Student thesis series INES nr 269
Biogenic volatile organic compound
emissions from Willow trees
Md. Ahsan Mozaffar
2013
Department of
Physical Geography and Ecosystem Science
Lund University
Sölvegatan 12
S-223 62 Lund
Sweden
Md. Ahsan Mozaffar (2013). Biogenic volatile organic compound emissions from Willow trees
Master degree thesis, 30 credits in Physical Geography and Ecosystem Analysis
Department of Physical Geography and Ecosystem Science, Lund University
ii
Biogenic volatile organic compound emissions from
Willow trees
....................................................................................
Md. Ahsan Mozaffar
Master degree 30 credits thesis in
Physical Geography and Ecosystem Analysis
Supervisor: Thomas Holst
Department of Physical Geography & Ecosystems Science
Lund University
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Abstract
In August 2012, emissions of biogenic volatile organic compounds (BVOCs) from willow
(Salix) trees were measured in Lund, southern Sweden. Sweden has more than 16000 ha of short
rotation willow plantations for biofuel production and willow trees are suspected to emit
substantial amounts of BVOC to the atmosphere. BVOC emission measurements were carried
out on two Salix fragilis trees and one Salix phylicifolia tree. The measurements were conducted
using a dynamic flow through chamber covered with Teflon film. Salix trees were found to emit
large amounts of isoprene. Especially Salix phylicifolia emitted 98% isoprene of total BVOC
emission, whereas Salix fragilis trees released about 85% of the total BVOC mass as isoprene.
Apart from isoprene, willow trees were found to emit significant amounts of ocimene as well.
Emission rates of different BVOCs were found to vary between different individuals of the same
species of willow trees as well as between the subspecies of willow trees. Emissions of BVOCs
were detected to depend on temperature and photosynthetically active radiation. Isoprene
emission potential for Salix phylicifolia was 56.4 µg g-1dw h-1 whereas 16.9 µg g-1dw h-1 and 44.4
µg g-1dw h-1 were measured for the Salix fragilis trees.
Keywords
Biogenic volatile organic compounds, Willow, Isoprene, BVOC, Climate, Salix, Atmosphere.
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CONTENTS
ABSTRACTS ................................................................................................................................................................. IV
1. INTRODUCTION ............................................................................................................................................ 1
1.1. INTRODUCTION TO BVOCS FROM PLANTS .............................................................................................................1
1.2. OBJECTIVES AND HYPOTHESIS ...............................................................................................................................1
2. BACKGROUND ............................................................................................................................................... 2
2.1. DIFFERENT TYPES OF BVOC FROM PLANT EMISSION .............................................................................................2
2.2. EMISSION SPECTRA OF BVOC FROM WILLOW ........................................................................................................3
2.3. EFFECTS OF BVOC ON ATMOSPHERIC CHEMISTRY.................................................................................................3
2.4. REGULATIONS OF BVOC EMISSIONS .....................................................................................................................4
2.5. BVOC EMISSION IN THE FUTURE ...........................................................................................................................4
3. MATERIALS AND METHODS ...................................................................................................................... 5
3.1. STUDY AREA AND SAMPLING DETAILS .....................................................................................................................5
3.2. EXPERIMENT ........................................................................................................................................................ 7
3.2.1. Design of enclosure systems .......................................................................................................................7
3.2.2. Enclosure purge air ....................................................................................................................................9
3.2.3. Environmental monitoring ..........................................................................................................................9
3.2.4. Sample collection and analysis .................................................................................................................10
3.3. FLUX CALCULATION ...........................................................................................................................................11
3.4. ISOPRENE EMISSION MODEL ................................................................................................................................12
3.5. MONOTERPENES AND SESQUITERPENES EMISSION MODEL ....................................................................................13
4. RESULTS ....................................................................................................................................................... 13
4.1. MEASURED EMISSION RATES ...............................................................................................................................13
4.2. CORRELATION WITH PAR AND TEMPERATURE...................................................................................................... 22
4.3. EMISSION POTENTIALS ........................................................................................................................................25
4.3.1. Isoprene emission potential ......................................................................................................................26
4.3.2. Monoterpenes and sesquiterpenes emission potentials .............................................................................27
5. DISCUSSION ................................................................................................................................................. 30
5.1. UNCERTAINTIES RELATED TO BVOC SAMPLING BY THE CHAMBER METHOD ..........................................................30
5.2. EMISSIONS OF BVOCS ........................................................................................................................................30
5.3. DEPENDENCE OF EMISSION ON TEMPERATURE AND PAR .....................................................................................31
5.4. DIFFERENT EMISSION RATES WITHIN SAME SPECIES ..............................................................................................31
5.5. DIFFERENT EMISSION RATES WITHIN SUBSPECIES .................................................................................................32
5.6. EMISSION POTENTIALS ........................................................................................................................................32
6. CONCLUSIONS ............................................................................................................................................. 34
ACKNOWLEDGEMENTS ................................................................................................................................ 34
REFERENCES ................................................................................................................................................... 35
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1. Introduction
1.1. Introduction to BVOCs from plants
Biogenic volatile organic compounds (BVOCs) are a versatile group of non-methane
hydrocarbons (NMHC) emitted by vegetation (Ekberg et al., 2011). Isoprene, monoterpenes,
sesquiterpenes and different types of alkanes, alkenes, carbonyls, alcohols, esters, ethers, acids
all fall into the BVOC category (Fehsenfeld et al., 1992; Kesselmeier and Staudt, 1999). BVOCs
are involved in plant growth, reproduction and defense mechanisms (Peñuelas and Staudt, 2010).
In addition, as a source of BVOC, terrestrial vegetation offers an important interaction between
the earth’s surface, atmosphere and the climate system (Laothawornkitkul et al, 2009). BVOC
play an important role in tropospheric chemistry (Atkinson, 2000) and could increase methane’s
lifetime (Pacifico et al., 2009), form tropospheric ozone and increase CO2 concentration
(Peñuelas and Staudt, 2010). Annually 1.2 Pg of carbon are released to the atmosphere as BVOC
globally, of which 500 Tg is isoprene (Guenther et al., 1995). Because of the presence of carboncarbon double bonds, isoprene is extremely reactive (Seinfeld and Pandis, 1998). However, the
isoprene emission capacity of vegetation is highly species specific (Hanson et al., 1999). Willow
(Salix sp.) is recognized as a high BVOC emitter and emits isoprene dominantly (Copeland et al.,
2012, Hakola et al., 1998, Olofsson et al., 2005, Shi et al., 2011). For the last 25 years willow has
been cultured as an agricultural crop for bioenergy production and played in important role in
Swedish energy sector by producing wood fuel (Mola-Yudego and Aronsson, 2008). During this
period, more than 16000 ha of short rotation willow plantations have been planted in Sweden,
which is around 0.5% of total cultivable land in Sweden and that made Sweden the main region
of commercial farming of willow in Europe (Mola-Yudego and Pelkonen, 2008). During the
1990s different measures were undertaken by Swedish government to promote willow plantation
in Sweden (Mola-Yudego and Pelkonen, 2008). A generic subsidy of 10000 SEK per ha was
provided to the farmers who transferred a part of their cereal crop land to willow plantation
(Rosenqvist et al., 2000) This thesis paper aims to characterize BVOC emissions from different
willows.
1.2. Objectives and hypothesis
BVOC emissions from plants are known to vary within different tree species and the
emission rates have been shown to depend on leaf temperature and intensity of solar radiation,
1
and emissions may be modified by different stresses e.g. drought, ambient CO2 concentration, O3
concentration etc. (Laothawornkitkul et al., 2009). In general, willows have been reported to
substantially emit isoprene (Olofsson et al., 2005; Shi et al., 2011; Hakola et al., 1998; Copeland
et al., 2012). The objectives of my study were: (1) to screen BVOC emission rates across
different willow trees within the same species (Salix fragilis), (2) to survey BVOC emission rates
over different subspecies of willow trees (Salix fragilis and Salix phylicifolia), and (3) to
estimate BVOC emission potential from willow trees by using Guenther’s emission algorithm
(Guenther et al., 1993; Guenther, 1997).
Emission rates of BVOC may vary within different subspecies of willow trees because of
their different environmental conditions e.g. light, temperature (Guenther et al. 1993), ambient
CO2 concentration, O3 concentration (Laothawornkitkul et al., 2009) and other factors like,
growth form (Haapanala et al. 2009), age (Kim, J., 2001), sex (Carvalho et al., 2005), water
availability (Faubert et al., 2011). BVOC emission rates may also vary within same species of
willow trees because of the above reasons. Isoprene emission should dominate the total BVOCs
emissions from willows.
2. Background
2.1. Different types of BVOC from plant emission
There is a spectrum of different compounds emitted from trees and these compounds may
vary according to their corresponding species. For example, mountain birch emits substantial
amounts of linalool, sabinene, α-farnesene (Haapanala et al., 2009) whereas scots pine
significantly emits 3-carene, α-pinene and β-pinene (Tarvainen et al., 2004). Isoprenoids
(isoprene and monoterpenes) are the most prominent compounds emitted as BVOC from the
plants. Other BVOCs from plants include sesquiterpenes as well as different types of alkanes,
alkenes, alcohols, carbonyls, acids, esters and ethers (Kesselmeier and Staudt, 1999). Low
molecular weight (Carbon<5) BVOCs like methanol, ethylene, formaldehyde, ethanol, acetone
and acetaldehyde are also emitted by plants (Kreuzwieser et al., 1999). Both above and below
ground plant organs emit BVOC, especially flowers and fruits emit the broadest kind of BVOC
(Dixon & Hewett, 2000; Knudsen et al., 2006). Various mixtures of terpenoids, including
isoprene, monoterpenes, sesquiterpenes and some diterpenes are release by the vegetative parts
2
of woody plants (Owen et al., 2001). On the other hand, large amounts of oxygenated BVOC and
some monoterpenes are released by the grass species (Fukui & Doskey, 2000). Tropical and
extra-tropical forests are the largest source of BVOC and emit BVOCs like isoprene, α-and βpinene and methanol (Guenther et al., 2006). Besides, some species are acknowledged as
specificly isoprene, monoterpene or sesquiterpene emitters as plant species have a specific
spectrum of BVOCs emissions (Duhl et al., 2008).
2.2. Emission spectra of BVOC from willow
Willow trees emit isoprene dominantly with other different types of BVOC (Olofsson et al.,
2005; Hakola et al., 1998). According to Copeland et al. (2012), coppice willow (Salix sp.) emits
isoprene, methanol, monoterpenes, acetic acid, acetaldehyde, acetone, methyl vinyl ketone and
methyl ethyl ketone. Besides, young leaves of tea-leafed willow have been reported to emit
terpenes, 1-butene, ethene, and propene, but mature leaves emit very high amount of isoprene
(approximately 50 µg g-1dw h-1) (Hakola et al., 1998). Moreover, Lindfors and Laurila (2000)
indicate willow as a high isoprene emitter compare to Betula sp. and Alnus sp.
2.3. Effects of BVOC on atmospheric chemistry
BVOCs are highly reactive compounds because of their unsaturated nature and are quickly
oxidized by hydroxyl (OH), ozone (O3) and nitrate radicals (NO3) (Ekberg et al., 2011).
Especially isoprene is extremely reactive because of the presence of carbon-carbon double bonds
(Seinfeld and Pandis, 1998). BVOCs play an important role in atmospheric chemistry as it has
both positive and negative radiative forcing on climate (Naik et al., 2004; Faubert et al., 2011).
Positive radiative forcing on climate change includes enhancing the concentration of three
crucial greenhouse gases: carbon dioxide (CO2), methane (CH4) and tropospheric ozone (O3)
(Laothawornkitkul et al., 2009). Oxidative reactions of BVOC with hydroxyl radicals and oxides
of nitrogen form tropospheric ozone (Peñuelas and Staudt, 2010). The competition between
BVOCs and methane for hydroxyl radicals may increases the lifetime of methane in the
atmosphere if BVOCs are emitted in large amounts (Kaplan et al., 2006). Besides, the end
product of BVOCs oxidative reactions in the atmosphere is carbon dioxide (Peñuelas and Staudt,
2010). Conversely, secondary organic aerosols (SOAs) are formed by the atmospheric reaction
of BVOCs (Laothawornkitkul et al., 2009), which have a negative radiative forcing on climate
change (IPCC 2007, Working group I, 2.4.1). SOAs induce a cooling effect in the atmosphere by
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creating cloud condensation nuclei raising the cloud cover that scatters sunlight and altering the
cloud albedo (Laothawornkitkul et al., 2009).
2.4. Regulations of BVOC emissions
Temperature and photosynthetic active radiation (PAR) have a great influence on BVOC
emission from plants (Guenther et al. 1993; Niinemets et al., 2004). Temperature affects the
availability of BVOC forming substrates (e.g. isoprenoid intermediates) and the activity of ratelimiting enzymes and by this way control emission rates of BVOC. Besides, physicochemical
constraints like availability of pyruvate caused by temperature also affect the emission of BVOC
from leaves (Niinemets et al., 2004). Under both optimum and stress conditions emissions of
many BVOCs (e.g. α-pinene, β-pinene, limonene etc.) are regulated by enzyme activities
(Fischbach et al., 2002). As all BVOCs are volatilized by higher temperature, their emission rate
is dependent on temperature (Kesselmeier and Staudt, 1999). For some compounds like isoprene,
which are not stored in leaves and are thus highly volatile; emission depends on light intensity
(Laothawornkitkul et al., 2009). BVOCs emission might depend on the storage compartments in
leaves. Compounds like monoterpenes can be stored in the storage pool inside the plants if
storage pools exist and their emission mainly depends on the temperature (Haapanala et al.,
2009). Biotic and abiotic stresses like herbivore-damage, drought, air pollution may also lead to
enhanced release of some BVOCs, like terpenes, methyl jasmonate and methyl salicylate
(Takabayashi et al., 1994). Distinct diurnal or nocturnal patterns of BVOC emissions are
observed from herbivore-damaged leaves (Loivamaki et al., 2007). Besides, the production and
emission of some BVOCs like isoprene depends on the carbon dioxide (CO2) concentration, soil
moisture availability and other environmental stresses, including ozone (O3) concentration
(Laothawornkitkul et al., 2009).
2.5. BVOC emission in the future
Emission rates of most BVOCs increase with the enhancement of temperature as high
temperature raises both the volatility of BVOC and biochemical reactions in the BVOC
producing metabolic pathways, and long-term changes of temperature also increase the emission
rates by lengthening of the growing season (Filella et al., 2007). According to the IPCC A1F1
(fossil fuel-intensive) scenario, mean earth surface temperature will increase by 2.4-6.4°C
towards the end of 21st century (IPCC, 2007; working group I, 10.5). Higher temperature
4
enhances chemical reaction rates, rises cellular diffusion rates and increases the vapor pressures
of volatile compounds. For these reasons, emissions of BVOCs are expected to increase in future
(Fuentes et al., 2000; Sharkey and Yeh, 2001). Many experiments (Faubert et al. 2011; Sharkey
et al., 1996) were undertaken to estimate how an enhance in temperature will increase BVOC
emission rates. Rising global mean temperature by 2–3°C, which is predicted to occur early this
century (IPCC 2007; Working group I, 11.2.3), could increase global BVOC emissions by 25–
45% (Penuelas & Llusia, 2003). On the other hand, at very high temperature (approximately
above 40oC), isoprene emission will decrease, especially in the tropics (Laothawornkitkul et al.,
2009). Besides, vegetation species composition and vegetation characteristics could change by
the changing climate and eventually alter the regional and global scale of BVOC emission
(Wilmking et al., 2004). In response to increasing surface temperature, precipitation frequency
and intensity are anticipated to change in the future. Drought is expected to occur in the tropics
(IPCC 2007;Working group I, 3.3), and moderate drought can reduce, raise or have no effect on
isoprene and monoterpenes emissions, but severe drought might inhibit photosynthesis and thus
significantly decrease BVOC emissions (Laothawornkitkul et al., 2009). Growing atmospheric
CO2 concentrations could enhance the productivity and standing biomass of plants by CO2
fertilization and indirectly increase the production and emission of BVOC (Korner, 2006).
3. Materials and Methods
3.1. Study area and sampling details
In August 2012, measurements of BVOC emissions were conducted on two trees of Salix
fragilis and one tree of Salix phylicifolia in the botanical garden (55° 42′N, 13° 12′E) of Lund
University, Sweden (Figure 1). Lund city is located in the southern part of Sweden which has
mean annual temperature of 7.9°C between 1961-1990 (data from Swedish meteorological
institute, SMHI: www.smhi.se). Annual precipitation during this period is moderate; with an
amount of 665.8 mm. August is the second warmest (16.5°C) and wettest (64.6mm) month
during this period. However, the average temperature during the measurement days was 18.5°C
(from my measurement) with some discrete rain events. Two trees of Salix fragilis with a height
of ~15m and one tree of Salix phylicifolia with a height of ~5m were sampled (Figure 2). Two
branches of each tree and two days on each branch were sampled from morning till afternoon.
However, the second branch of the second Salix fragilis tree was broken by wind during the
5
second day of measurements, and sampling was discontinued after this. All the experimental
branches were at about 2 m height from the ground surface. Two branches of first Salix fragilis
tree’s position were in a more shaded zone compared to the two branches of second Salix fragilis
tree (Figure 2). Besides, two branches of Salix phylicifolia were not shaded by other trees and
had sunlight throughout the day.
Figure 1. Map of Lund University showing botanical garden and black star mark in the botanical
garden roughly pointing at the measurement site (Courtesy: Department of Physics, Lund
University).
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Figure 2. Photograph of three experimental willow trees (August, 2012) at Lund University
botanical garden: (a) first Salix fragilis tree; the experimental branches of this tree were shaded
by surrounding trees in the morning, (b) second Salix fragilis tree; it had more sunlight almost
all the time of the day compared to the first Salix fragilis tree, (c) Salix phylicifolia tree; it was
not surrounded by any other tree and had direct sunlight throughout the day.
3.2. Experiment
3.2.1. Design of enclosure systems
A schematic diagram of flow-through chamber technique used in this study to measure
BVOC emissions is shown in figure 3. This chamber was fixed on a mast at the same height to
the experimental branch of the tree and branch was enclosed inside the chamber (Figure 4). The
chamber is made of Teflon and stainless steel to avoid adhesion of BVOC on its surface.
Besides, the chamber is wrapped with Teflon film which is transparent to PAR
(Photosynthetically Active Radiation, 400-700nm). There are several openings on both ends of
this cylindrical chamber; one end has a small round opening connected with a sealable opening,
where the trunk side of the branch was introduced. There are three small opening on the other
end of the chamber and also a big lid, which is open all the times except when the measurement
took place. Among the three small openings, one is used for placing temperature and humidity
sensors inside the chamber, one is for inlet purge air and other one is for the sample outlet. The
volume of the cylinder was ~20L, which was enough to fit with various sampling and analytical
7
standards. For instance, a sufficient amount of leaf biomass (~10 g) was enclosed that allows a
good mixing ratio of BVOC and purge air and for this, emitted BVOCs were within a range that
permits relatively high time resolution (hourly) collection of samples. Besides, this volume was
large enough for maintaining minimal contacts with the leaves of a small branch of a Salix tree.
Figure 3. Schematic picture of flow through chamber measurement of BVOC emission from tree
branch.
Symbols are as follows: PAR= light sensor, P=pocket pump, F= charcoal filter,
MFM=mass flow meter, T=temperature sensor, RH= relative humidity sensor.
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Figure 4. Photograph of an experimental branch of a Salix fragilis tree enclosing inside the
chamber (August, 2012).
3.2.2. Enclosure purge air
A Charcoal filter with MnO2 scrubber at its outlet was used to create hydrocarbon and
ozone free purge air and this charcoal filter was connected to the outlet of a diaphragm pump
(Diaphragm gas pump; KNF, UK) and this pump provided ~ 5 l min-1 ambient air to the chamber
via the charcoal filter. In addition, a flow meter was used to observe and control purge air flow
into the chamber. This ~5 l min-1 continuous purge air flow into the ~20 l chamber prohibits any
artificial BVOC concentration buildup in the enclosure system and keeps an almost constant
BVOC-air mixing ratio (see Ortega and Helmig, 2008). The enclosure system was not
completely airtight and excess air can easily get out from the chamber. Moreover, this excess air
prevented contamination of the chamber from outside. The inflow tube inside the chamber was
perforated in different places for good mixing of air inside the enclosure.
3.2.3. Environmental monitoring
A shaded temperature and humidity probe (CS215; Campbell Scientific, INC. Utah, USA)
was measuring the air temperature and humidity inside the chamber. Another temperature and
humidity probe (CS215; Campbell Scientific, INC. Utah, USA) with a radiation shield was
placed just beside the chamber to measure ambient air temperature and humidity. Besides, PAR
was measured by quantum sensor (LI-190; LI-COR INC. USA) placed beside the enclosure. A
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data logger (CR 1000; Campbell Scientific Ltd, UK) was continuously recording one minute
average values of these environmental variables during the measurement days.
3.2.4. Sample collection and analysis
The branch setup inside the enclosure was made at least 12h before (overnight) the
measurements, so that induced emissions caused by tempering on the leaves and branch was
reduced. Besides, great care was taken when branches were put inside the chamber and it was
made as gentle as possible. Each morning before measurements started, the chamber was flushed
with purge air for 10 minutes to remove BVOCs from the chamber. Air from the enclosure was
drawn through the sampling tubes by a pump at a rate of ~220 ml min-1 and collected on
adsorbent cartridges (see Ekberg et al., 2011 or Haapanala et al., 2009). Cartridges filled with
Tenax TA (a porous organic polymer) and Carbograph 1TD (graphitized carbon black) were
used for adsorbing volatile and semi-volatile organic compounds which were coming with the
pumped air from the chamber. Sample cartridges were changed every hour and typical sample
volume on each cartridge was ~13 liter. After collecting each sample, long-term storage caps
were tightened on the cartridge connectors and the sample cartridges were kept at a cool place.
To observe the BVOC compounds and background concentrations in the inflow air, an additional
pump (SKC Pocket pump 210-1000, SKC Inc., PA, USA) was connected to the inflow line to
collect inflow air at a rate of ~220 ml min-1 in the inlet air sample cartridge. All sample
cartridges were stored in the refrigerator at around 4°C temperature until their contents were
identified and quantified in the department’s laboratory. Gas chromatography-mass spectrometry
(GC-MS, Shimadzu QP2010 Plus, Shimadzu Corporation, Japan) was used to analyze the sample
air in the cartridges. After thermodesorption at 280˚C and cryofocusing at 0˚C on a Tenax TA
cold trap using an automated thermal desorber (Turbomatrix 650 ATD, PerkinElmer, Waltham,
MA, USA), samples were analyzed by GC-MS. Helium was used as carrier gas and BVOCs
chromatographic separation was carried out by using an Agilent EZ-Guard VF-5ms capillary
column (30 m*0.25 mm, film thickness 0.25 µm, Agilent Technologies, Santa Clara, USA) with
the following temperature program: initial temperature 40˚C held for 1 minute, then raised by
15˚C/min to the final temperature 250˚C which was held for 2 minutes. The MS ion source
(electron impact) temperature was kept at 250˚C and the interface at 275˚C. Full scans for ion
fragments m/z 47-350 were carried out and compound identification took place by comparison of
analyte mass spectra with mass spectra in the NIST 08 library. Total ion counts (TIC) of pure
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standards of isoprene, α-pinene, β-pinene, 3-carene, eucalyptol, limonene and caryophyllene
were used for the quantification of compounds. Compounds for which no pure standards were
available were quantified by assuming a TIC integration equation based on the average factor
and constant of the above standards. Isoprene concentration in 22 cartridges out of 73 was so
high and flushing the detector. Among these 22 sample cartridges, 13 cartridges were from the
first Salix fragilis tree, 4 cartridges were from the second Salix fragilis tree and 5 cartridges were
from the Salix phylicifolia tree. In addition, ocimene concentration in 44 cartridges out of 73 was
high and flushing the detector. Among these 44 sample cartridges, 23 cartridges were from the
first Salix fragilis tree, 20 cartridges were from the second Salix fragilis tree and 1 cartridge was
from the Salix phylicifolia tree. Thus sample inside the cartridges was diluted to 3.3, 10, 13.2 and
25% depending on the concentration, and finally identified compounds concentration was
corrected by the dilution factor (A. Ekberg 2012, pers. comm.).
All leaves on the sampling branch inside the chamber were collected at the end of
measurements on each branch. Pictures of fresh leaves together with Swedish coins (5 and 10
SEK) as a standard for area were taken and leaf areas were estimated by using Adobe Photoshop
software by counting pixels of green leaves and coins. Area of each Swedish coin was known
and coins had different colour compared to the leaves. So, leaves pixels could be easily separated
from the coins pixel. Finally, the number of pixels of coins was compared with pixel of leaves to
measure the area of leaves. Weight of the fresh leaves from each branch was recorded. After
measuring freshweight, the leaves were placed in the oven at 70°C. Approximately 24h later, dry
weight of the leaves for each branch was measured.
3.3. Flux calculation
The outcomes from gas chromatography-mass spectrometry were the individual mass
weights of a spectrum of BVOC compounds from each cartridge in the µg range. Then emission
rates (ER) were calculated by equation 1;
ER =
(Cout − Cin) * V
Gdw * t
(1)
where ER is the emission rate (µg gdw-1 h-1), Cin is the concentration (µg m-3) of a compound
in the purge air collected in the inlet air sample cartridge, Cout is the concentration (µg m-3) of
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the BVOC in the enclosure collected by the sample cartridge, V is the volume (m3) of purge air
that passed through the chamber, Gdw is the dried mass weight (g) of leaves enclosed and t is the
duration (h) of the measurement that was one hour each. Finally, resulting emission rates units
are µg gdw-1 h-1.
3.4. Isoprene emission model
There are different models for isoprene emission estimation from vegetation (Guenther et
al.,1993; Guenther, 1997; Niinemets et al., 1999) but for this thesis I applied the most used G97algorithm (Guenther et al.,1993; Guenther, 1997). This model describes emission of isoprene
which is dependent on both temperature and light intensity given by,
I=IS * CL* CT
(2)
where I is the isoprene emission rate (µg g-1dw h-1) from the chamber measurements at
temperature T(K), IS is isoprene emission rate at a standard temperature, TS (303K) and at
standard PAR flux (1000μmol m-2s-1), the factor CL describes the light dependency of isoprene
emission by
CL =
αC L1 L
(3)
1+ α 2 L2
where L is PAR (μmol m-2s-1), α (0.0027) and CL1 (1.066) are empirical coefficients which
are determined by nonlinear best fit procedures using eucalyptus, sweet gum, aspen, and
velvet bean emission measurements (see Guenther et al.1993).
The factor CT describes the temperature dependency of isoprene emission (µg g-1dw h-1) by
(
C
T
)
 CT1 T − T S 

exp
 RT T 
S


=
 CT 2 T − T M
C T 3 + exp
 R T ST
(
)
(4)


where R(8.314 JK-1) is the universal gas constant, CT1 (95000 J mol-1),CT2 (230000 J mol1
), CT3 (0.961) and TM (314K) are all empirical coefficients which are determined by nonlinear
12
best fit procedures using
eucalyptus,
sweet
gum, aspen,
and velvet bean emission
measurements (Guenther et al.1993).
3.5. Monoterpenes and sesquiterpenes emission model
Some BVOCs like monoterpenes and sesquiterpenes are stored inside the plant and
emission from the storage pools is given by a temperature dependent algorithm (Guenther et al.,
1993) in a simple exponential model of the form,
E=ES * exp (β(T-TS))
(5)
where E is the emission rate (µg g-1dw h-1) at temperature T(K), ES is the emission rate at
standard temperature
TS (303K) and
β(0.09K-1 for monoterpenes and 0.143K-1 for
sesquiterpenes) is an empirical coefficient describing the strength of the temperature dependence
(Guenther et al., 1993; Haapanala et al., 2009).
4. Results
4.1. Measured Emission rates
There were 31 compounds detected both from Salix fragilis and Salix phylicifolia species
(Table 1). Among them, 10 monoterpenes (e.g. ocimene, C10H14O, 3-carene); 9 sesquiterpenes
(e.g. caryophyllene, α-Farnesene (Z,E)); 9 other VOCs (compounds not sharing the molecular
formula of isoprene, monoterpenes or sesquiterpenes; e.g. toluene, acetophenone, pentadecane);
2 unknown (compounds whose molecular formula could not be identified ) and isoprene.
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Table 1. A list of all the compounds that were measured from the Salix trees. All compounds are
classified in five major classes: Isoprene, Monoterpenes, Sesquiterpenes, Other VOCs and
Unknown. Bold numerals in the table are the % of total BVOC emission by mass and italic
numerals are the % of the corresponding groups emission by mass.
Compound
% of emission
2nd Salix fragilis
84
Isoprene
1st Salix fragilis
88
Monoterpenes
6
7
2
C10H16
Ocimene
C10H14O
C10H16 2,4,6-Octatriene, 2,6-dimethyl-,
(E,Z)C10H14 E,E-2,6-Dimethyl-1,3,5,7-octatetraene
3-carene
α-pinene
β-pinene
Eucalyptol
Limonene
Sesquiterpenes
3
45
17
2
6
42
17
3
11
80
4
1
2
31
~0
~0
~0
~0
4
4
28
~0
~0
~0
~0
6
2
2
~0
~0
~0
~0
~0
Salix phylicifolia
98
Copaene
C15H24
Caryophyllene
α-Caryophyllene
α-Farnesene
α-Farnesene (Z,E)
Naphthalene, 1,2,3,5,6,8a-hexahydro-4,7dimethyl-1-(1-methylethyl)-, (1S-cis)Farnesyl cyanide
Caryophyllene
Other VOCs
2
1
8
2
4
39
2
1
1
2
3
3
81
5
2
1
1
8
12
45
27
1
41
2
~0
4
2
4
~0
~0
Toluene
Acetophenone
Nonanal
Benzoic acid
Tridecane
Tetradecane
Pentadecane
Hexadecane
Heptadecane
Unknown
42
12
8
5
3
5
18
3
4
~0
25
4
28
23
1
4
10
3
2
1
61
7
5
16
~0
1
8
1
1
~0
14
Emission rates of different compounds were found to differ not only between species but
also vary from tree to tree and even from branch to branch (see Figure 5 to Figure 9). For
instance, the first Salix fragilis tree emitted 86% isoprene of total BVOCs blend (Figure 5). On
the other hand, the second Salix fragilis tree emitted 84% isoprene of total BVOCs emission.
Although, this is not a significant difference but still different from each other. Furthermore, the
Salix phylicifolia tree released 98% of the total mass emission as isoprene, only 2% as
monoterpenes and hardly any sesquiterpenes. On the other hand, the first Salix fragilis was found
to emit 6% monoterpenes and 4% sesquiterpenes of total measured emissions. Similarly, second
Salix fragilis released 7% monoterpenes and 6% sesquiterpenes. Average of three trees, together
they released 92% isoprene, 4% monoterpenes, 3% sesquiterpenes and 1% other VOCs.
Sesquiterpenes
3%
Monoterpenes
4%
Other
VOCs
1%
Unknown
0%
Other
VOCs
2%
Sesquiterpenes
4% Unknown
0%
Monoterpenes
6%
b
a
Isoprene
88%
Isoprene
92%
Monoterpenes
2%
Other Sesquiterpenes
VOCs
0%
0%
Unknown
0%
Sesquiterpenes Other
VOCs Unknown
6%
1%
2%
Monoterpenes
7%
d
c
Isoprene
84%
Isoprene
98%
Figure 5. Relative mass importance of emissions for (a) all three Salix trees together, (b) first
Salix fragilis, (c) Salix phylicifolis and (d) second Salix fragilis.
15
In the same way, emission rates of different monoterpenes were detected different from
three Salix trees. 45% ocimene, 31% 3-carene and 17% C10H14O of total monoterpenes mass
spectrum were emitted from the first Salix fragilis, and 42% ocimene, 28% 3-carene and 17%
C10H14O of total monoterpenes mass spectrum were released from the second Salix fragilis
(Figure 6). Conversely, 80% ocimene, only 2% 3-carene and 4% C10H14O of total monoterpenes
mass emission were found to release from Salix phylicifolia. So, the most important
monoterpenes emitted from Salix are ocimene, C10H14O and 3-carene.
α-pinene
0%
β-pinene
0%
Eucalyptol
0%
EucalyptLimonen
β-pinene
C10H16
0% α-pinene ol
e
3%
0%
0%
0%
Limonene
C10H16
0%
7%
3-carene
24%
3-carene
31%
C10H14
3%
C10H16
2%
b
a
Ocimene
50%
C10H14O
14%
Ocimene
45%
C10H14
2%
C10H16
2%
C10H14
O
17%
α-pinene Limonene
Eucalypto 0%
0%
l
0%
3-carene α-pinene β-pinene Limonene
C10H14O
2%
0%
0%
0%
4% C10H16
Eucalypto
l
C10H14 1%
C10H16
0%
2%
11%
β-pinene
0%
C10H16
6%
3-carene
28%
d
c
Ocimene
42%
C10H14
4%
Ocimene
80%
C10H16
3%
C10H14O
17%
Figure 6. Relative mass importance of monoterpenes emissions for (a) all three Salix tree
together, (b) first Salix fragilis, (c) Salix phylicifolis and (d) second Salix fragilis.
16
In the case of sesquiterpenes, the first Salix fragilis tree emitted 39% α-farnesene (ZE), 41%
caryophyllene and 2% naphthalene, 1,2,3,5,6,8a-hexahydro-4,7-dimethyl-1-(1-methylethyl)-,
(1S-cis)- of total sesquiterpenes mass emission (Figure 7). The second Salix fragilis tree released
81% α-farnesene(ZE), 4% caryophyllene and 5% naphthalene, 1,2,3,5,6,8a-hexahydro-4,7dimethyl-1-(1-methylethyl)-, (1S-cis)- of total sesquiterpenes mass emission. However, 45% αfarnesene (ZE), 0% caryophyllene and 27% naphthalene, 1,2,3,5,6,8a-hexahydro-4,7-dimethyl-1(1-methylethyl)-, (1S-cis)- of total sesquiterpenes mass emission were emitted from the Salix
phylicifolia tree. Therefore, the dominating sesquiterpenes emitted from these Salix are αfarnesene (ZE), naphthalene, 1,2,3,5,6,8a-hexahydro-4,7-dimethyl-1-(1-methylethyl)-, (1S-cis)and caryophyllene.
17
Farnesyl
cyanide
1%
Naphthale
ne
5%
Copaene
1% C15H24
1%
Caryophyl
αlene Caryophyl
3%
lene
Caryophyl
lene
C15H24
8%
1%
Copaene
2%
3%
αFarnesene
3%
Caryophyl
lene
11%
Caryophyl
lene
41%
b
a
αFarnesen
e (Z,E)
72%
Farnesyl
cyanide
4%
Caryophyll
ene
1%
αCaryophyll
ene
8%
αFarnesen
e (Z,E)
39%
Naphthal
ene
2%
Farnesyl
cyanide
1%
Caryophyll
C15H24
ene
1%
Copaene
0%
2%
αCaryophyl
lene
2%
αFarnesene
4%
Farnesyl
C15H24 Caryophyll
cyanide Caryophyll 1%
ene
0%
ene
2%
Naphthale 4% Copaene
1%
ne
5%
αCaryophyll
ene
3%
αFarnesene
3%
αFarnesene
12%
Naphthale
ne
27%
d
c
αFarnesene
(Z,E)
81%
αFarnesene
(Z,E)
45%
Figure 7. Relative mass importance of sesquiterpenes emissions for (a) all three Salix trees
together, (b) the first Salix fragilis, (c) Salix phylicifolis and (d) the second Salix fragilis.
Among Other VOCs, dominating compounds from the first Salix fragilis were toluene
(42%), pentadecane (18%), acetophenone (12%), benzoic acid (5%), and nonanol (8%) (Figure
8). On the contrary, emissions of Other VOCs compounds from the second Salix fragilis were
18
predominantly toluene (25%), pentadecane (10%), benzoic acid (23%), acetophenone (4%) and
nonanol (28%). In addition, Salix phylicifolia tree emitted toluene (61%), pentadecane (8%),
benzoic acid (16%), acetophenone (7%) and nonanol (5%). So, the most important Other VOCs
compounds from Salix species are toluene, benzoic acid, pentadecane, nonanol and
acetophenone.
Hexadecane
Pentadecane 3%
11%
Tetradecane
3%
Tetradecane Hexadecane
3%
5%
Heptadecane
3%
a
Toluene
37%
Pentadecane
18%
Heptadecane
4%
b
Toluene
42%
Benzoic acid
17%
Tridecane
1%
Pentadecane
8%
Tetradecane
Tridecane
3%
Nonanal
18%
Acetophenone
7%
Hexadecane
1%
Benzoic acid Nonanal
8%
5%
Heptadecane
1%
Pentadecane Hexadecane
10%
3%
1%
Tridecane
0%
Acetophenone
12%
Heptadecane
2%
Tetradecane
4%
Tridecane
1%
Benzoic acid
16%
Toluene
25%
d
c
Benzoic acid
23%
Toluene
61%
Nonanal
5%
Nonanal
28%
Acetophenone
4%
Acetophenone
7%
Figure 8. Relative mass importance of other VOCs emissions for (a) all three Salix trees
together, (b) the first Salix fragilis, (c) Salix phylicifolis and (d) the second Salix fragilis.
Even BVOC emission rate varied between the branches of the same tree (Figure 9). The
branches of Salix phylicifolia showed biggest difference in emission among the three trees. The
19
second branch of Salix phylicifolia had almost three times higher average emission rate than the
first branch of it. Similarly, two branches of second Salix fragilis also showed different emission
pattern, first branch had more than three times higher emission rate than the first branch. Two
branches of the first Salix fragilis tree had almost similar emission patter but second branch had
higher emission rate than the first branch. Total BVOC emission from the first branch of the first
Salix fragilis tree was ranging from 2.2 to 18.1 µg g-1dw h-1 at first day of measurement (Figure
9). For the same branch, emissions at second day of measurement were higher and varied
between 15.1 to 18.6 µg g-1dw h-1. From the second branch of the first Salix fragilis tree, total
BVOC emissions were ranging from 1.3 to 22.4 µg g-1dw h-1 at first day of measurement and at
the second day of measurement, it varied between 3.1 to 48 µg g-1dw h-1. For the second Salix
fragilis, the BVOC emission rate was ranging from 21.9 to 85.9 µg g-1dw h-1 from the first branch
at the first day of measurement and 30.9 to 109.8 µg g-1dw h-1at the second measurement day.
Besides, the second branch of the second Salix fragilis tree had an emission rate of 11.3 to 37.8
µg g-1dw h-1 during the measurement day. For Salix phylicifolia, the BVOC emission rate was
ranging from 13.9 to 40.9 µg g-1dw h-1 from the first branch at the first day of measurement and
4.9 to 56.9 µg g-1dw h-1 at the second measurement day. But for the second branch, the BVOC
emission varied between 15.1 to 210.1 µg g-1dw h-1 at the first measurement day and 18.4 to 215.6
µg g-1dw h-1 at the second measurement day. Variations in temperature and PAR values were the
reasons behind the variations of BVOC emission from the branches (Figure 9 to Figure 11). I
will discuss these matters in details in section 4.2.
20
250
Emission rate(µg g-1dw h-1)
200
Emission at 1st day of measurement
Emission at 2nd day of measurement
150
100
50
0
Local time
1st S. fragilis
1st branch
1st S. fragilis
2nd branch
2nd S. fragilis
1st branch
2nd S. fragilis
2nd branch
S. phylicifolia
1st branch
S. phylicifolia
2nd branch
Figure 9. Emission of total BVOCs from two branches of three Salix trees each during August
2012. Gray line with gray circular marker represents the emission at first day of measurement
and red line with red cube marker represents the emission at second day of measurement.
Salix trees were found to emit large amount of isoprene (Table 2), especially Salix
phylicifolia, which had an average isoprene emission rate of 59.5 µg g-1dw h-1 during the
measurement days. In addition, the average isoprene emission rate for the second Salix fragilis
was 46 µg g-1dw h-1, which was higher than the average isoprene emission rate from the first Salix
fragilis. Average isoprene emission rate from the first Salix fragilis was only 14.1 µg g-1dw h-1.
Besides isoprene, monoterpene and sesquiterpene emissions were not negligible (except for
average sesquiterpene emissions from Salix phylicifolia). Monoterpene and sesquiterpene
average emission rates from the first Salix fragilis tree were 0.9 and 0.6 µg g-1dw h-1, but it was
relatively higher for the second Salix fragilis tree, 3.8 and 3.3 µg gdw-1 h-1. On the other hand,
Salix phylicifolia emitted relatively very low amount of monoterpenes and sesquiterpenes, on
average only 0.9 and 0.1 µg g-1dw h-1. After isoprene, ocimene was the second highly emitted
compound from all the trees. The first Salix fragilis emitted 2.7% ocimene (average 0.4 µg g-1dw
h-1) of total BVOC emission whereas the second Salix fragilis emitted 3% ocimene (average 1.6
µg g-1dw h-1) of the total BVOC emissions. In addition, only 1.2% ocimene (average 0.7 µg g-1dw
h-1) of total mass emission was emitted from Salix phylicifolia.
21
Table 2. Average emission rates (µg g-1dw h-1) of isoprene, monoterpenes and sesquiterpenes
from Salix species during August 2012. Average temperature (˚C) and PAR (μmol m-2s-1) are also
given.
3 trees
Average of 2 Salix
1st Salix
2nd Salix
Salix
average
fragilis trees
fragilis
fragilis
phylicifolia
Temperature (˚C)
24
23.9
21.4
26.4
24.9
PAR(µmol m-2s-1)
531.3
477.3
285.1
669.6
696.5
Isoprene
38.4
30
14.1
46
59.5
Monoterpenes
1.7
2.3
0.9
3.8
0.9
Sesquiterpenes
1.1
1.9
0.6
3.3
0.1
4.2. Correlation with PAR and temperature
Emission rates of total BVOCs were obviously correlated with the temperature and PAR
values during the measurement time (Figure 9 to Figure 11). Patterns of total BVOC emission
rates curve almost always followed the peaks and valleys of temperature and PAR curves (Figure
9). Branch-wise emission curve showed this relation more clearly (Figure 10 and Figure 11).
Branch-wise BVOC emissions followed almost identical patterns with the corresponding
temperature and PAR values, but it is difficult to say which has the strongest influence on
emissions of BVOCs. Emission rates were comparatively high during noon when both the
temperature and PAR were high and comparatively low during morning and afternoon when
both the temperature and PAR were low (Figure 10 and Figure 11).
22
Figure 10. Temperature, PAR and emission rates of all compounds are plotted together to
observe the correlation among them. Roman numerals at the top indicate six branches of the
Salix: I and II for the first Salix fragilis; III and IV for the second Salix fragilis; V and VI for Salix
phylicifolia.
23
250
35
30
25
150
20
100
15
10
50
T (°C)
Emission rate(µg g-1dw h-1)
200
40
1st day emission
2nd day emission
1st day temperature (°C)
2nd day temperature (°C)
5
0
0
1st S. fragilis
1st branch
1st S. fragilis
2nd branch
Local time
2nd S. fragilis
1st branch
2nd S. fragilis
2nd branch
S. phylicifolia
1st branch
S. phylicifolia
2nd branch
Figure 11. Emission of total BVOCs from two branches of three Salix trees during August 2012.
Gray line with gray circular marker represents the emission at first day of measurement and red
line with red cube marker represents the emission at second day of measurement. Gray line
with gray rectangular marker represents the temperature at first day of measurement and red
line with red triangular marker represents the temperature at second day of measurement.
200
1600
1st day emission
2nd day emission
1st day PAR (µmol m-2 s-1 )
2nd day PAR(µmol m-2 s-1 )
1400
1200
1000
Emission rate(µg g-1dw h-1)
150
800
100
600
400
50
200
0
0
Local time
1st S. fragilis
1st branch
PAR(µmol m-2 s-1)
250
1st S. fragilis
1st branch
2nd S. fragilis
1st branch
2nd S. fragilis
2nd branch
S. phylicifolia
1st branch
S. phylicifolia
1st branch
Figure 12. Emission of total BVOCs from two branches of three Salix trees during August 2012.
Gray line with gray circular marker represents the emission at first day of measurement and red
line with red cube marker represents the emission at second day of measurement. Gray line
24
with gray rectangular circular marker represents the PAR at first day of measurement and red
line with red triangular marker represents the PAR at second day of measurement.
4.3. Emission potentials
Measured BVOC emissions were standardized for temperature and light conditions (at 30˚C
and 1000 μmol m-2s-1) according to Guenther’s algorithm (Guenther et al., 1993; Guenther,
1997). Standardized basal emission rates were calculated separately for isoprene, monoterpenes
and sesquiterpenes (Table 3). Isoprene can not be stored inside the plant because there is no
storage pool for isoprene in the plant (Kesselmeier and Staudt, 1999). So, isoprene’s emission
rate depends on synthesis and thus on both temperature and light. A temperature and light
dependent algorithm (based on Guenther et al., 1993; Guenther, 1997) was used to calculate
isoprene emission potentials. Monoterpenes and sesquiterpenes can be stored in the storage pool
of the plant and their emission rate dependents on temperature. So, an only temperature
dependent algorithm (Guenther et al., 1993) was used for calculating monoterpenes and
sesquiterpenes emission potential. For comparative reasons, the standardized basal emission
rates were calculated for all three trees separately and all three trees together as well.
Table 3. Standardized basal emission rates (µg g-1dw h-1) of isoprene (by using a temperature and
light dependent algorithm at 30⁰C temperature and 1000 µmol m-2 s-1 incident photon flux
density), monoterpenes and sesquiterpenes (by using a temperature dependent algorithm at
30⁰C temperature).
3 trees
2 Salix fragilis
1st Salix
2nd Salix
Salix
average
trees average
fragilis
fragilis
phylicifolia
Isoprene
49.2
30.6
16.9
44.4
56.4
Monoterpenes
2.7
3.7
1.9
5.6
1.3
Sesquiterpenes
2.2
3.6
1.8
5.5
0.1
Compounds
25
4.3.1. Isoprene emission potential
The standardized basal emission rate of isoprene from the first Salix fragilis tree was 16.9
µg g-1dw h-1. For the second Salix fragilis it was much higher than the first Salix fragilis tree, 44.4
µg g-1dw h-1. Salix phylicifolia also showed comparatively higher isoprene basal emission rate of
56.4 µg g-1dw h-1. Combining averaged standardized basal emission rate of isoprene for all three
was 49.2 µg g-1dw h-1. For comparison, modelled values of isoprene emission calculated by
Guenther’s algorithm are plotted against the measured values of isoprene emission (Figure 13).
All slope values in the equations were less than 1; therefore modelled emissions of isoprene were
underestimated compared to the measured emissions. Isoprene emission model performance was
best for second Salix fragilis tree (coefficient of determination, r2 =0.82 and slope =0.72),
although offset was high (11.29). Isoprene emission model performance was worst for all three
trees together (r2 =0.09, offset = 39.14 and slope =0.26). Some gray triangular markers are used
in the scatter plot of all three together. These gray triangular markers indicate some measured
emission values of isoprene. These measured emission rates were so high because of the high
temperature and PAR values (around 27˚C and 1000 μmol m-2s-1) during their measurement
time. Without these high measured emission values, model performance of isoprene emission
would be better for all three trees together.
26
0
25 50 75 100 125 150 175 200
Measured emission (µg
Modelled emission (µg gdw-1 h-1)
a
200
175
150
125
100
75
50
25
0
25
50
70
60
50
40
30
20
0
)
20
30
40
50
60
70
g-1dw h-1)
105
y = 0.72x + 11.29
R² = 0.82
90
75
60
45
30
15
0
0
g-1dw h-1)
10
Measured emission (µg
b
75 100 125 150 175 200
Measured emission (µg
y = 0.92x + 3.91
R² = 0.35
10
0
g-1dw h-1
y = 0.56x + 22.95
R² = 0.76
0
c
Modelled emission (µg g-1dw h-1)
y = 0.26x + 39.14
R² = 0.09
Modelled emission (µg gdw-1 h-1)
Modelled emission (µg g-1dw h-1)
200
175
150
125
100
75
50
25
0
d
15
30
45
60
75
Measured emission (µg
90
105
g-1dw h-1)
Figure 13. Modelled emission values of isoprene are plotted against measured values; (a) all
three Salix trees together, (b) first Salix fragilis, (c) Salix phylicifolis and (d) second Salix fragilis.
Some gray triangular markers are used at plot (a). These gray triangular markers indicate some
measured emission values of isoprene. These measured emission rates were so high because of
the high temperature and PAR values (around 27˚C and 1000 μmol m-2s-1) during their
measurement time. These emission values probably reduce the R2 value of this plot. In all plots,
R2 indicates coefficient of determination and y represents straight line equation.
4.3.2. Monoterpenes and sesquiterpenes emission potentials
In case of monoterpenes, standardized basal emission rate for the first Salix fragilis tree was
1.9 µg g-1dw h-1. For the second Salix fragilis tree, it was comparatively higher, 5.6 µg g-1dw h-1.
The emission potential for Salix phylicifolia was calculated as 1.3 µg g-1dw h-1. The modelled
standard emission rate for monoterpene for all three trees together was was 2.7µg g-1dw h-1.
27
Except for the first Salix fragilis, in all cases modelled emissions overestimated measured
emission (Figure 14). In general, monoterpene emission models performed well for three Salix
trees seperately and also for all three trees together. Best model performance was found for the
8
6
4
y = 1.33x + 0.42
R² = 0.85
2
0
0
4
6
8
Measured emission (µg
a
Modelled emission (µg g-1dw h-1)
2
10
4
3
y = 1.19x + 0.22
R² = 0.90
1
0
c
1
2
3
4
5
3
2
y = 0.75x + 1.23
R² = 0.59
1
0
b
5
0
4
0
g-1dw h-1)
6
2
Modelled emission (µg g-1dw h-1)
10
Modelled emission (µg g-1dw h-1)
Modelled emission (µg g-1dw h-1)
Salix phylicifolia tree (r2 =0.90, slope =1.19 and offset = 0.22).
6
2
3
Measured emission (µg
4
g-1dw h-1)
10
8
6
4
y = 1.43x + 0.06
R² = 0.58
2
0
0
Measured emission (µg g-1dw h-1)
1
d
2
4
6
Measured emission (µg
8
10
g-1dw h-1)
Figure 14. Modelled emission values of monoterpenes are plotted against measured values; (a)
all three Salix tree together, (b) first Salix fragilis, (c) Salix phylicifolis and (d) second Salix
fragilis. In all plots, y represents straight line equation and R2 indicates coefficient of
determination.
The modelled emission rates for sesquiterpenes were comparable with monoterpenes
modeled emission except for the Salix phylicifolia tree, which had a standardized sesquiterpene
emission rate of 0.1 µg g-1dw h-1. The first Salix fragilis tree showed sesquiterpene modelled
emission rate of 1.8 µg g-1dw h-1 and the second Salix fragilis had sesquiterpene modeled emission
28
rate of 5.5 µg g-1dw h-1. The modelled standard emission rate for sesquiterpene for all three trees
together was 2.2 µg g-1dw h-1. In all cases modelled emissions overestimated (by a factor of 2
almost) measured emission except for the second Salix fragilis tree (Figure 15). The best model
performance for sesquiterpene was found for all trees together (r2 =0.74, slope =1.35 and offset =
0.70), then Salix phylicifolia (r2 =0.72, slope =1.59 and offset = 0.01), after that first salix fragilis
(r2 =0.61, slope =1.77 and offset = 0.83), and finally second Salix fragilis (r2 =0.28, slope =0.79
and offset = 2.88) tree.
8
6
4
y = 1.35x + 0.70
R² = 0.74
2
0
2
4
6
8
Measured emission (µg
10
4
3.5
3
2.5
2
1.5
y = 1.77x + 0.83
R² = 0.61
1
0.5
0
12
g-1dw h-1)
b
0.45
0.4
0.35
0.3
0.25
0.2
y = 1.59x + 0.01
R² = 0.72
0.15
0.1
0.05
1
2
3
4
Measured emission (µg
5
6
gdw-1 h-1)
12
10
8
6
4
y = 0.79x + 2.88
R² = 0.28
2
0
0
0
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
c
4.5
0
0
a
Modelled emission (µg gdw-1 h-1)
Modelled emission (µg gdw-1 h-1)
10
Modelled emission (µg gdw-1 h-1)
Modelled emission (µg g-1 dw h-1)
12
Measured emission (µg
gdw-1 h-1)
d
2
4
6
8
Measured emission (µg
10
12
gdw-1 h-1)
Figure 15. Modelled emission values of sesquiterpenes are plotted against measured values; (a)
all three Salix tree together, (b) first Salix fragilis, (c) Salix phylicifolis and (d) second Salix
fragilis. In all plots, y represents straight line equation and R2 indicates coefficient of
determination.
29
5. Discussion
5.1. Uncertainties related to BVOC sampling by the chamber method
Some source of uncertainties related to the measurement of BVOC by the chamber method
used in my study need to be discussed. During the BVOC collection in the sample cartridge, it is
possible that some of the BVOCs emitted from the plants could adhere on the plant inside the
chamber, so BVOC sampling could be underestimated (Niinemets et al., 2011). Sample
cartridges may not be totally blank before sampling; it could contain some compounds before
sampling. One hour sampling time was used per cartridge. For this reason isoprene and ocimene
concentration of some samples (22 cartridges out of 73 for isoprene and 44 cartridges out of 73
for ocimene) were flushing the detector. Even after dilution (3.3, 10, 13.2 and 25% depending on
the concentration) isoprene and ocimene mass were flushing the detector. So, measured isoprene
and ocimene mass from those cartridges were the maximum mass that the detector can detect and
emission rate could be higher than the measured value. So, one hour sampling per cartridge
probably underestimated the emission for isoprene and ocimene. If sampling time were less than
one hour, BVOCs which were emitted very small amount (α-pinene, β-pinene, eucalyptol,
limonene, C15H24, copaene, tridecane, hexadecane ), would not be detected.
Light sensor may not measure the actual PAR on the leaves because one leaf may shade on other
leaf. Another source of uncertainty about BVOC sampling was the temperature and relative
humidity inside the chamber; both were higher than the ambient condition (Kolari et al., 2012).
The elevated temperature could volatilize compounds that would actually be stored inside the
plant. In addition, higher relative humidity inside the chamber formed condensation water
droplets which might have caught hydrophilic compounds emitted from the tree. However, this
chamber method is the most convenient method to use in the field experiment (Faubert et al.
2011).
5.2. Emissions of BVOCs
Isoprene was the highest emitted BVOC from willow trees, as expected. Willow tree emits
isoprene dominantly with other BVOCs (Olofsson et al., 2005). According to Kesselmeier and
Staudt (1999), common bioenergy trees like willow and aspen are strong isoprene emitter. Salix
phylicifolia has been found to emit around 59.5 µg g-1dw h-1 of isoprene. Hakola et al. (1998) also
found approximately 50 µg g-1dw h-1 of isoprene emitted from Salix phylicifolia during August
30
when the temperature exceeds 20⁰C in Finland. Next to isoprene, ocimene has been also reported
as highly emitted (almost 3% by mass) compound from Salix species, which is similar to Owen
et al. (2001). Compare to the isoprene emission, very small amount of monoterpene and
sesquiterpene were emitted from all the Salix trees (see table 2). Hakola et al. (1999) also
reported very low amount of monoterpene emission from Salix trees, only 0.33 µg g-1dw h-1.
Besides, monoterpenes and sesquiterpenes were emitted very small amount from different Salix
trees (see table.3, Karl et al., 2009). As in this experiment, Chai et al. (2011) has found a group
of different compounds were emitted from Salix tree in China e.g. isoprene, α-pinene, β-pinene,
3-carene, limonene, ocimene. These compounds were also emitted by Salix phylicifolia reported
by Hakola et al. (1998).
5.3. Dependence of emission on temperature and PAR
Measured BVOC emission rates were found to vary from branch to branch depending on
the temperature and PAR, as anticipated earlier. According to Guenther et al. (1993),
temperature and photosynthetic active radiation have a great influence on BVOC emission from
plants. As had been shown in Figure 9; first branch of first Salix tree had slightly higher
emission rates at second day of measurement related to first day measurements, which is also
true for the rest of the branches of measured Salix trees. That is because of average temperature
and PAR at second day of measurement was higher than the first day of measurement (Figure 11
and Figure 12). Temperature affects the availability of substrate and the activity of rate-limiting
enzymes and by this way control emission rates. Besides, physicochemical constraints caused by
temperature also affect the emission of BVOCs from leaves (Niinemets et al., 2004). In addition,
some compounds like isoprene, which are not stored in leaves and they are highly volatile. For
this reason their emission depends on light intensity (Laothawornkitkul et al., 2009). Moreover,
as all BVOCs are volatilized by the higher temperature, their emission rate is dependent on
temperature (Kesselmeier and Staudt, 1999). But it is difficult to say which factor has the greater
influence on BVOC emissions, perhaps it is their combined effect.
5.4. Different emission rates within same species
Standardized emission rates of different BVOCs for two Salix fragilis trees were found
different. Staudt et al. (1997) also reported significant intra-specific variability in base emission
rates for some plant species, especially Pinus spp. Intra-specific variability in base emission rates
31
was also observed by Carvalho et al. (2005). Environmental conditions, growth form, sex
(Carvalho et al., 2005), age could be different for them and these factors may cause the emission
differences. According to Kim (2001), BVOC emission depends on the age of the tree. External
variables may also affect the plant’s metabolism and influence BVOC emission (Carvalho et al.,
2005).
5.5. Different emission rates within subspecies
As anticipated before, basal emission rates of different BVOC compounds were different
between Salix phylicifolia and Salix fragilis trees. Karl et al. (2009) also observed BVOC
emission differences between different Salix species (see Table 3 at Karl et al., 2009). In
addition, Helming et al, (2007) reported different basal emission of monoterpene from different
pine trees (Pinus sabiniana, Pinus sylvestris, Pinus teada). Haapanala et al. (2009) also noticed
that the difference of emission rates between different phenotypes of mountain birch might be
caused by of hybridization which affects the growth from. In this case, tree size, age, leaf
structures of Salix phylicifolia were totally different from the Salix fragilis trees. Salix
phylicifolia tree was relatively young and smaller in size. These variations may be one of the
reasons resulting the emissions differences between the trees. Salix phylicifolia tree was in a
higher and more open place compare to the Salix fragilis trees. Therefore, Salix phylicifolia tree
got relatively high amount of PAR throughout the day and had young fresh leaves related to the
Salix fragilis trees. Hakola et al., 1998 also reported that leaf age has a great influence on BVOC
emission.
5.6. Emission potentials
Isoprene emission potential from Salix phylicifolia was found 56.4 µg g-1dw h-1 during
August, 2012. Although Hakola et al. (1998) measured it as 32 µg g-1dw h-1. But she and her
group estimated isoprene emission potential from May to September and they also indicated that
the isoprene emission rates were low all the months except August and during August isoprene
emission rate increased as high as 76 µg g-1dw h-1. Besides, first Salix fragilis had isoprene
emission potential of 16.9 µg g-1dw h-1. Most of the Salix Species has isoprene emission potential
around this value. For instance, isoprene emission potential of Salix caprea was18.9 µg g-1dw h-1
(Karl et al., 2009) and for Salix alba it was 18 µg g-1dw h-1 (Pio et al., 1993). But isoprene
emission potential of the second Salix fragilis tree was more than twice compare to the first Salix
32
fragilis. Emission potential could vary within the same species of tree depending on different
factors like growth form, age, sex (Carvalho et al., 2005). For example, Pio et al. (1993) found
isoprene emission potential of Salix alba was 18 µg g-1dw h-1, whereas Karl et al. (2009) found it
37.2 µg g-1dw h-1 for Salix alba.
Karl et al. (2009) worked on different Salix trees and estimated monoterpenes emission
potentials, which are similar to this experiment. He found Salix alba has monoterpenes emission
potential of 1.1 µg g-1dw h-1 and Salix cinerea, Salix eleagnos both have monoterpenes emission
potential of 0.8 µg g-1dw h-1. In addition, sesquiterpenes emission potential of Salix alba, Salix
cinerea, Salix eleagnos and Salix caprea were 0.1 µg g-1dw h-1 (Karl et al., 2009) which is similar
to my estimation for Salix phylicifolia.
33
6. Conclusions
I measured branch scale emissions of different BVOC compounds from Salix trees, a
bioenergy plant. Emissions rates of different BVOC compounds were found to be different
between two Salix fragilis trees because of different temperature and light conditions. May be
other reasons were involved for the variation of emissions but it is difficult to say based on my
small experiment and the limited dataset. BVOC emission pattern of Salix phylicifolia were
found to differ from the Salix fragilis trees. Perhaps dissimilarities in environmental conditions,
growth form, leaf structures, age etc. have influenced their emission rates of BVOC. Long term
multi-seasonal measurements on several trees are necessary to clearly find out the reasons why
emissions rates were different. Standardized emission potentials of different compounds were
found different between the trees. Emission potential of isoprene for Salix phylicifolia was 56.4
µg g-1dw h-1 whereas 16.9 µg g-1dw h-1 and 44.4 µg g-1dw h-1 were measured for the Salix fragilis
trees. But one thing was common between the different trees; isoprene emission dominates the
total mass emission. Temperature and light intensity were noticed to have great influence
BVOCs emission from all the trees. As isoprene is a highly reactive compound and has great
impact on atmosphere and climate system, we should think about alternative bioenergy plants
which emit less or no isoprene at all. Non-isoprene emitting poplars are an option for the future
biofuel plantation.
Acknowledgements
I like to express my sincere gratitude to my supervisor Thomas Holst for suggesting the subject
and for his time, idea, quick feedback, and guidance during the course of this study.
This study was performed in the botanical garden of Lund University. I acknowledge the support
provided by the employees of botanical garden and this work would have not been possible
without their support.
34
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tropical wetlands using multiple modelling approaches
257
Mohammed S. Alwesabi (2012) MODIS NDVI satellite data for assessing
drought in Somalia during the period 2000-2011
258
Christine Walsh (2012) Aerosol light absorption measurement techniques:
A comparison of methods from field data and laboratory experimentation
259
Jole Forsmoo (2012) Desertification in China, causes and preventive actions in
modern time
260
Min Wang (2012) Seasonal and inter-annual variability of soil respiration at
Skyttorp, a Swedish boreal forest
261
Erica Perming (2012) Nitrogen Footprint vs. Life Cycle Impact Assessment
methods – A comparison of the methods in a case study.
262
Sarah Loudin (2012) The response of European forests to the change in
summer temperatures: a comparison between normal and warm years, from
1996 to 2006
263
Peng Wang (2012) Web-based public participation GIS application – a case
study on flood emergency management
264
Minyi Pan (2012) Uncertainty and Sensitivity Analysis in Soil Strata Model
Generation for Ground Settlement Risk Evaluation
265
Mohamed Ahmed (2012) Significance of soil moisture on vegetation
greenness in the African Sahel from 1982 to 2008
266
Iurii Shendryk (2013) Integration of LiDAR data and satellite imagery for
biomass estimation in conifer-dominated forest
267
Kristian Morin (2013) Mapping moth induced birch forest damage in northern
Sweden, with MODIS satellite data
268
Ylva Persson (2013) Refining fuel loads in LPJ-GUESS-SPITFIRE for wetdry areas - with an emphasis on Kruger National Park in South Africa
269
Md. Ahsan Mozaffar (2013) Biogenic volatile organic compound emissions
from Willow trees
42