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Daytime Photochemical Pollutant Transport over a Tributary Valley Lake in Southwestern British Columbia. McKendry, Ian G.; Steyn, Douw G.; Banta, Robert M.; Strapp, W.; Anlauf, K.; Pottier, J. 2011

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APRIL 1998 393MCKENDRY ET AL.q 1998 American Meteorological SocietyDaytime Photochemical Pollutant Transport over a TributaryValley Lake in Southwestern British ColumbiaI. G. MCKENDRY AND D. G. STEYNAtmospheric Science Programme, University of British Columbia, Vancouver, British Columbia, CanadaR. M. BANTANOAA/Environmental Technology Laboratory, Boulder, ColoradoW. S TRAPP AND K. ANLAUFCloud Physics Research Division, Atmospheric Environment Service, Downsview, Ontario, CanadaJ. POTTIEREnvironmental Conservation Branch, Environment Canada, Vancouver, British Columbia, Canada(Manuscript received 10 February 1997, in final form 20 June 1997)ABSTRACTTethersonde, lidar, aircraft, and surface chemistry measurements from an intensive field campaign (Pacific’93) in the Lower Fraser Valley (LFV) demonstrate the daytime advection of pollutants into a lake-filled valleyadjoining a broad urbanized coastal valley. On three separate days (immediately before, during, and after apollutant episode), elevated concentrations of ozone (O3) in the narrow tributary valley could be attributed tothe advection of pollutants northward from sources in the LFV (primarily metropolitan Vancouver). On 5 August,the highest concentrations of O3observed in the region during the entire episode were observed over the tributarylake. Simple Lagrangian mass budget calculations suggest that the unusually high concentrations observed on5 August over the lake were physically reasonable and consistent with the known chemistry of the air advectedinto the valley. They also indicate that reductions in O3flux divergence during the overlake trajectory in thePitt Valley, primarily as a result of reduced surface deposition, may contribute to the relatively high concentrationsobserved in the tributary valley. Observations immediately after the episode show that chemically aged pollutedair masses can persist within the tributary valleys from the previous day. These results have implications forthe understanding of air pollution in other regions of complex terrain and show that the predominance of daytimeupvalley pollutant transport in such tributary valleys is likely to have significant impacts on the local ecologyand visibility.1. IntroductionLarge cities and their surrounding regions often ex-perience degraded air quality due to the formation ofphotochemical smog resulting primarily from automo-bile emissions of oxides of nitrogen and hydrocarbons.These ‘‘precursor’’ pollutants are transformed in thepresence of sunlight into a variety of pollutants [in-cluding ozone (O3) and fine particles] that can be del-eterious to human health and vegetation (Seinfeld 1989).The Lower Fraser Valley (LFV), with rapidly growingmetropolitan Vancouver (population approximately 1.7Corresponding author address: Dr. I. G. McKendry, Departmentof Geography, 251–1984 West Mall, University of British Columbia,Vancouver, BC, V6T 1Z2, Canada.E-mail: ian@geog.ubc.camillion) at its western end, experiences such photo-chemical pollution during summer months. The O3con-centrations exceed the national ambient air quality hour-ly objective of 82 ppb at one or more of the 24 mon-itoring stations within the LFV on approximately 8 daysper summer (Pryor et al. 1995). Such exceedances aretypically single-day events and are associated with astrong ridge of high pressure developed over the region(McKendry 1994; Pryor et al. 1995). Primary emissionsources are mainly associated with the transportationsector in the western portion of the LFV.Around coastal cities, the transport and dispersion ofpollutants is often strongly affected by local winds, suchas sea and land breezes. In complex terrain, valley andslope circulations also play important roles (Oke 1987).Consequently, in the daytime, urban pollutants may beadvected inland from coastal plains into adjoining val-leys by upvalley winds and sea breezes (e.g., Kurita et394 VOLUME 37JOURNAL OF APPLIED METEOROLOGYFIG. 1. Map of the Lower Fraser Valley showing sites mentioned in the text and surface back-trajectories ending at 1600 LST over Pitt Lake for 2, 5, and 6 August 1993.al. 1985; Douglas and Kessler 1991). At night, the re-verse occurs, and pollutants are advected out of valleysand perhaps offshore by downvalley winds and landbreezes (Banta et al. 1997; Douglas and Kessler 1991).These diurnally reversing circulations create potentialfor not only seriously degraded air quality at remotesites but also for pollutants from one day to recycle andcontribute to ground-level concentrations over urban-ized areas the following day. Despite observational stud-ies in a number of urbanized settings, the role of valleysadjoining coastal urbanized lowlands in the regionalbudget of pollutants is not well understood. Of particularinterest is the extent to which nearby valleys act as asink for pollutants emanating from an urbanized coastand whether they contribute to the recirculation of pol-lutants via diurnally reversing circulations (Banta et al.1997).Previous studies of pollutant transport in the vicinityof Vancouver have largely ignored tributary valleys andfocused on the role played by thermotopographic cir-culations (notably land/sea and valley breezes) operat-ing within the LFV, the broad, steep-sided, east–west-oriented coastal valley in which the city is located (Steynand McKendry 1988). In the LFV (Fig. 1), pollutanttransport during episodes of poor summertime air qual-ity is dominated by the sea breeze that develops duringstagnating anticyclonic conditions (McKendry 1994;Steyn and Faulkner 1986) and advects the urban plumeeastward from Vancouver. Consequently, the highestconcentrations of O3within the surface monitoring net-work (which is wholly located within the LFV) havehistorically occurred in the Pitt Meadows region (down-wind of major sources of precursors in Vancouver) andthe eastern LFV. Recent modeling studies indicate thatdaytime flow into the north–south-oriented tributaryvalleys along the northern edge of the LFV also carrieswith it pollution emanating from metropolitan Vancou-ver (Steyn and Miao 1995). In particular, the largelyunpopulated, forested, and lake-filled Pitt Valley im-mediately downwind of the city (Fig. 1) produces strongdaytime upvalley and nocturnal downvalley flows insimulations. As a result, simulated pollutant trajectoriesmove into and out of this valley in a diurnal cycle (Steynand Miao 1995). Other results (Banta et al. 1997) fromPacific ’93 (the intensive field campaign on which thisstudy reports) show that Pitt Valley produces strongnocturnal drainage with a pollutant signature consistentwith an urban (i.e., greater Vancouver) source. Inter-estingly, this flow is found to be depleted in O3,aneffect attributed to nocturnal deposition of O3alongvalley slopes and also observed in valleys in Switzerland(Broder and Gygax 1985). A potentially significant site-specific aspect of the LFV region is the presence of longfinger lakes in virtually all of the adjoining valleys. ForO3, water surfaces are known to reduce deposition ratesconsiderably below those observed over terrestrial sur-faces (Galbally and Roy 1980). Consequently, over-water trajectories may affect the budget of pollutantswithin the boundary layer.A moderate episode of photochemical air pollutionoccurred in southwestern British Columbia from 1 to 6August 1993 during an intensive field campaign de-signed to improve understanding of the meteorology andchemistry of photochemical pollution in the region. Theobjective of this paper is to decribe a set of observationsfrom this period that corroborate modeling results(Steyn and Miao 1995), which highlighted the potentialfor daytime advection of the Vancouver urban plumeinto the Pitt Valley. By exploiting the same rich Pacific’93 dataset used by Banta et al. (1997), this study com-plements their description of nocturnal flows in the samevalley by confirming pollutant transport into the PittValley and elucidating processes affecting daytime con-centrations within the valley. Finally, a simple massAPRIL 1998 395MCKENDRY ET AL.budget approach is used to assess whether high con-centrations observed over Pitt Lake are physically rea-sonable and to provide a first-order assessment of therelative roles of the photochemical production and sur-face deposition in influencing observed concentrationswithin the valley. The air pollution meteorology of thePitt Valley described here, and by Banta et al. (1997),highlights the important effect of complex terrain ininfluencing the mass budget of photochemical pollutantsalong urbanized coasts.2. MethodsPacific ’93 was a multiagency field campaign de-signed to elucidate the meteorology and chemistry ofphotochemical air pollution episodes in the LFV andenvirons (Steyn et al. 1997). The National Oceanic andAtmospheric Administration/Environmental Technolo-gy Laboratory (NOAA/ETL) scanning Doppler lidarwas located to the south of Pitt Lake adjacent to PittMeadows Airport and was able to scan the atmosphereover a range extending into the mouth of the Pitt Valley(Fig. 1). A description of the lidar and its applicationin complex coastal terrain is provided in Banta et al.(1993), and Banta et al. (1997). Particularly useful inthe context of this study were a series of vertical slice,or range–height indicator (RHI) scans, directed towardthe Pitt Valley. In RHI mode, the lidar remains at aconstant azimuth angle while scanning in elevation. Thisproduces a high-resolution vertical slice (distance versusheight) of wind velocity and backscatter data (an indexof aerosol loading) to a horizontal distance of approx-imately 15 km from the lidar.Vertical tethered balloon soundings of meteorologicalvariables and O3to approximately 1000 m above groundlevel (AGL) were conducted at Harris Road, at themouth of the Pitt Valley, and Little Goose Island on PittLake (Fig. 1), during the photochemical pollution epi-sode from 1 to 6 August 1993. The island location per-mitted examination of the valley circulations at a rangeconsiderably beyond that captured by lidar at the valleyentrance. Vertical sounding operations at each site con-sisted of deployment of an Atmospheric InstrumentationResearch Inc. (AIR) tethersonde (TS-3A-SPH) withozonesonde (OZ-3A-T) beneath a 5-m3helium-filledballoon and were restricted to daylight hours to meetaircraft safety considerations. In total, 37 soundingswere made at Harris Road in the 6-day period, while atthe Pitt Lake site, 16 soundings were made mostly on2 and 5 August. Tethersonde flights were intermittantat both sites due to logistical problems and strong windsthat occasionally halted operations. Further details areprovided in McKendry et. al. (1997).Measurements of a wide variety of chemical specieswere also made at Harris Road during Pacific ’93. Thispermitted analysis of the chemical signature of pollut-ants transported into and out of the Pitt Valley, as wellas a check on ground-level O3measurements from theozonesondes. The range of ‘‘fast chemistry’’ instru-mentation at Harris Road is described in detail by Li etal. (1997) and Steyn et al. (1997). For the species usedin this study, nitrogen oxide (NO) and nitrogen dioxide(NO2) were measured by O3-chemiluminescence, O3byUV absorption, carbon monoxide (CO) by gas corre-lation, and peroxyacetal nitrate (PAN) by gas chroma-tography using electron capture detection.Aircraft [National Research Council (NRC) Convair580] observations during Pacific ’93 are described indetail in Steyn et al. (1997). Of the multitude of flightlegs undertaken, low-level observations (300–500 m)over Pitt Lake and vertical profiles in the vicinity of PittMeadows provide valuable ancillary data relevant tounderstanding air quality and meteorology in the trib-utary valleys. In particular, simultaneous observationsof O3,NOy, NO, and aerosol (in the size range 0.2–3mm) along a horizontal track are utilized to give a broad-er perspective on air quality in the area than possiblewith tethersonde observations alone.To assess whether concentrations observed over PittLake were physically reasonable and to provide a first-order estimate of the relative roles of the photochemicalproduction and surface deposition in influencing ob-served concentrations within the valley, a simple massbudget (within a Lagrangian framework) was calculatedfor 5 August 1993. This approach is based on a bound-ary layer slab model (Lenschow et al. 1981) and as-sumes that the air column sampled by tethersonde atHarris Road is subsequently sampled by the second teth-ersonde at Little Goose Island (25 km to the northeast).The mass budget approach and calculations are de-scribed in detail in the appendix. To apply this modelin the context of the Pitt Valley, several assumptionswere necessary. First, horizontal homogeneity and zeromean vertical velocity were deemed to be reasonable,simplifying assumptions given the breadth of the valley(measurements are made on the valley floor several ki-lometers from the base of slopes) and zero slope (PittLake is near sea level) along the section of the valleyconsidered here. Second, in using only two measure-ment sites, it is assumed that only the along-valley com-ponent of horizontal flux divergence is resolved in theair columns moving across Harris Road and past thePitt Lake site. We consider this to be a reasonable first-order approximation in this long, rather narrow valley.3. Resultsa. OverviewDuring the intensive observational period from 1 to6 August 1993, upper-level synoptic flow conditionsfavored the development of above average O3concen-trations but were not conducive to the development ofa major O3episode (Pottier et al. 1997). The latter gen-erally occurs when an upper-level ridge is positionedover the region producing stagnation, subsidence, and396 VOLUME 37JOURNAL OF APPLIED METEOROLOGYFIG. 2. Time series of meteorological variables and chemical spe-cies at Harris Road for 1–6 August 1993 showing (a) upvalley windcomponent (upvalley is positive with valley oriented 2358–458), (b)surface air temperature, (c) ozone (O3) concentration, (d) NOx/NOyratio, (e) NO concentration, and (f) CO concentration. Note missingmeteorological data on 6 August.the ancillary conditions necessary for elevated O3(McKendry 1994; Pottier et al. 1997). In this case, theupper-level ridge was positioned to the west of BritishColumbia producing a predominately northerly flow.However, surface heating during the period was suffi-cient to produce a thermal trough along the west coast,a characteristic feature of elevated O3episodes (Mc-Kendry 1994). Because of the relatively weak longwaveridge of high pressure during the episode described here,traveling shortwave ridges and troughs propagatingalong the edge of the ridge induced significant day-to-day variability in the local meteorological conditions(e.g., stability, subsidence, and winds) that modulatepollutant concentrations (Pottier et al. 1997; Hayden etal. 1997).The course of meteorological variables and releventpollutants at Harris Road during the observational pe-riod is shown in Fig. 2. Maximum temperatures (Fig.2b) and O3concentrations (Fig. 2c) occurred on 4 Au-gust after a 3-day period of successively higher daytimemaximum temperatures. The component of wind ve-locity up Pitt Lake (positive values represent upvalleyflow with the Pitt Valley axis assumed to be aligned2258–458) shows a consistent pattern of upvalley windsduring the afternoon. This pattern is representative oflong-term wind observations from the mouth of the PittValley that show a summer afternoon (1200–1800 LST)upvalley flow frequency of 77%. The relatively shortperiod of downvalley flow in the evening shown in Fig.2a is also consistent with long-term observations. Bantaet al. (1997) note that, during this period, surface windsat Harris Road were generally light and variable over-night (probably due to localized drainage flows) butwere overlaid by a persistent downvalley flow. Strongdownvalley flow on the early evening of 4 August wasassociated with a localized thunderstorm that movedalong the northern edge of the LFV. A shift to low-levelsoutheasterly flow on the evening of 5/6 August wasassociated with the propogation of low-level coastallytrapped disturbances into the region and marked the endof the moderate air pollution episode (McKendry et al.1997).Carbon monoxide (Fig. 2f) is a species that is rela-tively chemically inert in the atmosphere and, conse-quently, provides an effective means of assessing theeffects of dilution in the atmosphere (assuming rela-tively consistent emission patterns). Relatively highconcentrations evident on 3 and 4 August and to a lesserextent on 5 August therefore reflect the reduced dilutionassociated with low mixing depths (Hayden et al. 1997)and relatively light winds under the ridge of high pres-sure. The reduction in dilution associated with the syn-optic-scale meteorological pattern is also apparent inphotochemically active precursor species (e.g., NO, Fig.2e). These show elevated concentrations on 3–5 Augustassociated with the injection of fresh precursors fromthe early morning traffic peak. Later in the day, con-centrations decrease due to photochemistry, reducedemission, and enhanced mixing. High concentrations ofprecursor species on 3–5 August were responsible forbuildup of O3concentrations during the moderate epi-sode.The ratio of NOx(the sum of NO and NO2)toNOy(the sum of all odd-nitrogen species, e.g., including PANand HNO3) provides a useful measure of the ‘‘chemicalage’’ of the pollutant mass (Banta et al. 1997). Whenthis ratio is high (greater than 0.9), it implies that mostof the NOyis reactive nitrogen oxides (NO and NO2)APRIL 1998 397MCKENDRY ET AL.and therefore the air mass contains high concentrationsof fresh pollution. Low values of NOx/NOy(less thanor equal to 0.6) suggest air that is chemically aged withmost reactive nitrogen oxides, having been transformedto stable end products. For most of the episode (1–5August) the ratio remained well above 0.6 (Fig. 2d),especially overnight (due to lack of photochemistry andlocal sources of NOx) and in the morning (fresh injec-tions of NOx). This suggests that during the daytime on1–5 August, polluted air masses passing across the Har-ris Road site and into Pitt Lake were photochemicallyactive. Only in mid- to late afternoon, when photo-chemistry had consumed precursors and created morestable end products, did ratios drop below 0.6. With thechange in weather conditions on the night of 5/6 August,the ratio shifts to quite a different pattern. On 6 August,values of NOx/NOydrop below 0.5 from midmorningthrough late afternoon. This suggests that the air passinginto Pitt Valley on this day was chemically aged andthat above-average O3concentrations were most likelyassociated with the downmixing of ozone from an el-evated layer persisting from the previous day (see Fig.6) rather than significant photochemical production(McKendry et al. 1997).Crude back-trajectories based on hourly surface windobservations for 2, 5, and 6 August 1993 beginning at0900, 1100, and 1000 LST and ending at 1600 LST areshown in Fig. 1 and provide a background to the detaileddiscussion presented below. On each of the days, thetrajectories indicate daytime flow into Pitt Valley fromthe LFV. On 2 August, back-trajectories for air parcelsarriving over Pitt Lake at 1600 LST indicate that pol-lutants observed in the Pitt Valley during the afternoonmost likely emanated from sources along the northernedge of the LFV, including the city of Vancouver. On5 and 6 August, the late afternoon trajectories reflectmore southwesterly to southerly flow and pass over pri-mary emission sources in the southern and eastern mu-nicipalities (e.g., Surrey, Delta, Port Coquitlam). Mod-erate winds associated with the latter two trajectoriesalso suggest the possibility that a proportion of the pol-lutant load may have emanated from over the southernGeorgia Strait where chemically aged pollutants mayaccumulate as a result of nocturnal offshore flow fromthe LFV (McKendry et al. 1997).A series of afternoon vertical profiles of lidar back-scatter and radial velocity at the mouth of the Pitt Lakevalley, 8–9 km to the north of Pitt Meadows Airport,not only confirm the upvalley flow depicted in Fig. 2but also show its vertical structure and aerosol loading(Fig. 3). Profiles all show low-level positive radial ve-locities, indicating flow into the valley. On 2 August,this upvalley flow lay beneath a layer of downvalleyflow aloft associated with synoptic-scale northeasterlyflow. Later in the episode, upper-level flow shifted tothe west and this downvalley component aloft was notobserved in the Pitt Valley (Pottier et al. 1997). From2 until 5 August (note data are missing for 3 August)the strong low-level upvalley flow was associated withthe high aerosol backscatter indicative of the advectionof polluted air into the valleys. High backscatter wasobserved on 5 August when highest ground-level O3concentrations were also observed in the LFV (Steynet al. 1997). On both 4 and 5 August the highest aerosolcontent was associated with the strong upvalley flowclosest to the surface. On 6 August, peak backscatteroccurred in an elevated layer of aerosol at 350–500 mAGL.In summary, daytime flow into the Pitt Valley fromthe LFV occurs frequently during summer days and ad-vects polluted air into the valley. However, the trajectoryanalysis highlights the extent to which air reaching thePitt Valley may follow quite different paths and there-fore exhibit different pollutant histories. Air quality inthe Pitt Valley during Pacific ’93 will now be examinedin greater detail with emphasis on 2, 5, and 6 Augustwhen intensive tethersonde observations were available.b. Observations of daytime advection of pollutantsinto tributary valleys1) 2 AUGUST 1993: EARLY EPISODEAn upper-level trough deepened over the LFV on 2August, producing a relatively strong northerly flowalong the Georgia Strait and west to northwesterly flowover the western LFV. Meanwhile, the eastern valleyexperienced northeasterly outflow winds and tempera-tures that reached 328C at Abbotsford during the after-noon.In Fig. 4, isopleths of upvalley wind velocity, poten-tial temperature, and O3concentration for the period0800–2000 LST are presented to show the temporalevolution of vertical structure over Pitt Lake. Earlymorning winds (0800–1000 LST) show the start ofupvalley flow near the surface and downvalley flow aloftwith a local maximum (up to6ms21) in the stronglystable layer from 200 to 400 m (Fig. 4a). The latterrepresents the decaying nocturnal flow regime describedby Banta et al. (1997). The shallow layer of upvalleyflow already present by 0830 LST grew deeper as themixed layer increased to a maximum depth of approx-imately 600 m in late afternoon. Above the mixed layer,the flow had a persistent downvalley component asso-ciated with predominant synoptic-scale northeasterlyflow. Throughout the day, the relatively cool lake sur-face (approximately 188C) maintained a shallow sur-face-based inversion. This stable surface layer may havecontributed to the low-level upvalley wind maximum of4–5ms21(100–300 m above lake level) during theafternoon. However, observations elsewhere in the LFVsuggest that this low-level ‘‘jet’’ was present across asignificant proportion of the region. In late afternoon(1855 LST), upvalley flow was abruptly replaced bydownvalley flow at the surface over Pitt Lake in whatseems to be a regular pattern within the Pitt Valley (Ban-398 VOLUME 37JOURNAL OF APPLIED METEOROLOGYFIG. 3. Profiles of radial wind velocity (solid lines: units are in meters per second with positivevalues representing flow away from the lidar and into Pitt Valley) and aerosol backscatter intensity(dashed lines: unit are in decibels) averaged over 1 km at 8–9 km from lidar site (Pitt MeadowsAirport, Fig. 1).ta et al. 1997). This cycle was evident on many daysduring Pacific ’93 and on all days of the photochemicalpollution episode.In the early morning, O3isopleths (Fig. 4b) showedrelatively clean air over Pitt Lake with little verticalvariation in O3concentrations. Concentrations then in-creased rapidly from 1300 to 1400 LST in associationwith a marked strengthening of the upvalley flow. Therapid increase on O3concentration likely represents theleading edge of the urban ‘‘plume’’ as it was advectedup the valley. In the midafternoon, O3concentrationsreached 60 ppb within the Pitt Valley in a layer from200 to 500 m above the lake. During this time, profilesalso showed a sharp decrease in O3concentration acrossthe top of the mixed layer (between 600 and 700 m).2) 5 AUGUST 1993: LATE EPISODEOn 4 August, the later afternoon was characterizedby unstable conditions associated with the passage of ashortwave trough that induced thunderstorm activityalong the northern edge of the LFV. However, by 5August, the LFV was again under the influence of syn-optic conditions conducive to the development of pho-tochemical pollution. Maximum surface air tempera-tures reached 298C at Vancouver airport and 308CatAbbotsford. Within the LFV surface-monitoring net-work, maximum hourly average O3concentrationsreached 83 ppb at Abbotsford in late afternoon. Nexthighest surface concentration was 73 ppb at Harris Roadat 1554 LST.Selected tethersonde profiles from Pitt Lake and Har-ris Road are shown in Fig. 5. (Note: due to a strongafternoon low-level jet, tethersonde operations were in-termittent and ultimately curtailed at both sites. Con-sequently, the time evolution of profile variables is notwell resolved for this case.) Early morning wind profilesfrom Pitt Lake (0635 LST, Fig. 5a) show downvalleyoutflow extending to approximately 700 m above thelake with strongest outflow immediately above the shal-low mixed layer (Fig. 5b). Ozone concentrations werelow at this time through the entire depth of the sounding(Fig. 5c) and suggest that the valley was flushed ofpollutants. This was most likely the result of the lateAPRIL 1998 399MCKENDRY ET AL.FIG. 4. Time–height plots showing isopleths of (a) the upvalleycomponent of wind speed (positive values are shaded and representupvalley flow), (b) O3(ppbv) concentration, and (c) potential tem-perature (K) based on tethersonde profiles from Little Goose Islandon Pitt Lake for 2 August 1993. Contours are based on data fromnine tethersonde flights giving 18 separate profiles.afternoon thunderstorm and associated strong outflowthe previous day (Fig. 2a). Winds switched to upvalleyflow in the Pitt Valley by 0730 LST and strengthenedto greater than 6 m s21by midafternoon (Fig. 5a, 1430LST). As there were no balloon operations at Pitt Lakein late morning, the manner in which O3concentrationsincreased is unclear. However, by 1430 LST, an ozoneconcentration of 111 ppb was observed at 300 m withinthe strong upvalley flow that halted tethersonde oper-ations. This was the highest concentration recorded inthe valley during Pacific ’93 and suggests that duringepisodes of elevated O3concentrations in the LFV, PittValley, and perhaps other tributary valleys may expe-rience the worst air quality in the region. MidafternoonO3profiles from Harris Road and Langley (Fig. 5c)show O3concentrations in the mixed layer of 60–70ppb, in broad agreement with concentrations at groundlevel but considerably less than those observed at LittleGoose Island. The extent to which the unusually highconcentrations observed in the tributary valley on thisday were physically reasonable is addressed in section3c below.3) 6 AUGUST 1993: FOLLOWING EPISODEOn 6 August, relatively cool conditions (low 20s de-grees Celsius) prevailed in the LFV after a coastallytrapped disturbance (bringing a low-level surge of coolmarine air) propagated northward through the region(McKendry et al. 1997). The combination of a completeseries of tethersonde observations from Harris Road anda late afternoon low-level aircraft mission across theLFV from White Rock northward across Pitt Lake (Fig.1) provided on this occasion an unprecedented view ofthe horizontal and vertical structure of pollutants andmeteorological variables within the valley system at theend of a pollutant episode.Temporal evolution of the vertical structure of wind,ozone, and potential temperature at the mouth of thePitt Valley (Harris Road) is shown in Fig. 6. Despite asurge of low-level southeasterly flow across the regionovernight, early morning winds at Harris Road showeda significant downvalley component (Fig. 6a). This flowpersisted above 500 m AGL until approximately middaybut was replaced by upvalley flow in a shallow layer(approximately 200 m deep) at the surface by 0730 LST.Within the downvalley flow, morning O3concentrationsabove 500 m exceeded 50 ppb. This suggests that thedownvalley flow was relatively polluted, most probablywith chemically aged air associated with photochemicalactivity the previous day (given that there are no sourcesof pollutants within Pitt Valley).Early afternoon winds (Fig. 6a) at Harris Roadshowed relatively strong upvalley flow near the surfaceoverlain by relatively light upvalley winds from 600 to800 m. The latter were associated with an inversion (Fig.6c) in which ozone concentrations reached 90 ppb in awell-defined elevated layer [origin of this layer is dis-cussed in detail in McKendry et al. (1997)]. Within thesame polluted layer, aircraft observations showed lowNO and NO2concentrations, suggesting that the layerwas chemically aged and had likely persisted from theprevious day (McKendry et al. 1997). Below this layer,O3concentrations reached 60 ppb in the moderatelystrong upvalley flow during midafternoon. The O3iso-pleths at this time (Fig. 6b) show concentrations in-creasing with height through the mixed layer, a patternattributed to vertical downmixing from the polluted lay-er above (McKendry et al. 1997). Injection of chemi-cally aged pollutants into the mixed layer from the layerabove probably contributed to the chemically aged sig-400 VOLUME 37JOURNAL OF APPLIED METEOROLOGYFIG. 5. Vertical profiles for 5 August 1993 of (a) upvalley wind component (U) at Little GooseIsland, Pitt Lake (PL), at 0635 and 1430 LST; (b) potential temperature (Q) at 0635 and 1430LST at PL and 1255 LST at Harris Road (HR); and (c) ozone concentration at 0635 and 1430LST at PL, 1548 LST at HR, and 1605 LST at Langley (L).nature (NOx/NOy) of the afternoon surface fast chem-istry measurements at Harris Road shown in Fig. 2.A late afternoon, low-altitude (500–600 m) flight intothe Pitt Valley (Fig. 7) complements the tethersondeobservations by showing the horizontal extent andchemical character of polluted air advected into the val-ley system late in the episode. Concentrations of bothaerosol and O3show an increase with distance into thevalley, a pattern frequently observed in the Pitt Valleyduring and subsequent to Pacific ’93. However, the sa-lient features of the transect in this case are the sharpincrease in both O3and aerosol concentrations at thesouth end of Pitt Lake, the associated sharp decrease inNO concentrations, and an initial drop, then increase,in NOyconcentrations. The same pattern was repeatedon the flight out of the valley at a significantly loweraltitude (400 m AGL, not shown here). This suggeststhat the aircraft remained within the mixed layer overPitt Lake and below the elevated layer shown in Fig.5. The abrupt transition at the south end of Pitt Lakesuggests a transition to a different air mass with differentinitial pollutant loadings. Given the late (midday) tran-sition to upvalley flow in the mixed layer apparent atHarris Road (Fig. 6a) together with the polluted natureof the morning outflow from the valley (Fig. 6b), it ispossible that the mixed layer to the north of the southend of Pitt Lake represented chemically aged air ad-vected into the valley the previous day, while pollutedair to the south was associated with the combination oflimited fresh photochemistry associated with the coolerconditions of 6 August and vertical downmixing fromthe chemically aged elevated layer.c. Mass budget calculations for 5 AugustObservations over Pitt Lake as described here forPacific ’93, and in subsequent field studies, suggest thatconcentrations of O3during daytime are usually higherover the tributary lakes than within the LFV at com-parable distances from the urban source. The first clearevidence of this pattern came from 5 August 1993 whenthe ozone concentration midway up Pitt Lake reached111 ppb, by far the highest concentration observed inthe entire region during Pacific ’93. The availability oftethersonde observations from Harris Road on this dayafforded the opportunity to carry out simple mass bud-get calculations (appendix) in order to 1) confirm thatwhat appeared to be unusually high O3values observedin Pitt Lake on 5 August were consistent with simplemass budget estimates and the observed chemistry ofair advected into the Pitt Valley and 2) given the knownlow O3deposition velocities during overwater trajec-tories (Galbally and Roy 1980), estimate the relativeroles of O3production and flux divergences in contrib-uting to the high O3concentrations.In the Lagrangian analysis, it was assumed that thesame column of air extending through the entire bound-ary layer observed by tethersonde at Harris Road atapproximately 1230 LST was observed 2 h later by thetethersonde ascent at the Pitt Lake site (25.5 km to thenortheast, assuming mean boundary layer winds of 4 ms21). Over the 2-h period, mean O3concentration in thePBL in the advected column increased from 70 to 100ppb, giving a mean change in concentration within thelayer of 30 ppb (or 15 ppb h21). In traveling betweenthe two sites it is estimated that 48 min was spent overland and 72 min over water. Furthermore, losses fromsurface deposition and entrainment at the top of theboundary layer were assumed to be distributed through-out the entire 600-m boundary layer.Assuming that the ozone concentrations in the columnof air traveling from Harris Road to Little Goose Islandincreased at 15 ppb h21, mass budget calculations (seeappendix) give an estimated flux divergence [due toeffects of entrainment at the top of the planetary bound-ary layer (PBL) and surface deposition] of 2.3 ppb h21,APRIL 1998 401MCKENDRY ET AL.FIG. 6. Time–height plots showing isopleths of (a) the upvalleycomponent of wind speed (positive values are shaded and representupvalley flow), (b) O3(ppbv) concentration, and (c) potential tem-perature (K) based on tethersonde profiles from Harris Road for 6August 1993.FIG. 7. NRC Convair aircraft observations of (a) dried aerosol (.0.2mm), (b) O3, (c) NOy, and (d) NO for the period 1614–1630 LST on6 August 1993 for a south to north flight from White Rock into PittValley Heights and landmarks are shown in (e). NO2measurementswere from an uncalibrated luminol instrument scaled to a slow responseNOymeasurement (Ecophysics) assuming NOywas all NO2.and by residual, a net photochemical production rate(Qs) of approximately 17.3 ppb h21. Chemical evidencefrom Harris Road, together with independent mass bud-get calculations (based on surface observations fromHarris Road and farther south) suggest that this estimateof Qsis reasonable (see appendix). Together, thesesources of evidence lend credibility to the rather highconcentrations observed over Pitt Lake on 5 August andsuggest that they were unlikely to result from significantmeasurement error.In addition to confirming the role of photochemicalproduction within the urban plume in contributing tohigh O3concentrations in the Pitt Valley, the Lagrangiananalysis suggests that relatively high O3concentrationsobserved within the tributary valleys can in part be at-tributed to low flux divergences during overlake trajec-tories. For a purely overland trajectory of the distancefrom Harris Road to Little Goose Island (25.5 km withflux divergence of 2.23 ng m23s21), the rate of changeof O3concentration (moving with the flow) is 13.3 ppbh21(a rate of 15 ppb h21was observed for the trajectoryover both land and water). For a solely overwater tra-jectory of 25.5 km (flux divergence estimated at 0.63ng m23s21), the O3concentration would increase by16.1 ppb h21, a rate of 2.8 ppb h21greater than for an402 VOLUME 37JOURNAL OF APPLIED METEOROLOGYoverland trajectory. This indicates that over the longfingerlike lakes that occupy the tributary valleys of theLFV (Fig. 1), low surface deposition velocities may beone factor contributing to observed O3concentrationsthat are higher than concentrations within the LFV at acomparable distance from the primary source of pol-lutants.4. Discussion and conclusionsUpvalley flows into tributary valleys of the broad LFVplay a significant role in the photochemical pollutioncycle of the region in general and in the air quality ofadjoining valleys in particular. The daytime circulationsdraw polluted air from the LFV into the smaller valleyswhere photochemical conversion processes continue toact on the pollutants. In this study, air quality and localdaytime circulations in a tributary to the LFV were de-scribed for 3 days with quite different meteorologicalcharacteristics (before, during, and immediately after anO3‘‘episode’’). On each of the afternoons, concentra-tions of O3within the remote Pitt Valley were observedto be as high as or higher than in the LFV at the sametime. Degraded air quality in the Pitt Valley is attributedprimarily to photochemical production occurring withinthe polluted air mass as it was advected by upvalleywinds from the source region in the western LFV and,to a lesser extent, by the reduction of depositional sinkactivity at the lake surface. Aircraft observations at theend of an episode also suggest that high concentrationsin the valley may be associated with chemically agedpollutants persisting from the previous day. A simpleanalysis of the O3mass budget within the Pitt Valleyon 5 August 1993 indicates that high concentrationsobserved over the lake were physically reasonable. Fur-thermore, the overwater trajectory of polluted air massesin the tributary valleys was found to be a minor factorcontributing to the observed high concentrations in thetributary valleys. This reduced depositional activity mayhelp explain the relatively high O3concentrations ob-served over lakes in the Swiss Alps (Wanner et al. 1993).In addition to qualitatively validating the modelingresults of Steyn and Miao (1995), these results showthat the tributary valleys of the LFV may play an im-portant role in the pollutant mass budget of the localregion. The LFV is connected to several other largertributary valleys including Stave Lake and the very largeHarrison Lake to the east. Together these large valleysmay process a significant volume of LFV air, producingexcess concentrations of O3during daytime but cleans-ing the outflow air of O3and other pollutants at night(Banta et al. 1997). Although even the sign of the neteffect over the diurnal cycle is in doubt, these processesmay play a significant role in the mass budget of pol-lutants in the region. Further observations and a moredetailed analysis of the processes contributing to thehigh concentrations in the valleys are needed as a pre-lude to modeling studies.Despite remaining uncertainties, significant impactson visibility and the local ecology of the forested wa-tersheds adjoining the LFV are likely as a result of thetransport and formation of photochemical oxidants intributary valleys. Further research is required to ascer-tain the spatial extent and magnitude of exposure toatmospheric oxidants in the region.Acknowledgments. This research was supported bythe Natural Sciences and Engineering Research Councilof Canada (NSERC), Atmospheric Environment Ser-vice, and the Canadian Institute for Research in At-mospheric Chemistry (CIRAC). We are grateful to Bob-by Downs, Kate Stephens, Magdalena Rucker, MarkusKellerhals, and John Pisano for assistance with tether-sonde operations. We also thank members of the lidarcrews, including Janet Intieri, Mike Hardesty, M. J. Post,Ron Willis, Lisa Olivier, and Cui-Juan Zhu for analysisof lidar data. The assistance provided by Don Hastieand Corina Arias is also greatly appreciated. Thanks toPaul Jance for the preparation of the figures.APPENDIXMass Budget CalculationsFor any atmospheric constituent, S (kg m23) assuminghorizontal homogeneity for the turbulent flux terms andnegligible mean vertical velocity, the budget is]S ]S ]S9w95 Q 2 u 2 , (A1)s]t ]x ]zwhere u is the mean wind aligned along the valley axis(in the x direction), w is the fluctuating component ofvertical velocity, z is the vertical coordinate, t is time,and Qsis the source or sink of S. For O3, the rate ofchange in a given volume at a fixed location (term 1)is a result of O3production or destruction within thevolume (term 2), the mean horizontal advection (term3), and the vertical flux divergence (term 4).In a Lagrangian frame of reference and assuming hor-izontal homogeneity in flux divergence, Eq. (A1) be-comesdS ]S9w95 Q 2 , (A2)sdt ]zwhere the rate of change of O3concentration (term 1)now refers to the changing concentration in a columnof fixed volume (unit cross section) moving with theflow.If the entire boundary layer is the volume consid-ered, flux divergence is calculated on the basis of fluxacross the top of the mixed layer and deposition of O3at the ground. The O3destruction rate at the groundFgis given byFg5 (O3),21rswhere rsis the surface resistance to O3uptake (GalballyAPRIL 1998 403MCKENDRY ET AL.TABLE A1. Estimates of parameters for mass budget calculations.In calculations it is assumed that 1 ppb O35 1.980 mgm23at 258C(Oke 1987), a good approximation in this context.ziwe(land)we(water)rs(land)rs(water)DO3U600 m0.02 m s210.01 m s21160sm211000 s m2120 mgm234ms21From tethersonde profileSteyn and Oke (1982)EstimatedGalbally and Roy (1980)Galbally and Roy (1980)From ozonesondeFrom tethersondeand Roy 1980). Previous studies (Galbally and Roy1980; Wesely et al. 1981) suggest that over grassland21rssurfaces during daytime in the midlatitude NorthernHemisphere is approximately 160 s m21, while overwater surface values are an order of magnitude high-21rser (approximately 1000 s m21). At the top of the bound-ary layer the flux of O3can be calculated byFt5 weDO3,where weis the entrainment velocity and DO3is thechange in O3concentration across the entrainment layer(Lenschow et al. 1981). Given typical entrainment ve-locities in the LFV of 0.02 m s21[Steyn and Oke (1982)calculated from dzi/dt, where ziis the height of themixed layer], then the flux divergence terms can beapproximated for the region over land and water.Parameters used in the mass budget calculations areshown in Table A1 and are based on best estimates fromthe literature or measured values for the region.Given these simple assumptions for the 2-h trajectoryand from (A2) the following is evident.R Rate of change of O3concentration in a fixed volumemoving with the flow from Harris Road to LittleGoose Island is given bydS23 21 215 8.25 ng m s 5 15 ppb h .dtR Amount of O3lost from the boundary layer acrossthe top of the mixed layer and to the surface bydeposition is estimated to be]S9w923 215 1.27 ng m s]z21 23 215 2.3 ppbh (2.23 ng m s over land,23 210.63 ng m s over water).R By residual [see Eq. (A2)], the net rate of pho-tochemical production of O3in the boundary layerin a fixed volume moving with the flow is there-foreQs5 9.52 ng m23s215 17.3 ppb h21.Chemical measurements at Harris Road on the earlyafternoon of 5 August provide important corroborativeevidence for net production rates estimated by the mod-el. First, relatively high concentrations of peroxy radi-cals (up to 20 pptv) and NO (up to 4 ppb) throughoutthe early afternoon suggest that the column of air con-sidered in the Lagrangian analysis was chemically activeand had a gross O3production rate on the order of 30ppb h21(D. Hastie 1996, personal communication). Netproduction rates of the magnitude estimated here (17.3ppb h21) would therefore not be unexpected when chem-ical destruction processes were considered. Second, thelocal rate of change of surface O3at Harris Road mea-sured by continuous monitor [the term on the left-handside of Eq. (A1)] was 9.7 ppb h21from 1230 to 1430LST. Given this value, Eq. (A1) can be solved inde-pendently for the net production rate (Qs) with advectioncalculated over the trajectory AB (Fig. 1) and the fluxdivergence over land estimated using parameters fromTable A1. With advection based on the observed averageearly afternoon O3gradient (AB) of 18 ppb, and a meanwind speed of 2.1 m s21(based on available surfacewind data in the vicinity of AB), Qsis 19.9 ppb h21.This value is in close agreement with Qscalculated usingEq. (A2) from Harris Road to Little Goose Island andsuggests that the Lagrangian analysis is consistent withobservations.REFERENCESBanta, R. M., L. D. Olivier, and D. H. Levinson, 1993: Evolution ofthe Monterey Bay sea-breeze layer as observed by pulsed Dopp-ler lidar. J. Atmos. Sci., 50, 3959–3982., and Coauthors, 1997: Nocturnal cleansing flows in a tributaryvalley. Atmos. Environ., 31, 2147–2162.Broder, B., and H. A. Gygax, 1985: The influence of locally inducedwind systems on the effectiveness of nocturnal dry depositionof O3. Atmos. Environ., 19, 1627–1637.Douglas, S. G., and R. C. Kessler, 1991: Analysis of airflow patternsin the south-central coast air basin during the SCCCAMP 1985intensive measurement periods. J. Appl. Meteor., 30, 607–631.Galbally, I. E., and C. R. Roy, 1980: Destruction of ozone at theEarth’s surface, Quart. J. Roy. Meteor. Soc., 106, 599–620.Hayden, K. L., and Coauthors, 1997: The vertical chemical and me-teorological structure of the boundary layer in the Lower FraserValley during Pacific ’93. Atmos. Environ., 31, 2089–2105.Kurita, H., K. Sasaki, and H. Muroga, 1985: Long-range transportof air pollution under light gradient wind conditions. J. ClimateAppl. Meteor., 24, 425–434.Lenschow, D. H., R. Pearson Jr., and B. B. Stankov, 1981: Estimatingthe O3in the boundary layer by use of aircraft measurements ofO3eddy flux and mean concentration. J. Geophys. Res., 86,7291–7297.Li, S.-M., K. G. Anlauf, H. A. Wiebe, J. W. Bottenheim, P. B. Shepson,and T. Biesenthal, 1997: Emission ratios and photochemical ef-ficiencies of nitrogen oxides, ketones and aldehydes in the LowerFraser Valley during the summer Pacific ’93 oxidant study. At-mos. Environ., 31, 2037–2048.McKendry, I. G., 1994: Synoptic circulation and summertime ground-level O3concentrations at Vancouver, British Columbia. J. Appl.Meteor., 33, 627–641., and Coauthors, 1997: Elevated O3layers and vertical down-mixing over the Lower Fraser Valley, B.C. Atmos. Environ., 31,2135–2146.Oke, T. R., 1987: Boundary Layer Climates. Methuen, 435 pp.Pottier, J. L., S. C. Pryor, and R. M. Banta, 1997: Synoptic variabilityrelated to boundary layer and surface features observed duringPacific ’93. Atmos. Environ., 31, 2163–2173.Pryor, S. C., I. G. McKendry, and D. G. Steyn, 1995: Synoptic-scale404 VOLUME 37JOURNAL OF APPLIED METEOROLOGYmeteorological variability and surface ozone concentrations inVancouver, British Columbia. J. Appl. Meteor., 34, 1824–1833.Seinfeld, J. H., 1989: Urban air pollution: State of the science. Sci-ence, 243, 745–752.Steyn, D. G., and T. R. Oke, 1982: The depth of the daytime mixedlayer at two coastal sites: A model and its validation. Bound.-Layer Meteor., 24, 161–180., and D. A. Faulkner, 1986: The climatology of sea breezes inthe Lower Fraser Valley, B. C. Climatol. Bull., 20, 21–39., and I. G. McKendry, 1988: Quantitative and qualitative eval-uation of a three-dimensional mesoscale numerical model sim-ulation of a sea breeze in complex terrain. Mon. Wea. Rev., 116,1914–1926., and Y. Miao, 1995: Mesometeorological modelling and trajec-tory analysis during an air pollution episode in the Lower FraserValley, B.C. Trans. Regional Photochemical Measurement andModelling Studies Conf., San Diego, CA, Air and Waste Manag.Assoc., 249–281., J. W. Bottenheim, and B. Thomson, 1997: Overview of tro-pospheric O3in the Lower Fraser Valley, and the Pacific ’93field study. Atmos. Environ., 31, 2025–2035.Wanner, H., T. Kunzle, U. Neu, B. Ihly, G. Baumbach, and B. Steis-slinger, 1993: On the dynamics of photochemical smog over theSwiss Middleland—Results of the first POLLUMET field ex-periment. Meteor. Atmos. Phys., 51, 117–138.Weseley, M. L., D. R. Cook, and R. M. Williams, 1981: Field mea-surement of small O3fluxes to snow, wet bare soil, and lakewater. Bound.-Layer Meteor., 20, 459–471.

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