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Modeling speciated terpenoid emissions from the European boreal forest
Atmospheric Environment 34 (2000) 4983} 4996
Modeling speciated terpenoid emissions from the European boreal forest V. Lindfors *, T. Laurila , H. Hakola , R. Steinbrecher, J. Rinne Finnish Meteorological Institute, Air Quality Research, Sahaajankatu 20 E, FIN-00810 Helsinki, Finland Fraunhofer-Institut fu( r Atmospha( rische Umweltforschung, Kreuzeckbahnstrasse 19, D-82467, Garmisch-Partenkirchen, Germany National Center for Atmospheric Research, P.O. Box 3000, Boulder CO 80307-3000, USA Received 20 December 1999; received in revised form 31 March 2000; accepted 10 April 2000
Abstract We present the "rst estimates of speciated monoterpene emissions from the North European coniferous forests. Measured emission factors and emission pro"les of boreal tree species (Picea abies, Pinus sylvestris, Betula pendula, Salix phylicifolia, Populus tremula, and Alnus incana) were used together with detailed satellite land cover information and meteorological data in an emission model based on the Guenther emission algorithms. The variation of the coniferous biomass within the boreal region (603N to 703N) was obtained from forest inventory data, and the seasonal variability of the deciduous biomass was taken into account through simple boreal climatology parameterisation. The annual biogenic emissions in the boreal zone are dominated by coniferous species, but in the summer months, the deciduous contribution to the monoterpene and isoprene emissions is considerable. Norway spruce (Picea abies) is the most important isoprene emitter in the north European boreal forests. The biogenic emission #uxes in the South boreal zone are approximately twice as high as #uxes in the North boreal zone. a- and b-pinene, carene, and cineole are the most abundant emitted terpenes, with a strong contribution of isoprene and linalool during the summer months. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Biogenic emissions; Boreal forest; Seasonal variation; Isoprene; Monoterpene
1. Introduction The boreal forest is characterized by high foliar biomass densities, which makes it an important source of biogenic volatile organic compounds (VOCs). All dominant boreal tree species Scots pine (Pinus sylvestris), Norway spruce (Picea abies), and birch (Betula pendula and Betula pubescens), are signi"cant monoterpene emitters (e.g. Simpson et al., 1999; Hakola et al., 1998). a- and b-pinene, and D-carene are the principal emitted compounds, but the monoterpene emission pro"les vary considerably between the tree species as well as seasonally. Norway spruce has been shown to be a low or moderate isoprene emitter (e.g. Steinbrecher and Rabong, 1994;
* Corresponding author. E-mail address: virpi.lindfors@fmi." (V. Lindfors).
Kempf et al., 1996), but its prevalence and high foliar biomass makes it a signi"cant isoprene emitting tree species in the North European boreal forests. The emission characteristics of European ecosystems are currently the object of intensive research e!orts. During the past several years, two large measurement campaigns have been carried out: biogenic emissions in the Mediterranean area (BEMA), which focused on biogenic emissions in the Mediterranean region (e.g. Seufert et al., 1997), and biogenic VOC emissions and photochemistry in the boreal regions of Europe (BIPHOREP), aimed at quantifying the biogenic VOC emissions from boreal forests (e.g. Laurila et al., 1997). However, there are still large gaps in the emission factor data available for detailed emission inventories, especially in remote ecosystems where the species distribution may di!er considerably from typical American or Central European vegetation. In Europe, for instance, more experimental
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 2 2 3 - 5
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data is needed to characterize the emissions from the hemiboreal forests as well as from some temperate forest ecosystems. Due to their high reactivity with ozone, and especially their aerosol forming capacity, the terpenoids emitted by vegetation are an important contributor to tropospheric chemistry (Calogirou et al., 1999; Carter, 1996; Gri$n et al., 1999a}c). We have combined the recent experimental information on the biogenic emission factors, monoterpene emission pro"les, and foliar biomasses of boreal tree species, obtained during the BIPHOREP measurement campaigns, with the best available emission algorithms and detailed land cover data to study the variation of the terpenoid emissions within the boreal zone. Model calculations are carried out over the growing season, taking into account the biomass variability and the changing monoterpene pro"les. The modeled emissions are compared with emission #ux and ambient air concentration measurements.
2. Materials and methods In order to estimate the emission characteristics of the boreal coniferous forests, we have calculated the regional biogenic emission #uxes over the growing season for typical South boreal, Middle boreal and North boreal areas using information from the Finnish forest and land cover data bases and hourly meteorological data. The studied area covers the latitudes 603N to 703N, as shown in Fig. 1. The regional grouping was done on the basis of the the Nomenclature des Unite& s Territoriales Statistiques of the European Union (N.U.T.S.) Level 3 area classi"cation of Finland. The main tree species in the studied region are pine, spruce, and birch. Pinus sylvestris is the dominant species on 64.5% of the Finnish forest land area, and the corresponding percentages for Picea abies, Betula pubescens, and Betula pendula are 25.7, 6.2, and 1.3%, respectively. Other deciduous trees, with only minor contribution are aspen (Populus tremula), willow (Salix sp.) and alder (Alnus sp.) (FFRI, 1997). In the northern parts of the country, the Norway spruce (P. abies) is partly substituted by Siberian spruce (Picea abies ssp. obovata) (HaK met-Ahti et al., 1992). The forest cover information used in this study was obtained from a 10;10 km grid analysis of LANDSAT TM-image-based land use classi"cation for Finland (Lindfors and Laurila, 2000). The data set includes 3 pure and 13 mixed forest classes, which we have reallocated to deciduous trees, pine, and spruce according to the Finnish forest statistics (Kauppi et al., 1995; FFRI, 1997) as described in Lindfors and Laurila (2000). The total forest areas and the relative contribution of the reallocated forests are given in Table 1 for the N.U.T.S. Level 3 regions.
Fig. 1. The model domain in the European boreal zone. The N.U.T.S. regions are identi"ed with numbers (see text), and the location of the PoK tsonvaara VOC sampling station in North Karelia (region 12) is indicated with a star.
A representative synoptic station was selected for each N.U.T.S. region, and meteorological data was obtained for the modeling period (April 1}October 31, 1997) from the database of the Finnish Meteorological Institute. The three-hourly values of temperature, relative humidity, cloudiness, and wind speed were interpolated linearly to construct continuous time series of hourly meteorological data for each area. The geographical coordinates of the synoptic stations are also given in Table 1. The 19 regions were classi"ed into the South, Middle and North boreal zones according to Solantie (1990) and Ahti et al. (1968). The classi"cation is indicated in Table 1. The VOC emissions from forest foliage were calculated according to the method described by Guenther (1997). The emission #ux F per ground area (in lg m\ h\) is given by F"eDc.
(1)
Here e is the normalized emission potential in lg g(dry weight)\ h\, D is the foliar biomass density in g(dry
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Table 1 The relative share (in per cent) of pine, spruce, and deciduous forests of the total land area of the 19 regions in Finland, based on the LANDSAT satellite data analysis Region
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
(S) (S) (C) (S) (S) (S) (S) (S) (S) (S) (S) (M) (S) (M) (C) (M) (M) (M) (N)
Synoptic station
Forest coverage, % of land area
Lat, 3N
Lon, 3E
Pine
Spruce
Deciduous
Total
60.32 60.51 60.15 61.47 60.82 61.42 60.97 60.90 61.73 61.73 63.02 62.67 62.40 63.10 63.10 63.10 65.37 64.28 67.37
24.95 22.27 19.88 21.80 23.50 23.42 25.63 26.93 27.30 27.30 27.80 30.93 25.68 23.03 23.03 23.03 27.02 27.67 26.65
20 24 28 25 22 30 24 21 17 21 18 24 28 25 23 30 27 29 28
22 18 9 29 28 28 28 32 31 28 28 25 31 29 27 22 25 30 23
8 5 6 7 10 7 8 9 8 10 19 11 12 9 12 11 22 14 16
50 47 42 62 60 64 61 63 56 59 65 60 71 63 61 62 74 73 67
The total forest coverage (in per cent) and the geographical coordinates of the synoptic station are also given for each region. The boreal zone classi"cation of the regions is indicated in parenthesis (S"South boreal, M"Middle boreal, N"North boreal, C"coastal) (see text).
weight) m\, and c is a non-dimensional environmental correction factor, which includes the e!ect of temperature and light conditions. The normalized emission potentials applied in this study are summarized in Table 2. The isoprene and monoterpene emission potentials of the coniferous species are based on the European ecosystem values recommended by Simpson et al. (1999) and the new data obtained by Steinbrecher et al. (1999) and Janson and DeServes (1999) during the BIPHOREP measurement campaigns. These values are also in accordance with the values given by Janson (1993) and Kempf et al. (1996). The emission potentials of deciduous species are based on the measurements of Hakola et al. (1998, 1999) carried out during the BIPHOREP campaign years. The measurements covered all summer months, and in this work we have tentatively assigned the deciduous monoterpene and isoprene emission potentials separately for early summer and late summer, as this type of behavior has been reported by several authors (e.g. Monson et al., 1994; Bertin et al., 1997; Hakola et al., 1998, 1999). The observed high early summer monoterpene emission rates of aspen and willow (Hakola et al., 1998) may be connected with the bud break process and not representative for the whole leaf expansion period. However, since
the biomass of these trees is not large this is not considered to be a source of signi"cant error in the emission calculations. The emission potentials of OVOCs are highly uncertain, pending further experimental data (e.g. Kesselmeier and Staudt, 1999) and therefore the default value 1.5 lg g(leaf biomass)\ h\ (Guenther et al., 1995; Simpson et al., 1999) is used for all species. The foliar biomass densities used in this inventory for the dominant boreal tree species are given in Table 3. According to Simpson et al. (1999), the default foliar biomass density of European deciduous trees is 320 g m\, which is adopted in this work for the deciduous species in the South and Middle boreal zones. The foliar biomass densities of Pinus sylvestris and the Picea species are highly variable with latitude. The values recommended to be used in areas north of 603N are 500 and 800 g m\ for Pinus and Picea species, respectively (Simpson et al., 1999). However, the forest areas in this study cover a wide range of latitudes (from 603N to 703N), and we have taken into account the coniferous foliar biomass variability within the study region, based on the Finnish forest inventory data and an analysis of the LANDSAT land cover data, as described by Lindfors et al. (1999) and Lindfors and Laurila (2000). Lindfors
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Table 2 The emission potentials (in lg g(leaf biomass)\ h\) of boreal tree species applied in this study Isoprene
Deciduous trees Betula pendula and Betula pubescens Populus, Salix sp. Alnus sp. Coniferous trees Pinus sylvestris Picea abies Picea a. ssp. obovata
Monoterpenes
OVOC
Early summer
Late summer
0.1 34 0.0
0.64 3.0 0.72
5.6 0.3 0.72
1.5 1.5 1.5
0.1 1.0 0.1
1.5 1.5 1.5
1.5 1.5 1.5
1.5 1.5 1.5
Simpson et al. (1999). Hakola et al. (1998, 1999). Steinbrecher et al. (1999). Kempf et al. (1996). Janson (1993).
Table 3 The average foliar biomass densities (in g(dry weight) m\) of the main boreal tree species applied in this study in di!erent parts of the boreal zone
South boreal Middle boreal North boreal
Pine
Spruce
Deciduous
300 300 200
900 900 750
320 320 210
et al. (1999) found the foliar biomass of pine to vary approximately from 400$300 to 200$200 g m\ and that of spruce from 1300$700 to 650$500 g m\, between the latitudes 603N and 703N. According to Lindfors et al. (1999) and Lindfors and Laurila (2000), the analysis of LANDSAT data yielded foliar biomass values of 300 g m\ (pine) and 900 g m\ (spruce) in southern Finland, and 200 g m\ (pine) and 750 g m\ (spruce) in northern Finland. The above "gures are also in accordance with the results of KellomaK ki (1999), who estimated that in Finland the average foliar biomass densities of pine and spruce are 284 g m\ and 1120 g m\, respectively. The values given in Table 3 for the coniferous species are those used in the Finnish biogenic emission inventory of Lindfors and Laurila (2000). The values given for deciduous species follow the recommendation of Simpson et al. (1999), with the latitudinal variation assumed to be similar to that of pine. In the North European forests, the seasonal variability of the deciduous foliage is pronounced, and in this work we have applied a simple parameterization for the leaf
development and senescence. The calculation is based on e!ective temperature sum (ETS'53C, in degree days) in spring and climatological observations in the autumn. In the beginning of the growing season the leaf development on each day (i) is represented by foliage percentage P(i) given by
P(i)" Sc log G
ETS(July31)!49 . ETS(i)!49
(2)
The threshold value for the initiation of leaf development is ETS"49 d.d. (Lappalainen, 1993). Maximum (P(i)"100%) foliage is assumed to be reached by July 31, facilitated by the scaling factor Sc which is "tted individually for each study location. Full foliage is maintained from the beginning of August until the start of senescence. The occurrence of leaf senescence and shedding is parameterized through observed dates of complete leaf shedding in di!erent parts of the study region. In Finland these dates range from September 20 in the very North to October 10 along the SouthWest coast (Havas and Sulkava, 1987). The senescence is assumed to start two weeks earlier, and P(i) is assumed to decrease linearly until the tree is bare (P(i)"0%). Thus, in our model the foliar biomass density D(i) of deciduous trees is given by D(i)"P(i)D ,
(3)
where D is the biomass density given in Table 3. The
foliar biomass density of coniferous trees is assumed to be constant ("D ) throughout the modeling period.
V. Lindfors et al. / Atmospheric Environment 34 (2000) 4983}4996
The monoterpene emissions of trees are controlled by the volatilization of hydrocarbons from storage pools inside the leaf. Thus, the monoterpene emission rates are strongly dependent on temperature (Ciccioli et al., 1997; Guenther et al., 1993; Hau! et al., 1999; Lamb et al., 1985; Schuh et al., 1997). The environmental correction factor for pool monoterpene emissions is usually parameterized as c(pool)"exp(b(¹!¹ )), (4) 1 where b ("0.093C\) is an empirical coe$cient, T (K) is the leaf temperature, and ¹ is the leaf temperature at 1 standard conditions ("303.15 K) (Guenther, 1993). This correction factor is generally also used for OVOCs, because experimental data on the OVOC emissions is still too scarce to facilitate the development of speci"c emission algorithms (Guenther et al., 1994; Simpson et al., 1999). Isoprene is emitted by vegetation directly after it has been synthesized in the plant. Isoprene synthesis is under enzymatic control, and strongly dependent on leaf temperature and light intensity (e.g. Monson et al., 1995). The environmental correction factor for this type of emissions is given by c(synthesis)"C C , (5) 2 * where C is the temperature correction and C is the 2 * light correction. The light correction has the form aC ¸ * C " , * (1#a¸
(6)
where L is the photosynthetically active photon #ux density (PPFD, lmol photons m\ s\), and a ("0.0027) and C ("1.066) are empirical coe$cients * (Guenther, 1997). The temperature correction is given by exp([C (¹!¹ )]/R¹ ¹) 2 1 1 C " . (7) 2 C #exp([C (¹!¹ )]/R¹ ¹) 2 2 + 1 Here T (K) is the leaf temperature, ¹ is the standard 1 temperature given above, R is the universal gas constant, and C ("95 000 J mol\), C ("230 000 J mol\), 2 2 C ("0.961), and ¹ ("314 K) are empirical coe$2 + cients (Guenther, 1997). In the emission model we have assumed the leaf temperature to be equal to the ambient temperature. Recently, it has been shown that the terpene emissions of some plant species are also light and temperature controlled (e.g. Schuh et al., 1997; Staudt et al., 1997; Steinbrecher, 1994; Steinbrecher and Hau!, 1996; Steinbrecher et al., 1999; Seufert et al., 1997; Komenda et al., 1999), indicative of both storage emissions and de novo biosynthesis. In this study we have parameterized the
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monoterpene emission rates of pine and spruce by the combined pool-synthesis model, as suggested by Schuh et al. (1997). The total monoterpene emission rate e is +2 given by e "e #e , (8) +2 '-17,2'1 where e and e are calculated using the .--* '-17,2'1 correction factors (4) and (5). Thus, the total emission rate is e "e exp(b(¹!¹ )) +2 . 1 1 aC ¸ exp([C (¹!¹ )]/R¹ ¹) 2 1 1 * #e 1 (1#a¸ C #exp([C (¹!¹ )]/R¹ ¹) 2 2 + 1 (9) where e is the pool emission potential and e the . 1 1 biosynthesis emission potential under standard conditions (303.15 K, 1000 lmol photons m\ s\). According to the results of Steinbrecher et al. (1999), the pool emission potential of Pinus sylvestris is 63% and that of Picea abies 64% of their respective total monoterpene emission potentials. Due to lack of experimental data on the light dependence of the monoterpene emissions of boreal deciduous species, they were parameterized according to the pool model using correction factor (4). In order to assess the individual monoterpene emissions, we have applied emission pro"les deduced from the experimental results of Hakola et al. (1998, 1999) and Steinbrecher et al. (1999). The pro"les are given in Table 4 for the tree species considered in this study. Early summer and late summer pro"les are used for the deciduous trees, and the pool emissions and biosynthesis emissions of coniferous trees are assigned according to their respective monoterpene pro"les. The calculation of the hourly emissions from each region was done using the FMI/BEIS emission model of the Finnish Meteorological Institute. The model is based on the updated version of the Biogenic Emissions Inventory System (BEIS) developed at the Environmental Protection Agency (EPA) of the U.S.A. (Birth and Geron, 1995; Geron et al., 1994; Pierce, 1996; Pierce et al., 1998). Details of the FMI/BEIS model are given in Lindfors and Laurila (2000). Using the land cover information and the meteorological time series, the emissions were calculated for each of the 19 areas on hourly and daily basis over the model period (April 1}October 31, 1997). Total emissions were obtained by summing the daily totals over the calculation period.
3. Model results The average monoterpene and OVOC emission #uxes from forests in the South boreal zone are approximately twice as high as #uxes in the North boreal zone (Table 5).
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Table 4 Monoterpene emission pro"les (in % of total monoterpene emission) of boreal tree species. &Pend' refers to Betula pendula and &pub' to Betula pubescens. &Pool' and &synth' refer to the pool emissions and synthesis emissions, respectively Birch (pend) Early summer emissions a-Pinene 21.6 b-Pinene 14.6 D-Carene 4.6 Camphene 3.1 Limonene 6.4 Sabinene 18.3 Myrcene Ocimenes 14.0 Caryophyllenes 3.4 1,8-Cineole 5.9 Linalool 8.0 Other Late summer emissions a-Pinene 8.2 b-Pinene 9.8 D-Carene 1.0 Camphene 0.1 Limonene 6.9 Sabinene 33.3 Myrcene Ocimenes 35 Caryophyllenes 0 1,8-Cineole 2 Linalool 3 Other
Birch (pub)
Willow, aspen
Alder
Pine pool
Spruce pool
Pine synth
Spruce synth
13.8 7.8 3.1 3.5 10.9 8.8
26.7 13.3 0.4 1.9 17.5 5.0
28.0 24.8 8.0 2.3 18.0 6.0
29.4 4.6 27.5 7.3 8 15.3 5.3
41.6 8.7 8.8 8.8 8 1 2.7
29.8 9.9 0 10.7 0 0 19.1
26.9 33.4 5.4 1.8 12.6 3.7 6.6
3.8 7.3 3.5 37.4
35.2
8.3 1.3
20.4
15.3
9.6
5
1.3 13.1 7.6 4.3 3.1 11.3 28.6
12.2 4.8 13.2 0.6 19.9 6.3
14.0 19.3 5.7 3.0 27.7 9.7
8.1 9.4 1.3 10.7 2.4
41.5
14.3 6.3
1.6
15.3
29.4 4.6 27.5 7.3 8 15.3 5.3
41.6 8.7 8.8 8.8 8 1 2.7
29.8 9.9 0 10.7 0 0 19.1
26.9 33.4 5.4 1.8 12.6 3.7 6.6
1.3
20.4
15.3
9.6
1.3
15.3
For the deciduous species, myrcene is included in the b-pinene contribution.
Table 5 Calculated total biogenic emission #uxes (in kg/km forest during the modeling period) from coniferous and deciduous forests in di!erent parts of the boreal zone Boreal zone
Isoprene
Monoterpene
OVOC
Total VOC
South boreal Middle boreal North boreal
139 108 48
939 805 458
938 785 474
2015 1698 980
For isoprene this di!erence is even larger, with the #uxes in the North only about one-third of those in the South. An example of the emission variability during the growing season is given in Fig. 2, where we have plotted the isoprene, total monoterpene and OVOC emission #uxes from deciduous trees, pine and spruce in North Karelia in the Middle boreal zone. Daily average temperature is also shown (Fig. 2c) to indicate the variation of meteorological conditions during the modeling period. The boreal biogenic emissions are dominated by conifers, but in the summer months the deciduous trees add
an important contribution to the total. Spruce is the principal isoprene emitter all through the growing season due to its much higher biomass compared to that of the deciduous isoprene emitters (Fig. 2a). However, because of the high emission factors of willow and aspen, the isoprene emissions from deciduous trees are far from insigni"cant, especially during warm and sunny periods. The monoterpene emission #uxes of the deciduous trees are almost as high as the coniferous monoterpene emissions during the latter part of the summer, when the deciduous foliar biomass is fully developed (Fig. 2b). In
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Fig. 2. Modeled daily average isoprene (a), total monoterpene (b) and OVOC (c) #uxes from deciduous and coniferous trees in North Karelia during the modeling period April 1}October 31, 1997. The daily average temperature is included in (c) with a solid line (right axis).
early summer, deciduous trees emit high amounts of monoterpenes during the warm spells, but the emissions are only about one-third of the total monoterpene emissions. The e!ect of foliar biomass on the emissions is clearly evident in the OVOC emissions (Fig. 2c), whose emission factors are assumed to be the same for all studied tree species. Thus, the OVOC emissions are dominated by spruce and pine, with only a minor contribution
((15%) from the deciduous species during the summer months. The highest monthly emissions of both isoprene and monoterpenes occur in July in all boreal zones (Fig. 3). The isoprene emissions decrease rapidly after the warmest summer period, whereas the monoterpene emissions remain relatively high in August and decrease more slowly than the isoprene emissions in September.
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pinene and carene, which contribute between 45 and 60% of total emitted monoterpenes. There is a strong cineole contribution in spring, and high sabinene emissions during the summer and autumn months. a-pinene and carene emissions are also highest in spring, while the relative abundance of b-pinene remains almost unchanged in all seasons. Linalool emissions are low in spring, show a maximum contribution in summer, and decrease again towards the autumn (Table 6). The seasonal Middle boreal zone averages of the terpenoid emissions are presented in Fig. 4. In the spring months a- and b-pinenes, carene, and cineole contribute close to 70% of the total terpenoid emissions, while isoprene contributes 8% of the total. In summer, the share of isoprene increases to 13%, and sabinene, ocimene, caryophyllene, and linalool emissions become relatively more important. In autumn, the emission pattern remains almost the same, with a decreasing contribution of isoprene and linalool, balanced by an increase in the relative amount of a-pinene.
4. Discussion Rinne et al. (1999, 2000a, b) have measured hydrocarbon #uxes at selected coniferous forest sites in the European boreal zone. According to their results, the total monoterpene #uxes varied, on the average, between 50 and 300 ng m\ s\ in the South boreal zone (Rinne et al., 2000a), 20 and 200 ng m\ s\ in the Middle boreal zone (Rinne et al., 1999), and 0 and 150 ng m\ s\ in the North boreal zone (Rinne et al., 2000b). These values are well within the range of our modeled monoterpene emission #uxes. Direct measurements of biogenic emission #uxes are, however, very scarce, and it is thus very di$cult
Fig. 3. Monthly average isoprene (a) and total monoterpene (b) emission #uxes in the South boreal, Middle boreal, and North boreal zones.
Even though there are considerable di!erences in the emitted monoterpene amounts in di!erent parts of the boreal zone, the monoterpene distributions are very similar (Table 6). However, there are strong month-to-month variations in the emission pro"les. Throughout the growing season, the emissions are dominated by a- and b-
Table 6 Average compound distribution of emitted monoterpenes (in % of total monoterpene emission) in di!erent parts of the boreal zone in spring (spr, April}May), summer (sum, June}August), and autumn (aut, September}October) months in 1997 Monoterpene
a-Pinene b-Pinene D-Carene Camphene Limonene Sabinene Myrcene Ocimenes Caryophyllenes 1,8-Cineole Linalool Other
South boreal zone
Middle boreal zone
North boreal zone
Spr
Sum
Aut
Spr
Sum
Aut
Spr
Sum
Aut
36.7 9.3 11.4 7.8 8.1 4.5 3.5 0.6 0.4 15.2 1.9 0.5
27.8 10.0 8.2 5.7 8.7 12.4 2.6 6.4 2.2 10.5 4.6 0.9
29.7 8.7 9.3 6.2 8.6 12.4 2.4 6.2 1.8 11.5 2.4 0.8
36.9 8.9 12.3 8.0 8.1 4.6 3.7 0.2 0.3 14.9 1.5 0.6
26.9 9.3 8.5 5.8 9.2 12.9 2.5 4.2 3.4 9.6 6.4 1.3
30.0 8.2 10.1 6.5 8.9 12.2 2.5 3.7 2.6 11.1 3.1 1.0
38.1 8.7 12.7 8.3 8.0 4.3 3.7 0.0 0.0 15.7 0.0 0.6
27.4 9.1 8.7 6.0 9.4 12.8 2.5 3.3 3.6 9.7 6.1 1.3
31.7 8.1 10.6 6.9 8.9 10.7 2.6 2.4 2.4 11.9 2.8 1.0
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Fig. 4. Middle boreal zone averages of terpenoid emission distributions in spring (April}May), summer (June}August), and autumn (September}October) months.
to verify this type of seasonal emission calculations using experimental data. One of the few available means for at least qualitatively validating the emission model results is to study the variability of observed ambient biogenic VOC concentrations in a background area. Given the short atmospheric lifetime of the biogenic compounds (minutes to hours), it is conceivable that in remote areas this variability is mostly due to the local emission patterns. The Finnish Meteorological Institute has carried out regular ambient VOC measurements at PoK tsoK nvaara (63307 N, 31304 E, 254 m a.s.l.), which is located in a forested environment in North Karelia (see Fig. 1). The monoterpene samples were collected two to three times a week from May to October into Tenax adsorbent and analyzed in the laboratory of the FMI using GC/MS, while isoprene was analyzed from canister samples using GC/FID (Hakola et al., 1998). 15-day running averages of the measured concentrations and modeled emission #uxes are shown in Fig. 5 for isoprene and total monoterpenes. Unfortunately, no measurements were available in April, but based on these results it appears that the onset of isoprene emissions and the rapid increase of monoterpene emissions in spring, as well as the phasing out of the emissions in autumn are adequately parameterized in the model. The calculated emission peaks occur in periods when high concentrations were observed in the ambient air
samples (Fig. 5). However, there is some disagreement between the behavior of the predicted monoterpene #ux and the measured concentration in August. During the summer months of 1997 North Karelia experienced rainy periods around midsummer (June 24th), in the second and the last week of July, and on several occasions in August, which also had and unusually cold spell (night temperatures fell close to freezing) from the 13th to the 18th (statistics of the Finnish Meteorological Institute). Rain is not taken into account in the emission model except indirectly through the cloudiness parameterization, but it may have a!ected either the emission process itself or the ambient air concentrations, and together with the sudden drop of the daily average temperatures this may partly explain the observed low concentrations of both isoprene and monoterpene around August 15. A more detailed analysis of the correspondence between the modeled #ux and the measured concentrations and an assessment of the accuracy of the predicted emission levels would require the use of a photochemical model in order to take into account the chemical reactions and the atmospheric dispersion and advection of the emitted compounds. However, even this rough comparison is encouraging, as it shows that the Guenther emission algorithms and the FMI/BEIS emission model are capable of producing realistic seasonal emission patterns in a typical boreal forest area. Compared to our earlier
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Fig. 5. Running 15-day averages of the modeled isoprene (a) and total monoterpene (b) #uxes (solid line, left axis) and observed ambient air concentrations (dashed line, right axis) in North Karelia during the modeling period April 1}October 31, 1997.
biogenic emission calculations (Lindfors and Laurila, 2000), the inclusion of a detailed deciduous biomass parameterization appears to have improved the agreement between the onset of the modeled isoprene emissions and the "rst observations of isoprene in ambient air in spring. Monoterpene emission measurements of Norway spruce and Scots pine (Janson, 1993) and #ux measurements over pine forests (Rinne et al., 1999, 2000a, b) have shown the strong dominance of a-pinene and carene in the coniferous monoterpene spectrum throughout the boreal zone. Our modeled #uxes also include the contribution of the deciduous emissions with relatively less a-pinene and higher amounts of e.g. sabinene, limonene, and ocimenes. In Fig. 6 we compare the modeled monoterpene emission distribution with the average measured concentrations in July 1997 in North Karelia. The most reactive emitted terpenoids (myrcene, ocimenes, caryophyllene, linalool,
and limonene (Atkinson, 1994)) are depleted in the ambient air compared to the emission spectrum, and the relative abundance of isoprene, a- and b-pinene, and carene, which have a longer atmospheric lifetime against oxidation (Atkinson, 1994), has increased. Besides forest emissions, the ambient concentrations are a!ected by emissions from other vegetation and the forest #oor (e.g. Wilske and Kesselmeier, 1999; Janson et al., 1999), which are not parameterized explicitly in the emission model. However, the overall agreement between the two distributions is quite reasonable, indicating that the monoterpene pro"les used in this work are representative of the boreal forest emissions, even though they are based on very limited sets of experimental data. Gri$n et al. (1999c) have estimated the relative species contribution to global monoterpene emissions. According to their estimate, a- and b-pinene correspond to 35 and
V. Lindfors et al. / Atmospheric Environment 34 (2000) 4983}4996
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Fig. 6. Relative abundance of terpenoids in the modeled emissions and measured ambient concentrations in North Karelia in July, 1997.
23% of the total emissions, respectively. Limonene contributes 23%, myrcene and sabinene both 5%, D-carene 4%, ocimene and terpinolene both 2%, and a- and cterpinene 1% of the total monoterpene emissions (Gri$n et al., 1999c). According to our results, the contribution of a-pinene varies from 27 to 38%, depending on the season and the boreal zone, which is in agreement with the results of Gri$n et al. (1999c). Our estimated b-pinene (8}10%) and limonene (8}9.4%) contributions are lower than those of Gri$n et al. (1999c), while we obtain higher contributions of D-carene (8}13%) and sabinene (4}13%). These di!erences are probably explained by the di!erent emission characteristics of the boreal forest ecosystems when compared to global averages. However, Gri$n et al. (1999c), have only included monoterpene species known to be important in secondary organic aerosol formation, and their inventory may not be directly comparable to our model results.
5. Conclusions We have calculated the biogenic terpenoid emission #uxes from typical South boreal, Middle boreal, and North boreal forests using the Guenther algorithms and experimental emission factor data. The accumulated emissions re#ect the strong dominance of the coniferous species with high a- and b-pinene and carene #uxes, but in the summer months the deciduous trees add an important contribution to the total, giving rise to substantial sabinene and linalool emission #uxes. Spruce is the principal isoprene emitter all through the growing season due to
its much higher biomass compared to that of the deciduous isoprene emitters. The biogenic emissions are strongly a!ected by the prevailing meteorological conditions. Thus, the average monoterpene and OVOC emission #uxes from forests in the South boreal zone are approximately twice as high, and the isoprene #uxes three times as high, as the corresponding #uxes in the North boreal zone. In spring a- and b-pinenes, carene, and cineole contribute close to 70% of the total terpenoid emissions, while isoprene contributes 8% of the total. In summer, the share of isoprene increases to 13%, and sabinene, ocimene, caryophyllene, and linalool emissions become more important. The highest monthly emissions of both isoprene and monoterpenes occur in July in all boreal zones. The isoprene emissions decrease rapidly after the warmest summer period, whereas the monoterpene emissions remain relatively high in August and decrease more slowly than the isoprene emissions in September. The Guenther emission algorithms and the FMI/BEIS emission model with the parameterized deciduous biomass development appear to produce realistic biogenic emission patterns when applied in the North European coniferous forest zone. However, a more comprehensive validation of the algorithms and the emission model requires further experimental data on the seasonal variation of the emission #uxes and the speci"c monoterpene emission spectra of the main boreal tree species. One of the main unknowns when considering emission modeling in cold climatic conditions is the applicability of the emission algorithms in temperatures close to or even below zero
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degrees centigrade, which also needs to be resolved using experimental data. The overall uncertainty of biogenic emission inventories may be estimated as high as a factor of "ve or even greater (Simpson et al., 1999). The error level varies when breaking down the total error to errors associated with a speci"c compound or compound group and the land use classi"cation. The error estimated resulting from a coarse land use, especially vegetation, classi"cation may be as high as a factor of three. When, as done in the present work, a detailed vegetation classi"cation down to the plant species level is used, based upon the latest information available, a conservative error in the order of 50% may be estimated. Further, the emission factors and emission algorithms used in this inventory are based on "eld measurements on plants in Finland, signi"cantly reducing the uncertainty of a factor of "ve associated with the emission factors of isoprene and monoterpenes. The errors of the emission factors for isoprene and monoterpenes used in this work are conservatively estimated to be not higher than 50%. Therefore the overall uncertainty of the presented emission inventory may amount to 70%.
Acknowledgements The "nancial support of the Nordic Council of Ministers and the &R&D Programme for Environment and Climate' of the European Commission (contract ENV4CT95-0022) is gratefully acknowledged.
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V. Lindfors et al. / Atmospheric Environment 34 (2000) 4983}4996 Janson, R., DeServes, C., 1999. Emissions of biogenic VOCs from boreal ecosystems. In: Laurila, T., Lindfors, V. (Eds.), Biogenic VOC Emissions and Photochemistry in the Boreal Regions of Europe } Biphorep, Air Pollution Research Report No 70. Commission of the European Communities, Luxembourg, pp. 45}55. 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. Kauppi, P.E., Tomppo, E., Ferm, A., 1995. C and N storage in living trees within Finland since 1950s. Plant and Soil 168}169, 633}638. Kempf, K., Allwine, E., Westberg, H., Claiborn, C., Lamb, B., 1996. Hydrocarbon emissions from spruce species using environmental chamber and branch enclosure methods. Atmospheric Environment 30 (9), 1381}1389. KellomaK ki, S., 1999. Calculation of foliage mass and foliage area. In: Laurila, T., Lindfors, V. (Eds.), Biogenic VOC Emissions and Photochemistry in the Boreal Regions of Europe } Biphorep, Air Pollution Research Report No 70. Commission of the European Communities, Luxembourg, pp. 113}125. Kesselmeier, J., Staudt, M., 1999. Biogenic volatile organic compounds (VOC): an overview on emission, physiology and ecology. Journal of Atmospheric Chemistry 33, 23}88. Komenda, M., Koppman, R., von Czapiewski, K., Wildt, J., 1999. Emissions of volatile organic compounds from pines (Pinus sylvestris): a comparison of laboratory and "eld studies. P1.30, Book of Abstracts of the Sixth Scienti"c Conference of the International Global Atmospheric Chemistry Project (IGAC), Bologna, Italy, September 13}17, 1999, pp. 44. Lamb, B., Westberg, H., Allwine, E., Quarles, T., 1985. Biogenic hydrocarbon emissions from deciduous and coniferous species in the U.S. Journal of Geophysical Research 90, 2380}2390. 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, University of Helsinki, Department of Botany, 115 pp. Laurila, T., Steinbrecher, R., Kesselmeier, J., Janson, R., KellomaK ki, S., 1997. Biogenic VOC emissions and photochemistry in the boreal regions of Europe. In: Larsen, B., Versino, B., Angeletti, G. (Eds.), The Oxidizing Capacity of the Atmosphere. Air Pollution Research Report 60. Commission of the European Communities, Luxembourg, pp. 300}303. Lindfors, V., Laurila, T., 2000. Biogenic volatile organic compound (VOC) emissions from forests in Finland. Boreal Environment Research 5 (2), in press. Lindfors, V., Rinne, J., Laurila, T., 1999. Upscaling the BIPHOREP results } regional emissions of non-methane hydrocarbons in the European boreal zone. In: Laurila, T., Lindfors, V. (Eds.), Biogenic VOC emissions and photochemistry in the boreal regions of Europe } Biphorep, Air Pollution Research Report No 70. Commission of the European Communities, Luxembourg, pp. 127}150. Monson, R.K., Harley, P.C., Litvak, M.E., Wildermuth, M., Guenther, A.B., Zimmermann, P.R., Fall, R., 1994. Environmental and developmental controls over the seasonal pattern of isoprene emission from aspen leaves. Oecologia 99, 260}270.
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Monson, R.K., Lerdau, M.T., Sharkey, T.D., Schimel, D.S., Fall, R., 1995. Biological aspects of constructing volatile organic compound emission inventories. Atmospheric Environment 29 (21), 2989}3002. Pierce, T.E., 1996. Update to PC-BEIS: release of Version 2.2. U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, USA. Pierce, T., Geron, C., Bender, L., Dennis, R., Tonnesen, G., Guenther, A., 1998. In#uence of increased isoprene emissions on regional ozone modeling. Journal of Geophysical Research 103 (D19), 25611}25629. Rinne, J., Hakola, H., Laurila, T., 1999. Vertical #uxes of monoterpenes above a Scots pine stand in the boreal vegetation zone. Physics and Chemistry of the Earth 24 (6), 711}715. Rinne, J., Hakola, H., Laurila, T., Rannik, UG ., 2000a. Canopy scale monoterpene emissions of Pinus sylvestris dominated forests. Atmospheric Environment 34 (7), 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. Seufert, G., Bartzis, J., Bomboi, T., Ciccioli, P., Cieslik, S., Dlugi, R., Foster, P., Hewitt, C.N., Kesselmeier, J., Kotzias, D., Lenz, R., Manes, F., Perez-Pastor, R., Steinbrecher, R., Torres, L., Valentini, R., Versino, B., 1997. The BEMA-project: an overview of the Castelporziano experiments. Atmospheric Environment 31 (SI), 5}18. Simpson, D., Winiwarter, W., BoK rjesson, G., Cinderby, S., Ferreiro, A., Guenther, A., Hewitt, C.N., Janson, R., Khalil, M.A.K., Owen, S., Pierce, T.E., Puxbaum, H., Shearer, M., Steinbrecher, R., TarrasoH n, L., OG quist, M.G., 1999. Inventorying emissions from nature in Europe. Journal of Geophysical Research 104 (D7), 8113}8152. Solantie, R., 1990. The climate of Finland in relation to its hydrology, ecology and culture. Finnish Meteorological Institute Contributions 2, Finnish Meteorological Institute, Helsinki, Finland. 130 pp. 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. Steinbrecher, R., 1994. Emission of VOCs from selected European ecosystems: the state of the art. In: Borrell, P.M., Borrell, P., Cvitas\ , T., Seiler, W. (Eds.), The Proceedings of EUROTRAC Symposium '94. SPB Academic Publ., The Hague, pp. 448}454. Steinbrecher, R., Rabong, R., 1994. Emission of terpenoids and C2}C6 hydrocarbons by Norway Spruce: Phytotron experiment and "eld studies. In: Borrell, P.M., Borrell, P., Cvitas\ , T., Seiler, W. (Eds.), The Proceedings of EUROTRAC Symposium '94. SPB Academic Publ, The Hague, pp. 502}505. Steinbrecher, R., Hau!, K., 1996. Isoprene and monoterpene emission from Mediterranean oaks. In: Borrel, P.M.,
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Regions of Europe } Biphorep, Air Pollution Research Report No 70. Commission of the European Communities, Luxembourg, pp. 29}43. Wilske, B., Kesselmeier, J., 1999. First measurements of the C and C -organic acids and aldehydes exchange between boreal lichens and the atmosphere. Physics and Chemistry of the Earth (B) 24 (6), 725}728.
Modeling speciated terpenoid emissions from the European boreal forest V. Lindfors *, T. Laurila , H. Hakola , R. Steinbrecher, J. Rinne Finnish Meteorological Institute, Air Quality Research, Sahaajankatu 20 E, FIN-00810 Helsinki, Finland Fraunhofer-Institut fu( r Atmospha( rische Umweltforschung, Kreuzeckbahnstrasse 19, D-82467, Garmisch-Partenkirchen, Germany National Center for Atmospheric Research, P.O. Box 3000, Boulder CO 80307-3000, USA Received 20 December 1999; received in revised form 31 March 2000; accepted 10 April 2000
Abstract We present the "rst estimates of speciated monoterpene emissions from the North European coniferous forests. Measured emission factors and emission pro"les of boreal tree species (Picea abies, Pinus sylvestris, Betula pendula, Salix phylicifolia, Populus tremula, and Alnus incana) were used together with detailed satellite land cover information and meteorological data in an emission model based on the Guenther emission algorithms. The variation of the coniferous biomass within the boreal region (603N to 703N) was obtained from forest inventory data, and the seasonal variability of the deciduous biomass was taken into account through simple boreal climatology parameterisation. The annual biogenic emissions in the boreal zone are dominated by coniferous species, but in the summer months, the deciduous contribution to the monoterpene and isoprene emissions is considerable. Norway spruce (Picea abies) is the most important isoprene emitter in the north European boreal forests. The biogenic emission #uxes in the South boreal zone are approximately twice as high as #uxes in the North boreal zone. a- and b-pinene, carene, and cineole are the most abundant emitted terpenes, with a strong contribution of isoprene and linalool during the summer months. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Biogenic emissions; Boreal forest; Seasonal variation; Isoprene; Monoterpene
1. Introduction The boreal forest is characterized by high foliar biomass densities, which makes it an important source of biogenic volatile organic compounds (VOCs). All dominant boreal tree species Scots pine (Pinus sylvestris), Norway spruce (Picea abies), and birch (Betula pendula and Betula pubescens), are signi"cant monoterpene emitters (e.g. Simpson et al., 1999; Hakola et al., 1998). a- and b-pinene, and D-carene are the principal emitted compounds, but the monoterpene emission pro"les vary considerably between the tree species as well as seasonally. Norway spruce has been shown to be a low or moderate isoprene emitter (e.g. Steinbrecher and Rabong, 1994;
* Corresponding author. E-mail address: virpi.lindfors@fmi." (V. Lindfors).
Kempf et al., 1996), but its prevalence and high foliar biomass makes it a signi"cant isoprene emitting tree species in the North European boreal forests. The emission characteristics of European ecosystems are currently the object of intensive research e!orts. During the past several years, two large measurement campaigns have been carried out: biogenic emissions in the Mediterranean area (BEMA), which focused on biogenic emissions in the Mediterranean region (e.g. Seufert et al., 1997), and biogenic VOC emissions and photochemistry in the boreal regions of Europe (BIPHOREP), aimed at quantifying the biogenic VOC emissions from boreal forests (e.g. Laurila et al., 1997). However, there are still large gaps in the emission factor data available for detailed emission inventories, especially in remote ecosystems where the species distribution may di!er considerably from typical American or Central European vegetation. In Europe, for instance, more experimental
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 2 2 3 - 5
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data is needed to characterize the emissions from the hemiboreal forests as well as from some temperate forest ecosystems. Due to their high reactivity with ozone, and especially their aerosol forming capacity, the terpenoids emitted by vegetation are an important contributor to tropospheric chemistry (Calogirou et al., 1999; Carter, 1996; Gri$n et al., 1999a}c). We have combined the recent experimental information on the biogenic emission factors, monoterpene emission pro"les, and foliar biomasses of boreal tree species, obtained during the BIPHOREP measurement campaigns, with the best available emission algorithms and detailed land cover data to study the variation of the terpenoid emissions within the boreal zone. Model calculations are carried out over the growing season, taking into account the biomass variability and the changing monoterpene pro"les. The modeled emissions are compared with emission #ux and ambient air concentration measurements.
2. Materials and methods In order to estimate the emission characteristics of the boreal coniferous forests, we have calculated the regional biogenic emission #uxes over the growing season for typical South boreal, Middle boreal and North boreal areas using information from the Finnish forest and land cover data bases and hourly meteorological data. The studied area covers the latitudes 603N to 703N, as shown in Fig. 1. The regional grouping was done on the basis of the the Nomenclature des Unite& s Territoriales Statistiques of the European Union (N.U.T.S.) Level 3 area classi"cation of Finland. The main tree species in the studied region are pine, spruce, and birch. Pinus sylvestris is the dominant species on 64.5% of the Finnish forest land area, and the corresponding percentages for Picea abies, Betula pubescens, and Betula pendula are 25.7, 6.2, and 1.3%, respectively. Other deciduous trees, with only minor contribution are aspen (Populus tremula), willow (Salix sp.) and alder (Alnus sp.) (FFRI, 1997). In the northern parts of the country, the Norway spruce (P. abies) is partly substituted by Siberian spruce (Picea abies ssp. obovata) (HaK met-Ahti et al., 1992). The forest cover information used in this study was obtained from a 10;10 km grid analysis of LANDSAT TM-image-based land use classi"cation for Finland (Lindfors and Laurila, 2000). The data set includes 3 pure and 13 mixed forest classes, which we have reallocated to deciduous trees, pine, and spruce according to the Finnish forest statistics (Kauppi et al., 1995; FFRI, 1997) as described in Lindfors and Laurila (2000). The total forest areas and the relative contribution of the reallocated forests are given in Table 1 for the N.U.T.S. Level 3 regions.
Fig. 1. The model domain in the European boreal zone. The N.U.T.S. regions are identi"ed with numbers (see text), and the location of the PoK tsonvaara VOC sampling station in North Karelia (region 12) is indicated with a star.
A representative synoptic station was selected for each N.U.T.S. region, and meteorological data was obtained for the modeling period (April 1}October 31, 1997) from the database of the Finnish Meteorological Institute. The three-hourly values of temperature, relative humidity, cloudiness, and wind speed were interpolated linearly to construct continuous time series of hourly meteorological data for each area. The geographical coordinates of the synoptic stations are also given in Table 1. The 19 regions were classi"ed into the South, Middle and North boreal zones according to Solantie (1990) and Ahti et al. (1968). The classi"cation is indicated in Table 1. The VOC emissions from forest foliage were calculated according to the method described by Guenther (1997). The emission #ux F per ground area (in lg m\ h\) is given by F"eDc.
(1)
Here e is the normalized emission potential in lg g(dry weight)\ h\, D is the foliar biomass density in g(dry
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Table 1 The relative share (in per cent) of pine, spruce, and deciduous forests of the total land area of the 19 regions in Finland, based on the LANDSAT satellite data analysis Region
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
(S) (S) (C) (S) (S) (S) (S) (S) (S) (S) (S) (M) (S) (M) (C) (M) (M) (M) (N)
Synoptic station
Forest coverage, % of land area
Lat, 3N
Lon, 3E
Pine
Spruce
Deciduous
Total
60.32 60.51 60.15 61.47 60.82 61.42 60.97 60.90 61.73 61.73 63.02 62.67 62.40 63.10 63.10 63.10 65.37 64.28 67.37
24.95 22.27 19.88 21.80 23.50 23.42 25.63 26.93 27.30 27.30 27.80 30.93 25.68 23.03 23.03 23.03 27.02 27.67 26.65
20 24 28 25 22 30 24 21 17 21 18 24 28 25 23 30 27 29 28
22 18 9 29 28 28 28 32 31 28 28 25 31 29 27 22 25 30 23
8 5 6 7 10 7 8 9 8 10 19 11 12 9 12 11 22 14 16
50 47 42 62 60 64 61 63 56 59 65 60 71 63 61 62 74 73 67
The total forest coverage (in per cent) and the geographical coordinates of the synoptic station are also given for each region. The boreal zone classi"cation of the regions is indicated in parenthesis (S"South boreal, M"Middle boreal, N"North boreal, C"coastal) (see text).
weight) m\, and c is a non-dimensional environmental correction factor, which includes the e!ect of temperature and light conditions. The normalized emission potentials applied in this study are summarized in Table 2. The isoprene and monoterpene emission potentials of the coniferous species are based on the European ecosystem values recommended by Simpson et al. (1999) and the new data obtained by Steinbrecher et al. (1999) and Janson and DeServes (1999) during the BIPHOREP measurement campaigns. These values are also in accordance with the values given by Janson (1993) and Kempf et al. (1996). The emission potentials of deciduous species are based on the measurements of Hakola et al. (1998, 1999) carried out during the BIPHOREP campaign years. The measurements covered all summer months, and in this work we have tentatively assigned the deciduous monoterpene and isoprene emission potentials separately for early summer and late summer, as this type of behavior has been reported by several authors (e.g. Monson et al., 1994; Bertin et al., 1997; Hakola et al., 1998, 1999). The observed high early summer monoterpene emission rates of aspen and willow (Hakola et al., 1998) may be connected with the bud break process and not representative for the whole leaf expansion period. However, since
the biomass of these trees is not large this is not considered to be a source of signi"cant error in the emission calculations. The emission potentials of OVOCs are highly uncertain, pending further experimental data (e.g. Kesselmeier and Staudt, 1999) and therefore the default value 1.5 lg g(leaf biomass)\ h\ (Guenther et al., 1995; Simpson et al., 1999) is used for all species. The foliar biomass densities used in this inventory for the dominant boreal tree species are given in Table 3. According to Simpson et al. (1999), the default foliar biomass density of European deciduous trees is 320 g m\, which is adopted in this work for the deciduous species in the South and Middle boreal zones. The foliar biomass densities of Pinus sylvestris and the Picea species are highly variable with latitude. The values recommended to be used in areas north of 603N are 500 and 800 g m\ for Pinus and Picea species, respectively (Simpson et al., 1999). However, the forest areas in this study cover a wide range of latitudes (from 603N to 703N), and we have taken into account the coniferous foliar biomass variability within the study region, based on the Finnish forest inventory data and an analysis of the LANDSAT land cover data, as described by Lindfors et al. (1999) and Lindfors and Laurila (2000). Lindfors
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Table 2 The emission potentials (in lg g(leaf biomass)\ h\) of boreal tree species applied in this study Isoprene
Deciduous trees Betula pendula and Betula pubescens Populus, Salix sp. Alnus sp. Coniferous trees Pinus sylvestris Picea abies Picea a. ssp. obovata
Monoterpenes
OVOC
Early summer
Late summer
0.1 34 0.0
0.64 3.0 0.72
5.6 0.3 0.72
1.5 1.5 1.5
0.1 1.0 0.1
1.5 1.5 1.5
1.5 1.5 1.5
1.5 1.5 1.5
Simpson et al. (1999). Hakola et al. (1998, 1999). Steinbrecher et al. (1999). Kempf et al. (1996). Janson (1993).
Table 3 The average foliar biomass densities (in g(dry weight) m\) of the main boreal tree species applied in this study in di!erent parts of the boreal zone
South boreal Middle boreal North boreal
Pine
Spruce
Deciduous
300 300 200
900 900 750
320 320 210
et al. (1999) found the foliar biomass of pine to vary approximately from 400$300 to 200$200 g m\ and that of spruce from 1300$700 to 650$500 g m\, between the latitudes 603N and 703N. According to Lindfors et al. (1999) and Lindfors and Laurila (2000), the analysis of LANDSAT data yielded foliar biomass values of 300 g m\ (pine) and 900 g m\ (spruce) in southern Finland, and 200 g m\ (pine) and 750 g m\ (spruce) in northern Finland. The above "gures are also in accordance with the results of KellomaK ki (1999), who estimated that in Finland the average foliar biomass densities of pine and spruce are 284 g m\ and 1120 g m\, respectively. The values given in Table 3 for the coniferous species are those used in the Finnish biogenic emission inventory of Lindfors and Laurila (2000). The values given for deciduous species follow the recommendation of Simpson et al. (1999), with the latitudinal variation assumed to be similar to that of pine. In the North European forests, the seasonal variability of the deciduous foliage is pronounced, and in this work we have applied a simple parameterization for the leaf
development and senescence. The calculation is based on e!ective temperature sum (ETS'53C, in degree days) in spring and climatological observations in the autumn. In the beginning of the growing season the leaf development on each day (i) is represented by foliage percentage P(i) given by
P(i)" Sc log G
ETS(July31)!49 . ETS(i)!49
(2)
The threshold value for the initiation of leaf development is ETS"49 d.d. (Lappalainen, 1993). Maximum (P(i)"100%) foliage is assumed to be reached by July 31, facilitated by the scaling factor Sc which is "tted individually for each study location. Full foliage is maintained from the beginning of August until the start of senescence. The occurrence of leaf senescence and shedding is parameterized through observed dates of complete leaf shedding in di!erent parts of the study region. In Finland these dates range from September 20 in the very North to October 10 along the SouthWest coast (Havas and Sulkava, 1987). The senescence is assumed to start two weeks earlier, and P(i) is assumed to decrease linearly until the tree is bare (P(i)"0%). Thus, in our model the foliar biomass density D(i) of deciduous trees is given by D(i)"P(i)D ,
(3)
where D is the biomass density given in Table 3. The
foliar biomass density of coniferous trees is assumed to be constant ("D ) throughout the modeling period.
V. Lindfors et al. / Atmospheric Environment 34 (2000) 4983}4996
The monoterpene emissions of trees are controlled by the volatilization of hydrocarbons from storage pools inside the leaf. Thus, the monoterpene emission rates are strongly dependent on temperature (Ciccioli et al., 1997; Guenther et al., 1993; Hau! et al., 1999; Lamb et al., 1985; Schuh et al., 1997). The environmental correction factor for pool monoterpene emissions is usually parameterized as c(pool)"exp(b(¹!¹ )), (4) 1 where b ("0.093C\) is an empirical coe$cient, T (K) is the leaf temperature, and ¹ is the leaf temperature at 1 standard conditions ("303.15 K) (Guenther, 1993). This correction factor is generally also used for OVOCs, because experimental data on the OVOC emissions is still too scarce to facilitate the development of speci"c emission algorithms (Guenther et al., 1994; Simpson et al., 1999). Isoprene is emitted by vegetation directly after it has been synthesized in the plant. Isoprene synthesis is under enzymatic control, and strongly dependent on leaf temperature and light intensity (e.g. Monson et al., 1995). The environmental correction factor for this type of emissions is given by c(synthesis)"C C , (5) 2 * where C is the temperature correction and C is the 2 * light correction. The light correction has the form aC ¸ * C " , * (1#a¸
(6)
where L is the photosynthetically active photon #ux density (PPFD, lmol photons m\ s\), and a ("0.0027) and C ("1.066) are empirical coe$cients * (Guenther, 1997). The temperature correction is given by exp([C (¹!¹ )]/R¹ ¹) 2 1 1 C " . (7) 2 C #exp([C (¹!¹ )]/R¹ ¹) 2 2 + 1 Here T (K) is the leaf temperature, ¹ is the standard 1 temperature given above, R is the universal gas constant, and C ("95 000 J mol\), C ("230 000 J mol\), 2 2 C ("0.961), and ¹ ("314 K) are empirical coe$2 + cients (Guenther, 1997). In the emission model we have assumed the leaf temperature to be equal to the ambient temperature. Recently, it has been shown that the terpene emissions of some plant species are also light and temperature controlled (e.g. Schuh et al., 1997; Staudt et al., 1997; Steinbrecher, 1994; Steinbrecher and Hau!, 1996; Steinbrecher et al., 1999; Seufert et al., 1997; Komenda et al., 1999), indicative of both storage emissions and de novo biosynthesis. In this study we have parameterized the
4987
monoterpene emission rates of pine and spruce by the combined pool-synthesis model, as suggested by Schuh et al. (1997). The total monoterpene emission rate e is +2 given by e "e #e , (8) +2 '-17,2'1 where e and e are calculated using the .--* '-17,2'1 correction factors (4) and (5). Thus, the total emission rate is e "e exp(b(¹!¹ )) +2 . 1 1 aC ¸ exp([C (¹!¹ )]/R¹ ¹) 2 1 1 * #e 1 (1#a¸ C #exp([C (¹!¹ )]/R¹ ¹) 2 2 + 1 (9) where e is the pool emission potential and e the . 1 1 biosynthesis emission potential under standard conditions (303.15 K, 1000 lmol photons m\ s\). According to the results of Steinbrecher et al. (1999), the pool emission potential of Pinus sylvestris is 63% and that of Picea abies 64% of their respective total monoterpene emission potentials. Due to lack of experimental data on the light dependence of the monoterpene emissions of boreal deciduous species, they were parameterized according to the pool model using correction factor (4). In order to assess the individual monoterpene emissions, we have applied emission pro"les deduced from the experimental results of Hakola et al. (1998, 1999) and Steinbrecher et al. (1999). The pro"les are given in Table 4 for the tree species considered in this study. Early summer and late summer pro"les are used for the deciduous trees, and the pool emissions and biosynthesis emissions of coniferous trees are assigned according to their respective monoterpene pro"les. The calculation of the hourly emissions from each region was done using the FMI/BEIS emission model of the Finnish Meteorological Institute. The model is based on the updated version of the Biogenic Emissions Inventory System (BEIS) developed at the Environmental Protection Agency (EPA) of the U.S.A. (Birth and Geron, 1995; Geron et al., 1994; Pierce, 1996; Pierce et al., 1998). Details of the FMI/BEIS model are given in Lindfors and Laurila (2000). Using the land cover information and the meteorological time series, the emissions were calculated for each of the 19 areas on hourly and daily basis over the model period (April 1}October 31, 1997). Total emissions were obtained by summing the daily totals over the calculation period.
3. Model results The average monoterpene and OVOC emission #uxes from forests in the South boreal zone are approximately twice as high as #uxes in the North boreal zone (Table 5).
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Table 4 Monoterpene emission pro"les (in % of total monoterpene emission) of boreal tree species. &Pend' refers to Betula pendula and &pub' to Betula pubescens. &Pool' and &synth' refer to the pool emissions and synthesis emissions, respectively Birch (pend) Early summer emissions a-Pinene 21.6 b-Pinene 14.6 D-Carene 4.6 Camphene 3.1 Limonene 6.4 Sabinene 18.3 Myrcene Ocimenes 14.0 Caryophyllenes 3.4 1,8-Cineole 5.9 Linalool 8.0 Other Late summer emissions a-Pinene 8.2 b-Pinene 9.8 D-Carene 1.0 Camphene 0.1 Limonene 6.9 Sabinene 33.3 Myrcene Ocimenes 35 Caryophyllenes 0 1,8-Cineole 2 Linalool 3 Other
Birch (pub)
Willow, aspen
Alder
Pine pool
Spruce pool
Pine synth
Spruce synth
13.8 7.8 3.1 3.5 10.9 8.8
26.7 13.3 0.4 1.9 17.5 5.0
28.0 24.8 8.0 2.3 18.0 6.0
29.4 4.6 27.5 7.3 8 15.3 5.3
41.6 8.7 8.8 8.8 8 1 2.7
29.8 9.9 0 10.7 0 0 19.1
26.9 33.4 5.4 1.8 12.6 3.7 6.6
3.8 7.3 3.5 37.4
35.2
8.3 1.3
20.4
15.3
9.6
5
1.3 13.1 7.6 4.3 3.1 11.3 28.6
12.2 4.8 13.2 0.6 19.9 6.3
14.0 19.3 5.7 3.0 27.7 9.7
8.1 9.4 1.3 10.7 2.4
41.5
14.3 6.3
1.6
15.3
29.4 4.6 27.5 7.3 8 15.3 5.3
41.6 8.7 8.8 8.8 8 1 2.7
29.8 9.9 0 10.7 0 0 19.1
26.9 33.4 5.4 1.8 12.6 3.7 6.6
1.3
20.4
15.3
9.6
1.3
15.3
For the deciduous species, myrcene is included in the b-pinene contribution.
Table 5 Calculated total biogenic emission #uxes (in kg/km forest during the modeling period) from coniferous and deciduous forests in di!erent parts of the boreal zone Boreal zone
Isoprene
Monoterpene
OVOC
Total VOC
South boreal Middle boreal North boreal
139 108 48
939 805 458
938 785 474
2015 1698 980
For isoprene this di!erence is even larger, with the #uxes in the North only about one-third of those in the South. An example of the emission variability during the growing season is given in Fig. 2, where we have plotted the isoprene, total monoterpene and OVOC emission #uxes from deciduous trees, pine and spruce in North Karelia in the Middle boreal zone. Daily average temperature is also shown (Fig. 2c) to indicate the variation of meteorological conditions during the modeling period. The boreal biogenic emissions are dominated by conifers, but in the summer months the deciduous trees add
an important contribution to the total. Spruce is the principal isoprene emitter all through the growing season due to its much higher biomass compared to that of the deciduous isoprene emitters (Fig. 2a). However, because of the high emission factors of willow and aspen, the isoprene emissions from deciduous trees are far from insigni"cant, especially during warm and sunny periods. The monoterpene emission #uxes of the deciduous trees are almost as high as the coniferous monoterpene emissions during the latter part of the summer, when the deciduous foliar biomass is fully developed (Fig. 2b). In
V. Lindfors et al. / Atmospheric Environment 34 (2000) 4983}4996
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Fig. 2. Modeled daily average isoprene (a), total monoterpene (b) and OVOC (c) #uxes from deciduous and coniferous trees in North Karelia during the modeling period April 1}October 31, 1997. The daily average temperature is included in (c) with a solid line (right axis).
early summer, deciduous trees emit high amounts of monoterpenes during the warm spells, but the emissions are only about one-third of the total monoterpene emissions. The e!ect of foliar biomass on the emissions is clearly evident in the OVOC emissions (Fig. 2c), whose emission factors are assumed to be the same for all studied tree species. Thus, the OVOC emissions are dominated by spruce and pine, with only a minor contribution
((15%) from the deciduous species during the summer months. The highest monthly emissions of both isoprene and monoterpenes occur in July in all boreal zones (Fig. 3). The isoprene emissions decrease rapidly after the warmest summer period, whereas the monoterpene emissions remain relatively high in August and decrease more slowly than the isoprene emissions in September.
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pinene and carene, which contribute between 45 and 60% of total emitted monoterpenes. There is a strong cineole contribution in spring, and high sabinene emissions during the summer and autumn months. a-pinene and carene emissions are also highest in spring, while the relative abundance of b-pinene remains almost unchanged in all seasons. Linalool emissions are low in spring, show a maximum contribution in summer, and decrease again towards the autumn (Table 6). The seasonal Middle boreal zone averages of the terpenoid emissions are presented in Fig. 4. In the spring months a- and b-pinenes, carene, and cineole contribute close to 70% of the total terpenoid emissions, while isoprene contributes 8% of the total. In summer, the share of isoprene increases to 13%, and sabinene, ocimene, caryophyllene, and linalool emissions become relatively more important. In autumn, the emission pattern remains almost the same, with a decreasing contribution of isoprene and linalool, balanced by an increase in the relative amount of a-pinene.
4. Discussion Rinne et al. (1999, 2000a, b) have measured hydrocarbon #uxes at selected coniferous forest sites in the European boreal zone. According to their results, the total monoterpene #uxes varied, on the average, between 50 and 300 ng m\ s\ in the South boreal zone (Rinne et al., 2000a), 20 and 200 ng m\ s\ in the Middle boreal zone (Rinne et al., 1999), and 0 and 150 ng m\ s\ in the North boreal zone (Rinne et al., 2000b). These values are well within the range of our modeled monoterpene emission #uxes. Direct measurements of biogenic emission #uxes are, however, very scarce, and it is thus very di$cult
Fig. 3. Monthly average isoprene (a) and total monoterpene (b) emission #uxes in the South boreal, Middle boreal, and North boreal zones.
Even though there are considerable di!erences in the emitted monoterpene amounts in di!erent parts of the boreal zone, the monoterpene distributions are very similar (Table 6). However, there are strong month-to-month variations in the emission pro"les. Throughout the growing season, the emissions are dominated by a- and b-
Table 6 Average compound distribution of emitted monoterpenes (in % of total monoterpene emission) in di!erent parts of the boreal zone in spring (spr, April}May), summer (sum, June}August), and autumn (aut, September}October) months in 1997 Monoterpene
a-Pinene b-Pinene D-Carene Camphene Limonene Sabinene Myrcene Ocimenes Caryophyllenes 1,8-Cineole Linalool Other
South boreal zone
Middle boreal zone
North boreal zone
Spr
Sum
Aut
Spr
Sum
Aut
Spr
Sum
Aut
36.7 9.3 11.4 7.8 8.1 4.5 3.5 0.6 0.4 15.2 1.9 0.5
27.8 10.0 8.2 5.7 8.7 12.4 2.6 6.4 2.2 10.5 4.6 0.9
29.7 8.7 9.3 6.2 8.6 12.4 2.4 6.2 1.8 11.5 2.4 0.8
36.9 8.9 12.3 8.0 8.1 4.6 3.7 0.2 0.3 14.9 1.5 0.6
26.9 9.3 8.5 5.8 9.2 12.9 2.5 4.2 3.4 9.6 6.4 1.3
30.0 8.2 10.1 6.5 8.9 12.2 2.5 3.7 2.6 11.1 3.1 1.0
38.1 8.7 12.7 8.3 8.0 4.3 3.7 0.0 0.0 15.7 0.0 0.6
27.4 9.1 8.7 6.0 9.4 12.8 2.5 3.3 3.6 9.7 6.1 1.3
31.7 8.1 10.6 6.9 8.9 10.7 2.6 2.4 2.4 11.9 2.8 1.0
V. Lindfors et al. / Atmospheric Environment 34 (2000) 4983}4996
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Fig. 4. Middle boreal zone averages of terpenoid emission distributions in spring (April}May), summer (June}August), and autumn (September}October) months.
to verify this type of seasonal emission calculations using experimental data. One of the few available means for at least qualitatively validating the emission model results is to study the variability of observed ambient biogenic VOC concentrations in a background area. Given the short atmospheric lifetime of the biogenic compounds (minutes to hours), it is conceivable that in remote areas this variability is mostly due to the local emission patterns. The Finnish Meteorological Institute has carried out regular ambient VOC measurements at PoK tsoK nvaara (63307 N, 31304 E, 254 m a.s.l.), which is located in a forested environment in North Karelia (see Fig. 1). The monoterpene samples were collected two to three times a week from May to October into Tenax adsorbent and analyzed in the laboratory of the FMI using GC/MS, while isoprene was analyzed from canister samples using GC/FID (Hakola et al., 1998). 15-day running averages of the measured concentrations and modeled emission #uxes are shown in Fig. 5 for isoprene and total monoterpenes. Unfortunately, no measurements were available in April, but based on these results it appears that the onset of isoprene emissions and the rapid increase of monoterpene emissions in spring, as well as the phasing out of the emissions in autumn are adequately parameterized in the model. The calculated emission peaks occur in periods when high concentrations were observed in the ambient air
samples (Fig. 5). However, there is some disagreement between the behavior of the predicted monoterpene #ux and the measured concentration in August. During the summer months of 1997 North Karelia experienced rainy periods around midsummer (June 24th), in the second and the last week of July, and on several occasions in August, which also had and unusually cold spell (night temperatures fell close to freezing) from the 13th to the 18th (statistics of the Finnish Meteorological Institute). Rain is not taken into account in the emission model except indirectly through the cloudiness parameterization, but it may have a!ected either the emission process itself or the ambient air concentrations, and together with the sudden drop of the daily average temperatures this may partly explain the observed low concentrations of both isoprene and monoterpene around August 15. A more detailed analysis of the correspondence between the modeled #ux and the measured concentrations and an assessment of the accuracy of the predicted emission levels would require the use of a photochemical model in order to take into account the chemical reactions and the atmospheric dispersion and advection of the emitted compounds. However, even this rough comparison is encouraging, as it shows that the Guenther emission algorithms and the FMI/BEIS emission model are capable of producing realistic seasonal emission patterns in a typical boreal forest area. Compared to our earlier
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Fig. 5. Running 15-day averages of the modeled isoprene (a) and total monoterpene (b) #uxes (solid line, left axis) and observed ambient air concentrations (dashed line, right axis) in North Karelia during the modeling period April 1}October 31, 1997.
biogenic emission calculations (Lindfors and Laurila, 2000), the inclusion of a detailed deciduous biomass parameterization appears to have improved the agreement between the onset of the modeled isoprene emissions and the "rst observations of isoprene in ambient air in spring. Monoterpene emission measurements of Norway spruce and Scots pine (Janson, 1993) and #ux measurements over pine forests (Rinne et al., 1999, 2000a, b) have shown the strong dominance of a-pinene and carene in the coniferous monoterpene spectrum throughout the boreal zone. Our modeled #uxes also include the contribution of the deciduous emissions with relatively less a-pinene and higher amounts of e.g. sabinene, limonene, and ocimenes. In Fig. 6 we compare the modeled monoterpene emission distribution with the average measured concentrations in July 1997 in North Karelia. The most reactive emitted terpenoids (myrcene, ocimenes, caryophyllene, linalool,
and limonene (Atkinson, 1994)) are depleted in the ambient air compared to the emission spectrum, and the relative abundance of isoprene, a- and b-pinene, and carene, which have a longer atmospheric lifetime against oxidation (Atkinson, 1994), has increased. Besides forest emissions, the ambient concentrations are a!ected by emissions from other vegetation and the forest #oor (e.g. Wilske and Kesselmeier, 1999; Janson et al., 1999), which are not parameterized explicitly in the emission model. However, the overall agreement between the two distributions is quite reasonable, indicating that the monoterpene pro"les used in this work are representative of the boreal forest emissions, even though they are based on very limited sets of experimental data. Gri$n et al. (1999c) have estimated the relative species contribution to global monoterpene emissions. According to their estimate, a- and b-pinene correspond to 35 and
V. Lindfors et al. / Atmospheric Environment 34 (2000) 4983}4996
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Fig. 6. Relative abundance of terpenoids in the modeled emissions and measured ambient concentrations in North Karelia in July, 1997.
23% of the total emissions, respectively. Limonene contributes 23%, myrcene and sabinene both 5%, D-carene 4%, ocimene and terpinolene both 2%, and a- and cterpinene 1% of the total monoterpene emissions (Gri$n et al., 1999c). According to our results, the contribution of a-pinene varies from 27 to 38%, depending on the season and the boreal zone, which is in agreement with the results of Gri$n et al. (1999c). Our estimated b-pinene (8}10%) and limonene (8}9.4%) contributions are lower than those of Gri$n et al. (1999c), while we obtain higher contributions of D-carene (8}13%) and sabinene (4}13%). These di!erences are probably explained by the di!erent emission characteristics of the boreal forest ecosystems when compared to global averages. However, Gri$n et al. (1999c), have only included monoterpene species known to be important in secondary organic aerosol formation, and their inventory may not be directly comparable to our model results.
5. Conclusions We have calculated the biogenic terpenoid emission #uxes from typical South boreal, Middle boreal, and North boreal forests using the Guenther algorithms and experimental emission factor data. The accumulated emissions re#ect the strong dominance of the coniferous species with high a- and b-pinene and carene #uxes, but in the summer months the deciduous trees add an important contribution to the total, giving rise to substantial sabinene and linalool emission #uxes. Spruce is the principal isoprene emitter all through the growing season due to
its much higher biomass compared to that of the deciduous isoprene emitters. The biogenic emissions are strongly a!ected by the prevailing meteorological conditions. Thus, the average monoterpene and OVOC emission #uxes from forests in the South boreal zone are approximately twice as high, and the isoprene #uxes three times as high, as the corresponding #uxes in the North boreal zone. In spring a- and b-pinenes, carene, and cineole contribute close to 70% of the total terpenoid emissions, while isoprene contributes 8% of the total. In summer, the share of isoprene increases to 13%, and sabinene, ocimene, caryophyllene, and linalool emissions become more important. The highest monthly emissions of both isoprene and monoterpenes occur in July in all boreal zones. The isoprene emissions decrease rapidly after the warmest summer period, whereas the monoterpene emissions remain relatively high in August and decrease more slowly than the isoprene emissions in September. The Guenther emission algorithms and the FMI/BEIS emission model with the parameterized deciduous biomass development appear to produce realistic biogenic emission patterns when applied in the North European coniferous forest zone. However, a more comprehensive validation of the algorithms and the emission model requires further experimental data on the seasonal variation of the emission #uxes and the speci"c monoterpene emission spectra of the main boreal tree species. One of the main unknowns when considering emission modeling in cold climatic conditions is the applicability of the emission algorithms in temperatures close to or even below zero
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degrees centigrade, which also needs to be resolved using experimental data. The overall uncertainty of biogenic emission inventories may be estimated as high as a factor of "ve or even greater (Simpson et al., 1999). The error level varies when breaking down the total error to errors associated with a speci"c compound or compound group and the land use classi"cation. The error estimated resulting from a coarse land use, especially vegetation, classi"cation may be as high as a factor of three. When, as done in the present work, a detailed vegetation classi"cation down to the plant species level is used, based upon the latest information available, a conservative error in the order of 50% may be estimated. Further, the emission factors and emission algorithms used in this inventory are based on "eld measurements on plants in Finland, signi"cantly reducing the uncertainty of a factor of "ve associated with the emission factors of isoprene and monoterpenes. The errors of the emission factors for isoprene and monoterpenes used in this work are conservatively estimated to be not higher than 50%. Therefore the overall uncertainty of the presented emission inventory may amount to 70%.
Acknowledgements The "nancial support of the Nordic Council of Ministers and the &R&D Programme for Environment and Climate' of the European Commission (contract ENV4CT95-0022) is gratefully acknowledged.
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