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Biogenic and anthropogenic fluxes of non-methane hydrocarbons over an urban-imapcted forest, Frankfurter Stadtwald, Germany
Atmospheric Environment 34 (2000) 3779}3788
Biogenic and anthropogenic #uxes of non-methane hydrocarbons over an urban-impacted forest, Frankfurter Stadtwald, Germany Rainer Steinbrecher *, Matthias Klauer , Karin Hau! , William R. Stockwell, Wolfgang Jaeschke, Thierry Dietrich, Fritz Herbert Fraunhofer-Institut fu( r Atmospha( rische Umweltforschung (IFU), Kreuzeckbahnstr. 19, D-82467 Garmisch-Partenkirchen, Germany Dessert Research Institute, Atmospheric Science Division, 2215 Raggio Parkway RENO, NV 89512, USA Zentrum fu( r Umweltforschung (ZUF), J.-W.- Goethe Unversita( t, Georg-Voigt-Str. 14, D-60325 Frankfurt/Main, Germany Institut fu( r Meteorologie und Geophysik, Robert-Mayer-Str. 1, D-60325 Frankfurt/Main, Germany Received 2 February 1999; received in revised form 15 October 1999; accepted 11 November 1999
Abstract In an urban-impacted oak/beech/pine forest (Frankfurter Stadtwald, 503 04 06 N; 83 40 17 E) trace gas distributions and #uxes of anthropogenic and biogenic non-methane hydrocarbons (NMHC) were determined for a bright weather period in August 1995. In general, ozone peaked at 70 ppb in the early afternoon. NO and NO reach values of up to V 25 ppb under low wind conditions and local automobile tra$c. Anthropogenic NMHC dominate with up to 8.0 ppb in the air above the forest. The dominating biogenic NMHC in ambient air above the forest was isoprene with peak values of 1.5 ppb during daytime. The #ux-gradient relationship with speci"c adapted and validated stability functions for this forest was used for calculating NMHC-#uxes. Transfer times of up to 100 s require a correction of the mixing ratios for HO-radical chemistry occurring along the gradient between 22 and 51 m for high reactive substances such as isoprene. The speci"c situation in the Frankfurter Stadtwald with high road tra$c inside the forest (up to 10,000 vehicles per hour) lead to sometimes signi"cant emission of anthropogenic NMHC as exhaust plumes were spread in the trunk space. Isoprene #uxes were high and amounted to 3.5 nmol m\ ground area s\ due to the high percentage of oaks growing in the forest but were at the lower end of estimates made by current biogenic NMHC emission inventories. The high isoprene emission #ux and ambient air mixing ratios underscore the importance of isoprene daytime and nighttime chemistry for the Frankfurt area. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Tall vegetation; Flux-gradient-relationship; Isoprene; Monoterpene; Photochemistry
1. Introduction High photooxidant concentrations, e.g. ozone, have been shown to be detrimental to both human health and plants (McKee, 1994; Sandermann et al., 1997). Ozone is not directly emitted by plants but produced rather photochemically in the presence of volatile organic compounds (VOC), especially non-methane hydrocarbons (NMHC), and nitrogen oxides (NO ) (Fehsenfeld V * Corresponding author. Fax #49-8821-73573. E-mail address: [email protected] (R. Steinbrecher).
and Liu, 1993). The importance of biogenic NMHC in tropospheric chemistry has been discussed extensively (Chameides et al., 1992; Fesenfeld et al., 1992) and recently, several papers described observations of the behaviour of biogenic NMHC such as isoprene at rural sites in the United States (Riemer et al., 1998; Helmig et al., 1998). Even at an urban-impacted rural site the isoprene chemistry is an important contributor to O production in urban anthropogenic VOC- and NO -rich plumes (Starn et al., 1998a). Therefore, there is V a pressing need to develop e!ective ozone control strategies and to investigate whether reduction of VOC or NO would be more e!ective in reducing ambient ozone V
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 5 1 8 - X
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levels in various regions (Blanchard et al., 1999; Sillman, 1999). To work out the most e!ective way to reduce photooxidant formation in an area, the various compounds and source strength of the di!erent precursors have to be known in the most exact way. Anthropogenic and biogenic emission inventories used in three-dimensional chemistry/transport modelling to predict the oxidant formation show a considerable source of uncertainty with an uncertainty factor of 2}3 and 5}10, respectively (Steinbrecher, 1994; Pierce et al., 1998). At present it is not possible to give a proper estimate of the overall accuracy of the available emission data due to systematic errors in the budgets, e.g. omitting signi"cant sources, omitting signi"cant compounds, inadequate emission factors, etc. To reduce the current large uncertainties in emission inventories, especially the biogenic VOC, "eld measurements have to be performed on the canopy scale in order to quantify the source pattern and source strength of the relevant compounds. Only recently have isoprene and monoterpene surface #uxes been reported for selected American and European vegetation due to the fact that estimation of surface #uxes of reactive biogenic compounds is still methodically di$cult (Goldstein et al., 1996,1998; Fuentes et al., 1996; Guenther et al., 1996; Reichmann et al., 1996; Cao et al., 1997; Schween et al., 1997a; Valentini et al., 1997). Flux measurements of VOC over forests in polluted areas in the midlatitudes are presently not available. For studying the VOC exchange between vegetation and the lower troposphere the #ux-gradient relationship can be used (Businger et al., 1971). Schween et al. (1997b) and Simpson et al. (1998) discussed the applicability of this relationship for the calculation of scalar #uxes for tall forest canopies. In principle, the #ux-gradient relationship is also valid for tall vegetation when the speci"c shape of the canopy and the height of the measuring point above the canopy are considered. It is still a debate whether this method can also be applied for more reactive compounds such as isoprene and monoterpenes without a correction since chemical degradation occurring along the gradient may result in an overestimation of the #uxes. This study aims at: (1) investigating the distribution of anthropogenic and biogenic NMHC over an urbanimpacted forest in central Europe, (2) deriving proper stability functions over tall vegetation for the correct application of the #ux-gradient relationship, (3) investigating the impact of photochemistry along a gradient on calculated #ux rates, and (4) determining surface #uxes of abundant anthropogenic and biogenic hydrocarbons.
The measurements were performed from a 51 m high meteorological tower equipped with meteorological instruments in six "xed and one variable height. Instruments for horizontal wind speed, air temperature, and air humidity were placed at 22, 25, 33, and 50 m height. At the same heights O (UPK, Bad Nauheim, Germany), NO, and NO mixing ratios were determined. CO and V VOC mixing ratios were determined at 22 and 51 m height. For the analysis of the trace gases in ambient air, the air was brought down to the analytical systems at the bottom of the tower using 60 m long of diameter Te#on tubing and pumping units. The air #ow through the tower lines was maintained at 3 to 4 l min\. Through bypass systems, the di!erent trace gas analysers were supplied with the necessary air #ow of about 1 l min\. Eddy correlation measurements were performed with a three-dimensional ultrasonic anemometer at 28 and 42 m height (Dietrich, 1998).
2. Experiment
2.3. NMHC analytic
2.1. Site description
C }C hydrocarbons at the two levels were analysed after simultaneously concentration of air samples on mixed bed tubes with an on-line gas chromatography system. The air samples from the two levels were sampled
The measuring site is located at 117.5 m altitude in the eastern part of the Frankfurter Stadtwald which is a
forest with an area of about 56 km located in the southern part of the city Frankfurt am Main, Germany. The geographic co-ordinates are 503 04 06 north 83 40 17 east. It is a mixed forest with 63% deciduous trees (oak 34%, beech 21%, others 8%) and 37% coniferous trees (31% pine, others 6%) and mean height of 12 m. At this site the mean precipitation sum during the vegetation period (170 days) amounts to only 310 mm with a mean air temperature of 16.13C. During drought periods in the summer the forest usually is water-stressed due to the sandy soil and the very deep ground water level (around 10 m) (Dietrich, 1998). The prevailing wind direction is south-west. The region &Rhein-Main' around Frankfurt is densely populated and characterised by heavy road and air tra$c (around 10,000 vehicles and 50 take o! and landings per hour, Amt fuK r Stra{en und Verkehrswesen Frankfurt, Germany). In the vicinity of the site, the Frankfurt airport and several highway crossings are located in the forest. These high anthropogenic activities result in a typical trace gas mixing ratio distribution for this highly polluted area (Jaeschke et al., 1997). In the summer months of July to September in 1995, in which the campaign was performed, a mean ozone mixing ratio of 25 ppb, a mean NO mixing ratio of 14 ppb and a mean NO mixing ratio (as NO ) of 21 ppbv are recorded V (Frankfurt Niederrad, Hessische Landesanstalt fuK r Umwelt, Wiesbaden, Germany). 2.2. Meteorological tower
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simultaneously with a #ow of 115 ml min\ regulated by two mass #ow controller (MKS Instruments, Germany) and concentrated for 28 min on the adsorbent Carbosieve III and Carbotrap (Supelco, Germany) at 203C. The trapped compounds were thermodesorbed at 2003C and cryofocused at !303C on a 18 cm;0.53 mm ID glass-capillary pre-column "lled with Carboback B (Supelco, Germany). The injection of the compounds on the capillary column (DB 1701: 10 m;0.25 mm ID, 1 lm df; Chrompack, Frankfurt, Germany) was achieved by heating up the pre-column to 3503C. The separation of the di!erent compounds was obtained with a linear temperature program starting at 353C and ending at 1503C, with a rate of 103C min\. All compounds were detected by #ame ionisation with a sensitivity of 15 pptv. Reproducibility of the GC-analytic was better than 5%. Compounds in air samples were identi"ed by co-elution of reference substances. For the calibration in the "eld, a gas standard of 9.9$0.3 ppb n-butane and 10.2$ 0.2 ppb benzene (National Institute of Standards and Technology, Gaithersburg, USA) was used. The benzene response factor calculated from the "eld measurements was used as reference for the response factors of toluene, m-, p-xylene, o-xylene, and b-pinene which were calibrated in the laboratory using a permeation tube system (Kin-Tek, Aerolaser, Garmisch-Partenkirchen, Germany). The quanti"cation of isoprene was achieved by a 10 ppb gas standard prepared by dilution with N from a 11$1.1 ppm isoprene gas standard (Messer Griesheim, Duisburg, Germany). The time resolution of the analytical cycle was 1 h. Before the VOC measurements started, it was ensured that the tubing had no e!ect on measured VOC mixing ratios by analysing in parallel ambient air with and without having passed the 60 m long tower tube. 2.4. Flux-gradient relationship for tall vegetation In order to calculate turbulent vertical #uxes for the campaign in August 1995, the gradient method has been used. This method is based on the Monin}Obukhov similarity hypothesis (Monin and Obhukov, 1954). According to that, in a constant-#ux layer the vertical #ux can be related to the vertical pro"le of the corresponding quantity like heat, momentum or gas concentrations. The #ux J is given by *s J"K , Q *z
(1)
where K is the turbulent di!usion coe$cient and *s/*z Q is the measured gradient of the quantity of interest. The turbulent di!usion coe$cient for gases is assumed to be equal to that for heat K (Monteith and Unsworth, 1994): F izu* K " (2) U (f)
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Here i is the von Ka`rma`n constant and u is the H friction velocity. The stability function U (f) has been determined in several "eld experiments almost entirely conducted over low vegetation with a roughness length of only a few centimetres (e.g. Businger et al., 1971). However, the application of such functions might fail for tall vegetation with a roughness length of about 1 m (Lenschow, 1995). Therefore, special stability functions for momentum U (f) and for heat U (f) were determined
using data from the two ultrasonic anemometers. These measurements were performed in 1994 from March to September. In August 1995 no eddy covariance data were available. 2.5. Photochemical box model The chemical degradation in relation to the transport time of the substances emitted or deposited to and from the canopy, respectively, occurring along the gradient between 22 and 51 m was investigated using reaction constants and HO-radical concentrations given by the new chemical box model, the `Regional Atmospheric Chemistry Mechanisma (RACM), explicitly considering the isoprene and monoterpene chemistry (Stockwell et al., 1997). It includes rate constants and product yields from the most recent laboratory measurements, and it has been tested against environmental chamber data.
3. Results and discussion 3.1. VOC mixing ratios and meteorological parameters The results of the measuring period in August 1995 for atmospheric trace gases and meteorological parameters are presented in Fig. 1a, 1b. The period investigated was characterised by bright weather with high temperatures of up to 323C. The wind speed reached values of up to 4.5 m s\ and the prevailing wind direction was changing from north east (453) at the "rst day to south-west (2453) at the second day. As the #ux-gradient relationship requires a certain wind speed ('1 m s\), fetch (in this case 4 km), and no in#uence of the tower itself on the turbulence structure of the atmosphere, a date selection has to be made according to the placement of the meteorological sensors at the tower, the vegetation cover and the wind speed. Therefore, only wind vectors between 200 and 2803 at wind speeds greater than 1 m s\ were considered in the #ux calculations. The NMHC mixing ratios in ambient air clearly demonstrated that the site is strongly in#uenced by tra$c. Benzene, toluene m-, p- and o-xylene showed maximum mixing ratios of about 3 ppb during the night when these substances were accumulating in the stable boundary layer as a result of ongoing tra$c on the roads crossing the forests. During daytime, mixing ratios decreased
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sharply as a result of increasing atmospheric turbulence and photochemical degradation of the substances. The mixing ratios of the anthropogenic NMHC were correlated with the mixing ratios of NO and NO , which V were also emitted mainly by cars. The diurnal cycle of NO and NO in general showed two maxima, one in the V morning and one in the late evening due to the rush hour of Frankfurt. However, on late Saturday 12th and Sunday 13th, the conditions were quite di!erent from the
days before as a result of local high tra$c on a parking lot nearby, combined with stable conditions of the boundary layer. The wind had calmed down during this period and the sky was overcast resulting in low turbulence and photochemistry which lead to high NO, NO and NMHC mixing ratios but very low ozone mixV ing ratios in the boundary layer. In contrast, ozone mixing ratios peak on the sunny days at 70 ppb. The shown diurnal cycles of ozone, NO and NO exhibit the V
Fig. 1. Diurnal courses of the mixing ratios and meteorological parameters in the Frankfurter Stadtwald during August 1995 at the ZUF-tower. (a) ozone, NO, NO (calibrated to NO ), (b) biogenic and (c) anthropogenic volatile organic compounds (VOC), and (d) air V temperature measured at 22 m height. Global radiation (d) was recorded at 3 m height and wind speed (e) was measured at 33 m height. Wind direction data (f ) were provided by the German weather service (DWD station Frankfurt/Main, Flughafen, about 5 km away from the tower). Blanks: no data available. In the stacked bar plots mixing ratios of single compounds are summed up for showing also the totals.
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Fig. 1. continued
classical behaviour in air polluted areas with maximum NO and NO mixing ratios leading to very low ozone V levels in the early morning around sunrise and late evening after sunset (Chameides et al., 1992). The biogenic NMHC in ambient air were dominated by isoprene resulting from isoprene-emitting oak tree species (Steinbrecher et al., 1997). The monoterpene bpinene was detected in much lower amounts (Fig. 1b). On 11 and 12th August the isoprene mixing ratios were lowest during the night at around 0.2 ppb. During daytime isoprene mixing ratios amounted to 1.5 ppb. For b-pinene the opposite diurnal cycles were observed with high mixing ratios in the night and very low mixing ratios during daytime. This "nding is consistent with observations at other sites (Enders et al., 1992; Schween et al.,
1997a; Riemer et al., 1998; Starn et al., 1998a,b). Isoprene and b-pinene are emitted from trees and this emission is correlated with temperature and light (for review see Steinbrecher, 1997; Steinbrecher and Ziegler, 1997). As biogenic isoprene emission peaked during daytime (see also Fig. 3), isoprene was able to accumulate in the air above the canopy despite turbulent transport and photochemistry. The source strength of b-pinene was much less resulting in low ambient air mixing ratios during daytime due to dilution and photochemistry. In the night, still ongoing b-pinene emission from resin reservoirs in pine trees lead to an accumulation of b-pinene in the stable boundary layer. Biogenic isoprene emission was almost zero in the night due to low physiological activity of the vegetation and mixing ratios in ambient air should drop
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to almost zero due to chemical degradation (Starn et al., 1998b). Isoprene deposition is negligible as pointed out by Guenther and Hill (1998). The authors report nighttime #uxes over an oak tree-dominated forest of near zero (0.04$0.12 nmol m\ s\). However, isoprene mixing ratios were about 0.25 ppb in the night. Although isoprene is mainly of biogenic origin, anthropogenic sources also have to be considered as isoprene is present in automobile exhaust (Du!y et al., 1999). A tracer for automobile exhaust is o-xylene. The mixing ratios of this compound were usually in range of 0.75 ppb except for the period when local air tra$c almost doubled observed mixing ratios. In consequence, isoprene mixing ratios also doubled in this speci"c night as compared to previous days. This implies that daytime isoprene mixing ratios were primarily a result of biogenic production, however, nighttime isoprene mixing ratios were of anthropogenic origin. 3.2. Stability functions The #ux-gradient relationship is based on the constant #ux assumption. The assumption of constant #ux is assumed to be ful"lled if the two momentum and heat #uxes provided by the two sonic anemometers in 28 and 41 m height di!er not more than 20% (uncertainty of the #ux determination) and the fetch requirement of 4 km (mean canopy height 12 m; measuring levels 22 and 51 m) is met. This is the case for wind velocities between 1 and 6.5 m s\ and appropriate wind directions between 2003 and 2803. The stability parameter f was also calculated from the sonic anemometer data. In case of constant #ux, site-speci"c stability functions for momentum U (f) and
heat U (f) have been calculated for a particular stability parameter f. Fluxes for a speci"c f were calculated by sliding averaging and then stability functions derived by regression analysis (s-method) (Fig. 2a and b). The values of the "tted function for a stable strati"ed surface layer (f'0) were higher than those given by Businger et al. (1971). The opposite was the case at unstable conditions (f(0). This "nding corresponded well with a greater turbulent mixing over a forest canopy during stable strati"cation and less turbulent mixing during unstable strati"cation compared with the surface layer over low vegetation. So the tall canopy acted as a sink for turbulence due to eddies penetrating into the space between single trees, as well as a source, due to the production of wakes in the lee of the trees. In a strongly turbulent, unstable strati"ed surface layer the sink e!ect dominates the source e!ect, whereas in a less turbulent, stable strati"ed surface layer the opposite is true. In Fig. 2c heat #uxes calculated by the new stability functions used in the #ux-gradient relationship and directly measured heat #uxes are compared. The slope of the regression line through the data points is nearly one with a regression coe$cient of 0.7. This result implies, that the
gradient method with the new stability functions for heat U (f) provides reliable turbulent heat #uxes at this site. U (f) is then used to calculate NMHC #uxes according to the #ux-gradient relationship. The uncertainty of NMHC #ux determination by this method is estimated to reach 50% (Schween et al., 1997a) when assuming an error in the mixing ratio measurements of 15%. 3.3. NMHC yuxes The #ux-gradient relationship requires no source or sink behaviour for the compounds of interest between the two measuring levels. However, for reactive NMHC this requirement may not be ful"lled. In order to investigate the importance of chemical degradation between 22 and 51 m for NMHC #uxes, the chemical loss rate for each compound was estimated. It has to be noted that only daytime values of 12 August were considered since wind direction and wind speed allowed #ux calculations only during this period. During daytime the lifetime of NMHC is mainly controlled by the HO-reaction (Stockwell et al., 1997). For an estimation of the e!ect of the HO-reactions on the #uxes, the characteristic loss rates c(q ) were determined for the di!erent NMHC accord ing to Eq. (3). The reactions follow a pseudo-"rst-order time law and the characteristic loss rate is given as c(q )"c(o)e\! GO .
(3)
The HO-reaction rate parameters, k , were calculated &for the measured temperatures (see Fig. 1d) from the Arrhenius expressions; C is the concentration of the HO-radical, q the transfer time between 22 and 55 m, and C(0) the initial mixing ratio of the speci"c compound. The HO-radical concentrations used in the calculations were adjusted according the RACM modelled diurnal cycles of HO-concentrations typical for the geographic location. Hydroxyl-radical concentrations of 5;10 molecules cm\ were used for data at 8:00 and 17:00 CET, respectively, 1;10 molecules cm\ for 9:00, 10:00 CET, 15:00 and 16:00 CET values and 2;10 molecules cm\ for 11:00 to 14:00 CET values (Stockwell et al., 1997). The reaction of the NMHC with ozone along the gradient between 22 and 51 m at transfer times of up to 100 s during daytime was not considered to be an important loss process because for the most reactive compound, b-pinene, the lifetime at highest observed ozone mixing ratios is 12 h. For the less reactive anthropogenic compounds the maximum loss due to HO-radical reaction is of the order of 0.1}0.6%. The losses for isoprene and b-pinene were an order of magnitude higher and amounted to 3.1 and 2.5%, respectively. In Fig. 3 calculated #uxes of biogenic and anthropogenic NMHC and the in#uence of chemical degradation along the gradient due to HO-radical reaction are shown.
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Fig. 2. Calculated stability functions for heat U (f) (a) and momentum U (f) (b) for the Frankfurter Stadtwald. Measurements were
performed in 1994 from 7 March to 29 September (n"142). The circles denote the values of the stability function averaged for a certain stability interval f, the error bars are the standard deviation for each average (n"8}30). The heat #uxes calculated by the gradient approach using the new stability function and the directly measured heat #ux by the eddy correlation technique are compared in (c).
If the mixing ratio di!erence between the two heights was increased due to chemical loss process along the gradient, the calculated #uxes according to Eq. (1) were overestimated. As the observed di!erences are usually very small, small changes in the absolute mixing ratios result
in signi"cant changed #uxes. This is the case for the most reactive compound isoprene and the uncorrected #ux is up to 14% higher if chemical degradation is not considered. However, for less reactive compounds corrected #uxes and uncorrected #uxes were comparable. It has to
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Fig. 3. Comparison of measured and corrected NMHC #uxes over the Frankfurter Stadtwald. The #ux correction due to chemical degradation along the gradient between 22 and 51 m was performed for the reaction with the HO-radical with HO-concentration of 5;10 molecules cm\, (8:00 and 17:00 CET), 1;10 molecules cm\ (9:00, 10:00 CET and 15:00, 16:00 CET) and 2;10 molecules cm\ (11:00 to 14:00 CET), respectively, to simulate the diurnal cycle of the HO-radical concentration in the ambient air (Stockwell et al., 1997). Fluxes are related to ground area. Blanks: mixing ratio di!erences were lower than the measuring accuracy of 5% of the GC analysis. * #uxes calculated at the 5% accuracy limit. In the stacked bar plots #ux rates of single compounds are summed up for showing also the totals.
be noted that the correction for chemical degradation along the gradient 22 and 51 m did not change the direction of the calculated #uxes and the forest was, as expected, a source for biogenic NMHC during daytime. The uncertainty in the #ux calculations amounted to $0.5 nmol for isoprene as demonstrated by negative and positive isoprene #uxes at 09.00 and 17:00 CET, respectively, calculated by using mixing ratio di!erences near the GC accuracy limit of 5%.
Isoprene #uxes amount to about 3 nmol m\ ground area s\ as a result of the isoprene emission from the oaks. Monoterpene emissions were very low and only b-pinene #uxes up to 0.1 nmol m\ s\ could be detected as a result of the small fraction of conifers growing in the foot print area. The maximum #ux was observed when the vegetation was most active during the day and turbulence was maximal. During nighttime #uxes could not be resolved due to limited meteorological conditions
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and the accuracy limit of the GC analysis. However, based on the "ndings of Guenther and Hill (1998) it is expected that during night #uxes of biogenic NMHC are near zero at this site. For this site, according to procedures used in BVOC emission inventories (Simpson et al., 1995) isoprene emission is calculated to amount to 7 nmol m\ s\ (for 303C and full sun, assuming that 50% of the oak leaves are fully active). Compared to the measured #uxes estimated #uxes are about a factor of two higher showing that present emission inventories possibly overestimate emission. The observed anthropogenic NMHC #uxes were small despite the high load of anthropogenic NMHC in the air above the forest. Surprisingly, the forest acted sometimes as a source for anthropogenic NMHC. The high road tra$c inside the forest may be an explanation for this phenomenon. The exhaust fumes spread in the trunk space and then were transported through the canopy to the boundary layer.
4. Conclusions This case study shows for the "rst time #uxes for isoprene, monoterpenes and major anthropogenic VOC in an urban-impacted oak/beech/pine forest in Europe. For this forest, speci"c stability functions have been calculated showing the necessity of adapting the classical #ux-gradient relationship to tall vegetation. For enhancing the resolution of the #ux determination the height gradient chosen was maximal at 39 m. This high gradient, however, may lead to transport times which are greater than the gas phase reaction times of the reactive isoprene. Therefore, #uxes may be overestimated by up to 14% if neglecting chemical degradation along the gradient. Fluxes of biogenic and anthropogenic NMHC were in the range of some nmol m\ s\. The high observed isoprene #ux during daytime underlines the importance of isoprene chemistry for photooxidant formation in the summertime in the Frankfurt area. However, measured isoprene #uxes were at the lower end of isoprene emission estimates made by landscape emission models. In general, the forest is no source or sink for anthropgenic NMHC. However, the speci"c situation in the Frankfurter Stadtwald with high road tra$c inside the forest lead to sometimes signi"cant emission of anthropogenic NMHC as exhaust plume were spread in the trunk space. This phenomenon needs further investigation with a detailed analysis of the transport and chemical reaction properties inside the canopy.
Acknowledgements We like to acknowledge the funding of the project by the German Department of Education and Research,
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projects 07EU766, 07EU722 and 07EU812. Beyond that we would like to thank Uwe Schickedanz (ZUF) for his assistance during the "eld campaign.
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Guenther, A., Baugh, W., Davis, K., Hampton, G., Harley, P., Klinger, L., Vierling, L., Zimmerman, P., Allwine, E., Dilts, S., Lamb, B., Westberg, H., Baldocchi, D., Geron, C., Pierce, T., 1996. Isoprene #uxes measured by enclosure, relaxed eddy accumulation, surface layer gradient, mixed layer gradient, and mixed layer mass balance techniques. Journal of Geophysical Research 101, 18555}18567. Guenther, A., Hill, A., 1998. Eddy covariance measurement of isoprene #uxes. Journal of Geophysical Research 103, 13145}13152. Helmig, D., Greenberg, J., Guenther, A., Zimmerman, P., 1998. Volatile organic compounds and isoprene oxidation products at a temperate deciduous forest site. Journal of Geophysical Research 103, 22397}22414. Jaeschke, W., Dietrich, T., Schickedanz, U., Klauer, M., 1997. Biogenic and anthropogenic trace gas #uxes between the atmosphere and a polluted mixed forest of central Europe. In: Slanina, S. (Ed.), Biosphere}Atmosphere Exchange of Pollutants and Trace Substances. Springer, Berlin, pp. 311}319. Lenschow, D.H., 1995. Micrometeorological techniques for measuring biosphere-atmosphere trace gas exchange. In: Matson, A., Harris, R.C. (Eds.), Biogenic Trace Gases; Measuring Emissions from Soil and Water. Blackwell Science, Cambridge, pp. 126}163. McKee, D.J., 1994. Tropospheric Ozone, Human Health and Agricultural Impacts. CRC Press, Boca Raton, FL. Monin, A.S., Obhukov, A.M., 1954. Basic Laws of Turbulent Mixing in the Atmosphere near the Ground. Physics Tr. Akad. Nauk., SSSR Geophiz. Inst. 24 (151), 1963}1984. Monteith, J.L., Unsworth, M.H., 1994. Principles of Environmental Physics. Edward Arnold, London. Pierce, T., Geron, C., Bender, L., Dennis, R., Tonnesen, G., Guenther, A., 1998. In#uence of increased isoprene emissions on regional ozone modelling. Journal of Geophysical Research 103, 25611}25629. Reichmann, A., Steinbrecher, R., Tranos, S., Zelger, M., 1996. Design and testing of a relaxed eddy accumulation system for VOC. In: Borrell, P.M., Borrell, P., Klly, K., Cvitas, T., Seiler, W. (Eds.), Proceedings EUROTRAC Symposium 1996: Emissions, Deposition, Laboratory Work and Instrumentation. Computational Mechanics Publications, Southampton UK, pp. 257}261. Riemer, D., Pos, W., Milne, P., Farmer, C., Zika, R., Apel, E., Olszyna, K., Kleindienst, T., Lonneman, W., Bertman, S., Shepson, P., Starn, T., 1998. Observations of non-methane hydrocarbons and oxygenated volatile organic compounds at a rural site in the south-eastern United States. Journal of Geophysical Research 103, 28111}28128. Sandermann, H., Wellburn, A.R., Heath, R.L., 1997. Forest Decline an Ozone. A Comparison of Controlled Chamber and Field Experiments. Ecological Studies, vol. 127, Springer, Berlin.
Schween, J.H., Dlugi, R., Hewitt, C.N., Foster, P., 1997a. Determination and accuracy of VOC-#uxes above the Pine/Oak forest at Castelporziano. Atmospheric Environment 31, 199}215. Schween, J.H., Zelger, M., Wichura, B., Foken, T., Dlugi, R., 1997b. Pro"les and #uxes of meteorological parameters above and within the Mediterranean forest at Castelporziano. Atmospheric Environment 31, 185}198. Sillman, S., 1999. The relation between ozone, NO and hydroV carbons in urban and polluted rural environments. Atmospheric Environment 33, 1821}1845. Simpson, D., Guenther, A., Hewitt, C.N., Steinbrecher, R., 1995. Biogenic emissions in Europe. Part I: Estimates and Uncertainties. Journal of Geophysical Research 100, 22875}22890. Simpson, I.J., Thurtell, G.W., Neumann, H.H., Den Hartog, G., Edwards, G.C., 1998. The validity of similarity theory in the roughness sublayer above forests. Boundary-Layer Meteorology 87, 69}99. Starn, T.K., Shepson, P.B., Bertman, S.B., Riemer, D.D., Zika, R.G., Olszyna, K., 1998b. Night-time isoprene chemistry at an urban-impacted forest site. Journal of Geophysical Research 103, 22437}22447. Starn, T.K., Shepson, P.B., Bertman, S.B., White, J.S., Splawn, B.G., Riemer, D.D., Zika, R.G., Olszyna, K., 1998a. Observations of isoprene chemistry and its role in ozone production at a semirural site during the 1995 Southern Oxidant Study. Journal of Geophysical Research 103, 22425}22435. Steinbrecher, R., 1994. Emission of VOC from selected European ecosystems: the state of the art. In: Borrell, P.M., Borrell, P., Cvitas, T., Seiler, W. (Eds.), Proceedings EUROTRAC Symposium 1994. SPB Academic Publishing, The Hague, pp. 448}454. Steinbrecher, R., 1997. Isoprene: production by plants and ecosystem-level estimates. In: Helas, G., Slanina, J., Steinbrecher, R. (Eds.), BVOC Emission by Vegetation. SPB Academic Publishing, The Hague, pp. 101}115. Steinbrecher, R., Hau!, K., Rabong, R., Steinbrecher, J., 1997. Isoprenoid emission of oak species typical for the Mediterranean area: Source strength and controlling variables. Atmospheric Environment 31/S1, 79}88. Steinbrecher, R., Ziegler, H., 1997. Monoterpene production by plants. In: Rennenberg, H., Eschrich, W., Ziegler, H. (Eds.), Trees * Contributions to Modern Tree Physiology. Backhuys Publishers, Leiden, pp. 119}138. Stockwell, W.R., Kirchner, F., Kuhn, M., Seefeld, S., 1997. A new mechanism for regional atmospheric chemistry modelling. Journal of Geophysical Research 102, 25847}25879. Valentini, R., Greco, S., Seufert, G., Bertin, N., Ciccioli, P., Cecinato, A., Brancaleoni, E., Frattoni, M., 1997. Fluxes of biogenic VOC from Mediterranean vegetation by trap enrichment relaxed eddy accumulation. Atmospheric Environment 31, 229}238.
Biogenic and anthropogenic #uxes of non-methane hydrocarbons over an urban-impacted forest, Frankfurter Stadtwald, Germany Rainer Steinbrecher *, Matthias Klauer , Karin Hau! , William R. Stockwell, Wolfgang Jaeschke, Thierry Dietrich, Fritz Herbert Fraunhofer-Institut fu( r Atmospha( rische Umweltforschung (IFU), Kreuzeckbahnstr. 19, D-82467 Garmisch-Partenkirchen, Germany Dessert Research Institute, Atmospheric Science Division, 2215 Raggio Parkway RENO, NV 89512, USA Zentrum fu( r Umweltforschung (ZUF), J.-W.- Goethe Unversita( t, Georg-Voigt-Str. 14, D-60325 Frankfurt/Main, Germany Institut fu( r Meteorologie und Geophysik, Robert-Mayer-Str. 1, D-60325 Frankfurt/Main, Germany Received 2 February 1999; received in revised form 15 October 1999; accepted 11 November 1999
Abstract In an urban-impacted oak/beech/pine forest (Frankfurter Stadtwald, 503 04 06 N; 83 40 17 E) trace gas distributions and #uxes of anthropogenic and biogenic non-methane hydrocarbons (NMHC) were determined for a bright weather period in August 1995. In general, ozone peaked at 70 ppb in the early afternoon. NO and NO reach values of up to V 25 ppb under low wind conditions and local automobile tra$c. Anthropogenic NMHC dominate with up to 8.0 ppb in the air above the forest. The dominating biogenic NMHC in ambient air above the forest was isoprene with peak values of 1.5 ppb during daytime. The #ux-gradient relationship with speci"c adapted and validated stability functions for this forest was used for calculating NMHC-#uxes. Transfer times of up to 100 s require a correction of the mixing ratios for HO-radical chemistry occurring along the gradient between 22 and 51 m for high reactive substances such as isoprene. The speci"c situation in the Frankfurter Stadtwald with high road tra$c inside the forest (up to 10,000 vehicles per hour) lead to sometimes signi"cant emission of anthropogenic NMHC as exhaust plumes were spread in the trunk space. Isoprene #uxes were high and amounted to 3.5 nmol m\ ground area s\ due to the high percentage of oaks growing in the forest but were at the lower end of estimates made by current biogenic NMHC emission inventories. The high isoprene emission #ux and ambient air mixing ratios underscore the importance of isoprene daytime and nighttime chemistry for the Frankfurt area. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Tall vegetation; Flux-gradient-relationship; Isoprene; Monoterpene; Photochemistry
1. Introduction High photooxidant concentrations, e.g. ozone, have been shown to be detrimental to both human health and plants (McKee, 1994; Sandermann et al., 1997). Ozone is not directly emitted by plants but produced rather photochemically in the presence of volatile organic compounds (VOC), especially non-methane hydrocarbons (NMHC), and nitrogen oxides (NO ) (Fehsenfeld V * Corresponding author. Fax #49-8821-73573. E-mail address: [email protected] (R. Steinbrecher).
and Liu, 1993). The importance of biogenic NMHC in tropospheric chemistry has been discussed extensively (Chameides et al., 1992; Fesenfeld et al., 1992) and recently, several papers described observations of the behaviour of biogenic NMHC such as isoprene at rural sites in the United States (Riemer et al., 1998; Helmig et al., 1998). Even at an urban-impacted rural site the isoprene chemistry is an important contributor to O production in urban anthropogenic VOC- and NO -rich plumes (Starn et al., 1998a). Therefore, there is V a pressing need to develop e!ective ozone control strategies and to investigate whether reduction of VOC or NO would be more e!ective in reducing ambient ozone V
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 5 1 8 - X
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levels in various regions (Blanchard et al., 1999; Sillman, 1999). To work out the most e!ective way to reduce photooxidant formation in an area, the various compounds and source strength of the di!erent precursors have to be known in the most exact way. Anthropogenic and biogenic emission inventories used in three-dimensional chemistry/transport modelling to predict the oxidant formation show a considerable source of uncertainty with an uncertainty factor of 2}3 and 5}10, respectively (Steinbrecher, 1994; Pierce et al., 1998). At present it is not possible to give a proper estimate of the overall accuracy of the available emission data due to systematic errors in the budgets, e.g. omitting signi"cant sources, omitting signi"cant compounds, inadequate emission factors, etc. To reduce the current large uncertainties in emission inventories, especially the biogenic VOC, "eld measurements have to be performed on the canopy scale in order to quantify the source pattern and source strength of the relevant compounds. Only recently have isoprene and monoterpene surface #uxes been reported for selected American and European vegetation due to the fact that estimation of surface #uxes of reactive biogenic compounds is still methodically di$cult (Goldstein et al., 1996,1998; Fuentes et al., 1996; Guenther et al., 1996; Reichmann et al., 1996; Cao et al., 1997; Schween et al., 1997a; Valentini et al., 1997). Flux measurements of VOC over forests in polluted areas in the midlatitudes are presently not available. For studying the VOC exchange between vegetation and the lower troposphere the #ux-gradient relationship can be used (Businger et al., 1971). Schween et al. (1997b) and Simpson et al. (1998) discussed the applicability of this relationship for the calculation of scalar #uxes for tall forest canopies. In principle, the #ux-gradient relationship is also valid for tall vegetation when the speci"c shape of the canopy and the height of the measuring point above the canopy are considered. It is still a debate whether this method can also be applied for more reactive compounds such as isoprene and monoterpenes without a correction since chemical degradation occurring along the gradient may result in an overestimation of the #uxes. This study aims at: (1) investigating the distribution of anthropogenic and biogenic NMHC over an urbanimpacted forest in central Europe, (2) deriving proper stability functions over tall vegetation for the correct application of the #ux-gradient relationship, (3) investigating the impact of photochemistry along a gradient on calculated #ux rates, and (4) determining surface #uxes of abundant anthropogenic and biogenic hydrocarbons.
The measurements were performed from a 51 m high meteorological tower equipped with meteorological instruments in six "xed and one variable height. Instruments for horizontal wind speed, air temperature, and air humidity were placed at 22, 25, 33, and 50 m height. At the same heights O (UPK, Bad Nauheim, Germany), NO, and NO mixing ratios were determined. CO and V VOC mixing ratios were determined at 22 and 51 m height. For the analysis of the trace gases in ambient air, the air was brought down to the analytical systems at the bottom of the tower using 60 m long of diameter Te#on tubing and pumping units. The air #ow through the tower lines was maintained at 3 to 4 l min\. Through bypass systems, the di!erent trace gas analysers were supplied with the necessary air #ow of about 1 l min\. Eddy correlation measurements were performed with a three-dimensional ultrasonic anemometer at 28 and 42 m height (Dietrich, 1998).
2. Experiment
2.3. NMHC analytic
2.1. Site description
C }C hydrocarbons at the two levels were analysed after simultaneously concentration of air samples on mixed bed tubes with an on-line gas chromatography system. The air samples from the two levels were sampled
The measuring site is located at 117.5 m altitude in the eastern part of the Frankfurter Stadtwald which is a
forest with an area of about 56 km located in the southern part of the city Frankfurt am Main, Germany. The geographic co-ordinates are 503 04 06 north 83 40 17 east. It is a mixed forest with 63% deciduous trees (oak 34%, beech 21%, others 8%) and 37% coniferous trees (31% pine, others 6%) and mean height of 12 m. At this site the mean precipitation sum during the vegetation period (170 days) amounts to only 310 mm with a mean air temperature of 16.13C. During drought periods in the summer the forest usually is water-stressed due to the sandy soil and the very deep ground water level (around 10 m) (Dietrich, 1998). The prevailing wind direction is south-west. The region &Rhein-Main' around Frankfurt is densely populated and characterised by heavy road and air tra$c (around 10,000 vehicles and 50 take o! and landings per hour, Amt fuK r Stra{en und Verkehrswesen Frankfurt, Germany). In the vicinity of the site, the Frankfurt airport and several highway crossings are located in the forest. These high anthropogenic activities result in a typical trace gas mixing ratio distribution for this highly polluted area (Jaeschke et al., 1997). In the summer months of July to September in 1995, in which the campaign was performed, a mean ozone mixing ratio of 25 ppb, a mean NO mixing ratio of 14 ppb and a mean NO mixing ratio (as NO ) of 21 ppbv are recorded V (Frankfurt Niederrad, Hessische Landesanstalt fuK r Umwelt, Wiesbaden, Germany). 2.2. Meteorological tower
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simultaneously with a #ow of 115 ml min\ regulated by two mass #ow controller (MKS Instruments, Germany) and concentrated for 28 min on the adsorbent Carbosieve III and Carbotrap (Supelco, Germany) at 203C. The trapped compounds were thermodesorbed at 2003C and cryofocused at !303C on a 18 cm;0.53 mm ID glass-capillary pre-column "lled with Carboback B (Supelco, Germany). The injection of the compounds on the capillary column (DB 1701: 10 m;0.25 mm ID, 1 lm df; Chrompack, Frankfurt, Germany) was achieved by heating up the pre-column to 3503C. The separation of the di!erent compounds was obtained with a linear temperature program starting at 353C and ending at 1503C, with a rate of 103C min\. All compounds were detected by #ame ionisation with a sensitivity of 15 pptv. Reproducibility of the GC-analytic was better than 5%. Compounds in air samples were identi"ed by co-elution of reference substances. For the calibration in the "eld, a gas standard of 9.9$0.3 ppb n-butane and 10.2$ 0.2 ppb benzene (National Institute of Standards and Technology, Gaithersburg, USA) was used. The benzene response factor calculated from the "eld measurements was used as reference for the response factors of toluene, m-, p-xylene, o-xylene, and b-pinene which were calibrated in the laboratory using a permeation tube system (Kin-Tek, Aerolaser, Garmisch-Partenkirchen, Germany). The quanti"cation of isoprene was achieved by a 10 ppb gas standard prepared by dilution with N from a 11$1.1 ppm isoprene gas standard (Messer Griesheim, Duisburg, Germany). The time resolution of the analytical cycle was 1 h. Before the VOC measurements started, it was ensured that the tubing had no e!ect on measured VOC mixing ratios by analysing in parallel ambient air with and without having passed the 60 m long tower tube. 2.4. Flux-gradient relationship for tall vegetation In order to calculate turbulent vertical #uxes for the campaign in August 1995, the gradient method has been used. This method is based on the Monin}Obukhov similarity hypothesis (Monin and Obhukov, 1954). According to that, in a constant-#ux layer the vertical #ux can be related to the vertical pro"le of the corresponding quantity like heat, momentum or gas concentrations. The #ux J is given by *s J"K , Q *z
(1)
where K is the turbulent di!usion coe$cient and *s/*z Q is the measured gradient of the quantity of interest. The turbulent di!usion coe$cient for gases is assumed to be equal to that for heat K (Monteith and Unsworth, 1994): F izu* K " (2) U (f)
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Here i is the von Ka`rma`n constant and u is the H friction velocity. The stability function U (f) has been determined in several "eld experiments almost entirely conducted over low vegetation with a roughness length of only a few centimetres (e.g. Businger et al., 1971). However, the application of such functions might fail for tall vegetation with a roughness length of about 1 m (Lenschow, 1995). Therefore, special stability functions for momentum U (f) and for heat U (f) were determined
using data from the two ultrasonic anemometers. These measurements were performed in 1994 from March to September. In August 1995 no eddy covariance data were available. 2.5. Photochemical box model The chemical degradation in relation to the transport time of the substances emitted or deposited to and from the canopy, respectively, occurring along the gradient between 22 and 51 m was investigated using reaction constants and HO-radical concentrations given by the new chemical box model, the `Regional Atmospheric Chemistry Mechanisma (RACM), explicitly considering the isoprene and monoterpene chemistry (Stockwell et al., 1997). It includes rate constants and product yields from the most recent laboratory measurements, and it has been tested against environmental chamber data.
3. Results and discussion 3.1. VOC mixing ratios and meteorological parameters The results of the measuring period in August 1995 for atmospheric trace gases and meteorological parameters are presented in Fig. 1a, 1b. The period investigated was characterised by bright weather with high temperatures of up to 323C. The wind speed reached values of up to 4.5 m s\ and the prevailing wind direction was changing from north east (453) at the "rst day to south-west (2453) at the second day. As the #ux-gradient relationship requires a certain wind speed ('1 m s\), fetch (in this case 4 km), and no in#uence of the tower itself on the turbulence structure of the atmosphere, a date selection has to be made according to the placement of the meteorological sensors at the tower, the vegetation cover and the wind speed. Therefore, only wind vectors between 200 and 2803 at wind speeds greater than 1 m s\ were considered in the #ux calculations. The NMHC mixing ratios in ambient air clearly demonstrated that the site is strongly in#uenced by tra$c. Benzene, toluene m-, p- and o-xylene showed maximum mixing ratios of about 3 ppb during the night when these substances were accumulating in the stable boundary layer as a result of ongoing tra$c on the roads crossing the forests. During daytime, mixing ratios decreased
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sharply as a result of increasing atmospheric turbulence and photochemical degradation of the substances. The mixing ratios of the anthropogenic NMHC were correlated with the mixing ratios of NO and NO , which V were also emitted mainly by cars. The diurnal cycle of NO and NO in general showed two maxima, one in the V morning and one in the late evening due to the rush hour of Frankfurt. However, on late Saturday 12th and Sunday 13th, the conditions were quite di!erent from the
days before as a result of local high tra$c on a parking lot nearby, combined with stable conditions of the boundary layer. The wind had calmed down during this period and the sky was overcast resulting in low turbulence and photochemistry which lead to high NO, NO and NMHC mixing ratios but very low ozone mixV ing ratios in the boundary layer. In contrast, ozone mixing ratios peak on the sunny days at 70 ppb. The shown diurnal cycles of ozone, NO and NO exhibit the V
Fig. 1. Diurnal courses of the mixing ratios and meteorological parameters in the Frankfurter Stadtwald during August 1995 at the ZUF-tower. (a) ozone, NO, NO (calibrated to NO ), (b) biogenic and (c) anthropogenic volatile organic compounds (VOC), and (d) air V temperature measured at 22 m height. Global radiation (d) was recorded at 3 m height and wind speed (e) was measured at 33 m height. Wind direction data (f ) were provided by the German weather service (DWD station Frankfurt/Main, Flughafen, about 5 km away from the tower). Blanks: no data available. In the stacked bar plots mixing ratios of single compounds are summed up for showing also the totals.
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Fig. 1. continued
classical behaviour in air polluted areas with maximum NO and NO mixing ratios leading to very low ozone V levels in the early morning around sunrise and late evening after sunset (Chameides et al., 1992). The biogenic NMHC in ambient air were dominated by isoprene resulting from isoprene-emitting oak tree species (Steinbrecher et al., 1997). The monoterpene bpinene was detected in much lower amounts (Fig. 1b). On 11 and 12th August the isoprene mixing ratios were lowest during the night at around 0.2 ppb. During daytime isoprene mixing ratios amounted to 1.5 ppb. For b-pinene the opposite diurnal cycles were observed with high mixing ratios in the night and very low mixing ratios during daytime. This "nding is consistent with observations at other sites (Enders et al., 1992; Schween et al.,
1997a; Riemer et al., 1998; Starn et al., 1998a,b). Isoprene and b-pinene are emitted from trees and this emission is correlated with temperature and light (for review see Steinbrecher, 1997; Steinbrecher and Ziegler, 1997). As biogenic isoprene emission peaked during daytime (see also Fig. 3), isoprene was able to accumulate in the air above the canopy despite turbulent transport and photochemistry. The source strength of b-pinene was much less resulting in low ambient air mixing ratios during daytime due to dilution and photochemistry. In the night, still ongoing b-pinene emission from resin reservoirs in pine trees lead to an accumulation of b-pinene in the stable boundary layer. Biogenic isoprene emission was almost zero in the night due to low physiological activity of the vegetation and mixing ratios in ambient air should drop
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to almost zero due to chemical degradation (Starn et al., 1998b). Isoprene deposition is negligible as pointed out by Guenther and Hill (1998). The authors report nighttime #uxes over an oak tree-dominated forest of near zero (0.04$0.12 nmol m\ s\). However, isoprene mixing ratios were about 0.25 ppb in the night. Although isoprene is mainly of biogenic origin, anthropogenic sources also have to be considered as isoprene is present in automobile exhaust (Du!y et al., 1999). A tracer for automobile exhaust is o-xylene. The mixing ratios of this compound were usually in range of 0.75 ppb except for the period when local air tra$c almost doubled observed mixing ratios. In consequence, isoprene mixing ratios also doubled in this speci"c night as compared to previous days. This implies that daytime isoprene mixing ratios were primarily a result of biogenic production, however, nighttime isoprene mixing ratios were of anthropogenic origin. 3.2. Stability functions The #ux-gradient relationship is based on the constant #ux assumption. The assumption of constant #ux is assumed to be ful"lled if the two momentum and heat #uxes provided by the two sonic anemometers in 28 and 41 m height di!er not more than 20% (uncertainty of the #ux determination) and the fetch requirement of 4 km (mean canopy height 12 m; measuring levels 22 and 51 m) is met. This is the case for wind velocities between 1 and 6.5 m s\ and appropriate wind directions between 2003 and 2803. The stability parameter f was also calculated from the sonic anemometer data. In case of constant #ux, site-speci"c stability functions for momentum U (f) and
heat U (f) have been calculated for a particular stability parameter f. Fluxes for a speci"c f were calculated by sliding averaging and then stability functions derived by regression analysis (s-method) (Fig. 2a and b). The values of the "tted function for a stable strati"ed surface layer (f'0) were higher than those given by Businger et al. (1971). The opposite was the case at unstable conditions (f(0). This "nding corresponded well with a greater turbulent mixing over a forest canopy during stable strati"cation and less turbulent mixing during unstable strati"cation compared with the surface layer over low vegetation. So the tall canopy acted as a sink for turbulence due to eddies penetrating into the space between single trees, as well as a source, due to the production of wakes in the lee of the trees. In a strongly turbulent, unstable strati"ed surface layer the sink e!ect dominates the source e!ect, whereas in a less turbulent, stable strati"ed surface layer the opposite is true. In Fig. 2c heat #uxes calculated by the new stability functions used in the #ux-gradient relationship and directly measured heat #uxes are compared. The slope of the regression line through the data points is nearly one with a regression coe$cient of 0.7. This result implies, that the
gradient method with the new stability functions for heat U (f) provides reliable turbulent heat #uxes at this site. U (f) is then used to calculate NMHC #uxes according to the #ux-gradient relationship. The uncertainty of NMHC #ux determination by this method is estimated to reach 50% (Schween et al., 1997a) when assuming an error in the mixing ratio measurements of 15%. 3.3. NMHC yuxes The #ux-gradient relationship requires no source or sink behaviour for the compounds of interest between the two measuring levels. However, for reactive NMHC this requirement may not be ful"lled. In order to investigate the importance of chemical degradation between 22 and 51 m for NMHC #uxes, the chemical loss rate for each compound was estimated. It has to be noted that only daytime values of 12 August were considered since wind direction and wind speed allowed #ux calculations only during this period. During daytime the lifetime of NMHC is mainly controlled by the HO-reaction (Stockwell et al., 1997). For an estimation of the e!ect of the HO-reactions on the #uxes, the characteristic loss rates c(q ) were determined for the di!erent NMHC accord ing to Eq. (3). The reactions follow a pseudo-"rst-order time law and the characteristic loss rate is given as c(q )"c(o)e\! GO .
(3)
The HO-reaction rate parameters, k , were calculated &for the measured temperatures (see Fig. 1d) from the Arrhenius expressions; C is the concentration of the HO-radical, q the transfer time between 22 and 55 m, and C(0) the initial mixing ratio of the speci"c compound. The HO-radical concentrations used in the calculations were adjusted according the RACM modelled diurnal cycles of HO-concentrations typical for the geographic location. Hydroxyl-radical concentrations of 5;10 molecules cm\ were used for data at 8:00 and 17:00 CET, respectively, 1;10 molecules cm\ for 9:00, 10:00 CET, 15:00 and 16:00 CET values and 2;10 molecules cm\ for 11:00 to 14:00 CET values (Stockwell et al., 1997). The reaction of the NMHC with ozone along the gradient between 22 and 51 m at transfer times of up to 100 s during daytime was not considered to be an important loss process because for the most reactive compound, b-pinene, the lifetime at highest observed ozone mixing ratios is 12 h. For the less reactive anthropogenic compounds the maximum loss due to HO-radical reaction is of the order of 0.1}0.6%. The losses for isoprene and b-pinene were an order of magnitude higher and amounted to 3.1 and 2.5%, respectively. In Fig. 3 calculated #uxes of biogenic and anthropogenic NMHC and the in#uence of chemical degradation along the gradient due to HO-radical reaction are shown.
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Fig. 2. Calculated stability functions for heat U (f) (a) and momentum U (f) (b) for the Frankfurter Stadtwald. Measurements were
performed in 1994 from 7 March to 29 September (n"142). The circles denote the values of the stability function averaged for a certain stability interval f, the error bars are the standard deviation for each average (n"8}30). The heat #uxes calculated by the gradient approach using the new stability function and the directly measured heat #ux by the eddy correlation technique are compared in (c).
If the mixing ratio di!erence between the two heights was increased due to chemical loss process along the gradient, the calculated #uxes according to Eq. (1) were overestimated. As the observed di!erences are usually very small, small changes in the absolute mixing ratios result
in signi"cant changed #uxes. This is the case for the most reactive compound isoprene and the uncorrected #ux is up to 14% higher if chemical degradation is not considered. However, for less reactive compounds corrected #uxes and uncorrected #uxes were comparable. It has to
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Fig. 3. Comparison of measured and corrected NMHC #uxes over the Frankfurter Stadtwald. The #ux correction due to chemical degradation along the gradient between 22 and 51 m was performed for the reaction with the HO-radical with HO-concentration of 5;10 molecules cm\, (8:00 and 17:00 CET), 1;10 molecules cm\ (9:00, 10:00 CET and 15:00, 16:00 CET) and 2;10 molecules cm\ (11:00 to 14:00 CET), respectively, to simulate the diurnal cycle of the HO-radical concentration in the ambient air (Stockwell et al., 1997). Fluxes are related to ground area. Blanks: mixing ratio di!erences were lower than the measuring accuracy of 5% of the GC analysis. * #uxes calculated at the 5% accuracy limit. In the stacked bar plots #ux rates of single compounds are summed up for showing also the totals.
be noted that the correction for chemical degradation along the gradient 22 and 51 m did not change the direction of the calculated #uxes and the forest was, as expected, a source for biogenic NMHC during daytime. The uncertainty in the #ux calculations amounted to $0.5 nmol for isoprene as demonstrated by negative and positive isoprene #uxes at 09.00 and 17:00 CET, respectively, calculated by using mixing ratio di!erences near the GC accuracy limit of 5%.
Isoprene #uxes amount to about 3 nmol m\ ground area s\ as a result of the isoprene emission from the oaks. Monoterpene emissions were very low and only b-pinene #uxes up to 0.1 nmol m\ s\ could be detected as a result of the small fraction of conifers growing in the foot print area. The maximum #ux was observed when the vegetation was most active during the day and turbulence was maximal. During nighttime #uxes could not be resolved due to limited meteorological conditions
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and the accuracy limit of the GC analysis. However, based on the "ndings of Guenther and Hill (1998) it is expected that during night #uxes of biogenic NMHC are near zero at this site. For this site, according to procedures used in BVOC emission inventories (Simpson et al., 1995) isoprene emission is calculated to amount to 7 nmol m\ s\ (for 303C and full sun, assuming that 50% of the oak leaves are fully active). Compared to the measured #uxes estimated #uxes are about a factor of two higher showing that present emission inventories possibly overestimate emission. The observed anthropogenic NMHC #uxes were small despite the high load of anthropogenic NMHC in the air above the forest. Surprisingly, the forest acted sometimes as a source for anthropogenic NMHC. The high road tra$c inside the forest may be an explanation for this phenomenon. The exhaust fumes spread in the trunk space and then were transported through the canopy to the boundary layer.
4. Conclusions This case study shows for the "rst time #uxes for isoprene, monoterpenes and major anthropogenic VOC in an urban-impacted oak/beech/pine forest in Europe. For this forest, speci"c stability functions have been calculated showing the necessity of adapting the classical #ux-gradient relationship to tall vegetation. For enhancing the resolution of the #ux determination the height gradient chosen was maximal at 39 m. This high gradient, however, may lead to transport times which are greater than the gas phase reaction times of the reactive isoprene. Therefore, #uxes may be overestimated by up to 14% if neglecting chemical degradation along the gradient. Fluxes of biogenic and anthropogenic NMHC were in the range of some nmol m\ s\. The high observed isoprene #ux during daytime underlines the importance of isoprene chemistry for photooxidant formation in the summertime in the Frankfurt area. However, measured isoprene #uxes were at the lower end of isoprene emission estimates made by landscape emission models. In general, the forest is no source or sink for anthropgenic NMHC. However, the speci"c situation in the Frankfurter Stadtwald with high road tra$c inside the forest lead to sometimes signi"cant emission of anthropogenic NMHC as exhaust plume were spread in the trunk space. This phenomenon needs further investigation with a detailed analysis of the transport and chemical reaction properties inside the canopy.
Acknowledgements We like to acknowledge the funding of the project by the German Department of Education and Research,
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projects 07EU766, 07EU722 and 07EU812. Beyond that we would like to thank Uwe Schickedanz (ZUF) for his assistance during the "eld campaign.
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