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Lower Tropospheric Ozone Measurements by Light Aircraft Equipped with Chemiluminescent Sonde. McKendry, Ian G.; Steyn, Douw G.; O'Kane, S.; Zawar-Reza, P.; Heuff, D. 1998

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136 VOLUME 15JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGYq 1998 American Meteorological SocietyLower Tropospheric Ozone Measurements by Light Aircraft Equipped withChemiluminescent SondeI. G. MCKENDRY AND D. G. STEYNAtmospheric Science Programme, Department of Geography, University of British Columbia, Vancouver, British Columbia, CanadaS. O’KANELevelton Associates, Richmond, British Columbia, CanadaP. Z AWAR-REZADepartment of Geography, University of Canterbury, Christchurch, New ZealandD. HEUFFDepartment of Mathematics, University of Canterbury, Christchurch, New Zealand(Manuscript received 11 October 1996, in final form 30 May 1997)ABSTRACTNovel use of a commercial, battery-powered, chemiluminescent ozonesonde on a light aircraft is described.This fast-response instrument, originally designed for balloon deployment into the stratosphere, is light, inex-pensive, robust (reuseable), reliable, and accurate. Integration with other lightweight components (data logger,global positioning system, pressure, temperature, and humidity sensors) renders the system suitable for use ina light (rental) aircraft with no modification of the aircraft required. The system is well suited to routinereconnaissance and vertical profiling in regions of complex terrain, and with well-designed field studies, massbudget analyses are feasible. The application and validation of the system is described for the Lower FraserValley, British Columbia, a region of complex coastal terrain where photochemical smog is a significant problemin the summer months.1. IntroductionFull understanding of the air pollution meteorologyof any region can only be based on consideration of theentire lower troposphere where the processes of trans-port and dispersion take place. This has been highlightedin intensive field campaigns in regions where photo-chemical air pollution is a problem. For example, studiesin northeastern North America (e.g., Clarke and Ching1983), the Swiss Alps (Neu et al. 1994), the Los AngelesBasin (McElroy and Smith 1993), and Vancouver, Brit-ish Columbia (McKendry et al. 1997), all describe en-vironments characterized by horizontal transport of pre-cursors and photochemical oxidants to remote/rural sitesand vertical down-mixing of pollutants from residual orelevated layers. In such circumstances, data from stan-dard surface monitoring networks may misrepresent theCorresponding author address: Ian G. McKendry, Department ofGeography, University of British Columbia, #217-1984 West Mall,Vancouver, BC V6T 1Z2, Canada.E-mail: ian@geog.ubc.catrue vertical and spatial distribution of pollutants in aregion and, furthermore, are of limited use in developingand validating photochemical models used to developabatement strategies. Simple methods of spatial surveyand vertical profiling are therefore of utmost importancein defining the dimensions of regional photochemicalpollution problems (perhaps as a prelude to surface net-work design) as well as providing data for both devel-opment of parameterizations of vertical mixing pro-cesses and model validation.This paper describes application of a commercial,battery-powered, chemiluminescent sensor for aircraftmeasurements of ozone (O3) in the lower troposphere.This fast-response instrument, originally designed forballoon deployment into the stratosphere, is light,cheap, robust (reuseable), reliable, and accurate. Dataacquisition is by standard data logger and notebookcomputer. These qualities render the system suitablefor use in a light (rental) aircraft with no modificationof the aircraft required. Consequently, the system of-fers distinct advantages over other airborne systemsthat are often expensive and require large modifiedFEBRUARY 1998 137McKENDRY ET AL.aircraft. Hence, it is particularly attractive for both rou-tine reconnaissance and intensive research campaigns.In addition to describing the instrumentation, its re-sponse characteristics, installation on the aircraft, andpostflight corrections, validation of the method bycomparison with surface monitors and tethered ozo-nesondes will be presented. Finally, application of thesystem will be demonstrated in the context of the Low-er Fraser Valley (hereafter referred to as LFV), BritishColumbia, a region of complex coastal terrain wherephotochemical smog is a significant problem in thesummer months.2. Methodsa. Measurements of ozone in the lower troposphereSeveral techniques have been used to measure ozoneconcentrations above the earth’s surface. These includeground-based and airborne remote sensing using lasertechnology (Schiff et al. 1994), deployment of bulkycommercial UV photometers or chemiluminescent sen-sors (with some modification to account for pressureeffects) on dedicated research aircraft (Kondo et al.1987; Beekman et al. 1995), and vertical profiling usinglightweight electrochemical sensors supported by bal-loons (Pisano et al. 1997) and kites (Balsey et al. 1994).Ozone flux measurements have also been made fromaircraft over a variety of ecosystems (e.g., Ritter et al.1994) using chemiluminescent sensors based on the de-sign of Pearson and Stedman (1980). Low-cost, light-weight electrochemical sensors (e.g., the ECC, Brewer–Mast, and Atmospheric Instrumentation Research son-des) have been traditionally used for ozone soundings(Beekman et al. 1995). However, recently, a cheap,lightweight (0.8 kg) chemiluminescent instrument hasbecome available for balloon deployment in the tro-posphere and stratosphere (Schurath et al. 1991; Gu¨stenet al. 1992). It is this commercially available instrumentthat forms the basis of the airborne system describedhere.b. Ozone sensorSpecifications of the GFAS (Gesellschaft Fu¨r Angle-wandte Systemtechnik) OS-B-2 ozonesonde are de-scribed in Schurath et al. (1991) and Gu¨sten et al.(1992). The instrument is based on the principle of sur-face chemiluminescence due to the reaction of ozonewith an organic dye (Coumarin 47). The organic dye isincorporated into the outer layer of a replaceable, com-mercially available chemiluminescent target. The inten-sity of chemiluminescence is measured by a side-win-dow photomultiplier tube and converted into an analogoutput signal. A crucial component of the ozonesondeis a miniaturized fan that ensures sufficient flow rate tomaintain the instrument in a regime, whereby chemi-luminescent intensity is independent of flow rate and isproportional only to the absolute concentration of ozonein the sample (the ‘‘flow-independent’’ regime). Theinstrument has a specified accuracy of 65% and showsgood agreement with Brewer–Mast electrochemicalsondes in profile intercomparisons extending into thestratosphere (Speuser et al. 1989). Before deploymentof the ozonesonde, a ‘‘preozonization’’ procedure en-sures activation of the target and the maximum sensi-tivity of the instrument. During this procedure, the ozo-nesonde is exposed to concentrations of 100–150 ppbof ozone for 1–1.5 h from a custom-designed GFAS‘‘control unit’’ (ozone generator). Once preozonizationis completed, the instrument is calibrated (this requiresthe availability of a secondary standard ozone monitor)and the sonde is then ready for use. The lifetime of thechemiluminescent targets is approximately 6 months instorage and 2000 ppb h once activated. The instrumentitself is extremely robust and therefore, provided thattargets are replaced periodically, it may be used re-peatedly.c. Preflight calibration and postflight correctionsAfter preozonization the OS-B-2 is calibrated by mea-suring the output voltage at a known concentration (pro-duced by the ozone generator) and measured by refer-ence monitor.Once calibrated, the sensitivity of the OS-B-2 is de-pendent upon temperature, pressure, and humidity(Schurath et al. 1991). With respect to temperature, theOS-B-2 is equipped with a heater that maintains thetemperature of the chemiluminescent target at either 08C(preferable for profiles through the troposphere andstratosphere) or 308C (preferable for boundary layermeasurements). Provided that the instrument is cali-brated with heater at the 308C setting, no temperaturecorrections are required during flights within the lowertroposphere. Decreasing ambient pressure will lead tohigher sensitivity values for the instrument. The cor-rection factor Fpfor the pressure dependence of thesensitivity is given bypmeasF 5 1 2 0.63 log ,p12pcalwhere pcalis the ambient air pressure during calibrationand pmeasis the ambient pressure during subsequent mea-surements. For measurements within the planetaryboundary layer (PBL), Fpmay be of order 0.97, whichrepresents only a 3% change in sensitivity from cali-bration (usually conducted at the surface). Consequent-ly, it too can be neglected. Finally, the sensitivity of theOS-B-2 changes with gross changes in absolute humid-ity as found between the troposphere and stratosphere.However, for measurements in the PBL this effect isalso negligible. In summary, when deployed in the PBL,corrections to the OS-B-2 signal for pressure, temper-ature, and humidity effects are small and can be ne-138 VOLUME 15JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGYTABLE 1. Instruments required, their weight (kg), and approximate cost ($CDN). Note that costs are approximate only; an ozone monitoris required for calibration procedures, and temperature and humidity sensors may be added to the system.Instrument Weight (kg) Cost (approx, $CDN)OS-B-2 Ozonesonde—GFASVM-K-2 control unit (ozone generator)—GFASIntellisensor II pressure sensor—Model AIR-AB-2ACR10X Data LoggerPower supply—12-V gel cellGPS—e.g., Trimble SveeSix Series (including magnetic mount antenna,baud rate converter, interface cable)0.86.00.30.88.00.4200050005002000803000glected. Outside the PBL these effects become signifi-cant and must be corrected for.d. Airborne system configurationIn adapting the GFAS OS-B-2 ozonesonde to deploy-ment in a light aircraft (in this case, a single-engine Cess-na 172), the following critical concerns were studied.1) ENSURE AN UNCONTAMINATED SAMPLETo avoid mounting the system on the aircraft exterior(an advantage when using rental aircraft), air is fun-nelled into the instrument that is clamped to the rearof the cockpit via Teflon intake tubing extendedthrough the open cockpit window. The intake is at-tached to the left-hand side of the cockpit windowwithin the propwash and away from the aircraft exhauston the right-hand underside of the aircraft. This ensuresa well-mixed, uncontaminated sample. Comparisonswith independent measurements during flight are de-scribed below and suggest negligible contamination ofthe sample.2) MAINTAIN FLOW RATES IN THE FLOWINDEPENDENT REGIMEIn connecting the OS-B-2 to Teflon tubing, the effi-ciency of the miniaturized pump is severely compro-mised by the aerodynamic resistance of the tubing.However, during flight, air is forced through the tubinginto the instrument. To ensure adequate, but not exces-sive, flow rates through the instrument, the intake tubingwas arranged in a bypass mode that permitted the ozone-sonde to bleed off a portion of the flow into the aircraft.Wind tunnel tests with this configuration showed thatthe critical flow rate of 15 l min21, which is requiredto maintain the instrument in the flow-independent re-gime at lower tropospheric pressures was reached ataircraft speeds of 22 m s21, well below those reachedduring takeoff, vertical profiling maneuvers (35 m s21),and horizontal flight (50 m s21).3) ENSURE SUFFICIENT RESOLUTION AT AIR SPEEDSOF APPROXIMATELY 50 MS21With a specified time constant of 1 s for the OS-B-2, the aircraft in horizontal flight can resolve fluctua-tions on wavelengths of approximately 100 m. This isdeemed sufficient for the purposes discussed here (ba-sic aerial reconnaissance). For vertical profiling, whereaircraft vertical velocity is of the order of 3 m s21,fluctuations at wavelengths of approximately 6 m areresolvable.e. System components and costsIn addition to the OS-B-2 ozonesonde, the systemincludes components for data acquisition, positioning,and monitoring of related meteorological variables.These components are listed in Table 1 with approx-imate costs. A high-resolution digital barometer is usedfor pressure measurements that may be used to correctozone measurements (see above) as well as establishaircraft altitude. Air temperature is measured by therm-istor at both the air intake and within the cockpit. Air-craft location may be determined by reference to pre-established landmarks or by commercial global posi-tioning systems (GPSs). A GPS compatible with theCampbell Scientific CR10X data logger is listed in Ta-ble 1. All variables, including position data, are loggedand stored at 3-s intervals on the data logger. A laptopcomputer may be used during flights to monitor allvariables, although this is not necessary. With availablememory in the CR10X (128K) and with a 3-s samplingrate, flights may extend for 2.5 h without downloadingfrom data logger to computer. A 12-V gel cell batteryis used to power both the data logger and OS-B-2.3. Field validationDuring operations in the LFV, validation of aircraftmeasurements was based on two sources of independentdata.a. Comparisons with surface monitors andelectrochemical ozonesondesFigure 1 shows an ensemble of comparative mea-surements on occasions when the aircraft (at an altitudeFEBRUARY 1998 139McKENDRY ET AL.FIG. 1. Comparison of midboundary layer measurements with air-craft-mounted OS-B-2 and surface monitors directly below. Com-parisons with an Atmospheric Instrumentation Research electrochem-ical sonde flown on a tethered balloon at approximately the sametime, height, and location as the aircraft are also shown.FIG. 2. Simultaneous, collocated afternoon boundary layer profilesfrom a tethered Atmospheric Instrumentation Research electrochem-ical sonde and the slowly spiraling aircraft.of ;500 m) flew directly over surface monitors or pasta tethered ozonesonde (Atmospheric InstrumentationResearch Inc.) at the same elevation. These observa-tions are derived from 5 days, reflecting the range ofozone concentrations typically encountered in the LFV.With the exception of the labelled early morning flightson 13 August 1994, all observations are associated withan afternoon convective boundary layer. For afternoonconditions, agreement between surface monitors andthe airborne sensor is generally good (r25 0.98). Air-craft concentrations are generally higher than surfaceconcentrations due to the effects of surface deposition.This effect is most pronounced on the morning of 13August when, at the surface, concentrations were verylow (due to deposition overnight) compared to the rel-atively high concentrations persisting within the resid-ual mixed layer aloft overnight (McKendry et al.1997). As expected, the agreement between observa-tions at the same elevation (electrochemical ozone-sonde on tethered balloon versus aircraft) is good, withno apparent bias.b. Simultaneous profilesA more rigorous test of the accuracy, and ability ofthe OS-B-2 to resolve ozone gradients during flight, isprovided by simultaneous, collocated ozone profiles bytethersonde–ozonesonde and slowly spiraling aircraft.In the case shown in Fig. 2, the aircraft spiralled aroundthe tethersonde during midafternoon in typical non-episode conditions. In this case, the mean differencebetween instruments was small and clearly demonstratesthe ability of the airborne OS-B-2 to capture the fine-scale vertical ozone structure in the PBL (e.g., the in-crease in concentrations evident at 450 m).4. Application in the Lower Fraser Valleya. BackgroundThe LFV of southwestern British Columbia (Fig. 3a)is one of three areas in Canada where the National Am-bient Air Quality Objective for ozone of 82 ppb (hourly)is frequently exceeded during the summer months. Suchevents are usually associated with stagnating anticy-clonic situations when local thermotopographic circu-lations such as sea/land and mountain/valley winds arewell developed, and pollutants emanating from greaterVancouver (Fig. 3a) are generally transported eastward(McKendry 1994; Steyn and McKendry 1988). Recentstudies show that during such episodes, pollutants areoften transported beyond the existing surface monitor-ing network within the LFV (e.g., into tributary valleys)and may also become trapped in layers aloft where theymay persist or become mixed to ground (McKendry etal. 1997). Understanding the vertical and horizontal dis-tribution of ozone is therefore essential to impact as-sessment, modeling, and mass budget analyses. The fol-lowing examples demonstrate how an airborne sensormay be used to complement standard surface-basedmeasurements.140 VOLUME 15JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGYFIG. 3. Observations on 19 July 1995 in the LFV and tributary valleys. (a) Map showing flightpathand 1500 PST ozone isopleths (ppb) derived from surface observation network, (b) aircraft-derivedozone concentrations along the flight path, and (c) aircraft altitude during flight.b. Horizontal surveys and vertical profilingOn 19 July 1995, ozone concentrations in the centralLFV reached 72 ppb under warm, anticyclonic condi-tions. Moderate (4–5 m s21) west to southwesterly windsprevailed during the afternoon in the lower troposphereover the LFV, while southerly up-valley winds wereevident in the tributary valleys to the north of the LFV.Such thermally driven flows are typical of ozone epi-sodes in the LFV and are described in detail in Steynand McKendry (1988). In Fig. 3a, ozone isopleths at1500 PST, based on the surface observing network,show the typically observed pattern of low concentra-tions over the western LFV and relatively high concen-trations downwind of the greater Vancouver source re-gion. In the period 1200–1500 PST, the aircraft flightpath included three legs within the LFV and three legsin the complex, forested terrain to the north of the LFV.Leg BC was within the Pitt Valley, a valley known tosuffer from degraded air quality due to the northwardadvection of photochemically active air from the LFV(McKendry et al. 1997). At the northern end of PittLake, the aircraft turned back toward C and began anFEBRUARY 1998 141McKENDRY ET AL.FIG. 4. Vertical profile at end of Harrison Lake in the vicinity ofpoint D on 19 July 1995 during the flight shown in Fig. 4.ascent to approximately 2500 m (Fig. 3a). It then trav-eled northeast (leg CD) to the northern end of HarrisonLake, where it then descended back into the middleboundary layer and flew southward over Harrison Lake,making the first known observations of ozone in thislocality (leg DE). Finally, the aircraft completed thesortie by traveling westward along the length of the LFV(legs EF and FA).The northward flight leg AB showed mean ozone con-centrations of 53 ppb across the LFV (Fig. 3b). Con-centrations measured by surface monitor at the entranceto Pitt Valley at this time (;1200 PST) were about 51ppb, in good agreement with those observed by the air-craft. Within the Pitt Valley (leg BC), concentrationsrose steadily to a peak of 79 ppb at the major bend inthe lake and then declined again toward the northernend of the lake. This pattern has been observed in manyflights into the Pitt Valley and suggests that the tributaryvalleys of the LFV may have the poorest air quality inthe region. Possible mechanisms for the high concen-trations observed include the effects of reduced surfacedeposition over water and the role of local thermoto-pographic circulations (McKendry et al. 1997).As the aircraft climbed at the northern end of PittLake, concentrations dropped from values of approxi-mately 50 ppb in the PBL to 35–40 ppb. At 2000 m,as the aircraft headed toward Harrison Lake on leg CD,concentrations abruptly increased to 67 ppb and re-mained in the range 65–70 ppb until immediately priorto the descent into Harrison Lake at D, when they againdropped to 40 ppb.The aircraft descent at D demonstrates the profilingcapability of the system in complex terrain and alsosheds light on the unusually high concentrations ob-served at an altitude of approximately 2500 on leg CD.The profile of ozone near D (Fig. 4) shows a distinctpolluted layer from 2000–2250 m with concentrationstwice those above and below. As observed in other stud-ies of elevated layers in the LFV (McKendry et al. 1997)and the Los Angeles Basin (McElroy and Smith 1993),this layer of ozone aloft was associated with an inver-sion. If it is assumed that the layer was horizontallywidespread, a likely explanation for the variations inozone concentrations observed on leg CD is that theaircraft initially flew within the elevated layer. As itapproached D it then briefly climbed out of the layer.On descent at D, the aircraft then encountered the layeragain, giving the profile shown in Fig. 4.Layers of ozone at this altitude have been previouslyobserved over the LFV (McKendry et al. 1997), how-ever, the mechanisms responsible for their formation arenot clear. Possible causes include the venting of pol-lutants from the LFV into the lower troposphere alongheated slopes (the ‘‘chimney effect’’) and plumes frombiomass burning. The latter have been observed to havehigh ozone concentrations and travel considerable dis-tances (Kirchhoff and Marinho 1994). A careful windanalysis and more comprehensive chemical measure-ments in such layers are required to determine theirsource.As the flight continued southward (DE) within thePBL over Harrison Lake (Fig. 3a), concentrations re-mained near background levels (35–40 ppb) until theaircraft encountered a frontlike discontinuity in whichconcentrations increased abruptly to 70 ppb in pollutedair traveling northward past Long Island (Figs. 3a,b).Concentrations over the lake farther south exceeded 80ppb and then dropped to 60 ppb near Agassiz. This isthe first known observation of the apparent transport ofpolluted air emanating from the LFV up Harrison Lake,a distance of approximately 110 km from Greater Van-couver. As in the case of the Pitt Valley, concentrationsover the water of Harrison Lake were generally higherthan those observed in the LFV.On the return flight within the LFV (leg EF), observedconcentrations in the PBL between Agassiz and Chil-liwack were mostly in the range 70–80 ppb. From Chil-liwack to the vicinity of Boundary Bay Airport (leg FG),concentrations decreased to the 60–70 ppb range. Thesevalues appear quite consistent with the observedground-level concentrations at approximately the sametime (Fig. 3a).This example demonstrates the extent to which air-craft can usefully extend observations into remote ter-rain beyond the surface monitoring network. In so do-ing, previously undocumented aspects of the regionaldistribution of pollutants and potentially importantstructures (such as elevated layers) may be identified.142 VOLUME 15JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGYc. Mass budget studiesIn addition to basic surveys of the horizontal andvertical distribution of ozone as described above, well-designed aircraft observations using the OS-B-2 systemcan provide the basis for more fundamental researchsuch as mass budget studies. In the latter technique, asimple mass budget model in which the boundary layeris treated as a slab (Lenschow et al. 1981) can be usedto investigate the relative importance of photochemicalprocesses, advection, and vertical fluxes in contributingto observed concentrations. For any atmospheric con-stituent, S (kg m23) assuming horizontal homogeneityfor the mean and turbulent flux terms, and negligiblemean vertical velocity the budget is]S ]S ]S9w95 Q 2 u 2 ,s]t ]x ]zterms 1 2 3 4where u is the mean wind aligned along the x axis, z isthe vertical coordinate, and Qsis the source or sink ofS (Lenschow et al. 1981). Therefore, for ozone, the ratechange in a given volume at a fixed location (term 1)is comprised of ozone production or destruction withinthe volume (term 2), the mean horizontal advection(term 3), and the vertical flux divergence (term 4). Withthe exception of term 2, and if necessary with estimatesof entrainment and deposition velocities, all the termscan be measured using the aircraft system describedabove.Term 3 is easily calculated from ozone gradients overhorizontal legs (e.g., FA in Fig. 3a) and using availablewind data. With successive flights over the same flightleg, term 1 may also be estimated on the basis of themean difference between successive time series. Finally,if the entire boundary layer is the volume considered,then flux divergence (term 4) is calculated on the basisof flux across the top of the mixed layer and depositionof ozone at the ground. Entrainment at the top of theboundary can be estimated on the basis of profile dataderived from the aircraft (or tethered balloon), whiledeposition may be estimated on the basis of publishedor measured values.5. ConclusionsA lightweight, fast-response chemiluminescent sys-tem for continuous monitoring of ozone and other me-teorological variables from light aircraft has been de-scribed and tested in applications in the LFV, BritishColumbia. This system represents an efficient, afford-able, and versatile alternative to more commonly used,but expensive, research configurations. Althoughmounted in a light aircraft in this application, it couldbe used on a range of light platforms (microlight, hangglider, drone), thereby permitting observations in en-virons inaccessible to larger, fast research aircraft (e.g.,close to ground or slopes, and in complex terrain). Asshown in the LFV, the system is a viable alternative toother methods of ozone profiling and is also particularlyuseful for routine aerial surveys of ozone distributionand process studies (e.g., mass budget analyses). In thesingle case discussed here, the system provided valuableinsights into the horizontal and vertical distribution ofozone in the complex terrain surrounding the LFV. Fi-nally, the system as described has the potential to beexpanded to include other lightweight instruments. Forexample, Pisano et al. (1996) describe a lightweight NO2sensor that could be integrated with the system. Theaddition of other chemical sensors would permit morerigorous analysis of mechanisms contributing to, andsources of, observed structures.Acknowledgments. We are grateful to Laurence Armi,whose experience with a portable aircraft data acqui-sition system and generous advice provided a focus forthe present work. We are also grateful to Dr. WilfriedHans (GFAS) for advice on adapting the sonde to air-craft applications. The Greater Vancouver Regional Dis-trict generously provided surface data and a monitor forcalibration. Several students ably assisted in tethersondeoperations. This work was supported by grants from theNatural Sciences and Engineering Research Council ofCanada.REFERENCESBalsley, B. B., J. W. Birks, M. L. Jenson, K. G. Knapp, J. B. Williams,and G. W. Tyrrell, 1994: Ozone profiling using kites. Nature,369, 23.Beekman, M., and Coauthors, 1995: Intercomparison of troposphericozone profiles obtained by electrochemical sondes, a groundbased lidar and an airborne UV-photometer. Atmos. Environ., 29,1027–1042.Clarke, J. F., and J. K. S. Ching, 1983: Aircraft observations of re-gional transport of ozone in the northeastern United States. At-mos. Environ., 17, 1703–1712.Gregory, G. L., E. V. Browell, and L. S. Warren, 1988: Boundarylayer ozone: An airborne survey above the Amazon Basin. 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