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The ambient concentrations of biogenic hydrocarbons at a northern European, boreal site
Atmospheric Environment 34 (2000) 4971}4982
The ambient concentrations of biogenic hydrocarbons at a northern European, boreal site Hannele Hakola*, Tuomas Laurila, Janne Rinne, Katri Puhto Air Quality Research, Finnish Meteorological Institute, Sahaajankatu 20 E, Fin-00810 Helsinki, Finland Received 13 December 1999; received in revised form 20 March 2000; accepted 24 March 2000
Abstract Concentrations of monoterpenes, 1,8-cineol and light hydrocarbons were measured in PoK tsoK nvaara, Ilomantsi, Eastern Finland during two growing seasons in 1997 and 1998. The measuring site was located on the top of a hill, outside a mixed forest. The monthly average summer concentrations of isoprene were 0.3}1.7 ppbC and monoterpenes and 1,8-cineol together 1.6}3.2 ppbC. Isoprene and a-pinene were the most abundant compounds throughout the growing season, but b-pinene, D-carene, camphene, 1,8-cineol, sabinene and limonene were found as well. Isoprene and sabinene concentrations started to increase later than the concentrations of other compounds, and were better correlated with each other than with other compounds. Diurnal variations of monoterpenes show a minimum in the daytime and a maximum at night, except sabinene at midsummer, that has maximum concentrations during the day. The "eld data support the idea that the e!ective temperature sum can be used to predict the initiation of emissions of isoprene and also terpene emissions from Betula pendula. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Isoprene; Monoterpenes; 1,8-cineol; Biogenic VOC; E!ective temperature sum (ETS); Ambient concentrations
1. Introduction The boreal vegetation zone is one of world's largest forest areas, covering a land surface of 15.8;10 km (Archibold, 1995). The area consists mainly of coniferous forests, but wetlands are also prominent features of boreal areas. The main tree species in the European boreal forest are Pinus sylvestris L. and Picea abies L. The main deciduous trees in Finland are Betula pendula and Betula pubescens, but Populus tremula and Alnus incana are also common. The boreal coniferous trees and birches are monoterpene emitters and isoprene is emitted at least by Picea abies, Populus tremula and Salix species (Janson, 1993; Steinbrecher et al., 1997; Hakola et al., 1998; Hau! et al., 1999; Janson et al., 1999; Rinne et al., 2000a, b). Wetlands are also a signi"cant source of isoprene (Janson and De Serves, 1998). The growing season
* Corresponding author. E-mail address: hannele.hakola@fmi." (H. Hakola).
lasts from May to September, and during that period the biogenic compounds are likely to have a profound e!ect on the atmospheric chemistry of remote boreal areas due to the large biomass involved. For the year 1997 (April}September) the total biogenic volatile organic compounds (VOC) from forests in the Middle boreal zone, in Eastern Finland, is estimated to be 1.72 ton km\ (Lindfors and Laurila, 2000). Monoterpenes contribute 0.72, isoprene 0.13 and other VOCs 0.82 ton km\ to the total amount. For the whole of Finland, biogenic emissions are calculated to be 318 kiloton per annum, which exceeds the annual anthropogenic VOC emissions of 193 kiloton (Mroueh, 1994). The biogenic emissions are a!ected by a number of factors; temperature, light intensity, plant phenology, injury, stress, etc. (Kesselmeier and Staudt, 1999) making emission inventories di$cult. Ambient measurements of biogenic compounds in di!erent environments can help in estimating the performance of such inventories. Ambient biogenic VOC measurements from European boreal forests have been reported by Hov et al. (1983) and Janson (1992). Hov et al. found terpene concentrations in coniferous
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forest air in Norway varying from 8.8 to 70.7 ppbC. Janson reported similar terpene concentrations in a pine forest in Sweden, maximum concentrations being &80 ppbC. In the atmosphere, most of the VOCs react rapidly with OH radicals, thus a!ecting the oxidative capacity of the atmosphere (Atkinson, 1994; Atkinson and Arey, 1998). Riemer et al. (1998) have measured di!erent VOCs and oxygenated VOCs in the southeastern USA and concluded that isoprene is the dominant sink for OH radicals. Similar results have also been obtained from Canadian boreal forests (Young et al., 1997). Laurila and Hakola (1996) reported light hydrocarbon (C }C ) measurements at two remote sites in Finland and discovered that isoprene, 1-butene, ethene and propene contributed signi"cantly to the total reactivity of light hydrocarbons, comprising more than half of the reactive air mass. Depending on the amount of nitrogen oxides, they can also a!ect ozone concentrations (Chameides et al., 1992). Even at normally clean locations with low NO V levels, ozone can be produced when occasionally polluted air masses are introduced into the area (Biesenthal et al., 1998). In addition to ozone formation VOCs also have a potential for forming secondary organic aerosols (Ho!mann et al., 1997). We report ambient concentrations of biogenic volatile organic compounds measured in the Middle boreal zone in Finland, at PoK tsoK nvaara, Ilomantsi (63308 N, 31303 E, 230 m above m.s.l.), during two growing seasons, 1997 and 1998, in order to study the variability of the VOC composition throughout the growing season.
The Tenax samples were analyzed using an HP 5890 gas chromatograph with an HP-1 column (60 m, i.d. 0.25 mm) and an HP-5972 mass selective detector in singular ion monitoring mode. The samples were concentrated in a Perkin-Elmer ATD-400 automated preconcentration unit. The calibration was performed using liquid standards in methanol solutions. The available standards were a-pinene, b-pinene, D-carene, d-limonene, camphene, an isomeric mixture of ocimenes, terpinolene and sabinene. b-Pinene and myrcene co-eluted and their sum is expressed as b-pinene. The canister samples were analyzed using an HP- 5890 gas chromatograph equipped with a #ame ionization detector and an Al O /KCl PLOT column (50 m;0.32 mm i.d.). The sample volume was about 0.5 l (the exact sample volume was determined by measuring the pressure in the canister prior to and after the analysis) and the samples were preconcentrated in two liquid nitrogen traps, in a stainless steel loop (1/8 ;125 cm) with glass beads and in a capillary trap. In order to dry it, the sample was passed through a 10 cm long stainless-steel tube "lled with K CO and NaOH. The calibration was performed using a gaseous standard from the National Physical Laboratory in the UK containing 27 hydrocarbons including isoprene, and analyzed as regular samples. The accuracy of the repeated Tenax calibration sample analysis was estimated to be about 6% for each terpene compound except limonene and 5% for light hydrocarbon analysis. Limonene was frequently found in blank tubes and therefore analytical error is bigger.
2. Experimental
3. Variability of the concentrations during the growing season
Samples were collected from an open "eld, on top of a hill, about 1.5 m above the ground. The distance from the forest was about 100 m. The main tree species growing on the area is Pinus sylvestris, but also spruces, birches and aspens are common. Close to the measuring site there are also willow bushes. The land cover is mainly forested, but with some "elds and cuttings. About 100 m from the measuring site there was a house and a narrow road leading to the house from a broader road (about 500 m) with a low tra$c intensity (50}100 cars day\). Three-litre samples were collected 3}5 times week\ into Tenax-TA adsorbent tubes using #ow-controlled pumps (Ametek Alpha-2). The sampling time varied from 11 a.m. to 4 p.m. Flow rates were 100 ml min\. In addition, larger sample volumes were collected for mass selective analysis in full scan mode for correct identi"cation of the compounds. Ozone was removed from the samples by introducing a MnO gauze in a Te#on holder in front of the Tenax tube. Light hydrocarbons (C }C ) were collected into evacuated stainless steel canisters (volume 0.85 l).
The measurements of the monoterpenes, isoprene and some light hydrocarbons are presented together with 30-day running medians in Fig. 1 and mean monthly values in Table 1. a-Pinene, b-pinene/myrcene, Dcarene, camphene and 1,8-cineol concentrations started to increase in late May/early June. The concentrations were the highest in June and July, and the correlation between a-pinene, b-pinene/myrcene, and D-carene was good (Table 2). The concentrations decreased in August and in September and October were low. Camphene concentrations showed a narrow peak in 1997 at the end of July, when they were high for only 2}3 weeks, then decreasing back to very low values. Limonene concentrations remained quite low throughout the growing season, a slight increase in the amount of limonene being seen in late autumn of 1997. The limonene increase took place at a time, when growing season was over and leaves had fallen. We can give no explanation for limonene increase; the following year it was not observed. Of the monoterpenes observed
H. Hakola et al. / Atmospheric Environment 34 (2000) 4971}4982
limonene is also the most reactive towards the OHradical and ozone, and these reactions get slower as autumn proceeds. Sabinene concentrations were low until the end of June increasing later than the concentrations of pinenes, 1,8cineol and 3-carene. Large sabinene and ocimene (mostly trans-ocimene) emissions have been measured from Betula pendula (Hakola et al., 1998) in southern Finland. The emissions also increased at the end of June, after the leaves had reached their full size, and remained high until the end of August. It is therefore likely that the sabinene in the air is from birches. Ocimenes are very reactive towards ozone and the OH radical (Table 3), and were
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only occasionally found in the ambient air samples, and then in very low concentrations. Sabinene concentrations were better correlated with isoprene than with other monoterpenes (Table 2). It is possible that the mechanism leading to the emission of sabinene is di!erent from that of the other monoterpenes, perhaps light dependent. Staudt et al. (2000) have shown that in the emissions of Pinus pinea some compounds (t-b-ocimene, 1,8-cineol and linalool) were emitted only during sunlit hours. Isoprene concentrations started to increase in the middle of June, 2}3 weeks later than most of the monoterpene concentrations, the maximum appearing
Fig. 1. Measurements and 30-day running median of ambient VOC concentrations at Ilomantsi, Finland.
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Fig. 1. (Continued).
concomitant with the sabinene maximum (Fig. 1). Monson et al. (1994) have found that the isoprene emission from aspen leaves growing on the Rocky Mountains of Colorado can be predicted as the time required after budbreak for the cumulative temperatures above 53C to reach approximately 3003 days. Hakola et al. (1998) have also studied the dependence of isoprene emission rates on phenology. Based on cuvette emission measurements, they found that isoprene emissions from Populus tremula and Salix phylicifolia began when the annual e!ective temperature sum (ETS) (not the temperature sum after budbreak used by Monson et al.) exceeded 210 and 2803
days for Salix phylicifolia and Populus tremula, respectively. The ETS is de"ned here as the sum of the daily mean temperatures exceeding a chosen threshold temperature (53C) and it is shown in Fig. 2 for years 1997 and 1998 together with isoprene and sabinene concentrations. Since isoprene data from the year 1996 is also available from the site, it is included in the "gure. This would result in the initiation of isoprene emissions from Populus tremula on 24, 14 and 18 June in 1996, 1997 and 1998, respectively. The isoprene emissions from Salix phylicifolia would begin on 14 June, 10 June and 13 June in 1996, 1997 and 1998, respectively. The "rst high
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Table 1 The monthly means of isoprene, monoterpenes and 1,8-cineol concentrations at Ilomantsi 1997
1998
pptv
May
June
July
Aug.
Sep.
Oct.
May
June
July
Aug.
Sep.
Oct.
a-Pinene Camphene Sabinene b-Pinene/myrcene 3-Carene Limonene 1,8-Cineol Isoprene
95 5 0 7 12 4 2 3
198 17 4 32 48 13 14 120
141 35 43 28 39 12 14 228
148 17 18 28 38 3 7 207
85 12 3 13 21 10 2 20
39 10 1 10 14 21 3 11
82 5 1 7 20 0 5 18
147 17 5 24 34 1 21 115
116 9 24 22 26 8 23 346
99 7 7 18 22 0 11 68
121 12 2 19 22 0 4 61
36 10 0 9 8 0 3 0
Table 2 Correlation coe$cient (r) between individual terpenoid compounds from the years 1997 and 1998
a-Pinene Camphene Sabinene b-Pinene/myrcene 3-Carene Limonene 1,8-Cineol Isoprene
a-Pinene
Camphene
Sabinene
b-Pinene
3-Carene
Limonene
1,8-Cineol
Isoprene
1
0.65 1
0.24 0.40 1
0.88 0.73 0.36 1
0.93 0.70 0.35 0.95 1
0.33 0.14 0.23 0.22 0.37 1
0.62 0.44 0.43 0.69 0.69 0.32 1
0.47 0.40 0.78 0.65 0.63 0.41 0.70 1
The data set includes 118 monoterpene measurements and 89 isoprene measurements.
Table 3 Rate constants k for the reactions with monoterpenes and 1,8-cineol with NO radical, OH radical and ozone (Atkinson, 1994)
a-Pinene Camphene Sabinene b-Pinene Myrcene 3-Carene Limonene 1,8-Cineol Trans-ocimene Ethene Propene 1-Butene Isoprene
OH
Ozone
NO
10k (cm molecule\ s\)
10k (cm molecule\ s\)
10k (cm molecule\ s\)
53.7 53 117 78.9 215 88 171 11.1 252 8.52 26.3 31.4 101
86.6 0.90 86 15 470 37 200 (0.15 540 1.59 10.1 9.64 12.8
6.16 0.66 10 2.51 11 9.1 12.2 0.00017 22 0.0002 0.0095 0.0121 0.68
isoprene concentrations were measured in 1996 on 23 June (142 pptv), in 1997 on 9 June (349 pptv) and in 1998 on 12 June (290 pptv) corresponding to ETS values of 271, 151 and 2063 days, respectively, although smaller
isoprene concentrations were already detected earlier than that. Picea abies is also a weak isoprene emitter, but the seasonal variability of the emissions is not well known. Since the forest close to the measuring site is
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Fig. 2. E!ective temperature sum at Ilomantsi and isoprene and sabinene measurements.
mainly pine mixed with aspens and birches, the isoprene emissions from Populus tremula and Salix phylicifolia would seem to explain a lot of the observed ambient isoprene concentrations. Using phenological data covering the whole of Finland, Lappalainen (1993) modelled the lea"ng and #owering of deciduous trees as a function of ETS. In Eastern Finland the lea"ng of Populus tremula starts approximately when the ETS value exceeds 145. The study did not include lea"ng of willows. Lea"ng was recorded as the time when the trees started to turn green and the "rst leaves were fully open. ETS 1453 days was reached in 8, 9 and 8 June in 1996, 1997 and 1998, respectively. The di!erence between isoprene emissions and the lea"ng is 1343 days. This is a lower value than the 3003 days observed by Monson et al. for aspens growing on the Rocky Mountains. The estimation suggests that the phenological maturing processes may be faster in more northern climate zones. Our site was also at a lower elevation, and the aspen trees were di!erent species. Lindfors and Laurila (2000) calculated isoprene emissions from the region where the measurement site is
located using meteorological time series for 1995}1997, regional average biomass data and the light and temperature algorithm by Guenther (1997). Biomass data and emission factors were assumed constant for the calculation period (April}October). The modeled isoprene emission variability was very similar to the observed ambient air concentrations. The only major discrepancy was that the model simulated substantial emissions already in May but in the concentration measurements they were not observed until June. This suggests that low concentrations before June are not due to low light and temperature conditions but phenological change in biomass or emission factors. The ETS has also been used to predict the monoterpene emissions of Betula pendula (Hakola et al., 2000). Based on cuvette measurements, conducted on trees growing in southern Finland, high emission rates of sabinene and the ocimenes were observed after the leaves had reached their full size. This took place after the ETS had exceeded 4003 days. Four-hundred degree days was exceeded in 1997 on 2 July, and in 1998 on 4 July. Sabinene concentrations increased on 1 and 4 July in 1997 and 1998, respectively, although low sabinene concentrations were already seen at mid June in both years. The ETS values on these days were 385 and 406, respectively. 1-Butene, ethene and propene have been shown to have biogenic sources (Goldstein et al., 1996; Hakola et al., 1998) although anthropogenic sources exist as well. In our data set the concentrations of these compounds showed only little seasonal variability during the period measurements were conducted. These compounds are very reactive towards OH radicals, and their concentrations already decrease from the higher winter values before May (Laurila and Hakola, 1996). Ethene and propene showed an additional, small summer maximum, suggesting a biogenic source. Laurila et al. (1999) applied factor analysis to three Finnish background stations measuring light hydrocarbons, one of which was Ilomantsi. They found that propene, ethene and 1-butene belong to the same factor which is related to both anthropogenic and biogenic origin. Of the light hydrocarbons, isoprene was alone in a purely biogenic group. Due to their di!erent sources, the relative abundances of the terpenoid compounds vary throughout the growing season. (Table 1, Fig. 3). In May, almost 70% of the terpenoid compounds is a-pinene, but in June isoprene concentrations increase, and in July isoprene comprises half of the total terpenoid amount. a-Pinene and isoprene are the most abundant compounds throughout the growing season, but the share of the other monoterpenes is substantial, the other important monoterpenes being bpinene/myrcene, D-carene and camphene. The amount of sabinene is signi"cant in July and August only. Although the concentrations of the terpenoid compounds are quite low, also much lower than
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Fig. 3. Monthly average concentration pro"les of monoterpenes, 1,8-cineol and isoprene at Ilomantsi 1997}1998.
concentrations of light hydrocarbons in summer, they do contribute signi"cantly to the local reactivity of the measured VOCs. Fig. 4 shows the monthly average mole fractions and reactivity-based relative abundances of the di!erent hydrocarbon groups. Based on OH-reactivities, concentrations were scaled to propylene equivalents according to Chameides et al. (1992). During the whole growing season terpenes contribute more than half of the total reactivity towards the OH radical. Our study did not include measurements of compound classes like aromatics (except benzene), aldehydes, ketones, alcohols, and organic acids. These compounds are also likely to have a profound e!ect on oxidative capacity. Ciccioli et al. (1999) have shown that oxygenated compounds contributed the most to the total amount of VOCs detected in some Italian forests. Ambient isoprene concentrations are dependent on temperature, as also shown by Jobson et al. (1994), Goldan et al. (1995), and McClenny et al. (1998). The regressions obtained from data sets from Fraserdale, Canada (Jobson et al., 1994) and from Alabama, USA (Goldan et al., 1995) are shown together with our data in Fig. 5a. The dependences of concentrations on temperature in all these data sets are quite similar, but the
concentrations are much lower in Ilomantsi than in the boreal forest in Canada, or in Alabama. McClenny's measurements from Tennessee are closer to what was found in this study. Monoterpene concentrations were also found to be exponentially dependent on temperature (Fig. 5b). The temperature dependence of the monoterpene emission (E) of plants is usually described by the equation E"E exp[b(¹!303C)], where E is the monoterpene emisson normalized to 303C, b is an empirical coe$cient and ¹ is the leaf temperature (Guenther et al., 1993). We found lower b-coe$cient (0.073C\) for the ambient air monoterpene concentrations than those observed in the emission rate measurements. Guenther et al. (1993) suggest that the best estimate of the temperature coe$cient on the basis of reported measurements is 0.093C\. In boreal areas also larger coe$cients have been found; Hakola et al. (1998) found b"0.243C\ for monoterpene emissions rates from Betula pendula in southern Finland and Rinne et al. (2000a) found b"0.1463C\ in canopy scale monoterpene emission measurements using the
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Fig. 4. Monthly mean concentrations and reactivity-scaled concentrations of total monoterpenes together with 1,8-cineol, isoprene, other light alkenes, alkanes and benzene together with acetylene in 1997 and 1998 at Ilomantsi.
micrometeorological gradient method over a Scots pine forest about 40 km southwest of our site. During middayafternoon, when the ambient samples were taken, e$cient mixing and rapid chemical reactions with the OH radical may act to reduce high ambient concentrations. The emissions are the highest when it is warm with plenty of sunshine; this is also when the destruction is fastest. To roughly estimate the e!ect of vertical mixing, hourly mixing heights were calculated for the nearest sounding station, which is located 300 km to the west of our site. The calculation procedure described in Karppinen et al. (1998), uses surface weather and sounding observations and an energy balance model. We assume that the mixing heights calculated for that site represent mixing conditions in Ilomantsi, because climatic and land-use conditions are similar. Afternoon mixing heights calculated for the study period varied between 300 and 3000 m. We could not "nd any clear dependence between afternoon mixing heights and total
monoterpene and isoprene concentrations. It seems that the in#uence of local emissions on ambient concentrations close to the surface is much greater than that of the afternoon mixing height. In summer at noon, the boundary layer is anyway well mixed, either by thermal or mechanical turbulence. The spatial distance between the measurement and the sounding site may, however, cause some additional uncertainty.
4. Diurnal variations of monoterpenes Diurnal variations of monoterpene and 1,8-cineol concentrations were measured four times during the summer of 1998; 13}14 May, 10}11 June, 30 June}1 July, and 22}23 September. For most of the monoterpenes, maximum concentrations were measured at night and minimum during the day. The same diurnal pattern has been measured earlier at least by Riba et al. (1987) and
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Fig. 6. Temperature and 30-day running medians of isoprene and the sum of monoterpenes and 1,8-cineol.
Fig. 5. (a) Ambient isoprene concentrations as a function of temperature from Ilomantsi. The regressions from measurements from Alabama, USA (diamonds) (Goldan et al., 1995) and Fraserdale, Canada (squares) (Jobson et al., 1994) are also shown. (b) Sum of monoterpene and 1,8-cineol concentrations as a function of temperature.
Fehsenfeld et al. (1992). This is contrary to the diurnal variation of the emissions (Steinbrecher et al., 1999). The low daytime concentrations observed are due to e$cient sink reactions with OH radicals and dilution by vertical transport. The compounds emitted during the night are trapped in a nocturnal inversion layer, and their concentrations build up until they are diluted in the morning by mixing. (Fig. 6) In the "rst measurements conducted in the middle of May, only a-pinene, b-pinene, D-carene, 1,8-cineol and camphene were present in concentrations above the detection limit. All of the compounds had a maximum during the night; 1,8-cineol reacts only slowly with the OH-radical, and the diurnal curve is less marked than for monoterpenes. During the night of 14 May, monoterpene concentrations rapidly decreased, with a-pinene,
b-pinene, and D-carene vanishing completely. There is no minimum observed in 1,8-cineol concentrations. aPinene, b-pinene, and D-carene react much faster with ozone and the NO radical than does camphene, and 1,8-cineol is quite unreactive (Table 3); the nighttime drop in the concentrations could thus be partly due to chemical reactions. The complex nighttime behavior of the monoterpene concentrations can be partly due to the fact that the measurement site is on the top of a hill. The hilltop can remain above the nighttime inversion of lower elevations, thus de-coupling the site from most monoterpene sources. The higher nighttime VOC concentrations in general, however, drop to lower daytime values, concomitant with an ozone increase caused by e$cient mixing and photochemistry. (Fig. 7) The two summer measurements in the middle of June and at the beginning of July are quite similar. Ozone data was lost in July due to a malfunction of the instrument. All other compounds again show a maximum in the night and minimum during the day, except sabinene. Sabinene reacts about twice as fast with the OH-radical than, for example a-pinene, and sabinene concentrations should therefore decrease like the others. The maximum sabinene concentration observed during the day indicates emission sources that are more e$cient during the day. Some monoterpenes are known to be produced by immediate light-induced synthesis (Schuh et al., 1997; Staudt et al., 1997). The last diurnal cycle measured at the end of the growing season, showed a longer nighttime maximum due to the longer nights. Ozone decreases during the night due to the reactions with monoterpenes and also due to deposition.
5. Summary Light hydrocarbon and monoterpene concentrations were measured during two growing seasons in Finland,
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Fig. 7. Diurnal variations of monoterpenes on 13}14 May, 10}11 June, 30 June, 1 July, and 22}23 September, 1998 at Ilomantsi.
H. Hakola et al. / Atmospheric Environment 34 (2000) 4971}4982
between the southern and middle boreal zone. Samples were collected in a "eld, about 100 m outside a mixed forest. The monthly average summer concentrations of isoprene were 0.3}1.7 ppbC and monoterpenes and 1,8cineol together 1.6}3.2 ppbC. The concentrations of a-pinene, b-pinene, camphene, D-carene and 1,8-cineol started to increase in May, about one month earlier than the concentrations of sabinene and isoprene. Sabinene and isoprene were also well correlated with each other suggesting that sabinene, like isoprene, might have a light-dependent source. a-Pinene and isoprene were the most abundant compounds throughout the growing season. Biogenic hydrocarbons contribute signi"cantly to the total reactivity towards the OH radical. During summer months they comprise almost all of the measured OH reactivity. Monoterpene and isoprene concentrations are dependent on the ambient temperature. The collected data support the idea that the e!ective temperature sum can be used to predict the initiation of both isoprene emissions and also monoterpene emissions from Betula pendula. Seasonal emission rate measurements from other trees, especially conifers, would be important to see if the ETS might also have wider use. Acknowledgements We wish to thank Kari LyytikaK inen in Ilomantsi and Anne-Mari MaK kelaK at the FMI for technical assistance and also Ari Karppinen at the FMI for calculating the mixing heights. This work was funded by the Academy of Finland and the European Commission DG XII (contract number ENV4-CT95-0022). Janne Rinne gratefully acknowledges the Maj and Tor Nessling Foundation for "nancial support.
References Archibold, O.W., 1995. Ecology of World Vegetation. Chapman & Hall, London, 510 pp. Atkinson, R., 1994. Gas-phase tropospheric chemistry of organic compounds. Journal of Physical and Chemical Reference Data, Monograph 2, 216 pp. Atkinson, R., Arey, J., 1998. Atmospheric chemistry of biogenic organic compounds. Accounts of Chemical Research 31, 574}583. Biesenthal, T.A., Bottenheim, J.W., Shepson, P.B., Li, S.-M., Brickell, P.C., 1998. The chemistry of biogenic hydrocarbons at a rural site in eastern Canada. Journal of Geophysical Research 103, 25487}25498. Chameides, W.L., Fehsenfeld, F., Rodgers, M.O., Cardelino, C., Martinez, J., Parrish, D., Lonneman, W., Lawson, D.R., Rasmussen, R.A., Zimmerman, P., Greenberg, J., Middleton, P., Wang, T., 1992. Ozone precursor relationships in the ambient atmosphere. Journal of Geophysical Research 97, 6037}6055.
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Ciccioli, P., Brancaleoni, E., Frattoni, M., 1999. Reactive hydrocarbons in the atmosphere at urban and regional scales. In: Hewitt, C.N. (Ed.), Reactive Hydrocarbons in the Atmosphere. Academic Press, London, pp. 160}207. Fehsenfeld, F., Calvert, J., Fall, R., Goldan, P., Guenther, A.B., Hewitt, C.N., Lamb, B., Liu, S., Trainer, M., Westberg, H., Zimmerman, P., 1992. Emissions of volatile organic compounds from vegetation and the implications for atmospheric chemistry. Global Biogeochemical Cycles 6, 389}430. Goldan, P.D., Kuster, W.C., Fehsenfeld, F.C., Monzka, S.A., 1995. Hydrocarbon measurements in the southeastern United States: the rural oxidants in the Southern Environment (ROSE) program 1990. Journal of Geophysical Research 100, 25945}25963. Goldstein, A.H., Fan, S.M., Goulden, M.L., Munger, J.W., Wofsy, S.C., 1996. Emissions of ethene, propene, and 1-butene by a midlatitude forest. Journal of Geophysical Research 101, 9149}9157. Guenther, A., 1997. Seasonal and spatial variations in natural organic compound emissions. Ecological Applications 7, 34}45. Guenther, A., Zimmerman, P.R., Harley, P.C., Monson, R.K., Fall, R., 1993. Isoprene and monoterpene emission rate variability: model evaluations and sensitivity analyses. Journal of Geophysical Research 98, 12609}12617. Hakola, H., Rinne, J., Laurila, T., 1998. The hydrocarbon emission rates of tea-leafed willow (Salix phylicifolia), silver birch (Betula pendula) and European aspen (Populus tremula). Atmospheric Environment 32, 1825}1833. Hakola, H., Rinne, J., Laurila, T., 2000. Hydrocarbon emissions from silver birch, downy birch and grey alder. Manuscript in preparation. Hau!, K., RoK ssler, J., Hakola, H., Steinbrecher, R., 1999. Isoprenoid emission in European boreal forests. In: Borrell, P.M., Borrell, P. (Eds.), Proceedings of EUROTRAC Symposium '98. WITpress, Southampton, pp. 97}102. Ho!mann, T., Odum, J.R., Bowman, F., Collins, D., Klockow, D., Flagan, R.C., Seinfeld, J.H., 1997. Formation of organic aerosols from the oxidation of biogenic hydrocarbons. Journal of Atmospheric Chemistry 26, 189}222. Hov, "., Schjoldager, J., Wathne, B.M., 1983. Measurements and modeling of the concentrations of terpenes in coniferous forest air. Journal of Geophysical Research 88, 10679}10688. Janson, R., 1992. Monoterpene concentrations in and above a forest of Scots pine. Journal of Atmospheric Chemistry 14, 385}394. Janson, R., 1993. Monoterpene emissions from Scots pine and Norwegian spruce. Journal of Geophysical Research 98, 2839}2850. Janson, R., De Serves, C., 1998. Isoprene emissions from boreal wetlands in Scandinavia. Journal of Geophysical Research 103, 25513}25517. Janson, R., De Serves, C., Romero, R., 1999. Emission of isoprene and carbonyl compounds from a boreal forest and wetland in Sweden. Agricultural and Forest Meteorology 98}99, 671}681. Jobson, B.T., Wu, Z., Niki, H., Barrie, L.A., 1994. Seasonal trends of isoprene, C }C alkanes and acetylene at a remote boreal site in Canada. Journal of Geophysical Research 99, 1589}1599.
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Karppinen, A., Kukkonen, J., Nordlund, G., Rantakrans, E., Valkama, I., 1998. A dispersion modelling system for urban air pollution. Publications on Air Quality, Vol. 28. Finnish Meteorological Institute, Helsinki. Kesselmeier, J., Staudt, M., 1999. Biogenic volatile organic compounds (VOC): an overview on emission, physiology and ecology. Journal of Atmospheric Chemistry 33, 23}88. Laurila, T., Hakola, H., 1996. Seasonal cycle of C }C hydro carbons over the Baltic Sea and Northern Finland. Atmospheric Environment 30, 1597}1607. Laurila, T., Hakola, H., Lindfors, V., 1999. Biogenic VOCs in Continental Northern Europe } Concentrations and Photochemistry. Physics and Chemistry of Earth (B) 24, 689}693. Lappalainen, H., 1993. Examples between climate and plant phenology; Examples of plant phenological events in Finland and their relation to e!ective temperature sum. Licentiate thesis, the University of Helsinki, Department of Botany, 115 pp. Lindfors, V., Laurila, T., 2000. Biogenic VOC emissions from forests in Finland. Boreal Environment Research 5, 1}19. McClenny, W.A., Daughtery Jr., E.H., Adams, J.R., Oliver, K.D., Kronmiller, K.G., 1998. Volatile organic compound concentration patterns at the New Hendersonville monitoring site in the 1995 Southern Oxidants Study in the Nashville, Tennessee area. Journal of Geophysical Research 103, 22509}22518. Monson, R.K., Harley, P.C., Litvak, M.E., Wildermuth, M., Guenther, A.B., Zimmerman, P.R., Fall, R., 1994. Environmental and developmental controls over the seasonal pattern of isoprene emission from aspen leaves. Oecologia 99, 260}270. Mroueh, U.-M., 1994. Inventory of volatile organic compound (VOC) emissions (1993). Technical Research Centre of Finland, VTT-Research Notes 1609, 215 (in Finnish). Riba, M.L., Tathy, J.P., Tsiropoulos, N., Monsarrat, B., Torres, L., 1987. Diurnal variation in the concentration of a- and b-pinene in the Landes Forest (France). Atmospheric Environment 21, 191}193. Riemer, D., Pos, W., Milne, P., Farmer, C., Zika, R., Apel, E., Olszyna, K., Kliendienst, T., Lonneman, W., Bertman, S., Shepson, P., Starn, T., 1998. Observations of nonmethane
hydrocarbons and oxygenated volatile organic compounds at a rural site in the southeastern United States. Journal of Geophysical Research 103, 28111}28128. Rinne, J., Hakola, H., Laurila, T., Rannik, UG ., 2000a. Canopy scale monoterpene emissions of Pinus sylvestris dominated forests. Atmospheric Environment 34, 1099}1107. Rinne, J., Tuovinen, J.-P., Laurila, T., Hakola, H., Aurela, M., HypeH n, H., 2000b. Measurements of hydrocarbon #uxes by a gradient method above a northern boreal forest. Agricultural and Forest Meteorology 102, 25}37. Schuh, G., Heiden, A.C., Ho!mann, Th., Kahl, J., Rockel, P., Rudolph, J., Wildt, J., 1997. Emissions of volatile organic compounds from sun#ower and beech: dependence on temperature and light intensity. Journal of Atmospheric Chemistry 27, 291}318. Staudt, M., Bertin, N., Hansen, U., Seufert, G., Ciccioli, P., Foster, P., Frenzel, B., Fugit, J.-L., 1997. Seasonal and diurnal patterns of monoterpene emissions from Pinus pinea (L.) under "eld conditions. Atmospheric Environment 32, 145}156. Staudt, M., Bertin, N., Frenzel, B., Seufert, G., 2000. Seasonal variation in amount and composition of monoterpenes emitted by young Pinus pinea trees } Implications for emission modeling. Journal of Atmospheric Chemistry 35, 77}99. Steinbrecher, R., Ziegler, H., EichstaK dter, G., Fehsenfeld, U., Gabriel, R., Kolb, Ch., Rabong, R., SchoK nwitz, R., SchuK rmann, W., 1997. Monoterpene and isoprene emissions in Norway Spruce forests. In: Slanina, S. (Ed.), Biosphere}Atmosphere Exchange of Pollutants and Trace Substances. Springer, Heidelberg, pp. 352}365. Steinbrecher, R., Hau!, K., Hakola, H., RoK ssler, J., 1999. A revised parameterization for emission modeling of isoprenoids for boreal plants. In: Laurila, T., Lindfors, V. (Eds.), Biogenic VOC Emissions and Photochemistry in the Boreal Regions of Europe. Air Pollution Research Report No 70, European Commission, Luxembourg, pp. 29}43. Young, V.L., Kieser, B.N., Chen, S.P., Niki, H., 1997. Seasonal trends and local in#uences on nonmethane hydrocarbon concentrations in the Canadian boreal forest. Journal of Geophysical Research 102, 5913}5918.
The ambient concentrations of biogenic hydrocarbons at a northern European, boreal site Hannele Hakola*, Tuomas Laurila, Janne Rinne, Katri Puhto Air Quality Research, Finnish Meteorological Institute, Sahaajankatu 20 E, Fin-00810 Helsinki, Finland Received 13 December 1999; received in revised form 20 March 2000; accepted 24 March 2000
Abstract Concentrations of monoterpenes, 1,8-cineol and light hydrocarbons were measured in PoK tsoK nvaara, Ilomantsi, Eastern Finland during two growing seasons in 1997 and 1998. The measuring site was located on the top of a hill, outside a mixed forest. The monthly average summer concentrations of isoprene were 0.3}1.7 ppbC and monoterpenes and 1,8-cineol together 1.6}3.2 ppbC. Isoprene and a-pinene were the most abundant compounds throughout the growing season, but b-pinene, D-carene, camphene, 1,8-cineol, sabinene and limonene were found as well. Isoprene and sabinene concentrations started to increase later than the concentrations of other compounds, and were better correlated with each other than with other compounds. Diurnal variations of monoterpenes show a minimum in the daytime and a maximum at night, except sabinene at midsummer, that has maximum concentrations during the day. The "eld data support the idea that the e!ective temperature sum can be used to predict the initiation of emissions of isoprene and also terpene emissions from Betula pendula. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Isoprene; Monoterpenes; 1,8-cineol; Biogenic VOC; E!ective temperature sum (ETS); Ambient concentrations
1. Introduction The boreal vegetation zone is one of world's largest forest areas, covering a land surface of 15.8;10 km (Archibold, 1995). The area consists mainly of coniferous forests, but wetlands are also prominent features of boreal areas. The main tree species in the European boreal forest are Pinus sylvestris L. and Picea abies L. The main deciduous trees in Finland are Betula pendula and Betula pubescens, but Populus tremula and Alnus incana are also common. The boreal coniferous trees and birches are monoterpene emitters and isoprene is emitted at least by Picea abies, Populus tremula and Salix species (Janson, 1993; Steinbrecher et al., 1997; Hakola et al., 1998; Hau! et al., 1999; Janson et al., 1999; Rinne et al., 2000a, b). Wetlands are also a signi"cant source of isoprene (Janson and De Serves, 1998). The growing season
* Corresponding author. E-mail address: hannele.hakola@fmi." (H. Hakola).
lasts from May to September, and during that period the biogenic compounds are likely to have a profound e!ect on the atmospheric chemistry of remote boreal areas due to the large biomass involved. For the year 1997 (April}September) the total biogenic volatile organic compounds (VOC) from forests in the Middle boreal zone, in Eastern Finland, is estimated to be 1.72 ton km\ (Lindfors and Laurila, 2000). Monoterpenes contribute 0.72, isoprene 0.13 and other VOCs 0.82 ton km\ to the total amount. For the whole of Finland, biogenic emissions are calculated to be 318 kiloton per annum, which exceeds the annual anthropogenic VOC emissions of 193 kiloton (Mroueh, 1994). The biogenic emissions are a!ected by a number of factors; temperature, light intensity, plant phenology, injury, stress, etc. (Kesselmeier and Staudt, 1999) making emission inventories di$cult. Ambient measurements of biogenic compounds in di!erent environments can help in estimating the performance of such inventories. Ambient biogenic VOC measurements from European boreal forests have been reported by Hov et al. (1983) and Janson (1992). Hov et al. found terpene concentrations in coniferous
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 1 9 2 - 8
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forest air in Norway varying from 8.8 to 70.7 ppbC. Janson reported similar terpene concentrations in a pine forest in Sweden, maximum concentrations being &80 ppbC. In the atmosphere, most of the VOCs react rapidly with OH radicals, thus a!ecting the oxidative capacity of the atmosphere (Atkinson, 1994; Atkinson and Arey, 1998). Riemer et al. (1998) have measured di!erent VOCs and oxygenated VOCs in the southeastern USA and concluded that isoprene is the dominant sink for OH radicals. Similar results have also been obtained from Canadian boreal forests (Young et al., 1997). Laurila and Hakola (1996) reported light hydrocarbon (C }C ) measurements at two remote sites in Finland and discovered that isoprene, 1-butene, ethene and propene contributed signi"cantly to the total reactivity of light hydrocarbons, comprising more than half of the reactive air mass. Depending on the amount of nitrogen oxides, they can also a!ect ozone concentrations (Chameides et al., 1992). Even at normally clean locations with low NO V levels, ozone can be produced when occasionally polluted air masses are introduced into the area (Biesenthal et al., 1998). In addition to ozone formation VOCs also have a potential for forming secondary organic aerosols (Ho!mann et al., 1997). We report ambient concentrations of biogenic volatile organic compounds measured in the Middle boreal zone in Finland, at PoK tsoK nvaara, Ilomantsi (63308 N, 31303 E, 230 m above m.s.l.), during two growing seasons, 1997 and 1998, in order to study the variability of the VOC composition throughout the growing season.
The Tenax samples were analyzed using an HP 5890 gas chromatograph with an HP-1 column (60 m, i.d. 0.25 mm) and an HP-5972 mass selective detector in singular ion monitoring mode. The samples were concentrated in a Perkin-Elmer ATD-400 automated preconcentration unit. The calibration was performed using liquid standards in methanol solutions. The available standards were a-pinene, b-pinene, D-carene, d-limonene, camphene, an isomeric mixture of ocimenes, terpinolene and sabinene. b-Pinene and myrcene co-eluted and their sum is expressed as b-pinene. The canister samples were analyzed using an HP- 5890 gas chromatograph equipped with a #ame ionization detector and an Al O /KCl PLOT column (50 m;0.32 mm i.d.). The sample volume was about 0.5 l (the exact sample volume was determined by measuring the pressure in the canister prior to and after the analysis) and the samples were preconcentrated in two liquid nitrogen traps, in a stainless steel loop (1/8 ;125 cm) with glass beads and in a capillary trap. In order to dry it, the sample was passed through a 10 cm long stainless-steel tube "lled with K CO and NaOH. The calibration was performed using a gaseous standard from the National Physical Laboratory in the UK containing 27 hydrocarbons including isoprene, and analyzed as regular samples. The accuracy of the repeated Tenax calibration sample analysis was estimated to be about 6% for each terpene compound except limonene and 5% for light hydrocarbon analysis. Limonene was frequently found in blank tubes and therefore analytical error is bigger.
2. Experimental
3. Variability of the concentrations during the growing season
Samples were collected from an open "eld, on top of a hill, about 1.5 m above the ground. The distance from the forest was about 100 m. The main tree species growing on the area is Pinus sylvestris, but also spruces, birches and aspens are common. Close to the measuring site there are also willow bushes. The land cover is mainly forested, but with some "elds and cuttings. About 100 m from the measuring site there was a house and a narrow road leading to the house from a broader road (about 500 m) with a low tra$c intensity (50}100 cars day\). Three-litre samples were collected 3}5 times week\ into Tenax-TA adsorbent tubes using #ow-controlled pumps (Ametek Alpha-2). The sampling time varied from 11 a.m. to 4 p.m. Flow rates were 100 ml min\. In addition, larger sample volumes were collected for mass selective analysis in full scan mode for correct identi"cation of the compounds. Ozone was removed from the samples by introducing a MnO gauze in a Te#on holder in front of the Tenax tube. Light hydrocarbons (C }C ) were collected into evacuated stainless steel canisters (volume 0.85 l).
The measurements of the monoterpenes, isoprene and some light hydrocarbons are presented together with 30-day running medians in Fig. 1 and mean monthly values in Table 1. a-Pinene, b-pinene/myrcene, Dcarene, camphene and 1,8-cineol concentrations started to increase in late May/early June. The concentrations were the highest in June and July, and the correlation between a-pinene, b-pinene/myrcene, and D-carene was good (Table 2). The concentrations decreased in August and in September and October were low. Camphene concentrations showed a narrow peak in 1997 at the end of July, when they were high for only 2}3 weeks, then decreasing back to very low values. Limonene concentrations remained quite low throughout the growing season, a slight increase in the amount of limonene being seen in late autumn of 1997. The limonene increase took place at a time, when growing season was over and leaves had fallen. We can give no explanation for limonene increase; the following year it was not observed. Of the monoterpenes observed
H. Hakola et al. / Atmospheric Environment 34 (2000) 4971}4982
limonene is also the most reactive towards the OHradical and ozone, and these reactions get slower as autumn proceeds. Sabinene concentrations were low until the end of June increasing later than the concentrations of pinenes, 1,8cineol and 3-carene. Large sabinene and ocimene (mostly trans-ocimene) emissions have been measured from Betula pendula (Hakola et al., 1998) in southern Finland. The emissions also increased at the end of June, after the leaves had reached their full size, and remained high until the end of August. It is therefore likely that the sabinene in the air is from birches. Ocimenes are very reactive towards ozone and the OH radical (Table 3), and were
4973
only occasionally found in the ambient air samples, and then in very low concentrations. Sabinene concentrations were better correlated with isoprene than with other monoterpenes (Table 2). It is possible that the mechanism leading to the emission of sabinene is di!erent from that of the other monoterpenes, perhaps light dependent. Staudt et al. (2000) have shown that in the emissions of Pinus pinea some compounds (t-b-ocimene, 1,8-cineol and linalool) were emitted only during sunlit hours. Isoprene concentrations started to increase in the middle of June, 2}3 weeks later than most of the monoterpene concentrations, the maximum appearing
Fig. 1. Measurements and 30-day running median of ambient VOC concentrations at Ilomantsi, Finland.
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Fig. 1. (Continued).
concomitant with the sabinene maximum (Fig. 1). Monson et al. (1994) have found that the isoprene emission from aspen leaves growing on the Rocky Mountains of Colorado can be predicted as the time required after budbreak for the cumulative temperatures above 53C to reach approximately 3003 days. Hakola et al. (1998) have also studied the dependence of isoprene emission rates on phenology. Based on cuvette emission measurements, they found that isoprene emissions from Populus tremula and Salix phylicifolia began when the annual e!ective temperature sum (ETS) (not the temperature sum after budbreak used by Monson et al.) exceeded 210 and 2803
days for Salix phylicifolia and Populus tremula, respectively. The ETS is de"ned here as the sum of the daily mean temperatures exceeding a chosen threshold temperature (53C) and it is shown in Fig. 2 for years 1997 and 1998 together with isoprene and sabinene concentrations. Since isoprene data from the year 1996 is also available from the site, it is included in the "gure. This would result in the initiation of isoprene emissions from Populus tremula on 24, 14 and 18 June in 1996, 1997 and 1998, respectively. The isoprene emissions from Salix phylicifolia would begin on 14 June, 10 June and 13 June in 1996, 1997 and 1998, respectively. The "rst high
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Table 1 The monthly means of isoprene, monoterpenes and 1,8-cineol concentrations at Ilomantsi 1997
1998
pptv
May
June
July
Aug.
Sep.
Oct.
May
June
July
Aug.
Sep.
Oct.
a-Pinene Camphene Sabinene b-Pinene/myrcene 3-Carene Limonene 1,8-Cineol Isoprene
95 5 0 7 12 4 2 3
198 17 4 32 48 13 14 120
141 35 43 28 39 12 14 228
148 17 18 28 38 3 7 207
85 12 3 13 21 10 2 20
39 10 1 10 14 21 3 11
82 5 1 7 20 0 5 18
147 17 5 24 34 1 21 115
116 9 24 22 26 8 23 346
99 7 7 18 22 0 11 68
121 12 2 19 22 0 4 61
36 10 0 9 8 0 3 0
Table 2 Correlation coe$cient (r) between individual terpenoid compounds from the years 1997 and 1998
a-Pinene Camphene Sabinene b-Pinene/myrcene 3-Carene Limonene 1,8-Cineol Isoprene
a-Pinene
Camphene
Sabinene
b-Pinene
3-Carene
Limonene
1,8-Cineol
Isoprene
1
0.65 1
0.24 0.40 1
0.88 0.73 0.36 1
0.93 0.70 0.35 0.95 1
0.33 0.14 0.23 0.22 0.37 1
0.62 0.44 0.43 0.69 0.69 0.32 1
0.47 0.40 0.78 0.65 0.63 0.41 0.70 1
The data set includes 118 monoterpene measurements and 89 isoprene measurements.
Table 3 Rate constants k for the reactions with monoterpenes and 1,8-cineol with NO radical, OH radical and ozone (Atkinson, 1994)
a-Pinene Camphene Sabinene b-Pinene Myrcene 3-Carene Limonene 1,8-Cineol Trans-ocimene Ethene Propene 1-Butene Isoprene
OH
Ozone
NO
10k (cm molecule\ s\)
10k (cm molecule\ s\)
10k (cm molecule\ s\)
53.7 53 117 78.9 215 88 171 11.1 252 8.52 26.3 31.4 101
86.6 0.90 86 15 470 37 200 (0.15 540 1.59 10.1 9.64 12.8
6.16 0.66 10 2.51 11 9.1 12.2 0.00017 22 0.0002 0.0095 0.0121 0.68
isoprene concentrations were measured in 1996 on 23 June (142 pptv), in 1997 on 9 June (349 pptv) and in 1998 on 12 June (290 pptv) corresponding to ETS values of 271, 151 and 2063 days, respectively, although smaller
isoprene concentrations were already detected earlier than that. Picea abies is also a weak isoprene emitter, but the seasonal variability of the emissions is not well known. Since the forest close to the measuring site is
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Fig. 2. E!ective temperature sum at Ilomantsi and isoprene and sabinene measurements.
mainly pine mixed with aspens and birches, the isoprene emissions from Populus tremula and Salix phylicifolia would seem to explain a lot of the observed ambient isoprene concentrations. Using phenological data covering the whole of Finland, Lappalainen (1993) modelled the lea"ng and #owering of deciduous trees as a function of ETS. In Eastern Finland the lea"ng of Populus tremula starts approximately when the ETS value exceeds 145. The study did not include lea"ng of willows. Lea"ng was recorded as the time when the trees started to turn green and the "rst leaves were fully open. ETS 1453 days was reached in 8, 9 and 8 June in 1996, 1997 and 1998, respectively. The di!erence between isoprene emissions and the lea"ng is 1343 days. This is a lower value than the 3003 days observed by Monson et al. for aspens growing on the Rocky Mountains. The estimation suggests that the phenological maturing processes may be faster in more northern climate zones. Our site was also at a lower elevation, and the aspen trees were di!erent species. Lindfors and Laurila (2000) calculated isoprene emissions from the region where the measurement site is
located using meteorological time series for 1995}1997, regional average biomass data and the light and temperature algorithm by Guenther (1997). Biomass data and emission factors were assumed constant for the calculation period (April}October). The modeled isoprene emission variability was very similar to the observed ambient air concentrations. The only major discrepancy was that the model simulated substantial emissions already in May but in the concentration measurements they were not observed until June. This suggests that low concentrations before June are not due to low light and temperature conditions but phenological change in biomass or emission factors. The ETS has also been used to predict the monoterpene emissions of Betula pendula (Hakola et al., 2000). Based on cuvette measurements, conducted on trees growing in southern Finland, high emission rates of sabinene and the ocimenes were observed after the leaves had reached their full size. This took place after the ETS had exceeded 4003 days. Four-hundred degree days was exceeded in 1997 on 2 July, and in 1998 on 4 July. Sabinene concentrations increased on 1 and 4 July in 1997 and 1998, respectively, although low sabinene concentrations were already seen at mid June in both years. The ETS values on these days were 385 and 406, respectively. 1-Butene, ethene and propene have been shown to have biogenic sources (Goldstein et al., 1996; Hakola et al., 1998) although anthropogenic sources exist as well. In our data set the concentrations of these compounds showed only little seasonal variability during the period measurements were conducted. These compounds are very reactive towards OH radicals, and their concentrations already decrease from the higher winter values before May (Laurila and Hakola, 1996). Ethene and propene showed an additional, small summer maximum, suggesting a biogenic source. Laurila et al. (1999) applied factor analysis to three Finnish background stations measuring light hydrocarbons, one of which was Ilomantsi. They found that propene, ethene and 1-butene belong to the same factor which is related to both anthropogenic and biogenic origin. Of the light hydrocarbons, isoprene was alone in a purely biogenic group. Due to their di!erent sources, the relative abundances of the terpenoid compounds vary throughout the growing season. (Table 1, Fig. 3). In May, almost 70% of the terpenoid compounds is a-pinene, but in June isoprene concentrations increase, and in July isoprene comprises half of the total terpenoid amount. a-Pinene and isoprene are the most abundant compounds throughout the growing season, but the share of the other monoterpenes is substantial, the other important monoterpenes being bpinene/myrcene, D-carene and camphene. The amount of sabinene is signi"cant in July and August only. Although the concentrations of the terpenoid compounds are quite low, also much lower than
H. Hakola et al. / Atmospheric Environment 34 (2000) 4971}4982
4977
Fig. 3. Monthly average concentration pro"les of monoterpenes, 1,8-cineol and isoprene at Ilomantsi 1997}1998.
concentrations of light hydrocarbons in summer, they do contribute signi"cantly to the local reactivity of the measured VOCs. Fig. 4 shows the monthly average mole fractions and reactivity-based relative abundances of the di!erent hydrocarbon groups. Based on OH-reactivities, concentrations were scaled to propylene equivalents according to Chameides et al. (1992). During the whole growing season terpenes contribute more than half of the total reactivity towards the OH radical. Our study did not include measurements of compound classes like aromatics (except benzene), aldehydes, ketones, alcohols, and organic acids. These compounds are also likely to have a profound e!ect on oxidative capacity. Ciccioli et al. (1999) have shown that oxygenated compounds contributed the most to the total amount of VOCs detected in some Italian forests. Ambient isoprene concentrations are dependent on temperature, as also shown by Jobson et al. (1994), Goldan et al. (1995), and McClenny et al. (1998). The regressions obtained from data sets from Fraserdale, Canada (Jobson et al., 1994) and from Alabama, USA (Goldan et al., 1995) are shown together with our data in Fig. 5a. The dependences of concentrations on temperature in all these data sets are quite similar, but the
concentrations are much lower in Ilomantsi than in the boreal forest in Canada, or in Alabama. McClenny's measurements from Tennessee are closer to what was found in this study. Monoterpene concentrations were also found to be exponentially dependent on temperature (Fig. 5b). The temperature dependence of the monoterpene emission (E) of plants is usually described by the equation E"E exp[b(¹!303C)], where E is the monoterpene emisson normalized to 303C, b is an empirical coe$cient and ¹ is the leaf temperature (Guenther et al., 1993). We found lower b-coe$cient (0.073C\) for the ambient air monoterpene concentrations than those observed in the emission rate measurements. Guenther et al. (1993) suggest that the best estimate of the temperature coe$cient on the basis of reported measurements is 0.093C\. In boreal areas also larger coe$cients have been found; Hakola et al. (1998) found b"0.243C\ for monoterpene emissions rates from Betula pendula in southern Finland and Rinne et al. (2000a) found b"0.1463C\ in canopy scale monoterpene emission measurements using the
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H. Hakola et al. / Atmospheric Environment 34 (2000) 4971}4982
Fig. 4. Monthly mean concentrations and reactivity-scaled concentrations of total monoterpenes together with 1,8-cineol, isoprene, other light alkenes, alkanes and benzene together with acetylene in 1997 and 1998 at Ilomantsi.
micrometeorological gradient method over a Scots pine forest about 40 km southwest of our site. During middayafternoon, when the ambient samples were taken, e$cient mixing and rapid chemical reactions with the OH radical may act to reduce high ambient concentrations. The emissions are the highest when it is warm with plenty of sunshine; this is also when the destruction is fastest. To roughly estimate the e!ect of vertical mixing, hourly mixing heights were calculated for the nearest sounding station, which is located 300 km to the west of our site. The calculation procedure described in Karppinen et al. (1998), uses surface weather and sounding observations and an energy balance model. We assume that the mixing heights calculated for that site represent mixing conditions in Ilomantsi, because climatic and land-use conditions are similar. Afternoon mixing heights calculated for the study period varied between 300 and 3000 m. We could not "nd any clear dependence between afternoon mixing heights and total
monoterpene and isoprene concentrations. It seems that the in#uence of local emissions on ambient concentrations close to the surface is much greater than that of the afternoon mixing height. In summer at noon, the boundary layer is anyway well mixed, either by thermal or mechanical turbulence. The spatial distance between the measurement and the sounding site may, however, cause some additional uncertainty.
4. Diurnal variations of monoterpenes Diurnal variations of monoterpene and 1,8-cineol concentrations were measured four times during the summer of 1998; 13}14 May, 10}11 June, 30 June}1 July, and 22}23 September. For most of the monoterpenes, maximum concentrations were measured at night and minimum during the day. The same diurnal pattern has been measured earlier at least by Riba et al. (1987) and
H. Hakola et al. / Atmospheric Environment 34 (2000) 4971}4982
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Fig. 6. Temperature and 30-day running medians of isoprene and the sum of monoterpenes and 1,8-cineol.
Fig. 5. (a) Ambient isoprene concentrations as a function of temperature from Ilomantsi. The regressions from measurements from Alabama, USA (diamonds) (Goldan et al., 1995) and Fraserdale, Canada (squares) (Jobson et al., 1994) are also shown. (b) Sum of monoterpene and 1,8-cineol concentrations as a function of temperature.
Fehsenfeld et al. (1992). This is contrary to the diurnal variation of the emissions (Steinbrecher et al., 1999). The low daytime concentrations observed are due to e$cient sink reactions with OH radicals and dilution by vertical transport. The compounds emitted during the night are trapped in a nocturnal inversion layer, and their concentrations build up until they are diluted in the morning by mixing. (Fig. 6) In the "rst measurements conducted in the middle of May, only a-pinene, b-pinene, D-carene, 1,8-cineol and camphene were present in concentrations above the detection limit. All of the compounds had a maximum during the night; 1,8-cineol reacts only slowly with the OH-radical, and the diurnal curve is less marked than for monoterpenes. During the night of 14 May, monoterpene concentrations rapidly decreased, with a-pinene,
b-pinene, and D-carene vanishing completely. There is no minimum observed in 1,8-cineol concentrations. aPinene, b-pinene, and D-carene react much faster with ozone and the NO radical than does camphene, and 1,8-cineol is quite unreactive (Table 3); the nighttime drop in the concentrations could thus be partly due to chemical reactions. The complex nighttime behavior of the monoterpene concentrations can be partly due to the fact that the measurement site is on the top of a hill. The hilltop can remain above the nighttime inversion of lower elevations, thus de-coupling the site from most monoterpene sources. The higher nighttime VOC concentrations in general, however, drop to lower daytime values, concomitant with an ozone increase caused by e$cient mixing and photochemistry. (Fig. 7) The two summer measurements in the middle of June and at the beginning of July are quite similar. Ozone data was lost in July due to a malfunction of the instrument. All other compounds again show a maximum in the night and minimum during the day, except sabinene. Sabinene reacts about twice as fast with the OH-radical than, for example a-pinene, and sabinene concentrations should therefore decrease like the others. The maximum sabinene concentration observed during the day indicates emission sources that are more e$cient during the day. Some monoterpenes are known to be produced by immediate light-induced synthesis (Schuh et al., 1997; Staudt et al., 1997). The last diurnal cycle measured at the end of the growing season, showed a longer nighttime maximum due to the longer nights. Ozone decreases during the night due to the reactions with monoterpenes and also due to deposition.
5. Summary Light hydrocarbon and monoterpene concentrations were measured during two growing seasons in Finland,
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Fig. 7. Diurnal variations of monoterpenes on 13}14 May, 10}11 June, 30 June, 1 July, and 22}23 September, 1998 at Ilomantsi.
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between the southern and middle boreal zone. Samples were collected in a "eld, about 100 m outside a mixed forest. The monthly average summer concentrations of isoprene were 0.3}1.7 ppbC and monoterpenes and 1,8cineol together 1.6}3.2 ppbC. The concentrations of a-pinene, b-pinene, camphene, D-carene and 1,8-cineol started to increase in May, about one month earlier than the concentrations of sabinene and isoprene. Sabinene and isoprene were also well correlated with each other suggesting that sabinene, like isoprene, might have a light-dependent source. a-Pinene and isoprene were the most abundant compounds throughout the growing season. Biogenic hydrocarbons contribute signi"cantly to the total reactivity towards the OH radical. During summer months they comprise almost all of the measured OH reactivity. Monoterpene and isoprene concentrations are dependent on the ambient temperature. The collected data support the idea that the e!ective temperature sum can be used to predict the initiation of both isoprene emissions and also monoterpene emissions from Betula pendula. Seasonal emission rate measurements from other trees, especially conifers, would be important to see if the ETS might also have wider use. Acknowledgements We wish to thank Kari LyytikaK inen in Ilomantsi and Anne-Mari MaK kelaK at the FMI for technical assistance and also Ari Karppinen at the FMI for calculating the mixing heights. This work was funded by the Academy of Finland and the European Commission DG XII (contract number ENV4-CT95-0022). Janne Rinne gratefully acknowledges the Maj and Tor Nessling Foundation for "nancial support.
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