Furthermore, TO (O3+DO3)
diurnal variation was rather smooth in the non-smog season due to weak
photochemical production of O3.
Diurnal O3 and DO3 concentrations during 1996–2015 in
Windsor for (a) all months, (b) smog season, and (c) non-smog season. CI stands for confidence interval.
Seasonal variation
Monthly O3 concentrations increased from January to May, reaching peak
values in June and July, then decreased from July until the minima in
December (Fig. 3 and Table S3). This seasonal pattern is similar to that of
solar radiation and ambient temperature, which control the photochemical
production rate of O3. A similar seasonal variation was observed for
TO, but DO3 followed an opposite trend than O3, i.e. higher in
the non-smog season (16.6 ppb) and lower during the smog season (13.1 ppb). Similar
to the diurnal variation, relative loss due to the titration effect appears
to be reduced when weather conditions favored O3 formation. The seasonal
O3 pattern observed in Windsor is consistent with the study by Gaudel
et al. (2018) reporting that in North America, the maximum O3 daytime
averages occurred in spring–summer and the minimum values were found in
autumn–winter during 2010–2014.
Monthly mean O3 and DO3 concentrations during 1996–2015
in Windsor.
Weekday–weekend variation
ANOVA indicates that O3 concentrations on weekends (25.9 ppb) were
statistically higher (p<0.05) than on weekdays (23.3 ppb). NO
concentrations were lower on weekends (6.5 ppb) than on weekdays (9.6 ppb)
due to less vehicular and industrial activities. Therefore, high O3
concentrations on weekends were likely attributed to decreased NO emissions
and the weakened titration effect as reported by other researchers (Koo et al.,
2012). This is supported by comparable TO concentrations between
weekdays and weekends (39.2 vs. 39.5 ppb, p<0.05) which remove
the titration effect. Differences in O3 levels between the weekday and
weekend were also reported in other studies, e.g., in Nepal (Pudasainee et
al., 2006) and Ontario, Canada (Huryn and Gough, 2014).
Long-term trendTrends of annual NOx, NMHC, ozone formation potential, O3 and
TO
During 1996–2015, annual mean O3 concentrations increased significantly
(0.452 ppb yr-1; Fig. 4a), while annual mean DO3 concentrations
decreased at a greater rate (-0.524 ppb yr-1). Consequently, TO
concentrations decreased slightly (-0.076 ppb yr-1,
but not significant at p<0.05 level). In
other words, O3 decreased slightly when the NO titration effect is
removed, suggesting that the decreased NO titration effect is one of the
reasons for the increased O3 concentrations in Windsor.
Annual mean concentrations for (a)O3, DO3, and TO, and
(b)NOx, NMHC, and ozone formation potential (OFP) in Windsor during
1996–2015.
Significantly decreasing trends were observed in Windsor for annual mean
NOx (-1.34 ppb yr-1), NMHC
(-2.98µg m-3 yr-1),
and OFP (-9.77µg m-3 yr-1) during the 20-year study period (Fig. 4b). The percent
decreases were 58 %, 61 %, and 73 % for NOx, NMHC, and OFP,
respectively, indicating effective emission control. It should be noted that
during 1996–2008, some pollutants were changing at greater rates compared
with the 20-year trend, including O3 (0.55 ppb yr-1), NMHC
(-4.34µg m-3 yr-1), and OFP (-13.5µg m-3 yr-1). After 2008,
concentrations of O3, NMHC, and OFP leveled off, while NOx and DO3
concentrations continued to decrease.
The decreased NO titration effect was further investigated by examining the
ratio of NO/NOx (Fig. S1). Significantly decreased NO
(-0.73 ppb yr-1) and NO2 (-0.66 ppb yr-1) were observed during the study
period. Furthermore, the NO/NOx ratio decreased from 0.34 in 1996 to
0.16 in 2015, with an average rate of -0.012 yr-1, supporting the decrease in
the NO titration effect in Windsor. Our results are consistent with studies
in other countries. For example, NO2/NOx ratio increased from 0.08 in
2005 to 0.15 in 2010 in Japan (Itano et al., 2014), implying a decreased
NO/NOx ratio. The NMHC/NOx ratios did not change much during the
20-year study period (min of 0.96, max of 1.3, and mean and median of 1.1). The low
ratios of VOC to NOx (<5) suggest that the study area is VOC
limited;
thus reduced NOx emissions may lead to increased O3 concentrations
(Sillman, 1999; USEPA, 2000).
Ozone and TO trends at various percentile levels
Figure S2 depicts long-term O3 and TO trends at 5th, 25th,
50th, 75th, and 95th percentiles in Windsor during 1996–2015.
The slopes of linear regression in three scenarios (i.e., all months, the
smog season, and the non-smog season) are summarized in Fig. 5 and Table S4.
Peak O3 concentrations (i.e., 95th percentile) decreased
during the smog season and for all months, suggesting that reduced precursor emissions
and photochemical production. O3 at all other percentile levels in all
three cases had increased, with higher rates at 25th and 50th
percentiles. The 25th percentile of O3 concentrations was
commonly considered to be a background value (Lin et al., 2000; Aleksic et al.,
2011; Parrington et al., 2013). Peak TO concentrations (95th
percentile) deceased, especially during the smog season, due to effective
emission control of O3 precursors. TO concentrations increased at all
other percentile levels during the non-smog season, when O3
photochemical production was limited, suggesting rising background O3
concentrations.
Slopes of long-term O3 and TO trends at various percentile
levels in all months, smog season, and non-smog season in Windsor during
1996–2015 (red border: significant at p<0.05; blue border:
significant at p<0.1; green border: not significant, i.e. p>0.1).
In the smog season, O3 concentrations increased at the
5th–75th percentile levels, while TO concentrations decreased with
a greater rate at higher percentile levels, suggesting that the decrease in
NO titration is one of the causes of increasing O3 in Windsor. In terms
of peak O3 concentrations (95th percentile), the decreasing rate
of TO (-1.0 ppb yr-1) is more than twice that of O3 (-0.45 ppb yr-1).
In other words, when the effect of NO titration is removed, peak O3
concentrations decreased more intensely due to reduced emissions of O3
precursors. During the non-smog season, the increasing rates of TO at
5–75th percentiles were much slower than those of O3. The results
suggest that the decreased NO titration effect could be one of the causes
for slower decrease in peak O3 in the smog season and increase in
O3 at low-to-high percentiles during both the smog and non-smog seasons.
The decreasing trend of the 95th percentile O3 levels in Windsor
is consistent with the decreasing concentrations at upper end of the
distribution across the United States (Simon et al.,
2015), which evaluated
maximum daily 8 h average O3 at over 1000 sites during 1998–2013,
when NOx and VOC emissions were decreasing. The declining peak O3 is
also evident in the study of the fourth-highest daily maximum 8 h
concentrations during 2000–2014 by Fleming et al. (2018), which indicated
that up to 70 % of North American stations experienced significant negative
trends (p<0.05). The results of the seasonal O3 trends in
Windsor are consistent with previous studies. For instance, Simon et
al. (2015) reported that the declining trends were more pronounced in summer
than winter and that increasing O3 trends at all percentiles were
found in both smog and non-smog seasons, except for decreasing peak values at
urban sites of the East North Central region (close to Windsor; Simon et
al., 2015). Moreover, Gaudel et al. (2018) reported the increasing O3
levels across North America in wintertime (December, January, and February).
Monthly and diurnal rates of change for ozone and TO
This section further investigates which hour or hours of a day and which
month or months of a year experienced greater or fewer changes in O3 concentrations
during the 20-year study period and to what degree those changes could be
explained by the change in the NO titration effect. The estimated
month-of-year slopes by Mann–Kendall and Sen's test during 1996–2015 are
shown in Fig. 6. The rates of change during the smog and non-smog seasons
are summarized in Fig. S3.
The increased O3 levels in the non-smog season
(mean of 0.58 ppb yr-1;
Fig. S3) suggest a reduced titration effect and rising background O3
levels,
since local photochemical production of O3 is limited. Analysis of
ambient data conducted by USEPA demonstrated that mid-tropospheric O3
concentrations in the US and globally have increased over the past two
decades by 0.4 ppb yr-1 (USEPA, 2015). Along the Pacific Coast, the rate of
increasing background O3 was estimated to be 0.5–0.8 ppb yr-1 during
1985–2002. This trend of ground-level O3 is consistent with the rate of
increase (0.51 ppb yr-1, 1994 to 2002) derived using aircraft measurements
(Jaffe et al., 2003). Another reason of increased O3 is the decreased
titration effect. A study in southeastern France demonstrated that the
decrease in the NO titration effect could be one of the reasons for
increased O3 concentrations in cold months (Sicard et al., 2011). The
slower increasing rate of O3 in smog season (0.32 ppb yr-1, Fig. S3) is
a result of increased background O3 levels, a decreased titration
effect,
and reduced local photochemical O3 production and regional
transport (MOECC, 2017). A similar trend of a greater rate of increasing
composite mean at 19 sites across Ontario in summer (49 %) than in winter
(14 %) during 1991–2010 was largely attributable to the reductions in
local NOx emissions and the rising global background ozone levels
(MOE, 2012).
O3 concentrations increased while DO3 concentrations decreased in
all months during 1996–2015 (Fig. 6). During the non-smog season, the increasing
rate of O3 (0.58 ppb yr-1; Fig. S3) was higher than the decreasing rate
of DO3 (-0.46 ppb yr-1). In other words, there was an additional
increase in O3 beyond the decreased titration effect. After the NO
titration effect is removed, TO concentrations increased in non-smog season
(0.13 ppb yr-1; Fig. 6), suggesting rising background O3 levels. In the smog
season, the increasing rate of O3 (0.32 ppb yr-1) was lower than the
decreasing rate of DO3 (-0.50 ppb yr-1). TO concentrations had
decreased in the smog season (-0.27 ppb yr-1; Fig. 6), being attributable to the
decreased regional O3 production.
Monthly rates of change during 1996–2015 for O3, DO3,
and TO.
On an hourly basis, greater increasing rates in O3 concentrations were
observed at evening and nighttime hours (18:00–03:00) in comparison with the early
morning and daytime (04:00–17:00), as shown in Fig. 7. The two minima in the
morning at 06:00 and 13:00 coincided with the lowest and highest O3
concentrations in a day, which were caused by different rates of change in
smog and non-smog seasons (see below). Overall, O3 increased while
DO3 decreased at all hours in a day during 1996–2015. The diurnal
pattern of increasing rates for O3 almost mirrored that of decreasing
rates for DO3. In other words, the increase in O3 concentrations
could be explained largely by the decreased NO titration effect. At most
hours, the increasing rates of O3 were higher than the decreasing rates
of DO3, especially in the morning hours (06:00–12:00). Overall, TO
concentrations increased slightly during daytime while decreasing a little in
the evening.
Rate of change by hour of day for all months during 1996–2015 for
(a)O3, (b) DO3, and (c) TO.
O3 and DO3 concentrations showed different diurnal patterns during
the smog season (Fig. 8). O3 concentrations increased while DO3
concentrations decreased at all hours as in the case of all months. During
the daytime (09:00–19:00), there was a sharp decline in the rates of change for
O3 until peak O3 hours (14:00–16:00) followed by a speedy recovery.
The peak hour O3 concentrations have not changed much during the last
20 years, and daytime ozone levels have increased with a much slower rate
(09:00–19:00, mean = 0.15 ppb yr-1) compared with the nighttime (20:00–08:00,
mean = 0.46 ppb yr-1). The daytime DO3 decreasing trend is similar,
however with less variability. The increasing rate of O3 is lower
than the decreasing rate of DO3, and TO concentrations decreased at all
hours, especially during the afternoon and early evening (14:00–19:00). This
suggested decreased photochemical O3 formation during the smog season
due to emission reduction.
During the non-smog season (Fig. 9), the rates of change in O3 and
DO3 were similar as in the case of all months, i.e. the increase in
O3 concentrations could be explained largely by the decreased titration
effect. Also similar to that of all months, the rates of change were lower
in the early morning (05:00–07:00). The greater rates of change were observed in
the late afternoon and evening (16:00–20:00), instead of at night, with all
months. The increasing rates of O3 were higher than the decreasing
rates of DO3 at all hours. The hour-of-day TO trend is overall
increasing with less diurnal variation, indicating rising background
O3 levels.
Rate of change by hour of day in smog season during 1996–2015 for
(a)O3, (b) DO3, and (c) TO.
Rate of change by hour of day in non-smog season during 1996–2015
for (a)O3, (b) DO3, and (c) TO.
Conclusions
This study investigates temporal variations and long-term trends (1996–2015)
of ground-level O3 and its precursors, NOx and VOCs, in Windsor,
Ontario, Canada. The driving force of the observed variations was assessed
by studying precursor emissions, photochemical production, NO titration, and
background O3 levels. One of the innovative approaches is the use of
TO and trend analysis for different percentiles levels in different seasons
and by hour of day.
O3 concentrations increased by 33 % during 1996–2015 (20.3 ppb in
1996 vs. 27 ppb in 2015) in Windsor, while concentrations of NOx (-58 %)
and NMHCs (-61 %) and OFPs (-73 %) decreased significantly during the
same time period, owing to effective emission control. Increased O3
concentrations were observed in all months in a year and all hours in a day
and at all percentile levels, with a few exceptions.
Our analysis revealed that the increased annual O3 concentrations in
Windsor were caused by the following reasons. First, there was decreased
O3 titration and local photochemical production of O3, both of
which being induced by reduced precursor emissions. The O3 loss due to
the titration decreased by 50 % in the 20-year study period, and the
declined O3 titration was observed in all months in a year and all
hours in a day. Therefore, the observed increase in O3 concentrations
can be largely explained by the decrease in the titration. By removing the
titration effect, TO concentrations increased in the non-smog season and
decreased in the smog season, resulting in a slightly decreasing trend of
annual means during 1996–2015 (-0.076 ppb yr-1). The declining photochemical
production of O3 is evident in decreased peak O3 levels (95th
percentile) in the smog season as opposed to increased O3
concentrations at all other percentile levels and all percentiles in the
non-smog season. Second, background O3 level was rising. This is
supported by increasing O3 concentrations in all months in a year and
all hours in a day and at all O3 percentile levels, with the exception
of peak O3 hours and the 95th percentile O3 levels in the
smog season. Furthermore, the increasing rates of O3 were higher than
the decreasing rate of DO3 at all hours in a day and all percentile
levels during the non-smog season, when O3 photochemical production is limited.
It is apparent that control measures implemented in Ontario and the
surrounding regions were effective in curbing NOx and VOC emissions during
the study period of 1996–2015. The reduced O3 precursors led to
decreasing peak O3 values in the smog season over the past 20 years.
However, those emission reductions also result in weakened O3 titration
effect in all months in a year and at all hours in a day. Meanwhile, the
background O3 concentrations appeared increasing in the study region,
with a greater impact on the low-to-median levels (i.e. 25th and 50th
percentiles) during the non-smog season and at night. The net effect of those
factors is decreasing peak O3 levels but an overall increasing annual
mean in Windsor. The increases in O3 concentrations in the non-smog season
(0.58 ppb yr-1), at night (20:00–08:00, 0.46 ppb yr-1), and at low-to-median
percentiles pose less of a risk on human health because those O3 levels are
relatively low. The decreasing peak O3 levels during the smog season
are
rather beneficial, considering the detrimental effects of human exposure to
high O3 concentrations.
Our long-term (1996–2015) trend analysis shows that annual O3, NMHC,
and OFP levels leveled off after 2008, while NOx concentrations and the
O3 titration effect appear to
be continuously decreasing. Considering that
O3 formation in Windsor remains to be VOC-limited, the weakened O3
titration by NO2 may lead to slightly increasing O3 annual means.
Moreover, the regional background levels are not expected to decline.
Therefore, it is anticipated that O3 concentrations in Windsor may
level off or increase slightly in the next few years under similar weather
conditions. Due to the complex nature of O3 formation, consumption, and
regional transport, it is clear that long-term regional and international
efforts are essential for lowering O3 concentrations and improving air
quality. Results of this study provide insight into the causes of changing
O3 levels in Windsor and how to mitigate O3 pollution and its
adverse effects on human health and the environment. Future studies are
warranted to quantify the background O3 level in Windsor area and its
long-term trend and to explore regional transport of O3 to Windsor.
Data availability
All data used in this study are publicly
accessible. The hourly O3, NO, NO2, and
NOx concentrations in Windsor can be accessed through the Ministry of
the Environment, Conservation and Parks Ontario website, http://airqualityontario.com/history/index.php
(last access: 26 May 2019); 24 h VOC data in Windsor can be accessed through
Environment and Climate Change Canada website, http://maps-cartes.ec.gc.ca/rnspa-naps/data.aspx
(last access: 26 May 2019).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-7335-2019-supplement.
Author contributions
TZ conducted data analysis and drafted the paper. XX
and YS designed the study and completed the paper.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
The authors would like to thank all who contributed to collection of air
quality data at MECP and ECCC and Yun Zhou at University of Windsor for
processing some VOC data. This project was funded by MECP and the Natural
Sciences and Engineering Research Council of Canada.
Financial support
This research has been supported by the Ontario Ministry of
the Environment, Conservation and Parks and the Natural
Sciences and Engineering Research Council of Canada.
Review statement
This paper was edited by Min Shao and reviewed by two anonymous referees.
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