Schauinsland Ozone Precursor Experiment (SLOPE96): Scientific Background and Main Results

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D1, PAGES 1553-1561, JANUARY 20, 2000

Schauinsland Ozone Precursor Experiment (SLOPE96)' Scientific background and main results

Andreas Volz-Thomas and Heiner Gei13 Institut ffir Chemie und Dynamik der Geosph•ireII, ForschungszentrumJfilich, Jfilich, Germany

Norbert Kalthoff Institut ffir Meteorologie und Klimaforschung,Forschungszentrum Karlsruhe, Karlsruhe, Germany

Abstract. The SchauinslandOzone PrecursorExperiment (SLOPE) was conductedin June 1996 as part of the former European Experimenton Transport and Transformation of EnvironmentallyRelevant Trace Constituentsin the TroposphereOver Europe (EUROTRAC) subprojectTropospheric Ozone Research(TOR) in order to studythe photochemicalformation of ozone and the degradationof the precursorsin the plume of the city of Freiburg on its way to Schauinsland.Chemical and meteorological measurementswere made at several surface sites in a valley that channelsthe airflow to Schauinsland,including vertical profilesat severallocations, and aboard a small aircraft. A tracer experimentwith SF6 was made in order to establishtransport and dispersionin the valley. Particular emphasiswas on the determinationof the OH concentrationfrom the cmdeca-• inofthe hydrocarbons presence ofduringhigh NOxtransport. concentrations The relatively(3-15highppb) OH indicates concentration thepotential of 7 xfor 106 unidentifiedprocesses in the HOx budget in the high-NOx regime which may have important consequencesfor the ozone budget and smogchemistry in the urban/suburban environment.

1. Introduction and thuslimits the productionof RO2 and HO2. The net effect of NO• on the radical balance and ozone formation depends The transition region between urban and rural areas is im- on the NO/NO: ratio, which is controlledby the UV radiation portant in understandingthe fast photochemistrythat leads to flux(JNo:) and by the concentrationsof peroxy radicals and the buildup of high concentrationsof ozone and other pho- 0 3. The radical chemistryin the high-NOx regime is further tooxidantsduring so-calledsummer smog episodes. While sev- complicated by the fact that photolysis of nitrous acid eral experimentshave investigatedthe budgets of ozone and (HONO), which is coemitted with NOx [Kesslerand Platt, radicals at low and moderate NO x concentrations,including 1984] and/or producedfrom NOx via heterogeneousreactions direct spectroscopicmeasurements of [OH] [e.g., Dom et al., [cf. Harrisonet al., 1996], is a potentiallyimportant sourceof 1988; Eisele et al., 1994; Mount and Williams, 1997; Holland et HOx in polluted air, in particular during the morning hours. al., 1998],little quantitativeexperimental information exists so In addition to chemistry, the local concentration of trace far on the ozone balance and the radical concentrations in the gasesis influencedby transport, dispersion,and deposition. high NO x regime. Which of the different processesdominates depends on the The OH concentrationand the production rate of ozone chemicallifetime of the speciesand on the meteorological both dependin a nonlinear fashionon the UV flux and on the conditions. While the concentrations of OH and other short- concentrationsof NOx and VOCs. The role of NO x in the lived radicalsare reasonablywell describedby the local chem- budgetsof HOx ([HOx] = [OH] + [HO2]) and ozone is am- ical fields,transport usually dominates the local rate of change bivalent. At low NOx concentrations,where recombination of of the longer-livedtrace gasessuch as ozone. For example,the the peroxy radicals constitutesthe major radical loss, [NO] daily ozone maximumwhich developsunder fair weather con- controlsthe recyclingof OH and is the rate-limiting factor in ditionsresults by approximately70% from downwardmixing of ozone production[Logan et al., 1981; Crutzen,1979]. At high ozone from the residual layer to the surfacelayer, while only NOx levels,on the other hand, radical lossesby recombination the remaining30% can be related to local chemicalformation of peroxy radicalsbecome insignificant,and HO2 is almost [Fiediet,1998]. Therefore informationabout the meteorologi- completelyrecycled to OH. In the presenceof sufficientNO, even more radicals can be produced from the hydrocarbon cal conditions like mixing layer height, stratification of the oxidation mechanismthan initially consumed,because of the boundary layer, and three-dimensionalstructure of the wind field is essential for the determination of the contributions of photolysisof the carbonylcompounds formed in the mecha- nism [Jeffriesand Tonnesen,1994; Jang et al., 1995]. This effect advection and diffusion in an experiment that aims at the is counteractedby the reaction of OH with NO:, which be- budgetsof trace gases.This is particularlythe casein complex comesthe predominantsink of HOx in the high-NO• regime terrain,where thermallyinduced periodic wind systems[Atkin- son, 1981; Whiteman,1990; Barry, 1992], like slope and valley Copyright2000 by the American GeophysicalUnion. winds, are important for the ventilation of the valley atmo- Paper number 1999JD900919. sphereand have a stronginfluence on the local changeof trace 0148-0227/00/1999JD 900919509.00 gases[e.g., Carroll and Baskerr,1979; Kurita et al., 1990;Millan,

1553 1554 VOLZ-THOMAS ET AL.: SLOPE96--SCIENTIFIC BACKGROUND AND MAIN RESULTS

1992; Bischoff-Gaufiet al., 1998]. In order to separatethe nent is much lesspronounced at Schauinsland,and the north- contributionsof transportand dispersionon the local concen- easterlycomponents are completelyabsent. Instead, there is a tration changesfrom the chemicaltransformation processes it strongpeak at northwesterlywinds (>40% duringdaytime), is advantageousto analyze the behavior of an artificial inert whichis dueto theup-valley wind system in the GrogesTal. tracer, in additionto measurementsof the chemicalcomposi- The northwestwind is, in general, associatedwith low wind tion of the air [e.g., Wilsonet al., 1983; Clementset al., 1989; speeds(90% <3 m s-1). At night,southeasterly wind direc- Allwine, 1993]. tionsare more frequent,and the northwesterlycomponent is With this in mind, and on the basisof earlier experiments, stronglyreduced. In winter (not shown),northwesterly winds the followingstrategy was selectedfor the SchauinslandOzone are rather seldom,mostly during frontal passages, and there is PrecursorExperiment (SLOPE96). The steepand narrowval- almostno differencebetween day and night. ley "GrogesTal" betweenthe cityof Freiburgand the Schauin- The chemicalsituation is illustratedby the averagediurnal slandobservatory was usedas approximationof a flow reactor variationsof the trace gasesin summerin Figure 3. Schauin- to study the chemicaland physicaldevelopment of the city slandis exposedto very clean air from southwesterlydirec- plume of Freiburgin a quasi-Lagrangianapproach by detailed tions.The nextlarger agglomeration in that directionis Lyon, meteorologicaland chemicalmeasurements, including a tracer about 300 km away. The data from the northwestsector show experimentwith SF6. The advantageof usinga valleyfor such a pronounced diurnal cycle, which is due to the fact that an experimentis the well-definedtransport path, whichmakes Schauinslandusually resides above the inversionat night.The it relativelyeasy to arrangethe measurementsystems, and that peak at noon is causedby the breakupof the inversionin the the exchangeof the transportedair masswith the surrounding morningand the establishmentof the valleywind systemin the environmentis restrictedto the vertical dimension.A partic- Groges Tal. The pronounceddiurnal variation in the north- ular advantageof the GrogesTal for a chemicalbudget study west sectoris a clear indicationfor the Rhine Valley beinga is its very thin population and the absenceof a transit road. strongsource for pollutantsand photooxidantsat Schauins- The details of the various measurements and the scientific land.As is exhibitedby the NOx andNO v datain Figure3, resultsare describedin subsequentmanuscripts of the special theseair masseshave been subjectedto relativelyfresh emis- section.This paper briefly outlines the historyand the layoutof sions,for example,from the Rhine Valley and the city of SLOPE96 and summarizes the main results and conclusions. Freiburg,leading to fairly large concentrationsof ozonein the courseof the day. The regular appearanceof the northwesterlywinds at 2. History of SLOPE Schauinslandled to the idea of utilizingthe valleyGroges Tal The field station Schauinsland(47ø54'N, 7ø48'E; 1230 m as an approximationof a flow reactor.A first experimentwas abovesea level (asl) [l/olz-Thomaset al., 1997])was operated conductedin September 1992 with ground-basedmeasure- between1989 and 1996as part of the EuropeanExperiment on mentsmade in Kappel, a smallvillage at the entranceof the Transport and Transformationof EnvironmentallyRelevant valley, and at Schauinsland(see Figure 4). Both siteswere Trace Constituentsin the TroposphereOver Europe (EURO- equippedwith instrumentation for NO, NOx,NOy, 03, JNo2, TRAC) subprojectTropospheric Ozone Research(TOR) and C2-C8 hydrocarbonsand meteorologicalparameters [Kleyet al., 1997].It is locatedin the southernpart of the Black [Krampand Volz-Thomas, 1997]. Forest mountains(Figure 1) on a saddleabout 400 m eastof Similar to the approachesused by, for example,Calvert the Schauinslandsummit (1284 m asl). The area around the [1976],Roberts et al. [1984],Satsumabayashi et al. [1992],Blake stationis characterizedby a generaldecline within about 10 km et al. [1993],Jobsonet al. [1994],and McKenna et al. [1995],the to the Rhine Valley in the west (250 m asl). The hills and OH concentrationwas estimated by simultaneouslyintegrating valleyssouth, west, and northwestof the stationare mostly the continuityequation (1) for a family of reactivehydrocar- coveredwith coniferousand deciduousforest. The remaining bons RH/, partsare coveredwith grasslands,interrupted by smalltree and bush stands. The site is shielded from direct influence of local O[RH,] = PR.,- LR.,- uV[RH,] - V(KV[RH,]) (1) traffic by a ridge. The road to the observatoryis closedfor c3t publictraffic for the last 1.5 km. The nearestsettlement (some 100 inhabitants)is about2 km away,and the nextmajor cityis where O/Otis the local rate of change,P and L are the local Freiburg (120,000inhabitants) in the Rhine Valley, about 11 productionand lossterms, and uV[RHi] and V[KV[RHi] ) is km northwest. Toward northwest the terrain declines with a the flux divergencedue to advectionand turbulence. slopeof more than 20% into the narrowvalley Groges Tal, in As hasbeen discussedin severalpublications [McKeen and which the SLOPE experimenttook place. Liu, 1993; McKenna, 1997; Kramp and l/olz-Thomas, 1997; Figure 2 showsthe summertimewind rosesfor Schauinsland Ehhalt et al., 1998],the successfulapplication of the so-called and for Feldberg, the highest mountain of the Black Forest hydrocarbonclock relies on the fulfillmentof severalassump- (1486m asl,about 10 km southeastof Schauinsland).Although tions:First of all, it is assumedthat the reactionwith OH (R1) not trulyrepresentative of the geostrophicflow, Feldberg is not constitutesthe major chemicalloss process for the hydrocarbons considered. muchinfluenced by local topographyand thusshould be rep- resentativeof the wind field in the upper planetaryboundary (R1) RH + OH + 02 --> RO2 layer (PBL) in the region,whereas the wind field at the TOR stationSchauinsland is stronglyinfluenced by localtopography In this sense,it must be emphasizedthat the result doesnot and the vicinityof the Rhine Valley. constitutea positiveidentification of OH in the atmosphere While the wind rose of Feldbergshows the expectedmaxi- but rather relies on the wisdom about the role of OH as the mum at southwesterlies(35.6% between230 ø and 190ø) with majoroxidant in the sunlitatmosphere [Levy, 1971]. The chem- secondarymaxima in the northeast,the southwesterlycompo- ical lossesare then givenby VOLZ-THOMAS ET AL.' SLOPE96--SCIENTIFIC BACKGROUND AND MAIN RESULTS 1555

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Figure 1. Map of the Upper Rhine Valley and the Black Forest mountainswith the locationof Schauinsland and the city of Freiburg.The insertgives an enlargedview of the area aroundFreiburg and the Zarten Basin, with the Groges Tal marked by the solid line.

L •H, = [ RHi] X [ OH] X k ]i (2) andafter integration, with k•i being the rate coefficientsfor (R1), which must be knownfrom laboratorymeasurements. Second, chemical productionand emissionsof the consideredhydrocarbons during ln([RHi]t2/[RHi]tl) =- fat [OH] Xku dt - •a t kd dt transportbetween the two measurementsmust be negligible. Last but not least, (1) can only be integrated if the dilution -[OH] x k]i X At -- kd X At (3) term is equal for all hydrocarbons, such that the term V (KV[RHi]) canbe approximatedby a first-orderloss process According to (3) the average OH concentrationcan be esti- with a dilution coefficientk d. With these assumptionsand mated from the slopeof a semilogarithmicplot of the ratios of after transformationinto Lagrangiancoordinates, we obtain the individualhydrocarbons versus the respectiverate coeffi- cients(k•i) with OH. The ratios[RHi]t2/[RHi]t• are formed d[RH,] from concentrationpairs that are measuredat Kappel at the = [RHi] X ([OH] x J•li-I'-J•d) (1') dt time t l and at Schauinslandat the time t 2. Precondition for 1556 VOLZ-THOMAS ET AL.: SLOPE96--SCIENTIFIC BACKGROUND AND MAIN RESULTS

day Schauinsland night o Feldberg o

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Figure 2. Wind rose at Schauinslandand Feldbergduring summer: (left) day and (right) night. determinationof the OH concentrationfrom the slopeis the Kappel and Schauinslandand by minimizingthe overallvari- knowledgeof the transporttime At = t 2 - t I betweenthe two ance in the semilogarithmicregressions made for all sets points of measurement.The first-orderdilution term, which is [RHi]tl+/Xt/[RHi]tl in the time intervalwhen the valleywind equivalentto a volumetricdilution factor (kdAt = In systempersisted [Kramp and Volz-Thomas,1997]. Ft:)), is derivedfrom the intercept with the ordinate. In 1992 In view of the large NOx mixingratio in the investigatedair the transporttime was estimatedfrom the localwind speedat mass(70 ppb at Kappel and 15 ppb at $chauinsland),the OH concentration(5-8 x 106 moleculescm -3) obtainedfrom the hydrocarbondecay was surprisinglylarge. An analysisof the radicalbudget showed that the large OH wasnot sustainedby NW-sector SW-sector 70 the primary sources,even when including an estimate for

_ 65 -- 03 (ppb) 65 HONO photolysis.As a possibleexplanation, it waspostulated 60 60 that amplificationprocesses during the oxidationof hydrocar-

55 55 bonsmay lead to the productionof more HOx radicals(in the

5O form of HO2) than originallyconsumed in the reactionof the

45 45 hydrocarbonswith OH. In order to close the budget it was

40 calculatedthat on the basisof the measuredhydrocarbons 2 HO 2 radicalshad to be formed in the courseof the degrada- -- NOy(ppb) 9 tion of an initial RO 2 radical.Such a large amplificationfactor 6 seemedunreasonable, and it was arguedthat biogenicVOCs other than isoprene,which were not measuredin 1992,might have contributedto the HOx recyclingand radical amplifica- tion, in additionto the radicalproduction from the ozonolysis -- NOx(ppb) 4 of olefinicVOCs, for example,terpenes. -- NO (ppb) Uncertaintiesin the 1992 experimentwere associatedwith 3 the assumptionsabout the generaltransport path, the trans- 2 2 port time, and the influenceof mixingprocesses, in particular 1 after total developmentof the boundary layer. In order to . 3OO • 5 •',d•"', ? ,'-:'T",," 7 ? r t-=,:• •--,- 300 reducethese uncertainties, tracer experimentswith SF6 were

250 conductedin the followingyears. The result of the preexperi- -- co (ppb) _ 250 mentsconfirmed the assumptionthat the transportof polluted 2OO 200 air from Freiburgto Schauinslandindeed proceededthrough the valleyGroges Tal. Transporttime and dilutionfactor were 150 150 found to be similar to those derived from the experimentin 1992. 0 3 6 9 12 15 18 21 0 0 3 6 9 12 15 18 21 0 On the basis of these encouragingresults, SLOPE96 was Time(CET) Time(CET) planned and conductedin June 1996 with two intensivemea- Figure3. Averagediurnal variation of 03, NOy, NOx, and suringcampaigns (June 4-6 and 27). The different research CO in summer for the two main wind sectors observed at groupsand measurementsystems that participatedin the cam- Schauinsland. paign are listed in Table 1, and Figure 4 gives a three- VOLZ-THOMAS ET AL.: SLOPE96--SCIENTIFIC BACKGROUND AND MAIN RESULTS 1557

Figure 4. Three-dimensionalmap of the Grof3esTal and partsof the Zarten Basinwith the measurement sitesduring SLOPE96 (for explanationof the sites,see Table 1).

dimensionalview of the valley with the measurementsites. calculationswere carriedout to supportthe interpretationof Detailed ground-basedchemical measurements were made at the meteorologicalphenomena and the behaviorof the tracer. Kappel and Schauinsland.A relativelydense meteorological The analysisconcentrated on the datafrom June5, 1996,when surfacenetwork was installedto analyze the developmentof clear air conditionsled to the developmentof necessaryup- the valleywind systemsin the differentvalleys. Vertical profiles valley winds, a prerequisite for the transport path from of ozone,NO2, and meteorologicalparameters were obtained Freiburgvia the Gro13esTal to Schauinsland. with remotesensing techniques (four SODARs and two ozone LIDARs) and from three tethered balloons.Airborne measurementsof hydrocarbons,ozone, NO2, and meteorological 3. Summary of the Publications on SLOPE96 datawere made aboarda smallairplane. An inert tracer (SF6) B. Kolahgaret al. (unpublishedmanuscript, 1999) describe wasreleased into the city plume of Freiburgto obtain quanti- the experimentalmethods used for determinationof VOCs at tative information on advection and dispersion,and model the two surface sites and aboard the small aircraft. At each site 1558 VOLZ-THOMAS ET AL.' SLOPE96--SCIENTIFIC BACKGROUND AND MAIN RESULTS

Table 1. Participantsin SLOPE96 and Descriptionof the MeasurementsSites Shown in Figure 4

Symbol Height, (Figure 4) m asl MeasuringSystem Institution Acronym

A 1230 TOR station Schauinsland ForschungszentrumJfilich--Institut ffir Chemie ICG2 (chemistry,meteorology) und Dynamik der Geosphare(ICG2) B 1060 meteorologicaltower Uni Paderborn UPB F 980 SODAR ForschungszentrumKarlsruhe--Institut ffir IMK Meteorologieund Klimaforschung G 995 meteorologicaltower Uni Freiburg GeologischesInstitut MIF C 980 meteorologicaltower Uni Paderborn UPB D 830 ozone-LIDAR, SODAR ForschungszentrumGeesthacht GKSS D 830 tetheredballoon (03, ForschungszentrumKarlsruhe--Institut ffir IMK meteorology) Meteorologieund Klimaforschung E 630 IFU measuringcontainer Fraunhoferinstitutffir Atmosph•irische IFU Umweltforschung H 495 tethered balloon Uni Freiburg GeologischesInstitut MIF (meteorology) I 392 launchsite for Zeppelin (03, Uni Stuttgart--Institut ffir Statik und Dynamik IVD meteorology) der Luft- und Raumfahrt J 385 tetheredballoon (03, NO2, Uni Stuttgart--Institut ffir Verfahrenstechnik IVD meteorology) und Dampfkesselwesen K 354 mobile laboratory(chemistry, ForschungszentrumJ•lich--ICG2 ICG2 meteorology) K meteorologicaltower, SODAR ForschungszentrumJfilich•ASS, Universit•it ASS KOln, MeteorologischesInstitut MIK K ozone-LIDAR Max-Planck-InstitutHamburg MPI L 330 SODAR Uni Freiburg GeologischesInstitut MIF M 327 radio sonde ForschungszentrumKarlsruhe•Institut ffir IMK Meteorologieund Klimaforschung X 327 SF6 release sites ForschungszentrumJfilich--ASS ASS ß SF6-samplingsites ForschungszentrumJfilich--ASS ASS

a self-built gas chromatographic(GC) systemwith cryogenic NOy compoundsis well predictedby the steadystate model preconcentrationwas operated in combinationwith a commer- when dry depositionfor HNO 3 is includedwith a first-order cial GC (Airmotec HC1010) that achievesa better time reso- lossterm corresponding to a depositionvelocity of 2-3 cms-•. lution (20 min) but suffersfrom coelutionand memoryeffects The local ozone productionrate as estimatedat Schauinsland for some compounds.Another Airmotec GC was operated from the photostationarystate of NOx showsa positivecorre- aboardthe aircraft(time resolution10 min). Measurementsof lationwith the product of UV radiation(JNo2) and precursor (C8-C•s) VOCs were made at the surfacesites by GC-MS on concentrations(NOx) with maximumvalues around 60 ppb samplescollected on charcoaladsorption tubes. The data qual- h -1. ity was explored on the basis of an instrumentcomparison M. M611mann-Coerset al. (unpublishedmanuscript, 1999) using calibration standardsand concurrentmeasurements in describethe planningand conductionof the tracerexperiment. ambient air. Particular emphasisof the comparisonwas to The optimumrelease sites were found in preexperimentscon- identify those compoundsthat were measuredby the three ducted between 1993 and 1995 and with the aid of model Airmotec instrumentswith a comparabilityof better than 10% simulationswith the KarlsruheAtmospheric Mesoscale Model to be used for the determination of OH. (KAMM) [Fiedler, 1993]. From simultaneousSF 6 and CO The measurementsof all systemswere combinedin order to measurementsmade in the samplesit is concludedthat an- obtain an estimate of the total VOC reactivityand the VOC/ thropogenicemissions in the GrogesTal are of negligibleim- NOx ratio. During the time when the city plume of Freiburg portancefor the concentrationsin the city plume of Freiburg. was transportedthrough the Groges Tal, about 50% of the Kalthoffet al. [this issue]discuss the resultsof the meteoro- VOC reactivitywas due to biogenicVOCs. The latter included logical measurementsand the tracer experiment. The data a relatively large fraction from terpenesand from oxygenated from meteorologicalsurface sites and vertical soundingsare compounds. used to analyzethe temporal developmentof the valley wind Piitz et al. [this issue] discussthe ground-basedmeasure- systemand the mixed layer heightwithin the different valleys. ments of inorganiccompounds and photolysisrates, and the The data showthat from 0900 to 1200 central European time chemical measurements made on the tethered balloons and (CET) up-valleywind systemsexisted in the GrogesTal and in aboardthe aircraft.Again, an instrumentcomparison was con- the Zarten Basin,leading to the transportof polluted air from ducted which was used for harmonization of the data sets for Freiburg via the Zarten Basin and the Groges Tal to the ozone and nitrogenoxides. Additional checkswere made using Schauinslandmountain. Around midday, however, a larger- situations in which the aircraft was in the vicinity of the scalewesterly flow above the Black Forest hills disturbedthe Schauinsland station and of the tethered balloon. up-valleywind systemin the GrogesTal. This wasof relevance From the measurementsof NO, NO2, NOy, PAN, and for the interpretationof the temporalbehavior of SF6 and NOx HNO 3 at Schauinsland,an estimate of the noontime OH con- at Schauinsland. centrationof 1 x 107cm -3 is derivedwith a simplequasi From the tracer, SF6,which was releasedbetween 0900 and steady state model. The temporal behavior of the different 1030 CET at three sitesin the Zarten Basin,the transporttime VOLZ-THOMAS ET AL.: SLOPE96--SCIENTIFIC BACKGROUND AND MAIN RESULTS 1559

1000- 000- 900- 900 - B00- B00- Marxenhof 700- Kappel 700- 600 - 600- 500- 500- 400- 400- 300- 300- 200- 200- 1 00- 100-

0 ' I ' I 0 ' I ' I ' I ' ! ' I ' I 8 9 11 12 13 14 8 9 10 tl 12 1,3 CET CET

1000- 1000- 900- 900-

800- B00- 700- Kohlengrund 700- Schauinsland 600- 600- 500- •- 500- 400- 400 - 300- 300- 200- 200- I n-}ll•$ur•rn•nts 100- simu lation 0 I ' BI ' I ' L--I ' _ I _ ' I 0 ' I ' I -•••'"-' I ' I ' I - ' I 9 10 11 12 13 14 B 9 10 11 12 13 14 CET CET

Figure 5. Comparisonof the measuredSF 6 concentrationsin the Groges Tal with the model simulations (Marxenhof:site E, Kohlengrund:site D) [Fledletet al., this issue]. and dilutionrate wascalculated (Figure 5). At the entranceof the formation of up-slopewinds on the Black Forest slopesof the Groges Tal the peak of 942 ppt appearedbetween 0930 the Rhine Valley. In the course of the morning, these winds and 1000 CET. At Schauinslandthe SF6 peak of 79 ppt was reached the edge of the Black Forest slopes and penetrated detected between 1100 and 1130 CET. The transport time farther eastward into the Black Forest. As a result of this betweenKappel and Schauinslandas derivedby fitting a mod- penetration of the larger-scalewesterly flow, the transport of ified Gauss function to the observedSF 6 concentrationswas air massesdid no longer take placevia the valley wind systems 90 _+_5 min, corresponding to a propagationrate of 1.5m s-•. from Freiburg via the Zarten Basin and the Groges Tal to The SF6 concentrationwas reducedduring transportbetween Schauinsland,but directly from the Rhine Valley over the the two sitesby approximatelya factor of 10. Black Forest slopesto Schauinsland. The NOx peak appearedbetween 0900 and 1030 CET at the l/olz-Thornasand Kolahgar[this issue]utilized the data ob- entranceof the Groges Tal in coincidencewith the SF6 peak. tained on June 5, 1996, for estimation of the average OH At Schauinsland,however, the NOx data revealedtwo peaks.A concentrationduring transportbetween Kappel and Schauin- first but minor peak was detectedbefore 1200 CET, together sland.With experimentalinformation about the chemicalcom- with the SF6 peak. The major NO• peak appearedmuch later position of the backgroundair from the airborne VOC mea- at about 1300 CET. This was causedby the changein wind surementsand the dilution factor from SF6, equation (3) was directionat 1200 CET, which led to advectionof polluted air expandedusing a simple mixing scheme: from the Rhine Valley. For this reason, only the data before 1200 CET could be used for calculation of OH concentrations from the hydrocarbondecay during transport in the Groges In f x [RHi]K-Jr-f'x [RH,]B ) = -k•, x [OH]x At (4) Tal. [RHi]s, [RHi]K, and [RHi] B are the mixingratios as measured Fledlet et al. [this issue]present model calculations,which at Schauinsland(t2) , at Kappel (t•), and in the air above supportthe observedmeteorological phenomena and the be- Kappel (t•). At is the transporttime betweenKappel (t•) and havior of the tracer in the investigatedarea. From the calcu- Schauinsland(t2) , and f = 0.1 is the reciprocal dilution lationsit is concludedthat the decreasein the SF6 concentra- factor. Both quantitiesare obtained from the SF6 maxima at tions in the Groges Tal resulted from flow splitting of the Kappel and Schauinsland(see above),and f' = (1 - f) is the airflowinto a sidevalley of the GrogesTal, up-slopewinds and fraction at which the backgroundair is admixed to the plume. mountainventing over the mountain ridges, and mixed layer The resultingOH concentrationof 7 x 106cm -3 is 30% growth during the transportof the tracer from the entranceof lower than that obtained without backgroundcorrection, that the GrogesTal to the Schauinsland(Figure 5). The observed is, usingequation (3), still almosta factor of 2 larger than what disturbanceof the valleywind in the GrogesTal at middaywas is calculatedwith a chemicalbox model constraintby the mea- causedby the formation of a larger-scalewesterly flow due to sured trace gas concentrationsand photolysisrates. The radi- 1560 VOLZ-THOMAS ET AL.' SLOPE96--SCIENTIFIC BACKGROUND AND MAIN RESULTS

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FritzPeak93 ii ß•ß • •' ß', • . . ,

..... o i i i i ii i i i i i i i i i i i i i i i i i i o,1 10 1()0 mixing ratio (ppb)

Figure 6. OH concentrationsas measuredat different locationswith different techniquesas a function of the prevailingNO 2 mixingratio (see Volz-Thomasand Kolahgar[this issue] for details). cal budgetcan be closedwithin the experimentaluncertainties, large influenceon the total VOC reactivityin the valley,even when an upper limit is adopted for the photolysisof nitrous at timeswhen the city plume passes. acid, in addition to the radical productionfrom photolysisof The tracer experimentand the interpretationof the meteo- ozone,H202, and carbonylcompounds as well as ozonolysisof rologicaldata also allowed to confirm the validity of the as- unsaturatedVOCs. BiogenicVOCs contributesignificantly to sumptionsmade for the estimationof [OH] from the experi- the recyclingof HOx and represent,on average,about 50% to ment in 1992. It seems therefore as if OH concentrations in the the total VOC reactivity measured at the surfacesites. The high-NOx regime might indeed be substantiallylarger than resultsfrom SLOPE96, in particularthe analysisof the tracer what is currentlyassumed. experiment,confirm the assumptionsmade in the analysisof With appropriatenesting (from 5 km to 250 m grid spacing), the earlier experimentconducted in 1992,when noontimeOH the transportmodel KAMM is capableof describingthe flow concentrationsof (6-8) x 106cm -3 werederived in thepres- and dispersionof the tracer in the narrowvalley, including the ence of NOx mixingratios between70 ppb at the entranceof observedasymmetry perpendicular to the valleyaxis. The com- the valley and 15 ppb at Schauinsland. prehensivedata set collected during SLOPE96 provides a A comparisonof the results obtained from SLOPE with uniqueopportunity for the testingof chemicaltransport mod- direct measurementsfrom different studiesis shownin Figure els. Future work will include modeling studiesto investigate 6. As discussedby Volz-Thomasand Kolahgar[this issue],the the potential causesfor the large OH concentrationsin the data qualitatively exhibit the theoretically expecteddepen- high-NOxregime. dence of [OH] on the NO 2 mixing ratio with a maximum around 1 ppb of NO2, but seemto give larger OH concentra- Acknowledgments.We would like to expressour thanks to the tions in the high-NOx regime than what is currentlybelieved. numerousinvestigators and coworkersinvolved in SLOPE96 for their enthusiasm.Particular thanksare due to thosewho have activelycon- tributed to the scientificinterpretation and the publicationsin this 4. Conclusions specialsection. We gratefullyacknowledge the supportthat was pro- The valley Groges Tal betweenFreiburg and Schauinsland vided by the local authoritiesof the city of Freiburg and the village Kappel, the staff of the Umweltbundesamtat Schauinsland,and the was used as an approximationof a chemicalflow reactor in inhabitantsof the Groges Tal. Many thanks also to Gerhard Smiatek order to studychemistry and transportin the city plume of of the FraunhoferInstitut in Garmisch-Partenkirchen,who kindlypro- Freiburg. Transport and dispersionwas quantified from a vided us with data for the map in Figure 1, and to SabineJansen for tracer experimentand detailedmeteorological measurements, her expert assistancein data handling and analysis.The work was includingvertical profilesat six different locations. supportedby the German ResearchMinister (BMBF) as part of EU- ROTRAC (contract07 EU 723/5A) and TFS (contract07 TFS 31/ Relatively large concentrationsof hydroxylradicals are es- i-i^.3). timatedfrom the decayof reactivehydrocarbons in the plume, which may point to as yet unknown processes.The radical budgetcan be closed,however, within the experimentaluncer- References tainties if sourcesof HOx from the ozonolysisof alkenesand Allwine, K. J., Atmosphericdispersion and tracerventilation in a deep the photolysisof carbonylcompounds, H202, and HONO are mountain valley,J. Appl. Meteorol.,32, 1017-1037, 1993. included at their maximum rates allowed from the measure- Atkinson, B. W., Meso-ScaleAtmospheric Circulations, 495 pp., Aca- ments or other estimates.Biogenic emissions seem to have a demic,San Diego, Calif., 1981. VOLZ-THOMAS ET AL.: SLOPE96--SCIENTIFIC BACKGROUND AND MAIN RESULTS 1561

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