Chapter 5
Lake Breeze and Oxidants Over
South-Eastern Ontario
Due to the thermal inertia of large bodies of water, lakes (or oceans) warm and cool
considerably slower than the adjacent land surfaces. On days with limited cloud
cover, solar radiation warms land surfaces more rapidly than the lake surface and a
temperature dierential between the land and lake surfaces develops. The dierence
in temperature between the land and the lake surfaces results in a dierence in the
temperature of the atmosphere above, which results in a slight perturbation to the
pressure eld. The response of the wind eld to the temperature-induced perturbations in the pressure eld gives rise to a ow from the lake towards the land at
the lowest levels of the atmosphere, and a corresponding return ow, from the land
towards the lake, aloft. At a certain distance inland, the onshore ow in the lowest
levels of the atmosphere rises and return towards the lake as part of the oshore ow
aloft, forming the lake breeze front (Simpson, 1977).
Interest in the inuence of lake breeze circulations on air quality in the Great
Lakes region has existed for more than 20 years. Observations in the early 1970's
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along the western shore of Lake Michigan found that high concentrations of ozone
observed at ground-level were associated with southeasterly or onshore ow as a result
of a lake breeze circulation (Lyons and Cole, 1973 1976). Lake breeze or sea breeze
circulations have also been found to have signicant eects on air quality along the
shore of Lake Erie in southwestern Ontario (Mukammal et al., 1982), in Los Angeles,
California (Lu and Turco, 1996), Athens, Greece (Lalas et al., 1983) and along the
Mediterranean coast of Spain (Martin et al., 1991 Millan et al., 1996).
The eects of lake and sea breeze circulations on air quality have been investigated
for a variety of locations with diering topography and meteorology. For example, a
modelling study by Lu and Turco (1996) of the eects of a sea breeze circulation on air
quality in Los Angeles has shown that the sea breeze can bring relatively unpolluted
air inland. The onshore ow of the sea breeze becomes progressively more polluted as
the airmass moves over regions with large anthropogenic emissions of hydrocarbons
and NOx, while vertical motions at the sea breeze front act to lift ozone and other
pollutants and result in layers with higher concentrations of oxidants aloft. These
elevated layers of high concentrations of oxidants may be mixed down to the surface
the following day.
For the case of the interaction of emissions from Toronto with a lake breeze over
Lake Ontario, the most applicable study is that performed by Lyons and Cole (1976).
They studied the eects of a lake breeze circulation over Lake Michigan on emissions
from Chicago and surrounding regions. Observations of the ground-level ozone concentration along the western shore of Lake Michigan showed that under conditions
of onshore ow during a lake breeze, ozone concentrations were highest from 1 { 8
kilometres inland from the lakeshore. Ozone concentrations were lower within one
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kilometre of the lakeshore and also decreased at distances greater than 10 km inland
from the lakeshore.
To explain the observations which showed the highest ozone concentrations occurring several kilometres inland from the lakeshore, Lyons and Cole suggested that the
bulk of the ozone, while over the lake, resides above the conduction inversion. As the
airmass is advected onshore, locations near the shoreline are exposed to the less polluted air which was found within the lowest layers of the atmosphere over the lake. As
the airmass moves further inland, the thermal internal boundary layer (TIBL) rapidly
deepens and the ozone aloft is eventually mixed down to the ground resulting in the
higher ground-level concentrations observed just inland of the shoreline. Still further
inland, ground-level ozone concentrations decrease due to the continued deepening of
the TIBL and loss processes which act on ozone such as dry deposition and titration
by freshly emitted NO.
Lyons and Cole (1976) proposed that the generation of elevated layers of high
ozone concentrations over the lake were the result of the advection of VOCs and NOx
out over the lake by the gradient ow or a land-breeze circulation during the early
morning. The ozone precursors, released as the nocturnal inversion is breaking down
but before the boundary layer has reached its full depth, will ow up and over the
conduction inversion present over the lake, leaving the air within the lowest 100 { 150
m of the atmosphere less polluted. With limited vertical mixing over the lake, the
VOCs and NOx will remain trapped aloft and during the afternoon ozone will be
photochemically produced. Since the ozone is physically separated from the surface,
dry deposition will not act to reduce ozone concentrations appreciably. With light
gradient winds, a lake breeze develops later in the day and the ozone which had
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formed over the lake is advected onshore and mixed down to the surface as described
above.
From an analysis of tetroon ights, Lyons and Cole suggested that recirculation
of pollutants in the lake breeze may be a possibility. Pollution advected inland by
the onshore ow of the lake breeze could move inland to the lake breeze front, rise,
and return towards the lake in the return circulation aloft. Eventually, the pollutants
could reenter the onshore ow of the lake breeze and return to land. In this manner,
pollutants could be advected along the western shore of Lake Michigan in a broad
helical trajectory by a combination of the lake breeze circulation and the gradient
wind. A modelling study using the Regional Atmospheric Modeling System (RAMS)
in conjunction with a Lagrangian particle dispersion model (Lyons et al., 1995) has
reproduced the recirculation phenomenon described by Lyons and Cole (1976), though
only approximately 30% of the particles were predicted to reenter the onshore ow
and complete a full recirculation.
Sillman et al. (1993) have shown that the suppression of vertical mixing and the
extremely slow rate of dry deposition of ozone and NOx species over a large body of
water is sucient to allow very high concentrations of ozone to be photochemically
generated over Lake Michigan. The model meteorology used by Sillman et al. did not
include a representation of the lake breeze, though advection of the Chicago urban
plume over Lake Michigan, particularly early in the morning before turbulent mixing
over land had distributed the precursors over a deeper layer, and the combination
of slow rates of vertical mixing and dry deposition were sucient for the model to
calculate high concentrations of ozone in a shallow layer over the lake. Therefore
it appears that a lake breeze circulation need not necessarily be present for high
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concentrations of ozone to be generated over large bodies of water, though lake breeze
circulations will be important in controlling the dispersion of pollutants over the lake.
The motivation for a modelling study of the lake breeze eects on oxidants over
Lake Ontario was provided by observations made as part of the SONTOS study
during the summers of 1992 and 1993. Figure 5.1 shows the concentration of ozone
and NOy measured at the Hastings site on August 26, 1993 (D. R. Hastie, private
communication). With the exception of a brief period at approximately 1830 GMT,
the concentration of ozone was observed to slowly increase during the afternoon.
Between 2040 GMT and 2120 GMT (16:40 and 17:20 EDT) ozone concentrations
increased from 46 ppb to 74 ppb and NOy concentrations increased from 2.2 ppb
to 4.0 ppb. Concurrent with the increase in concentration of ozone and NOy , the
concentration of NOx, CO, SO2 , PAN and HCHO were also observed to increase.
Figure 5.1: The concentration of (a) ozone and (b) NOy as observed at the Hastings
measurement site from 0600 GMT, August 26 to 0600 GMT, August 27, 1993.
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Meteorological parameters measured at the Hastings site showed that a decrease
in the dry bulb temperature of 2.5 C and an increase in the specic humidity by
20% was associated with the change in trace species concentrations noted above.
The change in meteorological parameters shows that the polluted airmass was cooler
and more moist than the less polluted airmass it replaced, suggesting that the more
polluted airmass had been modied through contact with a large body of water.
It was also possible to identify portions of the lake breeze front on visible satellite
imagery for August 26, as it appeared to be marked by a line of broken cumulus
clouds. An analysis of the visibly satellite imagery shows that the lake breeze front
passed over the Hastings measurement site between 2000 and 2200 GMT (Hastie et al.,
1998). Eppley radiometer measurements showed a brief decrease in solar radiation
as the chemical and meteorological measruments recorded the change in airmass,
presumably as a result of clouds associated with the lake breeze front.
Taken together, the meteorological and chemical measurements made at the Hastings site, as well as the position and inland progress of the lake breeze front deduced
from satellite imagery, suggest that a lake breeze circulation brought a more polluted
airmass north from Lake Ontario to the Hastings measurement site. Given that a
westerly to southwesterly gradient ow existed across southern Ontario on August
26, it seems probable that the polluted airmass observed at Hastings originated from
Toronto and surrounding urbanized regions around the western end of Lake Ontario.
It is hypothesized that, in a situation similar to that described by Lyons and Cole
(1976) for the western shoreline of Lake Michigan, VOCs and NOx emitted from
Toronto during the early morning are advected over Lake Ontario by the existing
gradient ow, possibly aided by a land breeze circulation. During the day, with little
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vertical mixing and small rates of dry deposition, the photochemical production of
ozone gives rise to high concentrations of ozone over the lake which are advected
inland by the lake breeze circulation later in the afternoon.
Behaviour of chemical and meteorological variables similar to that seen on August
26 was observed on three additional days during the six week intensive measurement
campaign in 1993. Given that Hastings is 40 km north of the Lake Ontario shoreline,
it appears that lake breeze circulations can signicantly, and regularly, aect air
quality along the northern shore of Lake Ontario for a considerable distance inland.
Further, since lake breeze circulations over the Great Lakes have been found to occur
on 25 { 40% of days during the summer (Comer and McKendry, 1993 Chermack,
1986 Lyons et al., 1972), it seems likely that the inuence of lake breeze circulations
on air quality for sites nearer the Lake Ontario shoreline is more frequent than that
observed at Hastings.
The MC2-online model had been used to test the hypothesis that high concentrations of ozone are generated over Lake Ontario and advected inland by the onshore
ow of a lake breeze. The model has been run for two cases in 1993 for which a
rapid increase in the concentration of ozone and related trace species were observed
to occur at the Hastings site late in the afternoon. The objectives of the modelling
study are twofold: 1) to see whether the model qualitatively agrees with the hypothesized mechanism by which high concentrations of oxidants are produced over Lake
Ontario and consequently advected northward by the lake breeze and 2) to assess
to what degree the model quantitatively agrees with the observed meteorological and
chemical elds.
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5.1 Method
The two cases which have been selected for the modelling study are August 8 and 26,
1993. For both study cases an initial run of MC2 without chemistry was performed
at a horizontal resolution of 42 km using CMC objective analysis to provide the
meteorological boundary conditions. Unfortunately, regional objective analyses in
sigma coordinates were not available for these days and it was necessary to use a
coarser resolution global analysis in pressure coordinates. The global analysis has a
horizontal resolution of 1.5 by 1.5 .
The 42 km resolution run was made on the standard 42 km resolution domain
discussed in Chapter 2, and used nudging to limit the growth of errors in the synoptic
scale features. The run was three days and three hours long, begun at 0000 GMT,
two days before the study day and ending at 0300 GMT on the day after the study
day.
A 21 km horizontal resolution run of MC2 with chemistry was made using meteorological boundary conditions interpolated from the previously completed 42 km
resolution run. Initial conditions and time-varying boundary conditions for the advected chemical species were taken from the global CTM as described in Chapter 2.
The 21 km resolution run began and ended at the same times as the 42 km resolution
run.
The nal run of the MC2-online model was made at a horizontal resolution of
5.292 km on an 83 83 grid point domain centered over Lake Ontario. The run was
27 hours long, beginning at 0000 GMT on the study day, and used initial and boundary conditions for the meteorological and chemical variables interpolated from the
21 km horizontal resolution run. Meteorological and chemical boundary conditions
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were updated every three hours, with linear interpolation used to derive boundary
conditions at intermediate times.
5.2 Case Study: August 8, 1993
The rst case chosen for study occurred on August 8, 1993. The concentrations of
ozone and NOy as observed at the Hastings eld measurement site are given in Figure 5.2. The observations show that within 15 minutes, beginning at 2221 GMT
(1821 EDT), the concentration of ozone increased from 47 to 59 ppb and the concentration of NOy increased from 2.6 to 4.3 ppb. While the increase in ozone on August
8 was rather modest, the time of day at which the increase occurred and the rapidity
of the change were similar to other cases observed at Hastings.
Figure 5.2: The concentration of (a) ozone and (b) NOy as observed at the Hastings
measurement site from 0600 GMT, August 8 to 0600 GMT, August 9, 1993.
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Figure 5.3 shows the synoptic conditions for 0000 GMT, August 8 and 0000 GMT,
August 9, as taken from the CMC objective analysis. A large area of high pressure
covered southern Ontario and much of the northeastern United States on August
8. Winds across southern Ontario were light and from the west. Winds recorded
at synoptic observing stations in southern Ontario, though away from Lake Ontario,
were generally less than 5 km hr;1 and had variable direction during the afternoon.
Daytime high temperatures were between 23 and 25 C.
Figure 5.3: Sea-level pressure and 850 mb winds taken from the CMC objective
analysis for 0000 GMT August 8, 1993 (panel a) and 0000 GMT August 9, 1993
(panel b).
Figure 5.4 shows the model calculated wind eld and CO concentration over the
5.3 km resolution domain at 1100 GMT, or 0700 EDT. The horizontal cross-section
shows the CO concentration in the lowest model level, approximately 10 m thick, and
the wind at 10 m above the surface. The near-surface winds over the western end of
Lake Ontario were generally from the north with speeds between 7 and 12 m s;1. The
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CO concentration eld, used as a long-lived tracer for anthropogenic emissions, shows
that emissions from Toronto were being advected to the south across the western end
of Lake Ontario. The vertical cross-section of the CO concentrations shows that the
plume of CO from Toronto was conned to within 200 m above the surface of the
lake.
As shown by the 10 m winds in Figure 5.4, a weak cyclonic circulation was calculated by the model to be centred over the southeastern portion of Lake Ontario. The
cyclonic circulation is associated with a weak low pressure system that is, erroneously,
calculated by the model to be present over eastern Lake Ontario at this time. Observations show that a weak low pressure system was centered over Lake Michigan at
1200 GMT, August 6, though by 1200 GMT, August 7 the low had substantially lled
in. The 42 km resolution run of MC2 more or less correctly captures the behaviour of
this centre of low pressure. However, the 21 km resolution run allows the low pressure
centre to persist, and as can be seen from the 5.3 km resolution wind elds, the low
is placed over the eastern end of Lake Ontario by the model during the early morning
of August 8. The increased pressure gradient across southern Ontario due to the low
pressure centre over eastern Lake Ontario, leads to the strong northerly gradient ow
predicted by the model for the early hours of August 8.
Figure 5.5 presents the observed 10 m winds from hourly synoptic observations and
the Hastings SONTOS site over southern Ontario. Winds at the surface are generally
from the west and windspeeds are low: typically less than 1.5 m s;1 . Though the
observations are scarce, there is some evidence of a land breeze circulation with winds
owing towards the lake at St. Catherines, Toronto Island and Kingston. Compared
with observations, the model calculated wind eld is too strong and has limited the
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Figure 5.4: Horizontal and vertical cross-sections of the CO concentration and wind
eld at 1100 GMT, August 8, 1993 calculated by the model at a horizontal resolution
of 5.3 km. The horizontal cross-section shows the CO concentration in the lowest 10
m of the model and winds at 10 m above the surface. The position of the vertical
cross-section is given by the thick line drawn across the horizontal cross-section panel.
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formation of a land breeze circulation.
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Figure 5.5: The observed wind at 10 m above the ground for 1100 GMT (0700 EDT)
on August 8, 1993. The station location is given by the solid dot at the end of
each barb. The direction of the wind is given by the direction of the barb and an
observation of calm conditions is denoted by an unlled circle. Wind speed is given
by the tick marks attached to each barb. Windspeeds less than 1.3 m s;1 are given by
a barb with no ticks. Windspeeds between 1.3 and 3.8 m s;1 are denoted by a barb
with a half tick mark and windspeeds between 3.8 and 6.4 m s;1 are shown by a barb
with a full tick mark. The stations plotted are: 1) Toronto International Airport
2) Toronto Island 3) Peterborough 4) Hastings 5) Waterloo 6) Hamilton 7) St.
Catherines 8) Kingston and 9) Ottawa.
The wind eld and CO concentration predicted by the model at 1600 GMT on
August 8 is given in Figure 5.6. By 1600 GMT the model has begun to produce
onshore ow associated with a lake breeze in the lowest layers of the model (panel a).
The strong northerly ow across the western end of Lake Ontario has been replaced
by light winds which, though not well represented in Figure 5.6, show several poorly
dened centres of divergent ow.
During the initial development of the lake breeze the gradient ow over the lake
weakens for levels less than 200 m above the surface. The eect on precursor emissions
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Figure 5.6: Horizontal and vertical cross-sections of the model calculated CO concentration and wind eld at 1600 GMT, August 8, 1993. The horizontal cross-sections
show these elds at 10 m (panel a) and 435 m (panel b) above the surface. The
position of the vertical cross-sections are denoted by the thick solid lines drawn over
the horizontal cross-sections.
202
can be seen in the distribution of CO near the surface (panel a). With the weakening
of the gradient ow over the lake, precursors emitted from Toronto have become
trapped over the lake and have formed a `pool' of precursors over the surface of the
lake and below approximately 200 m. With the strengthening of the onshore ow
of the lake breeze, the precursors are advected both towards the shore around the
western end of Lake Ontario, and in an easterly direction along the long axis of the
lake.
The onshore ow component of the lake breeze is initially quite shallow and deepens with time. As can be seen in the east{west vertical cross-section of Figure 5.6
(panel a), the advection of precursors towards the east only occurs for levels below
approximately 100 m above the surface. The initially shallow onshore components
of the lake breeze circulation results in a vertically thin layer of precursors advected
towards the east. At greater heights above the surface, shown in panel (b) of Figure 5.6, the direction of the gradient wind has been largely unaected by the lake
breeze, and emissions from Toronto are still advected towards the south.
The north{south vertical cross-section shown in panel (b) of Figure 5.6 shows that
the depth of the Toronto plume decreases towards the south. Portions of the Toronto
plume near the southern shore of Lake Ontario had been emitted from Toronto earlier
in the morning and were trapped by a shallow boundary layer before being advected
over the lake. As the boundary layer increased in depth during the morning, emissions
from Toronto were mixed over a deeper and deeper layer before being advected over
the lake. The changing vertical depth of the plume over the lake is a reection of the
boundary layer depth over Toronto at the time the emissions occurred.
The observed 10 m winds at 1600 GMT are shown in Figure 5.7. Away from
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Lake Ontario the windspeeds were generally less than 2 m s;1 and the winds were
from a westerly to northwesterly direction. Formation of a lake breeze over Lake
Ontario is evidenced by the onshore ow observed at St. Catherines, Toronto Island
and Kingston. The winds observed at Kingston, for example, were from the southwest
at 4 m s;1 .
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Figure 5.7: The observed wind at 10 m above the ground for 1600 GMT (1200 EDT)
on August 8, 1993.
A comparison of the model wind elds with observations shows that the model
predicts winds inland from Lake Ontario to be from a northerly to northeasterly
direction, while observations show winds were from a more westerly direction. Errors
in the direction of the gradient wind calculated by the model appear to be largely
the result of the erroneous centre of low pressure, positioned just to the southeast of
Lake Ontario at 1600 GMT. Eects of the low pressure centre on the model solution
decrease with time as the low moves further to the east and the central pressure is
calculated by the model to increase. Windspeeds calculated by the model north of
Lake Ontario are between 2 and 3 m s;1 and agree reasonably well with the observed
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windspeeds.
Winds observed at Toronto Island were calm or light from the north until 1300 GMT,
0900 EDT. Beginning at 1300 GMT the winds were from the south to southwest at
speeds from 1 to 2 m s;1 . Observations at St. Catherines and Kingston show onshore
ow beginning an hour later, at 1400 GMT. The MC2 model does not show the onset
of an onshore ow until 1600 GMT, and appears to be late in developing a lake breeze
circulation on this day.
Figure 5.8 shows the model CO concentration and wind elds at 2100 GMT
(1700 EDT). The 10 m winds, given in the horizontal cross-section of panel (a),
show the development of a mesohigh over the extreme southwestern corner of Lake
Ontario. A broad band of southwesterly winds, with speeds between 4 and 6 m s;1 ,
exist over much of the lake. From the vertical cross-sections shown, the onshore ow
of the lake breeze is approximately 300 - 400 m deep at 2100 GMT. The winds at 435
m above the surface, shown in the horizontal cross-section of panel (b), are, therefore,
near the top or slightly above the onshore ow of the lake breeze circulation.
The `pool' of CO which was present over the western end of Lake Ontario at
1600 GMT, just as the lake breeze was beginning to form, has been dispersed by the
strengthening lake breeze circulation. A signicant fraction has been advected inland
around the western end of Lake Ontario, while a second portion of the CO has been
advected towards the northeast over the lake. The plume of CO advected towards
the northeast can be seen in the horizontal cross-section of panel (a) to the southeast
of Toronto. Vertically, this plume is less than 100 m thick and is not separated from
the surface of the lake.
As shown by the north-south vertical cross-section of panel (b), the plume of CO
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Figure 5.8: Horizontal and vertical cross-sections of the model calculated CO concentration and wind eld at 2100 GMT, August 8, 1993. The horizontal cross-sections
show these elds at 10 m (panel a) and 435 m (panel b) above the surface. The
position of the vertical cross-sections are denoted by the thick solid lines drawn over
the horizontal cross-sections.
206
from Toronto has been undercut by less polluted air within the northward, onshore
ow of the lake breeze. The less polluted air originated along the southern shore
of Lake Ontario, between 200 and 500 m above the surface. Entrained into the
lake breeze as the circulation strengthened between 1600 and 2000 GMT, the less
polluted air was advected towards the north and eventually brought to the surface
by the subsidence associated with the lake breeze. At this point in time, the bulk
of the emissions from Toronto are being forced over top of the onshore ow of the
lake-breeze, resulting in the formation of an elevated plume.
The model CO and wind elds at 0000 GMT, August 9 are illustrated in Figure 5.9.
As above, the CO and wind elds are shown for two horizontal cross-sections, at 10
and 435 m above the surface. Note that the scale used to contour the CO concentration has changed from that used above. The 10 m wind eld shows that the mesohigh
remains in the southwest corner of Lake Ontario, with the near-surface component
of the lake breeze circulation consisting of a band of southwesterly to westerly winds
which cover most of the lake. The onshore ow of the lake breeze is now approximately 500 m deep and the winds at 435 m above the surface (panel b) show a
considerable onshore component.
One of the dominant features of the CO concentration eld at 10 m is the increase
in concentration of CO over regions with high emissions. The nocturnal inversion has
begun to form over land and, with slower rates of vertical mixing, the CO concentration rapidly increases for regions where emissions occur.
The concentration of CO in the lowest model layer over the western end of Lake
Ontario has increased from approximately 140 ppb at 2100 GMT to 180 ppb at
0000 GMT, August 9. The increase is caused by the entrainment of emissions from
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Figure 5.9: Horizontal and vertical cross-sections of the model calculated CO concentration and wind eld at 0000 GMT, August 9, 1993. The horizontal cross-sections
show these elds at 10 m (panel a) and 435 m (panel b) above the surface. The
position of the vertical cross-sections are denoted by the thick solid lines drawn over
the horizontal cross-sections.
208
Toronto into the return ow of the lake breeze. As was noted above, during the
late afternoon emissions from Toronto, advected to the south by the gradient ow,
are forced to rise over the onshore ow of the lake breeze. A certain fraction of the
emissions that rise over the onshore ow are entrained in the return ow, eventually
to enter the onshore ow. The increase in the near-surface CO concentration over the
western end of Lake Ontario is caused by this phenomenon.
Panel (a) of Figure 5.9 shows a north{south vertical cross-section through a shallow layer with higher concentrations of CO. This shallow plume, calculated by the
model to be less than 100 m thick, is being advected inland along the north shore
of Lake Ontario by the lake breeze circulation. The plume was originally a portion
of the `pool' of CO which formed over the western end of Lake Ontario as the lake
breeze circulation was beginning at 1600 GMT.
The CO distribution at 435 m above the ground, panel (b) of Figure 5.9, shows
the eects of the increasing depth of the lake breeze onshore ow. As was discussed
above, at 2100 GMT the onshore ow only weakly aected the wind at 435 m and CO
emitted from Toronto was advected to the south, across the lake, in the gradient ow
(for the moment we are ignoring the fraction of the plume entrained within the lake
breeze circulation). As the lake breeze onshore ow continues to deepen with time,
portions of the plume which had been advected to the south, in the gradient ow, are
caught in the onshore ow and advected to the northeast. The gradual deepening of
the onshore ow appears to result in the entrainment of a signicant amount of ozone
precursors into the onshore ow of the lake breeze for situations where the gradient
ow advects material from urban areas over the lake.
The observed 10 m winds at 0000 GMT, August 9 are shown in Figure 5.10.
209
Winds observed at Toronto Island and St. Catherines were calm, suggesting that the
lake breeze circulation had weakened considerably by this time. In fact, observations
at St. Catherines show the wind becoming oshore one hour later.
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Figure 5.10: The observed wind at 10 m above the ground for 0000 GMT, August 9,
1993 (2000 EDT, August 8).
While observations suggest a weakening of the lake breeze circulation, the model
continues to show strong onshore ow associated with the lake breeze at this time. A
tendency of models to be too slow in terminating onshore ow was noted by Lyons
et al. (1995), where it was found that the termination of onshore ow predicted by
RAMS, for a case study over Lake Michigan, was as much as 3.5 hours later than
observed.
The observed winds at Peterborough changed from northwesterly at 2200 GMT
to southwesterly at 2300 GMT. The change in wind direction observed at Peterborough occurred at approximately the same time as the putative lake breeze front was
observed to arrive at Hastings, 30 km to the west of Peterborough. The observed surface winds at Peterborough and the observed chemical and meteorological variables
210
at Hastings suggest that the lake breeze front had penetrated at least 40 km to the
north of the Lake Ontario shoreline by 2300 GMT. From Figure 5.9, it can be seen
that the inland penetration of the lake breeze is signicantly underpredicted by the
model for this day.
The model lake breeze never penetrates far enough inland to arrive at the Hastings
eld measurement site. At its closest, the model lake breeze front is approximately
20 km south of Hastings. As was discussed earlier, the incorrect treatment of a low
pressure centre contributed to the model calculating a northerly gradient ow, during
the early morning hours, that was stronger than observed. Later in the period the
inuence of the low pressure system on the 5.3 km resolution results has weakened,
however the model continues to predict northwesterly ow from Georgian Bay. Typical 10 m windspeeds calculated by the model north of Lake Ontario are between 5
and 7 m s;1 , while observations show variable winds, though generally from a westerly
direction, with speeds less than 2 m s;1 . Errors in the gradient ow to the north of
Lake Ontario undoubtedly contribute to limiting the inland penetration of the lake
breeze front on this day.
Figure 5.11 shows the model calculated ozone concentration and wind eld at
2100 GMT for the model levels at 10 m (panel a) and 220 m (panel b) above the
surface. The ozone concentration in the lowest model level shows concentrations
greater than 70 ppb along the north shore of Lake Ontario, just to the east of Toronto.
This area of higher ozone concentrations is associated with the thin layer of CO, and
other ozone precursors, advected to the northeast from the `pool' of precursors which
formed over the western end of Lake Ontario. As can be seen from the vertical crosssection in panel (a), the higher ozone concentrations are found at heights below 250 m,
211
with even higher ozone concentrations below 100 m. The ground-level concentration
of ozone decreases rapidly as this plume is advected onshore due to increased vertical
mixing over land.
Figure 5.11: Horizontal and vertical cross-sections of the model calculated ozone
concentration and wind eld at 2100 GMT, August 8, 1993. The horizontal crosssections show these elds at 10 m (panel a) and 240 m (panel b) above the surface.
The position of the vertical cross-sections are denoted by the thick solid lines drawn
over the horizontal cross-sections.
A second region of high ground-level ozone concentrations is predicted at the
western end of Lake Ontario. This region of high ozone concentrations is associated
with the onshore ow of the lake breeze bringing higher ozone concentrations from
over the lake. Panel (b) of Figure 5.11 shows the model ozone concentration and wind
eld at 240 m above the surface and a north{south vertical cross-section across the
western end of Lake Ontario. The vertical cross-section shows that over the lake, the
212
highest concentrations of ozone are found aloft. As was discussed earlier, the ozone
precursors emitted from Toronto and advected southward in the gradient ow are
forced to rise over the onshore ow of the lake breeze which is bringing less polluted
air northward along the surface of the lake. The resulting layer of ozone aloft is
advected towards land by the lake breeze and mixes down to ground-level due to
increased vertical mixing over land.
5.2.1 Forward Trajectory Analysis
For the purpose of visualizing how the lake breeze aects the dispersion of emissions
from Toronto, three-dimensional forward trajectories have been generated from the
model calculated wind elds. Figure 5.12 shows seven dierent forward trajectories
from Toronto. The rst forward trajectory was begun at 1000 GMT and a new
trajectory was started every hour. This set of trajectories were all begun at a height
of 20 m above the surface.
Trajectory (a) left Toronto in the early morning, before the lake breeze had developed, and continued south across the western end of Lake Ontario. Trajectory (b)
initially followed a similar path as trajectory (a), though became caught in the lake
breeze circulation as it developed and was eventually advected towards the north-east
within the onshore ow of the lake breeze. Several of the subsequent trajectories (trajectories c, d, e and f) became entrained within the lake breeze in a similar manner as
trajectory (b), though were initially advected towards the northwest by the onshore
ow of the lake breeze. This set of trajectories ascended at the lake breeze front,
to the west of where they were released, returned towards the center of the lake in
the return ow aloft, then re-entered the onshore ow of the lake breeze and were
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Figure 5.12: Three-dimensional forward trajectories from Toronto for August 8, 1993.
The trajectories were begun at one hour intervals between 1000 and 1600 GMT and
were released at a height of 20 m above the surface. The colour of the line represents
the height of the trajectory in metres above ground level.
advected towards the northeast.
Trajectory (g) shows an example of emissions from Toronto entrained into the
onshore ow of the lake breeze aloft. Trajectory (g) initially moved towards the
south, then was forced to rise over the lake breeze front before descending while over
the lake and eventually becoming entrained in the onshore ow of the lake breeze.
Note that many of the forward trajectories complete at least one full recirculation
within the lake breeze circulation, however trajectories calculated in the manner used
here ignore the eects of turbulent mixing on the dispersion of pollutants. While
many of the trajectories show a tendency to undergo recirculation within the lake
breeze, the fraction of precursors that undergo recirculation may be considerably
smaller than one (Lyons et al., 1995)
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5.2.2 Eects of Dry Deposition
The slow rate of dry deposition for many species over water has been suggested as
a contributing factor in the production of high concentrations of ozone over lakes
(Sillman et al., 1993). The slow rate of dry deposition results from the low solubility
of certain species and the high degree of atmospheric stability over water, due to
the formation of a conduction inversion during the daytime. Model calculated dry
deposition rates for ozone over Lake Ontario are not larger than 0.04 cm s;1 . For a
layer of ozone 100 m deep, a deposition velocity of 0.04 cm s;1 gives a time constant
for the loss of ozone by dry deposition of approximately 70 hours. Thus, over the
course of a day, loss of ozone by dry deposition to the lake surface is negligible, even
for the case of a shallow layer of higher ozone concentrations in physical contact with
the lake surface. For elevated layers of ozone, loss by dry deposition will be even
smaller.
Similarly, dry deposition velocities for NO2 and PAN are smaller over water than
over land. Typical midday deposition velocities for NO2 over land are calculated to
be between 0.4 and 0.6 cm s;1 , while over water the NO2 deposition velocities are
no larger than 0.002 cm s;1. The dierence in dry deposition velocities between land
and water surfaces for PAN is not as large as that calculated for NO2, with PAN
deposition velocities typically a factor of three times smaller over water than over
land. Henry's Law constants for NO2 and PAN show that NO2 is considerably less
soluble than PAN and therefore NO2 has a smaller deposition velocity over water
surfaces.
For the more water soluble oxidation products such as HNO3 and H2O2 , the lower
rates of dry deposition calculated over the lake are the result of greater aerodynamic
215
resistance, due to the increased stability over the lake. During the daytime, model
dry deposition velocities for HNO3 range from 0.01 to 0.4 cm s;1 over the lake, while
over land the range is from 1.5 to 4.0 cm s;1 . For H2O2 , deposition velocities over
water are calculated to vary between 0.01 and 0.4 cm s;1 , while over land the range
is from 0.7 to 1.5 cm s;1 .
From the brief analysis of dry deposition velocities over water, it appears that
the loss of ozone and NO2 by dry deposition will be negligible for an airmass over
water. As suggested by Sillman et al. (1993), higher rates of dry deposition would
most certainly have resulted in considerably lower concentrations of ozone over the
lake. This is particularly true for shallow plumes of ozone, predicted by the model
to be less than 100 m deep, for which appreciable ozone deposition velocities would
have had a signicant eect. The model also calculates the generation of elevated
plumes of ozone, and for these elevated plumes it can reasonably be expected that
higher deposition velocities will have less of an eect since the plumes are physically
separated from the surface.
5.3 Case Study: August 26, 1993
The second lake-breeze case study occurred on August 26, 1993. Chemical observations at the Hastings eld site showed that within 10 minutes, beginning at 2039 GMT
(1639 EDT), the concentration of ozone increased from 46 to 59 ppb. The concentration of ozone continued to increase, though more slowly, until 2144 GMT, at which
time a concentration of 79 ppb was observed. Likewise, the concentration of NOy
increased from 2.2 to 3.8 ppb during the rst 10 minutes of the event, then continued
to increase to 4.6 ppb at 2144 GMT. See Figure 5.1 for the observed ozone and NOy
216
concentrations at Hastings on August 26.
Figure 5.13 shows the synoptic conditions for 0000 GMT, August 26 and 0000 GMT,
August 27, 1993 as taken from the CMC objective analysis. A large ridge of high
pressure lay just to the west of Lake Ontario at 0000 GMT, August 26 and winds
from from the northwest. Over the course of the day the ridge moved across southern
Ontario to lie to the south and east of Lake Ontario, and the 850 mb winds backed
to become southwesterly by 0000 GMT, August 27.
Figure 5.13: Sea-level pressure and 850 mb winds taken from the CMC objective
analysis for 0000 GMT August 26, 1993 (panel a) and 0000 GMT August 27, 1993
(panel b).
Surface observations, from stations away from Lake Ontario, showed that winds
were from a west to southwesterly direction during the afternoon of August 26 with
speeds less than 2 m s;1 . Daytime high temperatures from 31 to 33 C were recorded
across southeastern Ontario. Kingston was an exception. With winds from the lake
for most of the day, the high temperature at Kingston was 26 C.
217
Figure 5.14 shows the CO concentration and wind eld in the second model layer,
approximately 20 m above the surface, at 1100 GMT. The wind eld aloft is quite
weak, with model winds at 1 km above ground less than 2 m s;1 . With no strong
synoptic forcing the wind eld near the surface is predicted by the model to be quite
complex. A region of strong convergence is predicted by the model along the southern
shore of Lake Ontario. A second region of strong convergence is predicted along the
northern shore of Lake Erie. Northerly ow across Lake Ontario bifurcates along the
southern shore with one branch moving to the southwest and the other branch moving
to the east. Emissions from Toronto are advected to the southwest and eventually
exit the 5.3 km resolution domain along the western edge of the model.
The observed surface winds at 1100 GMT are shown in Figure 5.15. The model
wind eld shows a signicant region of convergence over Lake Ontario as part of a
land breeze circulation. Observations at Toronto Island and St. Catherines seem to
support the occurrence of a land breeze with both stations reporting oshore ow
with windspeeds of 1.7 m s;1. Aside from the probable occurrence of a land breeze
circulation, the observations are not suciently numerous to support or refute other
features of the model wind eld.
Figure 5.16 presents horizontal and vertical cross-sections of the CO concentration
and wind eld calculated by the model for 1600 GMT. The 10 m winds show a centre
of divergence to the south and east of Toronto and onshore ow over all shores of
Lake Ontario. Inland penetration of the lake breeze appears to be particularly deep
to the northwest of Lake Ontario, in the vicinity of Toronto. As will be seen in the
results for subsequent hours, the deep inland penetration of the onshore ow over this
region collapses and a new lake breeze front forms nearer the lakeshore. It is unclear
218
Figure 5.14: Horizontal and vertical cross-sections of the CO concentration and wind
eld at 1100 GMT, August 26, 1993. The horizontal cross-section shows the CO
concentration and wind eld at 20 m above the surface. The position of the vertical
cross-section is given by the thick line drawn across the horizontal cross-section panel.
219
.
.
.
.
.
Figure 5.15: The observed wind at 10 m above the ground for 1100 GMT (0700 EDT)
on August 26, 1993.
whether such behaviour is realistic and the factors which have caused it have not been
analyzed. One possible cause may be instabilities in the model solution caused by the
application of boundary conditions, since the region in which this feature occurred is
quite close to the western boundary of the model domain.
The concentration of CO in the lowest model layer shows a plume from Toronto
spreading along the northern shore of the lake. A vertical cross-section through the
eastern edge of the plume shows weak onshore ow, approximately 100 m deep. Over
the lake the plume is less than 200 m deep, while over land, due to greater vertical
mixing, the plume is approximately 500 m deep.
Panel (b) of Figure 5.16 shows the concentrations of CO and the wind eld at 435
m above the surface. The vertical cross-section in panel (b) shows that the boundary
layer over Toronto at 1600 GMT is calculated by the model to be approximately 600
m deep. Winds at 435 m are from the west and are advecting the Toronto plume
along the northern shore of Lake Ontario.
220
Figure 5.16: Horizontal and vertical cross-sections of the model calculated CO concentration and wind eld at 1600 GMT, August 26, 1993. The horizontal cross-sections
show these elds at 10 m (panel a) and 435 m (panel b) above the surface. The
position of the vertical cross-sections are denoted by the thick solid lines drawn over
the horizontal cross-sections.
221
A comparison of the model calculated winds at 10 m above the ground with
observations at 1600 GMT, shown in Figure 5.17, suggest no signicant errors in the
model simulation of the surface winds. An exception to the general agreement between
observations and the model is the observed wind at St. Catherines. Observations show
that the wind at St. Catherines at 1600 GMT was from the south-southwest at 4 m
s;1 , while the model is calculating light onshore, or northerly, ow associated with
the lake breeze. The disagreement suggests that the model lake breeze front was
too far inland along the southwestern shoreline of Lake Ontario. However, the winds
observed at St. Catherines do become onshore the following hour.
..
..
..
.
.
.
.
.
.
Figure 5.17: The observed wind at 10 m above the ground for 1600 GMT (1200 EDT)
on August 26, 1993.
The CO concentration and wind eld calculated by the model for 2000 GMT are
shown in Figure 5.18. The model shows a well developed lake breeze circulation with
strong onshore ow over all shores of the lake. A mesohigh is centered over the western
half of Lake Ontario. The model lake breeze front has moved inland approximately 15
to 20 km along the north shore of Lake Ontario in the vicinity of Hastings. Analysis
222
of the inland penetration of the lake breeze front from visible satellite imagery for
this day suggests that the front was approximately 30 km north of the Lake Ontario
shoreline by 2000 GMT (Hastie et al., 1998).
Figure 5.18: Horizontal and vertical cross-sections of the model calculated CO concentration and wind eld at 2000 GMT, August 26, 1993. The horizontal cross-sections
show these elds at 10 m (panel a) and 435 m (panel b) above the surface. The
position of the vertical cross-sections are denoted by the thick solid lines drawn over
the horizontal cross-sections.
An interesting feature of the spatial distribution of CO in the lowest model level
is the lower concentrations present over the lake and the much higher concentrations
just inland over the northwestern shore of Lake Ontario. Panel (b) of Figure 5.18
shows the model CO concentration and wind eld at 435 m above the surface. The
winds at this level are from the west along the northern shore of Lake Ontario and act
to advect the Toronto plume to the east. Near the surface, in the lake breeze onshore
223
ow, the winds are from the southwest and undercut the Toronto plume. The onshore
circulation of the lake breeze is advecting less polluted air onshore, underneath the
eastward progressing plume from Toronto. As the onshore ow moves further inland,
vertical mixing rapidly mixes the higher concentrations of CO down to ground-level,
giving rise to the strong horizontal concentration gradients seen at the surface.
Though the model resolution is not sucient to resolve such a feature, the onshore
advection of less polluted air near the surface, with higher concentrations of pollutants
aloft, could be expected to result in a ground-level concentration pattern similar to
that found by Lyons and Cole (1976) along the western shore of Lake Michigan:
within 1 km of the shoreline concentrations of ozone were found to be lower than
those observed further inland. The explanation proposed by Lyons and Cole was
that an elevated plume of ozone, which existed over the lake, was mixed down to
the surface by a deepening TIBL as the onshore ow moved further inland. Though
there are no observations of a similar phenomenon occurring along the shores of Lake
Ontario, the large scale features predicted by the model for this case would result in
a similar pattern in the ground-level concentration.
The nal hour analyzed is 0000 GMT, August 27 (2000 EDT, August 26). The
CO and wind elds calculated by the model for this time are shown in Figure 5.19.
The wind elds and the spatial distribution of CO are largely a natural evolution of
the patterns found in the model output for 2000 GMT. The plume of CO has advected
further to the east along the north shore of Lake Ontario and continues to be undercut
by less polluted air in the onshore ow of the lake breeze. In regions where the
onshore ow is undercutting the Toronto plume the horizontal concentration gradient
at ground-level is not as strong as was seen earlier. With slower vertical mixing over
224
land, the onshore ow of less polluted air is able to penetrate further inland before
the higher concentrations of CO aloft are mixed down.
Figure 5.19: Horizontal and vertical cross-sections of the model calculated CO concentration and wind eld at 0000 GMT, August 27, 1993. The horizontal cross-sections
show these elds at 10 m (panel a) and 435 m (panel b) above the surface. The
position of the vertical cross-sections are denoted by the thick solid lines drawn over
the horizontal cross-sections.
Note that the concentration of CO in the plume is decreasing as the plume is
advected further to the east, presumably as a result of the entrainment of less polluted
air into the solenoidal circulation of the lake-breeze.
The northern end of the vertical cross-section drawn through panel (a) corresponds
to the approximate location of the Hastings observation site. As can be seen from the
surface wind eld, the model is not able to reproduce the observed inland penetration
of the lake breeze front. At 0000 GMT, August 27 the model lake breeze front was
225
still approximately 15 km south of Hastings. Note that the lake breeze front was
observed to arrive at Hastings at 2030 GMT.
The vertical cross-sections presented above suggest signicant horizontal and vertical motions aect the Toronto plume as it is advected towards the east along the
north shore of Lake Ontario. For example, the vertical cross-section in panel (a) of
Figure 5.19 shows the lake breeze inow layer is approximately 500 m deep. Above
the onshore ow the return circulation extends to approximately 1000 m above the
surface. Vertical motions, such as ascent at the lake breeze front and subsidence
behind the front, are also evident in the ow elds.
Figure 5.20 shows a series of forward trajectories from Toronto, released at one
hour intervals beginning at 1200 GMT. The forward trajectories suggest a solenoidal
circulation as the plume is advected towards the east. As rst proposed by Lyons and
Cole (1976), with gradient ow roughly parallel to the lakeshore material is advected
in the general direction of the gradient ow, though undergoes solenoidal circulations
due to the inuence of the lake breeze circulation.
Figure 5.21 shows the concentration of ozone calculated for the lowest model level
at 2000 GMT, August 26 and 0000 GMT August 27. The spatial distribution and
interaction of ozone with the lake breeze is quite similar to that described above
for CO: ozone is photochemically generated within the Toronto plume and advected
towards the east, along the north shore of Lake Ontario by a combination of the
gradient ow and lake breeze circulation. The model calculates a ground-level ozone
concentration of 84 ppb approximately 15 km to the south of Hastings at 0000 GMT,
August 27 (2000 EDT).
226
Figure 5.20: Three-dimensional forward trajectories from Toronto for August 26,
1993. The trajectories began at one hour intervals between 1200 and 1600 GMT and
were released at a height of 160 m above the surface. The height of the trajectory, in
metres above ground level, is given by the colour of the line.
5.3.1 Aircraft Observations
The SONTOS 1993 study did include a series of aircraft ights, during which ozone,
NO2 and NOx were measured. Hydrocarbon samples were also taken, though these
are not discussed here. One of these ights occurred on August 26, and the observed
ozone and NOx concentrations from this ight have been compared with the model
calculations.
The aircraft ight on August 26 was divided into two parts a ight from Guelph
to Peterborough, and a return ight from Peterborough to Guelph. Figure 5.22 shows
the path of the Guelph to Peterborough ight, which took place between approximately 1615 and 1800 GMT (1215 and 1400 EDT).
Figure 5.23 shows the observed and modelled concentrations of ozone and NOx,
and the altitude of the aircraft, as a function of time during the ight. To the west and
south of Toronto, both over land and over Lake Ontario, the aircraft measurements
227
Figure 5.21: Horizontal and vertical cross-sections of the model calculated O3 concentration and wind eld at 2000 GMT, August 26 (panel a) and 0000 GMT, August
27, 1993 (panel b). The horizontal cross-sections show these elds at 10 m above the
surface. The position of the vertical cross-sections are denoted by the thick solid lines
drawn over the horizontal cross-sections.
228
Figure 5.22: Path for the Guelph to Peterborough ight of August 26, 1993. The
labelled points along the path correspond to features in the observed ozone concentration and are referenced below.
showed a relatively homogeneous distribution of ozone, with concentrations between
80 and 90 ppb. While slightly higher concentrations of ozone were observed over Lake
Ontario south of Toronto, denoted by point (b), these concentrations were not greatly
dierent from those found between Guelph and Binbrook.
The most signicant feature in the observed ozone concentration eld was found
over Lake Ontario to the east of Toronto, where ozone concentrations between approximately 90 and 105 ppb were observed. The aircraft was approximately 300 to
400 m above the lake during this portion of the ight. The region with these higher
concentrations of ozone lie between points (c) and (d) on Figure 5.22. Ozone concentrations abruptly decreased as the aircraft passed point (d), though it is not clear
whether this was a result of horizontal or vertical gradients in the ozone eld, as the
229
Figure 5.23: The observed and modelled concentration of ozone (panel a) and NOx
(panel b), in ppb, as a function of time for the Guelph to Peterborough aircraft ight
of August 26, 1993. The aircraft altitude is also shown on both panels. The labelled
points along the time axis of the graph correspond to the position of the aircraft as
given by the labelled points along the aircraft path shown in Figure 5.22.
altitude of the aircraft was increasing during this time. Lower ozone concentrations,
between 50 and 60 ppb, were observed north of Lake Ontario.
The model ozone and NOx elds have been interpolated, in space and time, onto
the aircraft position and plotted as a function of time alongside the observations in
Figure 5.23. Upwind of Toronto the model underpredicts ozone: from takeo until
approximately 1735 GMT the model ozone is approximately 15 { 20 ppb lower than
the observed concentration. Nearer to Binbrook, denoted by point (a), the model
ozone is in better agreement with observations, as the model predicts higher ozone
concentrations associated with emissions from Hamilton and surrounding regions.
230
The model predicts ozone concentrations that are signicantly lower than observed
for almost the entire time the aircraft is over Lake Ontario. Over the western end
of Lake Ontario, denoted by point (b), the model underpredicts ozone by between
15 and 20 ppb, while within the area of higher ozone concentrations to the east of
Toronto, ozone is underpredicted by as much as 40 ppb.
Aircraft observations of NOx show two distinct regions over the lake where NOx
concentrations were found to be higher than the background concentration: over the
western end of Lake Ontario, between points (a) and (b), and within the region of
higher ozone concentrations to the east of Toronto, noted above, between points (c)
and (d).
The model NOx concentration over the extreme western end of Lake Ontario agrees
reasonably well with the observations, though further to the east, particularly south
of Toronto, denoted by point (b), the model underpredicts NOx. The model shows
emissions from urbanized regions around the western end of Lake Ontario, advected
over the lake by the westerly gradient ow at levels greater than 300 m above the
ground. At lower levels, the onshore ow of the lake breeze brings less polluted air
from over the lake, towards land.
The model also predicts higher NOx concentrations to the east of Toronto, associated with the Toronto urban plume being advected along the north shore of Lake
Ontario. As discussed above, the model predicts that the Toronto urban plume is advected to the east within the solenoidal circulation of the lake breeze (see Figure 5.18).
Observations show a rapid increase in the concentration of NOx as the aircraft passed
point (c). The model predicts a similar feature in the NOx eld, though the Toronto
plume is positioned slightly to the south of where observations suggest it is.
231
The return ight, from Peterborough to Guelph, took place between approximately 1930 and 2100 GMT (1530 and 1700 EDT), and followed an almost identical
path as the ight to Peterborough. The path of the return ight is shown in Figure 5.24.
Figure 5.24: Path for the Peterborough to Guelph ight of August 26, 1993. The
labelled points along the path correspond to features in the observed ozone concentration and are referenced below.
As above, the observed and modelled concentrations of ozone and NOx are presented as a function of time during the aircraft ight in Figure 5.25. As shown in
Figure 5.24, the aircraft began at Peterborough and initially ew to the northeast,
passing over the Hastings eld site, before heading south towards Lake Ontario. Approximately 10 km south of Hastings, denoted by point (e), ozone measured onboard
the aircraft increased rapidly from 60 to 90 ppb. It seems plausible that the airmass
encountered by the aircraft at point (e) was the same airmass that resulted in the
232
increase in ozone and other trace species concentrations at the Hastings eld site one
hour later. For the remainder of the ight ozone concentrations were observed to
range between 75 and 90 ppb, with the highest concentrations south of Toronto, in
the vicinity of point (g).
Figure 5.25: The observed and modelled concentration of ozone (panel a) and NOx
(panel b), in ppb, as a function of time for the Peterborough to Guelph aircraft ight
of August 26, 1993. The aircraft altitude is also shown on both panels. The labelled
points along the time axis of the graph correspond to the position of the aircraft as
given by the labelled points along the aircraft path shown in Figure 5.24.
The model ozone concentration, interpolated onto the aircraft position, shows
little evidence of a more polluted airmass moving north with the lake breeze front.
The model ozone does show a gradual increase after the aircraft passes point (f),
though much of the increase occurs while the aircraft is ying to the west, towards
Toronto.
233
Referring to Figure 5.21 above, which shows the model ozone eld at 2000 GMT,
approximately the time of the Peterborough to Guelph ight, it can be seen that the
model calculates much of ozone in the Toronto plume to be well to the west of the
Hastings-Peterborough area at 2000 GMT. Though a lake breeze front is calculated
by the model to lie to the south of Hastings, the airmass behind the front is not
signicantly polluted. These results suggest that the eastern edge of the Toronto
plume is placed too far to the west by the model. Whether this is caused by a too
slow eastward advection of the Toronto plume by the model or other causes is not
clear.
It is not until approximately 2200 GMT that the model calculates the Toronto
plume to have advanced far enough east to result in ozone concentrations which are
greater than 80 ppb as far east as Hastings. Note that the eastward advection of the
Toronto plume discussed here is somewhat separate from the northward, or inland,
penetration of the plume with the lake breeze front.
A comparison of the modelled and observed NOx, shown in panel (b) of Figure 5.25, shows that the model signicantly overpredicts the concentration of NOx
along the northern edge of Lake Ontario, between Peterborough and Toronto. The
model calculates the Toronto plume to lie across the path of the airplane, which it
clearly does not. Underprediction, by the model, of the inland penetration of the
lake breeze, discussed above, may be contributing to the large error in NOx, though
it is not clear if this is the cause of the discrepancy. Note that while the model ozone
agrees fairly well with the observations for the portion of the ight over Lake Ontario, the magnitude of the error in NOx would suggest the agreement for ozone is
fortuitous.
234
5.4 Summary
The MC2-online model has been used to study the interaction of the Lake Ontario
lake breeze with the Toronto plume on two dierent days. On both of the days
chosen for study, observations at the Hastings eld site showed the inuence of the
lake breeze circulation on air quality.
For the rst day studied, August 8, 1993, the simulation develops signicant errors
at the synoptic scale, which adversely aect the development of the lake breeze. While
the model results for August 8 appear to be a poor simulation of the meteorology,
and by extension the chemistry, of August 8, an analysis of the model results does
suggest some possible ways in which lake breeze circulations aect urban plumes and
the photochemical formation of ozone.
The MC2-online simulation for August 8 shows gradient ow from the north during the early morning. Before the onshore ow of the lake breeze circulation begins,
precursors are advected over the lake and remain conned within 100 to 200 m of the
surface. During the early morning, as the gradient wind, or land breeze circulation,
weakens, during the initial development of onshore ow of the lake breeze, the precursors already over the lake are temporarily trapped over the lake. As the onshore
ow of the lake breeze strengthens, the precursors trapped near the lake surface are
advected onshore. For the simulation of August 8, one centre of divergence formed
almost directly to the south of Toronto, within the plume of precursors which had
been advected over the lake earlier in the morning. The formation of the centre of
divergence results in a bifurcation of the plume, with part of the material advected
inland around the western end of Lake Ontario, and the remaining material advected
to the north-east, along the longer axis of the lake. Subsidence near the centre of
235
divergence brings less polluted air to the surface over the lake.
Once the onshore ow of the lake breeze is established, over the western end of
Lake Ontario the northerly gradient ow is opposed by the southerly onshore ow of
the lake breeze. Emissions from Toronto are forced to rise over the lake breeze front
and continue to the south under the inuence of the gradient ow and the lake breeze
return ow. The forced ascent over the lake breeze front results in the formation of
some elevated plumes over the lake.
As the depth of the onshore ow of the lake breeze increases with time, emissions
from Toronto which had been forced to rise over the lake breeze front are entrained
in the onshore ow. For the simulation of August 8, a centre of divergence forms over
the extreme southwest corner of Lake Ontario and a band of southwesterly winds
develops, which ow inland along the north shore of Lake Ontario to the east of
Toronto. Portions of the Toronto plume are carried to the south by the gradient
wind, become entrained within the southwesterly onshore ow of the lake breeze and
are advected towards the northeast. Signicant fractions of the Toronto plume are not
entrained within the lake breeze circulation and are advected to the south, completely
crossing the lake.
The model calculated distribution of ozone shows the formation of a vertically thin
layer with high concentrations of ozone associated with the portions of the Toronto
plume advected onto the lake in the early morning. This thin layer of ozone has only
a minimal eect on ozone concentrations inland as it is rapidly mixed and diluted as
it is advected inland.
The model does predict signicant formation of ozone in elevated layers over the
western end of Lake Ontario, due to precursors from Toronto which were forced to
236
rise over the lake breeze front and subsequently entrained within the lake breeze
circulation. These elevated layers of ozone are largely advected towards the northeast
by the onshore ow of the lake breeze, though they do not have sucient time to
advect inland and remain over the lake at the end of the simulation.
The model simulation of the second day chosen for study, August 26, 1993, does
not show any signicant errors in the synoptic features. Winds during the morning
are light and from the west, slowly backing to a more southwesterly direction during
the afternoon.
The model predicts the formation of a signicant land breeze during the early
morning of August 26. A strong region of convergence is calculated by the model
along the southern shore of Lake Ontario, with northerly wind crossing the lake.
Over the western end of Lake Ontario, the northerly winds are deected towards the
southwest, with the result that precursors emitted from Toronto early in the morning
are advected to the southwest, and do not `pool' over the lake.
Later in the morning, as the lake breeze circulation develops, the westerly gradient
ow advects the Toronto plume to the east, along the north shore of Lake Ontario. A
combination of the westerly gradient ow, and the solenoidal circulation of the lake
breeze, results in helical trajectories for material in the plume as it is advected along
the north shore of Lake Ontario.
Near the surface, the onshore ow of the lake breeze brings less polluted air,
from the centre of the lake, inland along the north shore of Lake Ontario. The less
polluted air in the lowest levels of the lake breeze onshore ow, undercuts the more
polluted air being advected to the east in the lake breeze solenoid. As the onshore
ow moves inland, greater vertical mixing brings the more polluted air aloft, down
237
to the surface. Though poorly resolved by the model, this scenario would seem to
result in a distribution of ground-level ozone similar to that observed by Lyons and
Cole (1976) along the western shore of Lake Michigan.
A comparison of the model lake breeze with a variety of observations suggests that
the model signicantly underpredicts the inland penetration of the lake breeze front
for this case. A comparison of the model ozone and NOx concentrations with aircraft
observations nds some anecdotal evidence supporting, qualitatively, the interaction
of the Toronto plume with the lake breeze predicted by the model. A more thorough
test of the model would require a more intensive aircraft campaign with ights which
more fully map the spatial distribution of the Toronto plume.
238