* Corresponding author. Atmospheric Environment 33 (1999) 3843}3857 Nonmethane hydrocarbons (NMHC) in the Greater Munich Area/Germany B. Rappenglu K ck*, P. Fabian Lehrstuhl fu K r Bioklimatologie und Immissionsforschung, Ludwig-Maximilians-Universita K t Mu K nchen, Am Hochanger 13, D-85354 Freising-Weihenstephan, Germany Received 11 May 1998; received in revised form 10 November 1998; accepted 11 November 1998 Abstract During several "eld campaigns in the years 1993}1997 quasi-continuous measurements of NMHC data were obtained at various locations (urban/suburban/rural) within the Greater Munich Area (GMA) by means of on-line gaschromato- graphic methods. Though limited to NMHC between C 6 and C 9 it comprises the "rst comprehensive data base for this region that features high temporal resolution. The results for the downtown area show relatively low NMHC values compared to other cities worldwide. Propene-eqivalent analysis suggests that aromatic compounds such as toluene and m & p-xylenes play a major role in the formation of urban photochemical smog in the GMA. Since aromatic compounds were found to be ubiquitous at all measurement sites (altogether 8 sites) the pattern of these NMHC were investigated thoroughly. The results suggest that aromatic compounds are most e!ective in the urban/rural transition zone where VOC-limitation of ozone formation can be expected. ( 1999 Elsevier Science Ltd. All rights reserved. Keywords: Photochemical smog; Urban atmosphere; Aromatic compounds; Gaschromatography 1. Introduction For decades now an increasing number of cities world- wide have been facing severe pollution problems during summertime. It is well known that apart from nitrogen oxides nonmethane hydrocarbons (NMHC) are driving forces in the photochemical formation of secondary pol- lutants such as ozone and PAN (Chameides et al., 1992; Bowman and Seinfeld, 1994). In urban areas a variety of anthropogenic sources lead to an enhancement of hydro- carbon levels in ambient air (Nelson et al., 1983; Seinfeld, 1989; Field et al., 1992) with tra$c being a major source in most cases (Bailey et al., 1990). In "eld campaigns on-line measurements of NMHC are scarce (e.g. see Der- went et al., 1995; Rappenglu K ck et al., 1998), especially investigations concerning diurnal variations of NMHC patterns within a greater urban area. However, on-line hydrocarbon measurements gain growing importance in air quality networks as demonstrated by the UK Hydro- carbon Network (1998) or the BTEX monitoring sites operated by the Swiss Protection Agency BUWAL. The metropolitan area of Munich has a well-estab- lished record of ambient air quality. Since 1974 a net- work of air monitoring sites that is run by the Bavarian Environment Protection Agency (BayLfU) exists throughout the city. Since then peak ozone values of up to 150 ppbv have been reported (Georgii and Neuber, 1986). Recent investigations included measurements of PAN and PPN : PAN reached maximum values of 6 ppbv (Rappenglu K ck et al., 1993; Kourtidis et al., 1993) and in some cases even more than 10 ppbv (Jakobi, 1994). 1352-2310/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 3 9 4 - X
Transcript
*Corresponding author.
Atmospheric Environment 33 (1999) 3843}3857
Nonmethane hydrocarbons (NMHC) in the GreaterMunich Area/Germany
B. RappengluK ck*, P. Fabian
Lehrstuhl fuK r Bioklimatologie und Immissionsforschung, Ludwig-Maximilians-UniversitaK t MuK nchen,Am Hochanger 13, D-85354 Freising-Weihenstephan, Germany
Received 11 May 1998; received in revised form 10 November 1998; accepted 11 November 1998
Abstract
During several "eld campaigns in the years 1993}1997 quasi-continuous measurements of NMHC data were obtainedat various locations (urban/suburban/rural) within the Greater Munich Area (GMA) by means of on-line gaschromato-graphic methods. Though limited to NMHC between C
6and C
9it comprises the "rst comprehensive data base for this
region that features high temporal resolution. The results for the downtown area show relatively low NMHC valuescompared to other cities worldwide. Propene-eqivalent analysis suggests that aromatic compounds such as toluene andm & p-xylenes play a major role in the formation of urban photochemical smog in the GMA. Since aromatic compoundswere found to be ubiquitous at all measurement sites (altogether 8 sites) the pattern of these NMHC were investigatedthoroughly. The results suggest that aromatic compounds are most e!ective in the urban/rural transition zone whereVOC-limitation of ozone formation can be expected. ( 1999 Elsevier Science Ltd. All rights reserved.
For decades now an increasing number of cities world-wide have been facing severe pollution problems duringsummertime. It is well known that apart from nitrogenoxides nonmethane hydrocarbons (NMHC) are drivingforces in the photochemical formation of secondary pol-lutants such as ozone and PAN (Chameides et al., 1992;Bowman and Seinfeld, 1994). In urban areas a variety ofanthropogenic sources lead to an enhancement of hydro-carbon levels in ambient air (Nelson et al., 1983; Seinfeld,1989; Field et al., 1992) with tra$c being a major sourcein most cases (Bailey et al., 1990). In "eld campaignson-line measurements of NMHC are scarce (e.g. see Der-
went et al., 1995; RappengluK ck et al., 1998), especiallyinvestigations concerning diurnal variations of NMHCpatterns within a greater urban area. However, on-linehydrocarbon measurements gain growing importance inair quality networks as demonstrated by the UK Hydro-carbon Network (1998) or the BTEX monitoring sitesoperated by the Swiss Protection Agency BUWAL.
The metropolitan area of Munich has a well-estab-lished record of ambient air quality. Since 1974 a net-work of air monitoring sites that is run by the BavarianEnvironment Protection Agency (BayLfU) existsthroughout the city. Since then peak ozone values of upto 150 ppbv have been reported (Georgii and Neuber,1986). Recent investigations included measurementsof PAN and PPN :PAN reached maximum values of6 ppbv (RappengluK ck et al., 1993; Kourtidis et al., 1993)and in some cases even more than 10 ppbv (Jakobi, 1994).
1352-2310/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved.PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 3 9 4 - X
PPN yielded 0.37 ppbv (Kourtidis et al., 1993). Thoughdata for total NMHC are available for some downtownsites dating back as far as 1978 it was not until 1986 that"rst canister measurements for speci"ed NMHC analysiswere taken in the Munich area (Kreuzig et al., 1986).These measurements were sporadic and not representa-tive since they did not cover either di!erent urban airquality regimes or seasonal beahaviour. Pilot quasi-con-tinous measurements of speci"ed NMHC were launchedby the BayLfU in 1991. The only measurement site waslocated close to a strongly frequented road. Though ofhigh technical quality the results are only representativefor a point site heavily impacted by tra$c sources. The"rst quasi-continuous NMHC measurements at morerepresentative sites in the Munich area have been re-ported by RappengluK ck (1994 and 1995).
Here we will give a comprehensive survey of several "eldcampaigns that were carried out by the Lehrstuhl fuK rBioklimatologie und Immissionsforschung (LfBIM) atvarious locations within the Greater Munich Area (GMA)during the years 1993}1997. These campaigns focused ontemporal high-resolved NMHC data using on-line gas-chromatographic (GC) techniques. Due to the large num-ber of samples on a round-the-clock and day-to-day basisthe results obtained are representative in statistical terms.In addition, this paper emphasizes diurnal and seasonalNMHC patterns using key species that were found to beubiquitous in the area such as aromatic compounds.
2. Experimental setup
Fig. 1 shows a map with all measurement sites in theGMA. All sites represented well-mixed open air condi-tions away from major anthropogenic sources of primarypollutants. Data was obtained at each site for at leastseveral weeks. Measurement periods were mainly carriedout from April until October (for individual measure-ment periods see Table 4). Only the rural site EIT workedthroughout the year.
At the sites EIT, MIM, WHS and MOHP SiemensRGC 402 gaschromatographic (GC) systems were instal-led. These GC devices have already been described indetail elsewhere (RappengluK ck et al., 1998). At all otherlocations a mobile station was used containing the com-mercially available portable GC Airmotec HC1010.Thorough descriptions are given in RappengluK ck andFabian, 1998.
3. Results and discussion
3.1. General results
Tables 1 and 2 give a summary of NMHC-data andthe speci"c NMHC pattern obtained at the centre site
UMW sorted by their median volume mixing ratio. Theresults were obtained during summer and spring time,respectively. Though these data sets only cover a limitedNMHC range due to technical restrictions of the Air-motec HC1010 some important features may be discern-ed. In all cases the aromatic compounds toluene, benzeneand m & p-xylene are the predominant species based ona volume mixing ratio basis. To a large extent summervalues are higher than in spring time, although tra$cemissions are expected to be much lower during vacationseason in August than in April. Possible reasons for thisdi!erent behaviour may be attributed to di!erent prevail-ing weather conditions. Weather in April is marked byadvective processes. Rapid changes of the general me-teorological situations resulting in higher wind velocitiesand well-mixed boundary layer conditions are predomi-nant. Higher temperatures during summertime in turnfavour the evaporation of NMHC. Along with con-vective weather patterns charcterized by slight diurnalvariations in terms of wind speed and wind directionaccumulation of primary species in the boundary layeris enhanced. In the Munich area a local wind ciculationcan develop during high pressure situations (BruK ndl etal., 1987). It is a weak mountain}valley breeze triggeredby the close location of the Munich area to the NorthernAlpine ranges.
3.2. Comparison with other cities
Compared to NMHC-results obtained in some othercities worldwide the Munich data set comprises mediumrange values. Key compounds are listed in Table 3. Onthe whole the burden of NMHC in Munich ambient airmay be best compared to suburban conditions in theGreater Athens area. Though median values are slightlylower in the suburban area of Athens, maximum valuesoften exceed the Munich results. Some species, however,show higher values in Munich. This applies for somexylene isomers and for methylcyclopentane. In Munichmaximum values of m & p-xylene may even surpassmaximum values of benzene and even reach toluenelevels, e.g. in August 1993. Median values are calculatedtaking all measurements (day and night/advective andconvective situations) into account, whereas maximumvalues tend to occur during stagnant weather patterns.Median values are much more representative for the totalcontribution of all NMHC sources to the mean NMHCmixture in an urban area. In this case a NMHC patterncomprising speci"c NMHC ratios can be found that istypical for any urban area. Maximum values along withNMHC ratios calculated for this speci"c situation, how-ever, may reveal local or speci"c emission sources,respectively. In Athens BTEX ratios are about thesame regardless whether median or maximum values areconsidered. In Munich, however, m & p-xylenes gainimportance in terms of their proportion within the BTEX
3844 B. Rappenglu( ck, P. Fabian / Atmospheric Environment 33 (1999) 3843}3857
Fig. 1. Map of all measurement sites. City sites: UMW: Umweltschutzreferat der Landeshauptstadt MuK nchen, MIM: MeteorologischesInstitut der UniversitaK t MuK nchen (roof level, about 30 m above ground), IHF: Institut fuK r Holzforschung der UniversitaK t MuK nchen,suburban city site: MPE: MuK nchen-Perlach, outer GMA: EBE: Ebersberberger Forst, WHS: Weihenstephan (Lehrstuhl fuK r Biok-limatologie und Immissionsforschung), MOHP: Meteorologisches Observatorium Hohenpei{enberg des Deutschen Wetterdienstes(DWD), EIT: Eitting.
B. Rappenglu( ck, P. Fabian / Atmospheric Environment 33 (1999) 3843}3857 3845
Table 1NMHC-results obtained at UMW}III (12.08.}26.08.1993)
!Number of data above the detection limit.n.d.: Not detected.
compounds when maximum mixing ratio values aretaken into account (i.e. under high-pollution scenarios).As stated above these species reach maximum values inthe order of toluene maximum values. These e!ects occurespecially during the summer period (UMW}III). Cor-responding values for early springtime (UMW}II) showless variations. Coincidentally also higher toluene/ben-zene ratios are observed in summertime (1.9 in August1993 contrary to 1.6 in April 1994). According to Nelsonet al. (1983) besides tra$c related emissions p & m-xylenes and toluene are also emitted through the useof solvents. Benzene, however, is usually not found insigni"cant amounts in solvent emissions. Evaporation ofsolvents, most e!ective at higher temperatures duringsummertime, may be responsible for this phenomenon.There are some indications in favour of this suggestion,although unambiguous source strengths considerationscannot be established based solely on our observeddata. According to investigations carried out by the
Bavarian environmental protection agency (BayerischesLandesamt fuK r Umweltschutz) about 17.2 kt NMHC areemitted by tra$c, whereas industrial releases of NMHCare about 4.0 kt annually in the GMA (BayLfU, 1992).Inventory data for benzene and toluene show that 1.05 ktbenzene and 1.05 kt toluene annually are tra$c relatedemissions. Yearly industrial emissions account only for0.02 kt for benzene and 0.17 kt for toluene, but 0.24 kt forxylene isomers. 90.2% of aromatic compounds are emit-ted by varnish facilities. Observations of aromatic com-pounds obtained at MIM site for late springtime arelisted in Table 4. This site is located at the roof level ofMunich. At this level total emissions of solvents in anurban area should be detected most adequately sincecontrary to ground-based tra$c sources solvents mayalso be released at higher levels of a building and in largequantities through chimneys on the rooftops. In fact inMunich this urban layer shows on one hand } as can beexpected } overall lower BTEX-values compared to the
3846 B. Rappenglu( ck, P. Fabian / Atmospheric Environment 33 (1999) 3843}3857
Table 2NMHC-results obtained at UMW}II (08.04.}02.05.1994)
!Number of data above the detection limit.n.d.: Not detected.
ground-based site UMW, but at the same time the por-tion of m & p-xylenes increases. The toluene/benzeneratio is 2.0.
Table 3 also reports results of 3-methylpentane andn-hexane for UMW}II and UMW}III. These species arephotochemically more reactive than benzene: OH-reac-tion rates for benzene, 3-methylpentane and n-hexane are1.23]10~12 cm3 molecules~1 s~1, 5.70]10~12 cm3
molecules~1 s~1, and 5.61]10~12 cm3 molecules~1 s~1,respectively (Atkinson, 1990). As a consequence, lowerambient urban values for the ratios 3-methylpentane/benzene and n-hexane/benzene are expected during sum-mertime when photochemical processes prevail. How-ever, in August 1993, vacation peak time in the GMAwhen tra$c reaches its annual minimum, the compounds3-methylpentane and n-hexane show both higher abso-lute and relative values compared to benzene than inApril 1994. According to Nelson et al. (1983) about 50%of urban 3-methylpentane and 43.3% of n-hexane levels
originate from fuel evaporation processes. With respectto our observations this enrichment of 3-methylpentaneand n-hexane is suggested to mainly derive from en-hanced gasoline evaporation during summertime.
As a consequence it must be concluded that in theGMA increasing gasoline evaporation during summer-time is an important NMHC source. However, it maynot be ruled out that during summertime also NMHCoriginating from solvent releases form an important frac-tion of the overall burden of NMHC in the GMA, inparticular during high pollution episodes, i.e. stagnantweather conditions. Often this coincides with photo-chemical active periods during summertime. However, itis essential to note that in general downtown NMHCmixing ratios tend to be low compared to other cities. Yetit must be thoroughly investigated how fast-reacting xy-lene isomers behave photochemically on the backgroundof low total NMHC values. These species may be impor-tant key compounds in the formation of ozone and PAN.
B. Rappenglu( ck, P. Fabian / Atmospheric Environment 33 (1999) 3843}3857 3847
Table 3Some key-NMHC obtained at various sites worldwide
B T E m & p-X o-X 3-Me-P Me-c-P Me-c-H n-H 1-H(ppbv) (ppbv) (ppbv) (ppbv) (ppbv) (ppbv) (ppbv) (ppbv) (ppbv) (ppbv)
! according to Chameides et al. (1992). Time of sampling 11 : 00}14 : 00 local time." according to Colbeck and Harrison (1985). Average values are given.# according to Sexton and Westberg (1984). Time of sampling 6 : 00}9 : 00 local time.$ according to Blake et al. (1993). VOC-values of one sample. Time of sampling unknown.% according to Derwent et al. (1995). Weekday mean concentrations during summer based on hourly on-line VOC-measurements.& according to Arendt and Pruggmayer (1982). Annual average values based on approx. 40 24-h samplings. First value obtained at
a suburban site, second value obtained at a downtown site.' according to Nelson et al. (1983). VOC-values of one sample. Time of sampling unknown.) according to Singh et al. (1985). Average values are given. In brackets maximum values.*according to Singh et al. (1992). Average values are given. In brackets maximum values.+ according to RappengluK ck et al. (1998). Median values are given. In brackets maximum values., this study (springtime: UMW}II data; summer: UMW}III data). Median values are given. In brackets maximum values.
3.3. NMHC ranking according to photochemicalimportance
In order to establish a NMHC ranking in terms oftheir possible photochemical signi"cance for the Municharea the concept of propene-equivalents as outlined byChameides et al. (1992) was applied. Though NMHCdata base did not include NMHC(C
6and therefore
presumably important ozone precursors such as etheneor propene had to be omitted this concept can be used atleast as a "rst guess tool to identify important species in
the data set available. Whenever possible, data for reac-tion rates were taken from Atkinson (1990). In cases ofcoelution, reaction rates were calculated as an averagevalue from the corresponding individual reaction rates.The results are given in Fig. 2. It clearly demontrates thesigni"cance of aromatic compounds: Among the "vemost important species are m & p-xylene, toluene andmesitylene. As described in detail elsewhere (Rappen-gluK ck et al., 1998) the aromatic compounds benzene,toluene, ethylbenzene, m & p-xylene and o-xylene(BTEX-compounds) comprise a very unique NMHC
3848 B. Rappenglu( ck, P. Fabian / Atmospheric Environment 33 (1999) 3843}3857
Table 4BTEX-results from all GMA measurement sites and periods. Di!erent periods at the same site are denoted by Roman numbers. Firstlines indicates median values, second lines maximum values (in brackets) and third lines list the number of data above the detection limit
Site Benzene Toluene Ethylbenzene m & p-xylene o-xylene(ppbv) (ppbv) (ppbv) (ppbv) (ppbv)
City sites1993 UMW 3.0 5.7 1.2 2.9 1.1(12.08}26.08.) III (14.9) (32.1) (9.5) (31.5) (10.1)
class. Fig. 2 demonstrates well the di!erences within theBTEX group as far as the photochemical signi"cance ofeach compound is concerned. By far, m & p-xylenes arethe most important species presumably second only toethene in urban areas. On the far end of the NMHCranking list there is benzene, a compound that is ofrelatively low importance in terms of photochemical ac-
ticity. Though found in considerable amounts in ambienturban air at times its low reaction rates makes it aNMHC specie of almost marginal importance in urban airat least as far as its photochemical relevance is concerned.However, benzene remains a hazardous compound dueto its well-documented cancerogenic potentials. In airquality studies there is often no comprehensive NMHC
B. Rappenglu( ck, P. Fabian / Atmospheric Environment 33 (1999) 3843}3857 3849
Fig. 2. Ranking of NMHC according to their photochemical signi"cance expressed as propene equivalents. NMHC-data was obtainedat UMW}II (08.04.}02.05.1994).
data set from C2}C
15with high temporal resolution
for extended measurement periods available. However,Fig. 2 demonstrates that focusing on BTEX-measure-ments alone will already provide a deep insight intoanthropogenic related NMHC patterns at di!erent loca-tions and under di!erent meteorological conditions.
3.4. BTEX-structures in the Greater Munich Area
In Table 4 all results of BTEX measurements made atthe various locations during the period 1993 } 1997 arelisted. Measurement site EIT was continuously operatingfrom February 1993 until July 1994. For comparisonreasons statistical data from EIT was taken for the total
period running from April 1993 until May 1994. FromTable 4 it is evident that BTEX-compounds are ubiqui-tous in the GMA. The only sites with very low values areEBE}I and MOHP. EBE}I is situated in a clearing of anextended forested area. In 1995 BTEX values were gener-ally lower than in 1993 at the same site due to poorweather conditions in August 1995 that did not allowprimary compounds to accumulate: Low-pressure sys-tems prevailed with low temperatures and rainfall. Inaddition, due to higher wind velocities atmosphericboundary conditions were well mixed. Please note thatduring EBE}I fewer samplings than during the precedingcampaigns were taken. The reason for this are longersampling cycles (1 h) since this campaign also focused onconcurrent terpene measurements. MOHP is located at
3850 B. Rappenglu( ck, P. Fabian / Atmospheric Environment 33 (1999) 3843}3857
the top of an isolated mountain (approx. 1.000 m M.S.L.).Often this top is above the inversion layer and is subjectto free tropospheric air. MOHP is a designated GlobalAtmospheric Watch (GAW) site operated by the Deut-scher Wetterdienst (DWD). As can be seen from theBTEX results the main feature of this site apart from lowmedian values are low maximum values. Obviously, nei-ther local sources nor transport processes lead to en-hanced BTEX-levels.
Table 4 consists of three sections according to di!erentgeographic locations within the GMA. The city sites aremarked by the highest BTEX-median values. In mostcases they are well above 1 ppbv. Maximum values mayreach up to 37.0 ppbv for toluene, for instance. Thesuburban city sites (MPE}I and MPE}II, rspectively)show median values below 1 ppbv, but at times is subjectto high BTEX-values. This happens when transport fromthe inner urban area occurs, predominantly during night-time hours (see RappengluK ck and Fabian, 1997). With theexception of EBE}I and MOHP, the data series of theouter GMA sites exhibit similar structures like the subur-ban sites. Though median values are generally well below1 ppbv, maximum values around 10 ppbv may be ob-served sometimes at EBE}III, WHS and EIT. These sitesare located northeast and east of the urban area ofMunich, respectively. Impacts of the urban plume underprevailing westerly and southwesterly winds must be heldresponsible for these phenomena as has recently beendemonstrated for the EIT site (RappengluK ck and Fabian,1999).
Within the BTEX-compounds m & p-xylene can beselected as a key compound in discriminating di!erentlocations within the GMA. This compound does nothave biogenic sources. As outlined above it is a fastreacting compound during daytime. Thus di!erences be-tween urban, suburban and rural sites should be signi"-cant. From Table 3 a general rule can be formulated: cityareas show median values for m & p-xylene above 1 ppb(even on the average urban roof level, as can be seen fromthe results obtained at MIM), at suburban city sitescorresponding values are at least 0.5 ppbv, whereas in theouter GMA m & p-xylene median values are below0.5 ppbv.
3.5. Diurnal variations of toluene
Though m & p-xylene median values appear to be anadequate quantity for site discrimination, this approachfails when diurnal variations are considered. At ruralsites xylene isomers are often depleted during daytimemaking it impossible to calculate mean diurnal vari-ations based on a minimum data set of 5 samplings per30 min above detection limit. In order to compare diur-nal patterns of various sites it is necessary to use toluene.Fig. 3 gives a survey of di!erent toluene diurnal struc-tures obtained in the GMA. Generally speaking, diurnal
patterns re#ect emissions, transport and dilution, andchemical removal. The emission of anthropogenic com-pounds will mostly follow tra$c patterns, while trans-port and dilution are in#uenced by the synoptic weathercirculation, and the spatial and temporal variation of themixing height. Since data sets usually comprise severalweeks signi"cant impacts of di!erent mixing hights canbe excluded. Chemical removal of BTEX will only occurthrough reaction with the OH radical during daytime.Obviously the toluene pattern at the downtown sites ofMunich is mainly controlled by the diurnal variation oftra$c, since the pronounced peaks occur during themorning and evening rush hours. This is still true forMIM site at the average urban roof level. The onlydi!erence to ground-based urban measurement sites arethe overall lower toluene values throughout the day. Thisis mainly due to vertical dilution. At the suburban sitesMPE}I and MPE}II, respectively, daytime maximumvalues usually occur after 18 : 00. As outlined in Rappen-gluK ck and Fabian (1997) this is due to local wind circula-tions that lead to a transport from the inner urban areasto the suburb areas at this time of the day. At WHS,a rural town site, the diurnal feature resembles thoseobserved in the urban area of Munich. Again, rush hourtimes can be discerned. However, usually daytime valuesat WHS are signi"cantly lower than nighttime valuesbetween midnight and 6 : 00. At the Munich sites thesevalues do not di!er a lot. The diurnal toluene pattern atthe rural sites are very uniform. Obviously no localsources contribute to the diurnal variation. However,enhanced levels of toluene at EBE}III in the eveninghours may be attributed to the impact of the urbanplume indicated by lower values for the benzene/tolueneratio (see Table 4) than during the other measurementperiods at the same site. This ratio is about the same asobserved at MPE}I and indicates the in#uence of tra$cemissions. Since EBE}III is located east of MPE}I sim-ilar impact of the urban plume as described previouslyfor MPE}I and MPE}II, respectively, can be expected.Please note that the toluene pattern at EIT di!ers signi"-cantly from all other sites. Toluene maximum values areexclusively observed during nighttime hours, althoughno toluene speci"c source is present at this site. Shortlyafter 6 : 00 in the morning toluene mixing ratios start todecrease. No typical daytime variations of sources (traf-"c/industrial releases) can be seen from the diurnal vari-ation. This diurnal variation is partly due to the impactof the nocturnal boundary layer. However, contrary tothe other rural sites in Southern Bavaria that only showslight diurnal variations, diurnal variations of toluene atEIT show di!erences between nighttime and daytimevalues that are sometimes even stronger than at theurban site UMW. Maximum values are always observedduring early morning hours. However, rush hour peaksas observed at UMW are not present, tra$c as a signi"-cant nearby emission source can be excluded at EIT.
B. Rappenglu( ck, P. Fabian / Atmospheric Environment 33 (1999) 3843}3857 3851
Fig. 3. Mean diurnal variation of toluene at all sites within GMA: (I) denotes city sites, (II) suburban sites of Munich along with WHS asan example for a rural town, and (III) rural sites. Time period of measurement campaigns: UMW}I: 18.10.}29.10.1993, UMW}II:08.04.}02.05.1994, UMW}III: 12.08.}26.08.1993, IHF: 30.09.}10.10.1993, MIM: 28.04.}24.05.1994, MPE}I: 26.05.}27.07.1995, MPE}II:28.09.}10.10.1995, WHS: 04.07.}25.07.1996, EBE}I: 27.07.}26.09.1995, EBE}II: 01.04.}18.04.1993, EBE}III: 29.05.}10.08.1993, EIT:April 1993}May 1994, MOHP: 06.05.}19.05.1997.
Additional investigations as presented in RappengluK ckand Fabian (1999) show that at EIT ambient air qualityis likely to be in#uenced by transport and/or dilutionfrom the surrounding region and, under particular me-terological conditions, by the superimposed transportfrom the Munich area. Thus at nighttime EIT site oftenexperiences the photochemically una!ected urban plumeof Munich, while at daytime EIT is subject to alreadyphotochemically processed air masses. These speci"c fea-ture is also demonstrated in the varying BTEX ratiosduring daytime (e.g. see Fig. 8).
3.6. Impact of photochemical processes on BTEXcompounds
In order to determine the impact of photochemicalprocesses on BTEX-compounds it is necessary toconsider speci"c BTEX-ratios. In our investigations weselected the benzene/toluene ratio (b/t) and the ethylb-enzene/m & p-xylene ratio (e/m & p-x). Whenever GC-analysis made it possible we also included the ethylben-zene/m-xylene ratio (e/m-x) (sites WHS, EIT and MOHPwhere the Siemens RGC 402 was installed). Figs. 4}6
show a compilation of the results. From Fig. 4 it isobvious that BTEX-ratios in downtown urban areas areprimarily controlled by emission processes. In the courseof the day they are rather constant. Only at UMW}Isome disturbances of this feature appear at early morninghours. This happens neither at UMW}II nor duringUMW}III. No conclusive reason for this behaviour hasbeen found so far. Fig. 5 depicts results from the subur-ban measurement site MPE}I and the rural town siteWHS. MPE}I is still in#uenced by emission processes.However, during daytime slight variations of the b/t-ratio occur. This an indication for photochemical pro-cesses that start to dominate BTEX-ratios. At WHS asigni"cant diurnal variation of b/t is visible. In additionboth e/m & p-x and e/m-x follow the b/t ratio. Since theethylbenzene often drops below the detection limit of theGC during daytime no corresponding ratio values areavailable for this time of the day. At the rural sites (Fig. 6)all ratios typically show diurnal variations with max-imum values during daytime when maximum photo-chemical activity takes place. However, this does notapply to MOHP. MOHP is located about 55 km awayfrom Munich. All other sites are much closer to the urban
3852 B. Rappenglu( ck, P. Fabian / Atmospheric Environment 33 (1999) 3843}3857
Fig. 4. BTEX ratios obtained at city sites (b/t denotes benzene/toluene and e/m & p-x denotes ethylbenzene/m & p-xylene, respectively).Time period of measurement campaigns: UMW}I: 18.10.}29.10.1993, UMW}II: 08.04.}02.05.1994, UMW}III: 12.08.}26.08.1993, IHF:30.09.}10.10.1993, MIM: 28.04.}24.05.1994.
Fig. 5. BTEX ratios obtained at suburban sites of Munich along with WHS (b/t denotes benzene/toluene, e/m & p-x denotesethylbenzene/m & p-xylene and e/m-x denotes ethylbenzen/m-xylene, respectively). Time period of measurement campaigns: MPE}I:26.05.}27.07.1995, WHS: 04.07.}25.07.1996.
B. Rappenglu( ck, P. Fabian / Atmospheric Environment 33 (1999) 3843}3857 3853
Fig. 6. BTEX ratios obtained at rural sites (b/t denotes benzene/toluene, e/m & p-x denotes ethylbenzene/m & p-xylene and e/m-xdenotes ethylbenzene/m-xylene, respectively). Time period of measurement campaigns: EBE}I: 27.07.}26.09.1995, EBE}II:01.04.}18.04.1993, EIT: April 1993}May 1994, MOHP: 06.05.}19.05.1997.
area. It can be expected from other investigations carriedout at the Schauinsland (Gilge et al., 1994), a mountainobservation site near the city of Freiburg/Germany, thatMOHP already lies in an area where NO
xlimitation of
ozone formation can be expected.
3.7. Case studies: concurrent BTEX-observations at twoselected sites
During April and August 1993 concurrent measurementsof BTEX and BTEX-ratios were obtained in the GMA. InApril 1993 two rural sites (EIT and EBE}II) could becompared. Fig. 7 shows that toluene levels are about thesame at both sites, but the diurnal variations of toluenedi!er: At EIT a signi"cant decrease of toluene occurs duringdaytime, whereas at EBE}II diurnal variations are muchsmoother. Regarding the BTEX-ratios, again, EIT exhibitsstrong variations. Toluene even becomes second to benzenein terms of concentration in the course of the day. Sinceethylbenzene and the xylene-isomers are depleted duringdaytime no ratios for these species could be calculated, butthe e/m-x ratio tends to follow the b/t-ratio. This impliesphotochemical processes. At EBE}II, too, variations arevisible, though they are less pronounced.
Fig. 8 compares the rural site EIT to the city siteUMW}III during August 1993. Regarding the diurnal
variation of toluene there are signi"cant di!erences. Asexpected toluene levels are much higher in urban area.During nighttime toluene at UMW}III is about 3 timeshigher than at EIT. During daytime the di!erence in-creases to about 10 times. E!ective depletion processestake place at EIT. Regarding other BTEX compounds itclearly can be seen that again toluene decreases rapidlyduring daytime and typically drops below benzene levels.In addition m and p-xylene depletes rapidly. After 11 : 00higher aromatic compounds are no longer detected. Dur-ing daytime the b/t ratio reaches maximum values thatare double the values observed during nighttime. Theresults from UMW}III, however, show typical urbanstructures during the same time period. Persisting emis-sions must beld responsible for this behaviour sinceBTEX ratio remains constant in the course of the day.The decrease in BTEX mixing ratios during daytime isprimarily due to atmospheric mixing processes and thevariation of mixing height. This applies in the same wayto EIT as well. However, these dynamic processes prim-arily have an impact on BTEX-mixing ratios rather thanBTEX-ratios.
Usually urban areas are marked by VOC-limitation ofozone formation due to relative high emissions of NO
x.
Ozone formation in rural areas, however, is determinedby NO
x-limitation. According to our "ndings MOHP
3854 B. Rappenglu( ck, P. Fabian / Atmospheric Environment 33 (1999) 3843}3857
Fig. 7. Concurrent measurements of BTEX and BTEX-ratios at EIT and EBE}II during 01.04.}18.04.1993 (rural sites).
must be declared a site where NOx-limitation is e!ective.
Our results imply that the urban/rural transition zones(EBE, EIT, WHS) of Munich are subject to VOC-limita-tion, at least as long as the urban plume stretches overthese areas. In these areas diurnal variations of BTEX-ratios can be observed. This is in good agreement withtheoretical assumptions made by Bowman and Seinfeld(1994). According to their investigations aromatic com-pounds, especially xylene isomers and toluene are moste!ective in terms of photochemistry in areas whereVOC-limitation prevails.
4. Conclusions
During several "eld campaigns in the years 1993}1997quasi-continuous on-line GC-measurements of NMHC
data was obtained at various locations (urban/suburban/rural) within the Greater Munich Area. Theresults show relatively low NMHC values compared toother cities worldwide. The data suggests that apartfrom tra$c emissions, fuel evaporation and solvent re-leases play an important part in summertime NMHCinventories. Aromatic compounds (BTEX) are sup-posed to be important ozone precursors in the Municharea.
Within the BTEX-compounds m & p-xylene could beselected as a key compound in discriminating di!erentlocations within the GMA. Urban sites show medianvalues for m & p-xylene above 1 ppb, suburban city sitescorresponding values of at least 0.5 ppbv, and rural sitesin the outer GMA below 0.5 ppbv.
Investigations of the diurnal variations of BTEXand BTEX-ratios suggest that the urban/rural transition
B. Rappenglu( ck, P. Fabian / Atmospheric Environment 33 (1999) 3843}3857 3855
Fig. 8. Concurrent measurements of BTEX and BTEX-ratios at EIT and UMW}III during 12.08.}26.08.1993 (rural/urban sites).
zone is a photochemical sensitive region. BTEX-ratiosshow diurnal variations contrary to urban sites orrural sites that are subject to NO
x-limitation. These
transition zones are marked by VOC-limitation, atleast as long as the urban plume stretches over theseareas. Future investigations should focus on theseareas since toluene and fast-reacting xylene isomersare known to be most e!ective in VOC-limitedareas.
Acknowledgements
NMHC-data from UMW, MPE, EBE and EIT wereobtained within the project KOVOX, correspondingdata from WHS and MOHP within the project OPAP.We gratefully acknowledge partial "nancial supportgranted by the Bavarian Ministry for Environment.
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