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