Meteorological conditions associated with ozone in southwestern Ontario, Canada

METEOROLOGICAL CONDITIONS ASSOCIATED WITH OZONE IN SOUTHWESTERN ONTARIO, CANADA

E. I. MUKAMMAL, H. H. NEUMANN Atmorpkic Environment Servia. Environment Canada, Toronto, Gum&

and

T. J. GILLESPIE University of Guclph, Guclph, Ontario, Canada

(Fit-a receiod 1 Decemk 1980 ad injndfmn 11 h’owmbrr 1981)

bf rurtrc ~athef eimpaion of wid sjxwd and dire&on fnqucncia for acvd ozone fxmaatdon chrsr. ti Duccl h&-tdatork and the amcentntion-WBFT rehtiomhip it&. Considcnbly hi&r

INlUODUCtlOW

white ban8 (Pharrolur uu&arf4 L) in southwortcm Ontario have shown k8f bmnzing aymptoln& attributed to injury by ozone. A study was umbrtaken fint, to quantify the nn?tcorobg!ic8i conditions associated withhighkwcbof- coluzntrat.ioll in surface air; Pndsccor@to examine the interrehtionship among ozone conccntratio& meteorological parameters and injurytowhitclmans.Thcfirstobja3ivcisakdcral hcrc,re, tkh&tcrcr~~~ the subw of another paper.

ModEmMA pr- from oxide8 of nitrogen and hydrocarbons emitted in indus- trialuasorbymotorvehidunuykcokdercdtobe the main cause of the contamination of surface air by ozone. However, Gidel(1980), Viaeeand Singh (1980) and Wolff et al. (1981) pracnt kdencc that tropospheric ozone due to the entrainment of stratospheric ozone at the tusk of tb: tropopause could contribute to total observed surface ozone. An upper air trough is a prerequisite for such oz0l?e intrusio& the extent of which depends on trough intensity, as determined by the maximum wind speed at the 5OOmb level. The stratospheric ozone amtribution h dimcult to cabzu- late and ertainly could not wholly account for conccn- trations as high a~ 8Oppb and above observed during @odes. The three scaka of motion, micro-, meso- andmBcro-,@xcrnthe lnmxmcntandpathoftbc~ Certainatmospkkmotioasdichtcthcdira3ioaand rate of bulk tran8por& while o&era galerate turbulent aldka which arc rwponsible for dilution of the gas as it moves with the mean motion.

During prolongal periods ofair stagnation, ozone may be trapped in the &west atmoeplsric layer, thus resulting in the build-up of abnormal concentration lcvcb. Loal cfkcts, due to phys@@kal and topo- graphical feature9 such as Ii&es aBd semi-pcrmancnt sue inversionr, are cffaxivc in reducing diffu- sion and tran8por-t of pollutantr.

During the summer months the area to the south and southwest of south- Gntario, Canada is oormallyundcrt&inftuc~~~ofawakpraaurc configuration with very littkcirculatkm as a result of a ~eaa#it~ude ridge extending from the Bermuda

Gccauo&ly a soutbwamrly Bow is established behind antkydones or ridgu which move in from the west or north-t and rometimes from the southwest, bringing into southern Ontario warm and moist air which has been stagnant for 8omc time over southern areas.

The association of high turf&x ozolx concentrations over the lower Great Lakes basin (and if&&

over much of the eastern portkms of the Unitod States, with southwesterly !lows on the kk mida of slow moving high pressure cent~4 croariing cutem United States) has been utablishcd in a number of studies (Mulnmrml. l%S; Wolfand Lioy, 1977; Vukovich et al_ 1977). In 8uch sittmtions an ozone river (Wolf and Lioy, 1980) stmehing from the northwat coast of the GulfofMexicotothebwcrGmatLakaandonto NcwE@andisformalapproxifB8telyalongthcair parcel tram* with an appuent corrdation be- twenthcpreunceofnmritimctropkalairand elevated ozone. The impkation is that high kvcb of

2096 E. 1. MUKAMMAL H. H. NEUMANN and T. J. GILLESPIE

ozone over this region are characteristic of a particular air mass and associated with accumulated emissions of a number of sources rather than individual emitters, although enhanced concentrations do occur im- mediately downwind of major sources. Air trajectory analyses for ozone episodes in southwestern Ontario (Chung, 1977; Anlauf et al., 1980) have shown that air parcels originated south or southwest of our region in synoptic situations identical to those favourable for formation of the ozone river. It seems reasonable to assume that the ozone problem in southwestern Ontario arises from an influx of ozone-contaminated air as was concluded by Stasiuk and Coffey (1974) to be the case for rural New York state, although some effect of local emissions was also detected by Chung (1977) northeast of the major urban centre of Toronto. Stasiuk and Coffey’s (1974) work also supported the theory of a polluted air mass by showing that for episodes, ozone concentrations were uniform throughout the depth of the mixing layer (about 2000m) during the afternoon and remained high during the night at elevations above the nocturnal inversion. The persistence of high ozone conantrations at night above the nocturnal inversion was also confirmed by Ripperton et of. (1976).

Stagnation does not appear to be a prerequisite for the occurrence of high ozone for eastern United States and the lower Great Lakes basin (Vukovich er al., 1977; Wolf and Lioy, 1980), although episodes may be more severe in such instances. AltshuIkr (1978) has catego- rized the occurrence of warm, stagnating anticyclones relative to ozone episodes in eastern United States. Even when the months of most frequent stagnation, August and September, were selected and the criteria for stagnation relaxed to allow brief periods of rain or strong winds aloft, only 27 y0 of ozone days with peaks above 80ppb were associated with stagnation.

Light winds in conjunction with stagnating anticyclones over eastern United States result in conditions favourable for the development of lake breezes along the Great Lakes. Mukammal (1965) has attributed some instances ofelevated ozone near Lake Erie to the onset of lake breezes. The effects of lake breeze circulations on pollutant diape& have b discus& in detail for Lake Michigan by Hewson and Olsson (1967). Lyons and Olsson, (1972,1973) and Keen and Lyons, (1978). What emerges conceptuplly from these studies is that trapping and recirculation of pollutants within the lake breeze cell occurs during the day with strati&d layers forming over the lakeond a fumigation zone occurring a short distance inland when layers aloft intersect the developing thermal internal boun- dary layer. In the pmence of a weak lo@tudinal general circulation, air pucels in the lake breeze circulation tend to follow a helical pattern, alternately fumigating over land and tben moving offshore. At night weak land breezes tend to move pollutants offshore, where they accumulate and become part of the lake breeze circulation during the day. In our study area no significant local sources of pollutant were

located along the north shore of Lalre Erie, and ozone episodes associated only with lake breezes here must have arisen from pollutants emitted into the circulation from across the border at major industrial complexes such as Cleveland, Toledo or even Detroit. Near Lake Ontario and possibly Lake Huron, contri- butions from Canadian sources would have been much more significant.

OZONE AND MEI-ROROLOCICAL MONITORING

Ozone concentrations were monitored by the University of GueIph, Ontario, Canada throughout the white bean growing area of southwestern Ontario from 15 June to 8 September 1978 (Fig 1) using Mast Model 724 OZOM meters fitted with sulphur dioxide filters*. At one station, Eiora, a Dwibi 1fKlEAH ozone monitor-? WBJ wed. AU sites were on farms, except for the Mount Forest matalbttion, which was located at the Atmospheric Environment Setvice weather station, near the edge of town. The Maat meters were cali- brated with a Monitor Labs 8500 Calibratot$about every second week. and Linear cat&rations of ozone concentrations vs rtxorder output were obtained. The Monitor Labscalibrator in turn had beencalibrated by a standard gas phase titration (Friedlander, 1977) at the beginning of the season. Ozone concentrations were derived from abstracted data baaed on calibration equations revised on each cahbration date. On the whok, the cahbrations were fairly subk throughout the season.

Additional ozone data were obtained from the Ontario Ministry of the Environment monitoring

stations (Air Quality Monitoring Reporta, 1978) using ethylene reaction cbemiluminescent ozone analysers. Only data from non-urban cantres were used, since scavengiug of ozone in urban ccntres has been noted in other studies (Staaiuk ad Worth, 1976), due to higher kveIsofnitricoxideincitksthMinruralJuaJaThese were: simcoe, Kit&ester (Waterloo), HoRand Marsh, Stoufivilk, Merlin, Petrolia and Huron Park (Fig. 1).

Ten climatologkal stations were eatabhabed at the sites with Maat ozone maters. Temperature and relative humidity were monitored with hygrotbermo- graphs, and wind speed and direction at 10m height were observed with MSC 45 anemometers (a 3cup continuous recording anemometer manufactured by the Atmospheric Environment Service). Other ~tcoroiogical data for southwestern Ontario were available from the synoptic stations of!Simco~, Mount Forest, Kitcbener (Waterloo) and Southampton and from other existing chmatolo&a1 stations, including Elora.

l hiast ckvelopment Company, Davenport, Iowa, U.S.A. t Daribi Environmental Corporation, Gkndak. Calif..

U.S.A. $ Monitor Labs Incorporated, Sen Diego, CA. U.S.A.

Meteorological conditions associated with ozone in southwestern Ontario, Canada 2097

SCALE h....! I

25 0 25 50 75 IO0 KM Q Pll-rSBUROH

Fig. 1. Locations of surface ozone monitoring and upper air radiosonde stations.

STATISTICAL RELATIONSHIPS BElWEEN OBSERVED

WINDS AND OZONE CONCENTRATIONS

The frequency distribution of hourly wind direction at specitied averages of hourly ozone concentrations (ppb) during peak ozone daylight hours (13tX&1900 EST inclusive) in July and August 1978 at Simcoe (Fig 2) shows that the highest percentage frequencies of surface ozone concentrations above 8Oppb were observed with southwesterly winds and the next highest with southerly winds. Ozone concentrations with wind flow from the NW, N, NE, E or SE did not exceed 80ppb, except for two occasions with northwesterly and one with southeasterly winds. The above daylight hours were chosen because ozone concentrations were generally high and winds were more representative of the actual circulation at these hours. The selection of Simcoe rather than Port Stanley was due to its complete records.

To examine the influence of wind speed upon ozone concentrations, the percentage frequency distributions

of hourly wind speed for wind directions (SE, S, SW and W) with the highest ozone concentrations were given in Fig 3(b) from 1300-1900 EST inclusive in July and August 1978 at Simcoe. The most frequent and highest ozone concentrations did not seem to occur with very light or with strong winds but were usually associated with relatively light to moderate flow. Ozone concentrations higher than 8Oppb occmred on only 2.4% of occasions with very light winds (0-IOkmh-‘) in comparison with 121% with light (ll-201cmh-1) and 12.0 % with moderate (21- 30 km h- ’ ) winds. On only 1.8 % of occasions did ozone concentration reach higher’than 80ppb with wind speeds 31-4Okm h-r. Much of the apparent paucity of occasions of ozone above 8Oppb at wind spaedsgreotetthan30kmh-‘is~uscoftherelative infrequency of winds in these classes compared to the lower wind speed clames. If the relative frequencies of ozone concentration above 80ppb in each wind class are compared, this is a musureofthechanceofhigh ozone with wind in a particular speed class, and such

2098 E. I. MUKAMMAL H. H. NEUMANN and T. J. GILLESPIE

30%-

LEGEND

Hourly ozone conceniramn fppb)

Y Hvaher than 120 (ppb) n

% .Sehvem 701.?ZO(ppb) g

Between 41-80 (ppb) q

Less than 41 (ppb) q 0 4% calm wmds

N NE E SE S SW W NW

WIND DIRECTION

Fig. 2 Frequency distribution of hourly wind direction at spa%ed averages of hourly ozone coneeatratiom (ppb) during peak oaone daylight hours (1300-1900 EST) in July

and August 1978 at Simcoc.

ozone concentrations are most likely for the 21-30kmh-’ class, followed in ordet by the 31-40kmh-1,11-20kmh-1andO-10kmh-1classes. Only one hour of winds greater than 40 km h - ’ was observed, and so a relative rank was not assigned to this class due to lack of sufficient data.

The percentage frequency distribution of wind speed for wind direction N, NE, E and NW at specified ozone concentrations for the same daylight hours, dates and place as above, given in Fig. 3(a), shows that ozone concentrations higher than 80ppb occurred on only 0.9% of occasions with the wind speed groups 0-1Okm h-’ and 21-30km h-l, respectively.

SURFACE SYNOPTIC PICTURE AND OZONE

As an example of the most prevalent conditions under which ozone concentrations were high, Fig. 4(a) shows the diurnal variation of average of hourly ozone concentrations, wind speed and daytime prevailing wind direction at Port Stanley and Simcoe for an episode on 7 July 1978. Port Stanley is a climatological station about 1 km north of Lake Erie, and Simcoe is a synoptic weather station about 1Okm north of L.ake Erie. Very high ozone concentrations were observed at Port Stanley between 13oOand 23OOEST, when hourly

59

>I Wnds SE. S, SW, W No OlCBseB 332

WlND SPEED (km’h) WIND SPEED (kmm)

Fig.3. Froqurmcrdimibutionofwigd~(Lmh-‘)forrpccificcrwioddirsctions withthchighl!standlowmtosDlre ammnuations duriq peak ozone daylight hours

(130049UI) in July and August 1978 at Simcoe.

TIME (EST) JULY 7.1970

PORT STANLEY

,.-\ ./-../- SIMCOE

b PREVAILING WING NE’.LV

01 . 1 0 04 OS 12 16 20 24

TIME (EST) JULY 4.1978

averages exceadcd 14Oppb and peaked to 192 ppb at 19OOEST. However, lowur uotuxntrations were recorded at Simcoc with an average hourly peak of 12Oppb at 19OOFST. Such a dazeam of ozone concentrations inland from Lake Erie &acted the influence of the lake ciruulation and wind direction during the summc~ (Mukamm4 l%s; Chung. 1977). Thesynopticpictureat1300ESTasaoc&dwiththis episode is shown in Fig. 5. A large anticyclone centred just off the eastern shores of the United States produced a south to southwesterly flow to the west aide ofthehi&prcssureamtrc.AtSimcoe,windswerelight south-southwesterly at 6rst and became m&rate southwesterly later. Hazy conditions and only l/10-3/10 cirrus and stmtouum ulus clouds pmvaikd. For this episode the distribution of the average daytime ozone conucntration (08W-2oEST) ovur southwestern Ontario is given in Fig 6. Ozone conantrations daucasal inland from the shores of the thruc Great Lakes Huron, Erie and Gntario. Howuver, a high ozone cell central over Elora with hourly average values a8 high as 126ppb was recorded at 2tXtOEST. No satisfactory explanation of the sourcea ofsuchahighccllhasbecnarrivcdat,althoughthc suggcutionisputfonvardthatlocalupwindaourcesin Kitchener (Waterloo) ware partially responsible.

In contrast to the episode of 7 July 1978, the occasion of 4 July 1978 with northerly trajuctories is given in Fig. 4(b). Light northeasterly winda prevailed and similar low ozone colKcluratiotu were expcricn- axlatbothSimcdeandPortSta&y.

Quite fruquantly the centre of high pressure be- comes quasi-statioMry rWar 8outhwcstcrn chltario, causing stagnant air which bar for some time been ovur areas to the south and southwest to become trappud under either low leve4 large scale subsidence inversions or local inversions over relatively cool lakes. Under such condition lake breeze and mego-scale systems dominate the weather picture and bscome the keymcchniamrbywhichozoneisadvectcdinlandby lakebrarzeandbroughtdowntothesurfacebylow level turbulancc. Fii 7 for 22 August at 1300 EST showsatypicalsurfaqcrpopticsituation.Thecentre ofthahighpruauruwujusttotbauxtthofJ_akeErie andthaarktcnaofalaknbraxewasco&mcdbythe fact that winds wcra northerly to the south and southerlytothenorthofLakcErie.Thcarcatothccast ofLakeHuronwasaboaffegtedbyalakebrcczc.At simcoe, winds e from north to north~terly to south to routhmrtaly at about loo0 EST and surface dewpoiMtempuratumurciacfiDm14to17”C,whilethe average hourly ozone conue&ation jumped from

2100 E. I. MUKAMMA~ H. H. NEUMANN and T. J. GILLESPIE

Fig 5. SuW syrqtic sitwth at 1300 EST on 7 July 1978.

LAKE HURON

LEGEND

LAKE ERG 70 ozom? concentrakun

i Wlnd amc1lal 17 Wind awed (kmlh)

31 191 \

19% go

i h 29 m

17 P

‘\ ’ ‘O?.?

‘1

Fig. 7. Surface sywptic situation at 1300 EST on 22 August 1978.

3O+O ppb to 70 ppb. Ozone distribution over the area on that day is shown in Fig. 8, which is typical of such weather conditions and characterrzad by ozone con- ctntrations decreasing with distance inland from Lake Erie and from Lake Huron, as shown by Mukammal (1965). On this day, hourly average ozone rose to 143 ppb at 2OOO EST at Port Stanley near the shores of Lake Erie and to 117ppb at about 1700EST at JJashwood near the shored of Lake Huron.

The above-mentioned features by no means en- comport all the conditions under which ozone in- creased. There were a few instances where ozone incraed to a high or moderate kvcls without the above charrrcteristics being well defined, due to the presence of synoptic fronts. An extreme cxampk was the night of 23-24 August 1978. when the distinct diurnal variation of low ozone concentration during the night was interrupted by the Mux of exceptionally high c~xzntrations. Table 1 gives oul~lc coacen- trations during the night at a few locations. At Port Stanky, hourly avcmge ozone peaked at 195ppb between midnight and 2a.m.

Incrcaed surf= ozone concentration has some- timebeen sI&Bocktcd with the occutreLlcc of thundcr- storms (Mu&ammal, l%S). However, no thunder- storms were detected in the region that night, although

afewcellswerereportedoverlOOkmtothenorthin association with a weak wave on the polar front. It appears that the noctumal maxima over southern ontario may have been associated with the passage of a weak warm front. Coincident with the ozone maxima wasanincreae inwindfromthea0uthwcstandrrkc in dewpoint. Another u&amctMtic cxampk was 15 July when the aWragc of hourly daytime (O8OWWOEsT) om11c concentrations across southwestern Ontario reached 72ppb as a low pressure tr0Ughcrosacdthcare.ninthca&noon_Onafcu other occpsions favoumbk weather conditions pre- vaikd but with little or moderately incnued ozone. An exampk of such a situation is 25 August which will be discussed later.

UPPER AIR DATA AND OZONE

Thusfaronlythcsurfaceweatherpicturchasbeen dealt with descriptively. To compkmcat the above fhdingsupperairdataaroundthcarmweredso cxamid to &pin a better quantitative undMst8ndiag of0ondition8pr0ducingabnomml-concen- trationkvels,whichiutummightcnabkthcmctcorol- ogist to predict cpkodes conducive to pleat injury by

2102 E. I. MUUMMAI., H. H. NEUMANN and T. J. GILLESPIE

LEGEND

70 Ozone ~on~ent~atlon

i Wnxd direclan

9 Wtndspeed(kmih)

Fig. 8. Distribution of the average of hourly daytia& ozone eoneentrations (ppb), wind spcut (km h - 1 ) and pfwdng wind dkaxion (OiW-=Esr) over southw Ontario on 22 August 1978.

Tab& 1. Average of hourly ozone concentrations (ppb) 23-24 August 1978

Time (EST) Station 16-18 18-20 20-22 22-24 24-02 02-04 04-06 08-10 12-14

stfathroy 138 140 104 79 104 85 47 48 Pt. Stanley 129 124 100 146 195 104 61 72 z cttllodcn 99 90 77 88 162 158 78 66 86 aeshwood 131 123 114 98 87 74 49 55 58

ozone. This c&d reduce vegetative dama#c by making possibk the initiation of emission control and/or issuaDeeOfad~wanl@s toagriculturalisu to take protactive measures, such as anti-oxidant spray, har- vesting of mature vegetation, withholding of irri- gation, etc.

In southwestern Ontario, the area of interest, there is no radio-sonde station but upper air data from three nearby stations (Flint, Pittsburgh and Bulfalo, U.S.A.) were available (Fig. 1). Of the 62 days in July and August, 21 were excluded from the analysis. These days had compkx frontal systems giving cloudy conditions with rain or showers over the area and were thus of minor signitia3nce to the particular study. T+diagramsofbothasccntsat 12OOaadtNXMGMT, corresponding to 0700 and 1900 EST, rqa%ively, of the above 3 radio-so& stations were examined. However, due to the early hour of the day the 0700 EST (1200 GMT) were more closely investigated because of their possibk utilization for prediction purposes in identifying the responsibk air masses.

In considering the more potent parameter in de- termining the diffar=nces of origin of various air masacs, the wet bulb potcmtinl temperature, which will hereafter be d&gnated WBPT, was cbc#en in pre- ference to the ordinary potential temperature. The reason is that although both nmain in&t for adiabatic chpnecs of pressure, the WBPT is not affected if the pressure changes are accom@ed by condensation or evaporation. It only varies if boat content of the air is cbangcd by mixing, by radiation or by conduction.

Avcmga WBFT at 07oOEST was computed for the layer between the surkc, or the top of any aurks- bad invenion. and UK! Bnt oaxrEwc bdow the 7OOmb level of either a temperature inversion or isothefmnlconditionsatbtl/orasbarpdropinWBPT bymorethan3”Covcroncaigdk8ntpzwwrabvelin the radio-sonde ascent. Such pressure interval6 were found to range from 5 to 57 mb in our study. If no such layer could be detined, the layer up to 700 mb was used. The exclusion of the surface WBPT in cases with a

Meteorological conditions associated with ozone in uouthwe3tcrn Ontario, canrda 2103

surface inversion was due to its unrepresentativeness of air masses at 0700 EST, as a result of surface cooling by radiation. In instances where no surface inversions were encountered, the surface WBFT was included in the computation (17,25 July; 1,26,27,30 August). The same procedure was followed with the few layer computations carrkd out for 1900 EST. Table 2 gives the average WBPT of the layer, the top of the layer, identitkation of inversion (I) or sharp drop in WBPT (D), time of ascent, wind speed and direction at the surface, at 850 and 7OOmb, average of hourly ozone concentration between 1200 and 2fKlOEST for four sites: Kitchener (Waterloo), Huron Park, Simcoe and Eetrolia to represent southwestern Ontario. The choice of the period of ozone averaging was due to

ozone being generally higher at these times during its diurnal cyck.

Of the 41 daily averages of WBPT for the layers given in Table 2, four were computed from the

1900 EST radio-sonde r&her than from the 0700 EST data, because the latter were not considered to be representative of the air mass responsible for the observed average oxone concentration. For exampk, on 22 August the computed WBPT of the layer for the 0700 EST ascent at Flint was only 13.9 “C, which is too low given the fact that the average of hourly ozone concentration for that day was 83 ppb. However, when the 19tXtESTdataatFlintforthesamedaywasused, the average WBPT of the layer was 20.4 “C, a value well in harmony with the rest of the data in Table 2. The

Table 2. Daily upper air and surface data

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

4 July

: 7

::

:: 18

z

ii 25 26 28 30 31

1 Aug 2 5

t 10 11 12 13 14 I5 17 18 20

z

i: 26 27 29 30 31

897 907 923 901 700

I I&D I&D

0700 14.5 41 9. 16.7 53 . . 18.5 83 . . 21.0 94 . . 9.9 25

1900 15.2 52 0700 17.6 69

w 12.4 30 *. 18.3 74 w 18.9 91 . . 22.3 97 w 23.0 94 9. 15.0 33 . . 17.7 67 . . 18.3 69

340

Ei 170 330 180 190 240 190 220 210 260 130 180 200

2 025 7 1 355 6 3 270 7 2220 4 3340 9 5220 8 6 235 18 2- - 4 245 5 4 245 11 3 210 3 4 285 9 1 015 2 1 205 4 5 235 12

355 6 21.9 345 7 24.1 290 5 28.2 205 11 30.2 320 11 21.6 275 8 24.8 265 11 26.3 335 9 26.2 270 6 29.6 245 I1 30.4 310 8 33.0 285 8 33.6 260 3 25.2 265 6 28.9 255 9 28.1 320 10 23.3 325 6 20.9 290 12 23.6 260 8 27.0 230 12 28.8 260 9 25.2

- - 26.3 260 IO 28.1 310 7 24.5 260 6 25.2 245 8 27.4 010 7 30.1 085 12 31.3 175 7 29.5 305 17 25.3 285 7 28.1

- - 23.4 - - 25.1 350 I2 27.0 225 6 29.0 335 I2 21.7 320 10 24.4 260 6 24.6 282 10 25.3 270 10 24.3 285 4 23.0

D I

850 D D I

850 850 831 819 818 981 859 700

I 9.

Pittnbur8 Flint

*.

Buffalo Flint

3.

dD I&D I&D

I I&D

f - I

:, I&D

D -

I:D D I D

w 11.5 28 330 2 350 9 . . 11.8 24 050 5 070 5

1900 16.5 50 220 250

t! 280 180 330 090

6 250 6 2 255 10 2 22s I2 2 180 4 2 240 6 2260 9 2 315 4 2 280 6 1 215 1 I 270 3 I 140 4 3200 6

- 300 21

2” 4 2 - 4 305 8 2 235 7 3 305 8 2 275 5 2 175 4 7 305 7 1 295 4 2 315 3

929 700

0700 16.9 61 . . 17.9 67

F’ittsbuq Bufhlo

Flint

. . 15.2 50 9. 17.6 71 . . 17.8 77 . . 14.0 28 . . 15.6 45

827 810 700 773 850 850 893 976 909 876 804 818 932 916 973

iFi 728

0

,. 17.3 63 180 . . 18.2 75 180

,, 9. 18.7 74 w 19.2 77 . . 16.5 35 . . 18.3 68 . . 11.9 21 9. 13.6 39

1900 20.4 83 0700 20.0 90

. . 16.9 35 7. 16.4 45 1, 16.8 51

1900 14.7 30 0700 14.6 27

. . 15.1 26

180 150 -

Buffalo Flint

Pittabuq Flint

9. 3. .t 11

Pittsburg FIint

9.

220 360 210 160 030 om 130 310

iii

D

: D&I

I I

iii

1,

. .

(1) Date (1978); (2) top of the layer in mb; (3) identification of inversion (I) or sharp drop in WBP’I’ (D) at ~ofthekya(4)rourcaofupper~&~(S)timcofuppa~Prant(~;(6)averogrretbulb po~~~~(WBPT)(“C)oftbek~(7)daytimeamyeo~aac CQllcentrations (ppb) (1200- 2200 BST), of Kitcbaw (Waterloo), Huron Park. Si aad Petrol& (8)-(10) wind dire&on (“) and speed (mr-‘) at the surfb, 850 ~IKI 7OOmb, mqcctivcly from upper air awent; (11) average maximum tcmpcratura of JCitcbencr (Waterloo), Huron Park, London. Strathroy and Simcuc.

2104 E. 1. MUKAMMA~ H. H. NEUMANN and T. J. GILLESPIE

spulation that there was a change of air mass during the 12 hours between the two ascents would appear to be borne out by the higher WBPT of the layer the next morning and the high ozone concentration the following day. Similar behaviour of the WBPT and ozone data for the following day occurred for 3 of the 4 cases when the 1900 EST data were used. It is interesting to note that weather conditions on most of those days fell into the category described earlier in Fig 7 (22 August) where lake breezes and meso-scale systems dominated the picture. Alternatively, it is possible that the air mass responsible for the relatively high ozone observed on three of five days was trapped under the inversions over Lakes Erie and Huron, was advected inland by lake breeze and brought down to the surface by low level turbulence.

Upper air data from Flint were normally used to develop Table 2. However, when consideration of air trajectories at 850 and 925 mb suggested that Flint was not representative of the air mass over southwestern Ontario, data from Pittsburgh and Buffalo were used. The trajectories were based on a model by Olson et al. (1978), which used objectively analyzed grid-point fields of meteorological variables from the Canadian Meteorological Centre. Vertical motions were computed using the “omega” equation and combined with horizontally analyzed winds to give the model a 3-D capability.

RELATIONSHlP BETWEEN OZONE AND WBPT

In Fig 9 a hyperbolic rather than linear function of the form Y = tanh X seems to be more appropriate to

express the relationship between values of ozone and WBPT given in Table 2. Thus, an equation of the form

Y= [

,#-A,B_,-IX-A@

e’X-A’B+e-lX-A’B 1 C+D

was formulated with the four constants (A,B,C,D) to allow for units and a translation to parallel axes with a new origin.

Iteration was used to solve for the best values of the constants A, B, C, D to give the least standard error of estimate. Best values of A = 17, B = 0.35, C = 38.0 and LJ = 60.5 were found to give a highly sign&ant correlation coefficient of 0.95 and an acceptable standard error of estimate of 8.3ppb. This gives upper and lower limits for average ozone concentrations of 100 and 23 ppb, respectively.

In general, the derived curve fitted the observed points fairly well, except for a few occasions when none of the available radio-sonde data were representative of air masses over southwestern Ontario. One notable exception was the 25 August, when two quasi- stationary fronts lay in a west to east direction over the Great Lakes Region. Under such conditions, upper air data separated by 12 h intervals and not observed over the area of interest could not identify the air mass responsible for the observed ozone concentration. Another conspicuous scattered point was that of 17 August, which was associated with the approach and passage of a cold front. Surface winds at Port Stanley, near the north shore of Lake Erie, were southwesterly 9-12 ms-’ until loo0 EST, then they veered to northerly at 16ms- 1 for 4h, but backed again to southwesterly 9-12ms- ’ after 14OOEST and became very

I I I I 1 I I I L I 1 I 1 I I 1 _I

10 12 14 16 16 20 22 24

AVERAGE WET BULB POTENTIAL TEMPERATURE (“C) OF A LAYER

Fig. 9. Relationship ktween average wet bulb potential tcmperatum (” C), of a layer and average of hourly daily ozone concentrations (ppb) (1200-2200 EST) at Kitchcncr (Waterloo), Huron Park, Simcoc and Petrolia.

Meteorological eouditioas asseciated with ozoae in southwestern olltalio, Canada 2105

light and variable late in the evening Upper winds at Flint at 07oOEST were 3u)“, 21ms-’ at 85Omb; and 305”, 17ms-’ at 7tlOmb. Here again, under these conditions it is extremely di5cult to identify the true air mass over the area of interest from a distant upper air station. Both scattered points of 12 July and 22 August were of the lake breeze type described earlier in Fig 7. As mentioned earlier, a change in air mass might have taken place or else the air trapped under the inversion over the lake and advected inland by the lake breeze might have been of a different origin from that identitkd from the 1900 EST ascent at Flint. There was no satisfactory explanation for the remaining few scattered points.

Some interesting relationships appear in Fig 9. At both ends of the curve there are very small variations in average ozone concentrations corresponding to large increases and decreases in WBPT. Also most points towards the left end of the curve, are associated with northwesterly to northerly winds of polar origin, whik those towards the right end are associated with south to southwesterly winds of maritime tropical or modified maritime polar air mass origin, as will be seen from the upper air winds given in Table 2 It is suggested that the left end of the curve with a low concentration represents background ozone levels, whereas the right end of the curve approaches what appears to be an upper limit of observed ozone concentration in south- em Ontario. The observed upper limit to ozone concentration is likely dependent upon prescursor

emission strength and the dilution by turbulent eddies as air is transported from source regions.

In another simpkr approach, the above pcopo- sitions may also be deduced. In Fig. 10 average maximum temperatures at London, Strathroy. Kitchener (Waterloo) and Simcoe given in Table 2 were plotted against the same average ozone concen tration used in Fig. 9. It is evident that, apart from the wide scatter of points, the freehand curve in Fig. 10 is similar to the derived curve in Fig 9. However, since the maximum temperature is greatly a&ted by cloud amount, height and type it cannot be considered a conservative parameter for ozone prediction purposes.

Given the air trajectory and upper air ascents from the three radio-sonde stations and the surface synoptic picture, the relationship shown in Fig. 9 may serve as a useful identifkr of speciiic air masses responsible for producing abnormal ozone concentmtion levels in southwestern Ontario, recalhng that the technique is not applicabk to days with compkz frontal systems over the area. This technique, mostly involving data available at an early hour of the morning (0700 ESTl, may be used as a warning of possible plant injury, provided the conditions are coupled with other re- quirements such as a susceptibk stage of development of the plant, the presence of a me&an& for bringing ozone down to the surface, and other micro- meteorological and physiological factors favoumbk for intake of ozone by the vegetation.

F I I 1 I I I I I I I 1 I I 1 I -

3 p -

$90- c g 80 - P 2 2 70-

’ !? 60-

Y 2 50- s Y 40- 1. % 0 JULYt97Bwi*,d&

2SA A AUGUST 1978 rri(h dale

Y 30 -

11 2 w 20-

-a-’

2 x)4

, I I I I I I 1 I I 1 I 20 1 1 1 22 24 26 28 30

) 32 34

AVERAGE MAXIMUM TEMPERATURE (“C)

Fig. 10. Rektiouship between average maximum tauperamres (“C) at Kit&aer (Waterloo), Huron F%rk, Lou&u, Strathroy and Simwe and average of hourly ozme conccntratiom (ppb) 1200-22OOEST) at Kitchew (Waterloo). Huron

Park, Simeee and Petrolis.

2106 E. I. MUKAMMAL, H. H. NEUMANN and T. 1. GILLESPIE

SUMMARY

Air masses associated with changes in ozone concentration in southwestern Ontario during July and August 1978 were identified, and a hyperbolic function relating average ozone concentration measured at several inland stations to estimated wet bulb potential temperature (WBPT) of the surface mixed layer derived from upstream radio-sonde ascents was de- veloped. The best fit curve showed a lower limit or background average daytime average ozone concentration near 23 ppb and an upper limit near 100 ppb. It was hypothetized that the upper limit determined by precursor emission strengths in the source regions south and southwest of Ontario and the dilution by turbulent eddies as the air moved from the source regions to the study area. The rationale for selecting WBPT as an indicator of ozone concentration was that WBPT is a conservative parameter and characteristic of air masses.

Although there were several recognizable meteorological regimes over southwestern Ontario associated with elevated ozone concentrations, those with southwesterly flows on the west side of slow moving high pressure centres crossing eastern United States or with weak pressure gradients favourable to the onset of lake breeze were the most frequent and resulted in the greatest ozone concentrations. Episodes of abnormally high ozone concentration followed the advection into the area of a modified maritime tropical air mass (WBPT about 21-24°C) and sometimes modified maritime polar air mass (WBPT 18-20°C).

Higher ozone concentrations were found near the shores of the Great Lakes during ozone episodes in southwestern Ontario as a result of thermally induced lake breezes or stable marine layers due to relatively cool lake surface water temperatures. Pollutants in- jected into such flows show limited mixing while over the lakes but give rise to a fumigation zone a short distance inland. In the case of lake breezes these pollutants may be recycled several times. No significant Canadian industrial or urban centres were located along the north shore of Lake Erie, and so the ozone maximum in this area was attributed to emissions across the lake in the United States, where several major industrial centres are located. Along the north shore of Lake Ontario and possibly for the east shore of Lake Huron, local sources are likely the major contributors to the ozone maxima.

Acknowledgement-We wish to thank the Ontario Ministry of Environment for the use of ozone data from their monitoring stations.

REFERENCES

Air Quality Monitoring Reports (1978) Ontario Ministry of Environment, Toronto, Canada.

Altshuller A. (1978) Association of oxidant episode-s with warm stagnating anticyclones, J. Air Pollut. Conrrol Ass. 28, 152-155.

Anlauf K., Olson M. and Wiebe H. (1980) Atmospheric transport of particulate sulphate and ozone into the Toronto region and its correlation with visibility. Atmos. Pollut. 8, 153-158.

Chung Y. (1977) Ground-level ozone and regional transport of air pollutants. J. appl. Met. 16, 1127-l 136.

Friedlander S. K. (1977) Ozone and other photochemical oxidants. National Research Council Committee on Medical and Biological Effects of Environmental Pollutants, Nat. Acad. Sci. Washington D.C. U.S.A.

Gidel L. (1980) General circulation model estimates of the net vertical flux of ozone in the lower stratosphere and the implications for the tropospheric zone budget. J. geophys. Res. 85, (No. 0). 4U49-4058.

Hewson E. and Olsson L. (1967) Lake effects on air pollution dispersion. J. Air Pollut. Conrrol Ass. 17, 757-760.

Keen C. and Lyons W. (1979) Lake/land breeze circulations on the western shore of Lake Michigan. J. appl. Mn. 17, _. 1843-1855.

Lyons W. and Olsson L. (1973) Detailed mesometeorological studies of air pollution disoersion in the Cbicaao lake breeze. Mon. &ath. Rev. 161, 387403. -

Lyons W. and Olsson L. (1972) Mzsoscak air pollution transport in the Chicago lake breeze. J. Air Pollut. Control Ass. 22, 876881.

Mukammal E. (1965) Ozone as a cause of tobacco injury. Agric. Met. 2, 145-165.

Olson M., Oikawa K. and Macafee A. (1978) A lrajecrory model applied to the long-range fransport of air pollutants. Atmospheric Environment Service, Environment Canada, Downsview, Ontario, Canada.

Ripperton L., Worth J., Vukovich F. and Decker C. (1976) Research Triangle Institute studies of high ozone concentrations in non-urban areas. International Conference on Photochemical Oxidant Pollution and its Control Proceedings VI: 413-424. EPA-6003-77-OOla.

Stasiuk W. and Coffey P. (1974) Rural and urban ozone relationships in New York State. J. Air Polk Control Ass. 24, 564-568.

Viezee W. and Singh H. B. (1980) The distribution of beryllium-7 in the troposphere: implications on stratospheric/tropospheric air exchange. Geophys. Res. Left. 7, 805-808.

Vukovich F., Bach W., Jr., Crissman B. and King W. (1977) On the relationships between high ozone and the rural surface layer and high pressure systems. Atmospheric Environment 11, 967-983.

Wolf G. and Lioy P. (1977) An investigation of long-range transport of ozone across the midwestern and CBstem United States. Atmospheric Enoironment 11, 797-802.

Wolf G. and Lioy P. (1980). Development of an ozone river associated with synoptic scale episodes in the eastern United States. Enuir. Sci. Technol. 14, 1257-1260.

Wolff G. T., Kelly N. A. and Ferman M. A. (1981) Source regions of summertime ozone and haze episodes in the Eastern United States. Environmental science Dept. (No. 110). General Motors Research Labs (3701). Warren, Ml 48090.

Documents

Meteorological conditions associated with ozone in southwestern Ontario, Canada