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Atmospheric Environment 65 (2013) 61e68

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Atmospheric Environment

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Volatile organic compound emissions from elephant grass and bamboo cultivarsused as potential bioethanol crop

E. Crespo a, M. Graus b,c, J.B. Gilman b,c, B.M. Lerner b,c, R. Fall b, F.J.M. Harren a, C. Warneke b,c,*

a Life Science Trace Gas Facility, Radboud University, Nijmegen, The NetherlandsbCooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USAcNOAA ESRL, CSD, Boulder, CO, USA

h i g h l i g h t s

< We determine the VOC emissions of two potential biofuels: elephant grass and bamboo.< VOC emissions after harvesting strongly depend on the seasonal stage.< Perennial grasses have lower emissions than woody species with implications for air quality.

a r t i c l e i n f o

Article history:Received 6 April 2012Received in revised form10 September 2012Accepted 5 October 2012

Keywords:Volatile organic compoundsBiofuelElephant grassBamboo

* Corresponding author. NOAA ESRL, CSD, Boulder,E-mail address: carsten.warneke@noaa.gov (C. Wa

1352-2310/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.atmosenv.2012.10.009

a b s t r a c t

Volatile organic compound (VOC) emissions from elephant grass (Miscanthus gigantus) and black bamboo(Phyllostachys nigra) were measured online in semi-field chamber and plant enclosure experimentsduring growth and harvest using proton-transfer reaction mass spectrometry (PTR-MS), proton-transferreaction ion-trap mass spectrometry (PIT-MS) and gas chromatography-mass spectrometry (GCeMS).Both cultivars are being considered for second-generation biofuel production. Before this study, noinformation was available on their yearly VOC emissions. This exploratory investigation shows that blackbamboo is a strong isoprene emitter (daytime 28,516 ng gdwt

�1 h�1) and has larger VOC emissions, espe-cially for wound compounds from the hexanal and hexenal families, than elephant grass. Daytimeemissions of methanol, acetaldehyde, acetone þ propanal and acetic acid of black bamboo were 618, 249,351, and 1034 ng gdwt

�1 h�1, respectively. In addition, it is observed that elephant grass VOC emissions afterharvesting strongly depend on the seasonal stage. Not taking VOC emission variations throughout theseason for annual and perennial species into account, may lead to an overestimation of the impact onlocal air quality in dry periods. In addition, our data suggest that the use of perennial grasses forextensive growing for biofuel production have lower emissions than woody species, which might beimportant for regional atmospheric chemistry.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Globally biogenic sources are the largest contributors of volatileorganic compounds (VOCs) into the atmosphere, and its impact onthe chemistry of the atmosphere has been subject of severalstudies (e.g.: de Gouw et al., 1999; Eerdekens et al., 2009). Only onregional scales can anthropogenic emissions dominate. Via theirphoto-oxidation, VOCs are involved in the formation of ozone,a toxic pollutant that also contributes to the greenhouse effect(Trainer et al., 1987; Chameides et al., 1988; Atkinson, 1998). In

CO, USA.rneke).

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addition, VOCs oxidation contributes to the formation of organicaerosols that can scatter or absorb solar radiation, modifying theEarth’s climate (Atkinson, 1998; Kanakidou et al., 2005; de Gouwet al., 2005). VOCs emitted by plants such as acetone, can reachthe upper troposphere where they are photolysed and contributeto the formation of HOx (HO and HO2) and PAN (peroxyacetic nitricanhydride), which can act as a temporary reservoir for nitrogenoxides (NOx) (Roberts et al., 2002). Isoprene is the single largestVOC emitted to the atmosphere by many plant species duringphotosynthesis (e.g.: Guenther et al., 1995; Kesselmeiers andStaudt, 1999) and leads to the formation of ozone when oxidizedin the presence of NOx (Biesenthal et al., 1997). Because of thelarge importance of biogenic VOCs, emission inventories need toinclude as many as possible regional plant species. Detailed VOC

E. Crespo et al. / Atmospheric Environment 65 (2013) 61e6862

emission measurements of planned crops such as Miscanthus willalso help to estimate the future impact of VOC on regional airquality.

There is strong interest in the use of biofuels as a replacementfor fossil fuels and interest is still growing. The fuel used in the UScontains about 10% ethanol and it is expected that this willincrease to 15% in the next few years. Several plant species arebeing considered or are already in use for starch ethanol (first-generation biofuels) and cellulosic ethanol (second-generation)production. Within those, cellulosic ethanol is gaining popularityas more sustainable and with greater environmental benefits overthe first-generation biofuels, which are food-based fuels andoften not cost competitive with existing fossil fuels, and in somecases not even ensuring energy sustainability when consideringthe whole life cycle of the biofuel production chain (Cocco, 2007).For second-generation biofuels, biomass from non-food crops,residuals parts of food crops, as well as biomass from industrywaste is used for production. Those feedstocks reduce the social/economic impact as compared to food-based fuels, as well asgreenhouse gas emissions by 85% over gasoline (U.S. Departmentof Energy studies, Argonne National Laboratory of the Universityof Chicago).

The shift to cellulosic ethanol consumption will likely cause anincrease in cultivation area dedicated to the non-food crops ofinterest. Several factors are important when choosing whichspecies to use to reach the general goal of sustainability and lowenvironmental impact (Sims et al., 2006; Carroll and Somerville,2009); those factors point to the use of perennials or woodyspecies as preferred species for cellulosic ethanol production.Because of the possible impact of VOC emissions on the regionalatmospheric air quality, it is important to estimate a priori the VOCcrop emissions during growing and from harvesting (i.e., duringdrying), as this should play a role in the decision of which speciesto use. Ashworth et al. (2012) have modeled the effects of realisticland use change scenarios for biofuel feedstock production in thetropics and mid-latitudes and found that changes in surfaceconcentrations of ozone and biogenic secondary organic aerosol(bSOA) are substantial at the regional scale.

It has been previously shown that VOC emissions by plants aretemporarily enhanced when physically wounded (Fall et al., 1999;de Gouw et al., 1999, 2000; Heiden et al., 2003; Loreto et al., 2006).Especially, C6 wound compounds (hexenals, hexenols and hexenylesters) show a strong increased emission immediately after cutting(Fall et al., 1999; de Gouw et al., 2000); these compounds are quitereactive in the atmosphere (Atkinson and Arey, 1998). When theleaves are left to dry, long-lasting VOC emissions are observed,which adds up to a much higher total C6 wound compound releasethan only from cutting (de Gouw et al., 1999).

In this study, we present measurements of two possiblecandidates for biofuel production, elephant grass (Miscanthusgigantus) and black bamboo (Phyllostachys nigra). Within perennialspecies, Miscanthus, together with switchgrass (Karp and Shield,2008), is one of the most likely candidates for cellulosic ethanolproduction. Those perennial grasses can be grown on marginallands and require lower input of water and fertilizer than otherbiofuel feedstocks such as corn (Carroll and Somerville, 2009).Next to grasses, the use of fast growing trees (such as poplars(Populus) and willows (Salix)) is also under consideration forcellulosic ethanol production (Karp and Shield, 2008; Fischer et al.,2005). Bamboo, though much less studied, is also a possiblecandidate for biofuel production because of its high biomass yield(El Bassam, 1998; Scurlock et al., 2000). Here, emissions fromuncut leaves as well as emissions during drying are quantified forboth plant species. From these data, yearly emissions from plan-tations are estimated.

2. Materials and methods

2.1. Plant material

The two species studied here, elephant grass (M. gigantus) andblack bamboo (P. nigra) were obtained from a nursery. Three potscontaining a bamboo plant and four pots containing elephant grassand were measured. From the four pots of elephant grass, two ofthem contained green grass, while the other two were end-of-season yellow grass that was starting to dry. Not many replicateswere done in this study (three bamboo, four elephant grass), but aswe will show, the differences between the green and yellowelephant grass and the bamboo are so significant that we can stilldraw meaningful conclusion from the measurements.

2.2. Chamber set-up and conditions

The bamboo plants were measured in a controlled and auto-mated indoor plant enclosure with about 320 l volume. The plantwas sealed at the stems inside the chamber, light was switched onand off at a regular 12 h interval, flow through the chamber wasmaintained at 12 l min�1 of room air cleaned by a catalyticconverter that removes the VOCs, but leaves the CO2 mixing ratiosbasically unchanged. The cutting and drying of the plants was doneinside the chamber, but the chamber had to be opened brieflyduring the cutting and it filled with laboratory air during thisprocess. Temperature, photosynthetically active radiation (PAR)and humidity were monitored continuously. VOCs and CO2 insidethe chamber were measured continuously together with routinelymonitoring the clean air purging the chamber.

For the elephant grass experiments a bunch of leaves stillattached to the plant were enclosed in a 24 � 24 � 34 cm Teflonchamber placed outdoors and sealed at the bottomwith Teflon film.Plants were watered every other day as needed. The air tempera-ture inside the chamber varied from 35 �C during day to 3 �C duringthe night. The relative humidity inside the chamber was 50e60%.For the drying experiments the same leaves and branches usedfor the growing experiments were cut from the plant and the stemswere placed in water for w1 h. After that they were moved to thelab and placed in the Teflon chamber and kept at a constanttemperature of 24 �C. For both drying and growing experiments,the chamber was continuously supplied with clean air, generatedby passing room air through a charcoal filter and a platinum cata-lyst heated to 350 �C, to remove VOCs at rates of 1e2 l min�1. Thefiltered air contained normal room-air CO2 concentrations.

2.3. Measurement of VOCs

VOCs for both experiments weremeasured with proton-transferreaction mass spectrometry (PTR-MS) (de Gouw and Warneke,2007). The black bamboo was also measured with a gaschromatography-mass spectrometer (GCeMS) instrument. Theelephant grass was also measured with proton-transfer reactionion-trap mass spectrometry (PIT-MS) (Warneke et al., 2005). PTR-MS and PIT-MS utilize proton-transfer reactions of H3Oþ to detectvarious atmospheric trace gases, usually at the MHþ ion. PTR-MSallows for the detection of numerous VOCs with high sensitivity(10e100 part per trillion volume (pptv) detection limit) and fastresponse time (1e10 s). The PTR-MS has a quadrupole as massselector, while in the PIT-MS the mass selector is an ion trap. Thedisadvantage of a quadrupole mass selector is that it only deter-mines one ion mass-to-charge ratio (m/z) at a time, while the iontrap measures full spectra. The ion trap also allows isolation ofa specific m/z value to perform MS/MS, which helps based on thefragmentation patterns to identify the compound (Warneke et al.,

E. Crespo et al. / Atmospheric Environment 65 (2013) 61e68 63

2005). The advantage of the quadrupole is a higher sensitivity. ThePTR-MS measured about 30 masses for 1 s each and the PIT-MS 1-smass scans were averaged to 1-min data. Both instruments used anE/N of about 115Td. Calibrations were done using VOC gas stan-dards for methanol, acetaldehyde, acetone, isoprene and mono-terpenes and literature values were used for thewound compoundsand acetic acid (Warneke et al., 2003), where fragmentation wastaken into account for the quantification. Measurement uncer-tainties of the PTR-MS and the PIT-MS for the oxygenated VOCs andisoprene are around 20% and about 50% for the wound compounds.Acetic acid is a “sticky” compound and shows significant memoryeffects in the plant chambers, which increases the uncertainty toa factor of 2.

To unambiguously identify the VOCs, the black bamboo in theindoor plant enclosure was also measured using an online GCeMSinstrument that identifies and quantifies up to hundred differentVOCs (Gilman et al., 2010). The GCeMS samples for 5 min every30min and has a lower detection limit of a few pptv. Duringmost ofthe plant measurements the mixing ratios were over the upperdetection limit of 10e20 ppbv and therefore we use the GCeMSdata only qualitatively for the identification of the mass signalsmeasured by PTR-MS and PIT-MS.

Once a plant was enclosed in the chamber, VOCs were moni-tored for 2e3 days. Afterwards the grass or bamboo was cut andmeasured during drying for up to five days. Once emissions duringdrying had decreased below the detection limit, the plant materialwas dried in an oven at 80 �C for three days to determine its dryweight. The specific leaf area (SLA) of black bamboo of197.3 cm2 g�1 was determined from the leaf area of 12 leaves (4from each plant) and their dry weight.

Fig. 1. 5-Day period of black bamboo emissions (3 days before cutting and 2 days ofdrying). Top panel: temperature, PAR and net assimilation; lower panels: methanol,acetaldehyde, acetone, isoprene and C6 wound compounds, fluxes measured with PTR-MS (left axis) and GCeMS data (right axis).

3. Results and discussion

3.1. Black bamboo (Phyllostachys nigra)

Fig. 1 shows a 5-day period (3 days before cutting and 2 days ofdrying) of one of the three black bamboo chamber measurements.Net assimilation, chamber temperature and PAR are shown in thetop panel. Fluxes of various VOCs (m/z) determined using the fastresponse PTR-MS measurements are shown on the left axis andmixing ratios of various VOCs measured by GCeMS, used foridentification of them/z detected by PTR-MS, on the right axis. VOCfluxes in nmol m�2 leaf area s�1 were calculated from the respec-tive concentration difference between the air in the well-mixedchamber and the scrubbed chamber purge air taking into accountthe purge air flowrate, correcting for dilution due to transpiration,and normalizing to the respective total leaf area.

For the healthy plant low overall emissions were seen in thedark and high emissions in the light periods. The main emissionsfor growing bamboo are isoprene (m/z 69), methanol (m/z 33),acetaldehyde (m/z 45), acetone þ propanal (m/z 59), acetic acid (m/z 61) and monoterpenes (m/z 137, not shown in Fig. 1). Emissions ofother monitored m/z were close to detection limits and are there-fore not reported here. Table 1 shows day- and nighttime emissionsfor uncut bamboo, averaged over the three replicas, and literaturevalues from poplar species. Table 1 and Fig. 1 indicate low but non-zero C6 wound compound emissions for uncut bamboo (detectedwith GCeMS and at several masses with PTR-MS,m/z 55,m/z 57,m/z 81, m/z 83, m/z 99, m/z 101 (Fall et al., 1999)), although the plantswere handled carefully not to inflict wounds. The emission ofwound compounds induced by temperature variations has beenpreviously observed by Loreto et al. (2006), and despite the fact thatin our case temperature variations were not too extreme, thisexplains the small wound compound emissions.

As can be seen in Fig. 1 and Table 1, black bamboo is a strongisoprene emitter, although bamboo isoprene emissions are stillalmost an order of magnitude lower than poplar species (Table 1).Acetaldehyde emissions from bamboo are similar to those ofcottonwood (Martin et al., 1999), but lower than for aspens (Secoet al., 2007). In the case of acetone, poplar showed only slightlyhigher emissions than bamboo, but was within the range of emis-sions from a boreal forest (Janson and de Serves, 2001). Acetic acidemissions were also determined using the PTR-MS. Acetic acid hasstrong memory effect in the chamber and tubing and therefore theuncertainty for the emissions is higher than for other compounds,but it is clear that black bamboo has substantial emission of aceticacid. Finally, small monoterpene emissions were also observed(Table 1).

When plants are cut and left to dry, it has been shown that theycontinue emitting for a period of time (de Gouw et al., 1999). Mainemissions released by bamboo during the first 48 h of drying can beseen in Fig. 1 and in Table 1, where one day and three day emissionsare summarized. The highest emissions correspond to the C6wound compounds from the hexenal and hexenol families and cis-3-hexenyl acetate (Fall et al., 1999). Next to those, m/z 33 (meth-anol), m/z 45 (acetaldehyde), m/z 59 (acetone) and ethanol alsoshowed high emissions within the first 24 h of drying. Ethanol andformic acid have the same mass and therefore cannot be quantifiedwith PTR-MS. Fluxes of ethanol were not quantified, because themixing ratios were above the upper limit of detection of the GCeMS. For uncut bamboo, using the GCeMS data VOC emissions atm/z 69 are attributed to isoprene; however during cutting and

Table 1Emissions of black bamboo.

Intact black bamboo[nmol m�2 s�1]

Cut black bamboo[nmol m�2]

Intact black bamboo[ng gdwt

�1 h�1]Cut black bamboo[ng gdwt

�1 ]Populus spp Drying haye drying

sorghumf sorghum rice

Day � SDNight � SD

P24 h � SD

P3 day � SD

DayNight

P24 h

P3 day

24 h Integrated,ng gdwt

�1 h�1Total emissions,ng gdwt

�1

Methanolm/z 33

0.551 � 0.2060.075 � 0.059

26,437 � 842844,245 � 7680

61863

14,26026,156

160 � 103

6.6 � 1.8 � 103

2.0 � 0.9 � 103

3.1 � 0.8 � 103

Acetaldehydem/z 45

0.206 � 0.1900.069 � 0.048

68,398 � 17,75986,856 � 2082

24949

50,03873,284

60 � 70a1

1100 � 500a2

1200 � 1400b

20e80 � 103

8.4 � 1.8 � 103

6.6 � 2.0 � 103

8.4 � 0.7 � 103

Acetonem/z 59

0.194 � 0.1380.046 � 0.059

12,343 � 795216,120 � 4716

35144

924413,373

550 � 400b 20e40 � 103

N/A0.4 � 0.3 � 103

1.7 � 1.1 � 103

Acetic acidm/z 61

0.473 � 0.2470.114 � 0.124

25,366 � 19,128 1034*

19518,24828,647

Isoprene(þC5 wound)m/z 69

7.804 � 2.3270.057 � 0.013

43,063 � 10,62234,526 � 11,266

28,516344

56,28858,067

1.6 � 1.4 � 103a

9.0 � 1.4 � 103b

150�1 � 103c

77�1 � 103c

600e50,000d

15 � 103

0.8 � 0.35 � 103

0.32 � 0.1 � 103

3.2 � 1.7 � 103

Sum of MTm/z 137

0.026 � 0.008BDL

1502 � 13451531 � 1334

119BDL

585587

10e8000d

Sum of GLVm/z99 þ 81þ57 þ 101þ

83 þ 103 þ 85

0.114 � 0.0530.046 � 0.073

110,430 � 14,922114,498 � 11,215

46532

223,972231,197

100e240 � 103

0.9 � 0.3 � 103

87 � 50 � 103

49 � 100 � 103

30e60 � 103

0.8 � 0.2 � 103

0.36 � 0.2 � 103

34 � 13 � 103

Standard deviations are for the three replicates. Comparison with emissions from poplar, sorghum and rice species is shown.a Martin et al. (1999) (1, cottonwood; 2, aspen).b Villanueva-Fierro et al. (2004), cottonwood.c Isebrands et al. (1999).d Hakola et al. (1998).e Karl et al. (2001).f Karl et al. (2005b).

0 6 12 18 24

0

100

200

300

ngr*

grD

W-1

*h-1

Time (hours)

m/z m/z

m/zm/z

33 (methanol) 45 (acetaldehyde)59 (acetone) 69

Fig. 2. Methanol, acetaldehyde, acetone and m/z 69 (isoprene and/or 1-penten-3-ol)emissions from uncut elephant grass (from the 5th experiment with PTR-MS). (Shadedarea represents the night (dark) period).

E. Crespo et al. / Atmospheric Environment 65 (2013) 61e6864

drying other VOCs contribute to m/z 69. GCeMS identificationshowed that they are C5 compounds as were also identified forother crops: methylbutanals (MBA) methylbutenols (MBO) andpentenols (Fall et al., 2001; Karl et al., 2001; Eller et al., 2011).

Wound compound emissions dropped down to their pre-cutemission rates after approximately 1 day, while methanol, acetal-dehyde and acetone emission rates were about one order ofmagnitude higher than pre-cut ones. Total emissions during thedrying process of bamboo are of the same order of magnitude ofthose of drying hay (Karl et al., 2001), and higher than sorghum andrice (Karl et al., 2005b) (see Table 1).

3.2. Elephant grass (M. gigantus)

Volatile emissions from grasses are relatively low compared toother plant groups (Guenther et al., 1995); this is also the case withuncut elephant grass. Methanol, acetaldehyde and acetone are themain constituents. In Fig. 2, an emission profile is shown with highdaytime emissions. Experiments were performed with grass at twodifferent seasonal stages, one with greenmid-season grass, and theother with end-of-season yellowing grass, which was alreadygetting dry. From the uncut grass, no clear difference was observedin VOC emissions between “green” grass and end-of-season“yellow” grass. In Table 2 emission rates are given and comparedto emissions from other grasses. Methanol, acetaldehyde andacetone emissions from uncut elephant grass are in the same orderof magnitude as the ones from uncut switchgrass (Eller et al., 2011),

Table 224-h Integrated emissions (in ng h�1 gdwt

�1 ) from uncut elephant grass and elephant grass drying, and peak emissions (in italic, in ng gdwt�1 ) from uncut elephant grass compared

to emissions from other grasses. Averages from two different season stages are shown (two replicates with green grass and two replicates with end-of-season yellow grass).Standard deviations are given in brackets.

Mass (compound) Uncut elephant grass, ng gdwt�1 h�1 Other uncut grasses,

ng gdwt�1 h�1

Drying elephant grass,ng gdwt

�1 h�1Drying switchgrass,c

ng gdwt�1 h�1

Green grass Yellow grass Green grass Yellow grass

m/z 33 (methanol) 80 (40) 130 (50) 1400 (200)a 140 (80) 69 (17) 380 (100)180 (60) 362 (20) 300 (160)c

m/z 45 (acetaldehyde) 110 (60) 70 (40) 54 (15)c 710 (140) 50 (20) 600 (200)425 (300) 270 (100)

m/z 59 (acetone) 140 (60) 50 (40) 62 (29)c 657 (190) 20 (3) 90 (30)294 (15) 120 (40)

m/z 69 5.3 (1.6) 5.9 (0.2) 40 (isop)a 62 (14) 6.9 (2) 50 (20)40 (3) 26 (15) 18 (9)c

C6 wound comp. 10b1

60b2P

Hexenals (m/z 57, m/z 81,m/z 99)

4 (5) 5 (1) 100 (22) 7.3 (1.3) 60 (60)(m81 þ m99)

Hexanal þ Phexenols

(m/z 83, m/z 101)15 (3) 11 (1) 130 (70) 10.4 (1.4) Hexenols 44 (62)

(m/z 83 þ m/z 101)

a Grassland site, Fukui and Doskey (1998).b1 Grass.b2 Clover, Kirstine et al. (1998).c Switchgrass,Eller et al. (2011).

E. Crespo et al. / Atmospheric Environment 65 (2013) 61e68 65

methanol emissions are one order of magnitude lower than fromgrassland, reported by Fukui and Doskey (1998). Emissions at m/z69 (likely C5 wound compounds and isoprene) were in the sameorder of magnitude as for switchgrass (Eller et al., 2011) andgrassland (Fukui and Doskey,1998). Differences can be explained byspecies dependency and by the differences in temperature and lightconditions. For example, switchgrass experiments were performedat constant temperature (30 �C) and light conditions (12 h), whilethe elephant grass experiments were performed under naturallight and ambient temperature conditions. Taking into account thatisoprene emissions are highly dependent on light and temperature(Sharkey et al., 1996; Harley et al., 1999), differences in emissionscould be expected. For those experiments, the contribution of m/z81 to the C6 wound compounds was discarded because a highbackground was observed at this mass.

During drying, elephant grass emissions are temporarilyenhanced (see Fig. 3). The main emissions within the first 24-hobservation period are summarized in Table 2, showing meth-anol, acetaldehyde, acetone, m/z 69s and C6 wound compounds.We observed a clear difference in volatile emission rates fromdrying green leaves (just after cutting) as compared to leaves thatwere cut in a yellow/dry stage. Grasses that were initially greenreleased higher amounts of volatiles for a longer time than yel-lowing grasses in the end-of-season stage. As can be seen in Table 2,the main emissions from drying green grasses in the first 24 h wereacetaldehyde and acetone. On the other hand for yellow grasses,the main emission was at m/z 101 at which several woundcompounds can be detected (cis-3-hexenol, trans-3-hexenol, trans-2-hexenol). Comparing average emissions during drying forelephant grass and switchgrass, we find that both grasses emitsimilar amounts of the measured VOCs in the first 24 h.

Emissions from bamboo are higher than elephant grass andswitchgrass from uncut plants methanol, acetaldehyde and acetone(Eller et al., 2011). Emissions from bamboo detected atm/z 69 are 20times higher than elephant grass emissions (this study, Tables 1 and2) and about five times higher than switchgrass emissions (Elleret al., 2011, Table 2). Bamboo is a high isoprene emitter incomparison to grasses (Fukui and Doskey, 1998; Eller et al., 2011).For other woody species (Villanueva-Fierro et al., 2004; Hakolaet al., 1998) a wide variation in isoprene emissions has beenobserved. Based on the expected seasonal variation on isoprene

emissions due to temperature variations (Guenther, 1997; Tani andKawawata, 2008), even higher emissions of isoprene could be ex-pected for bamboo during the summer months as compared to themeasurements presented here. During drying, average emissions ofbamboo (Table 1) are approximately one order of magnitude higherthan for the perennial grasses considered for biofuel production,elephant grass (this study, Table 2) and switchgrass (Eller et al.,2011, Table 2).

4. Implications

Many factors play a role when deciding which species to use forbiofuel production (Carroll and Somerville, 2009; Karp and Shield,2008), and air quality and climate impacts should be one of them.From this and other studies, it is clear that emissions from agri-cultural crops and grasslands are in general lower than emissionsfromwoody species, especially in the case of isoprenoids (Fukui andDoskey, 1998). Even though grasslands can emit considerableamounts of volatiles other than isoprenoids, especially duringharvesting (Kirstine et al., 1998; Fukui and Doskey, 1998; Eller et al.,2011; this study), they might be preferable due to their low VOCemissions for extensive growing for biofuel production over woodyspecies.

The season of elephant grass lasts approximately from Apriluntil FebruaryeMarch (48 weeks) and the estimated yield forelephant grass is about 30,000 kg ha�1 y�1 (average over threeyears, Heaton et al., 2008). The harvest of the grass normally takesplace at the end of thewinter (FebruaryeMarch), but can be done atany time point once the shoot has died completely. When the shootdies depends on the geographic location, but usually it will godormant after the first killing frost, at the end of Octoberebeginning of November. This means that the harvest is done,when the grass is “yellow”. If non-dormant “green” grass is used fordrying experiments to estimate post-harvesting emissions, thetotal quantities will be overestimated. To demonstrate the impor-tance of this difference of using green or yellow grass we estimatetotal emissions from elephant grass in Table 3. In column (A)a whole green season (AprileFebruary) is assumed (defined as 48weeks of background emissions), with one week of post-harvestemissions after cutting green grass. In column (B) a green seasonis assumed from April until mid-October (30 weeks) followed by

0 12 24 36 48

0

250

500

750

ngr*

(grD

W)

*h 2nd exp PIT-MS (green leaves) 4th exp PTR-MS (yellow leaves)

Time (hours)

m/z 59

0 12 24 36 48 600

10

20

30

ngr*

(grD

W)

*h

Time (hours)

m/z 99

0 12 24 36 480

20

40

60

ngr*

(grD

W)

*h

Time (hours)

m/z 83

0 12 24 36 48

0

50

100

ngr*

(grD

W)

Time (hours)

m/z 57

Fig. 3. Emissions of acetone,m/z 57,m/z 99 (trans-2-hexenal þ cis-3-hexenal) andm/z 83 (cis-3-hexenol þ trans-3-hexenol þ trans-2-hexenol) during drying from elephant grass intwo different time points of the season. Black symbols: grass was green before cutting; measured with PIT-MS. Red symbols: grass was turning yellow before cutting; measuredwith PTR-MS. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

E. Crespo et al. / Atmospheric Environment 65 (2013) 61e6866

a yellow (dry) period till mid-February (18 weeks) after which it isharvested, with one week of post-harvest emissions. The totalestimated yearly emissions of the main VOCs for both situations arecalculated. The total emissions for yellow grass are estimated to besomewhat lower than for green grass: C5 wound compounds and

Table 3Total emissions in kg ha�1 y�1 for the major emissions during growing and harvesting30,000 kg ha�1 y�1. (A) Elephant grass emissions assuming that grass stays green for thegrass emissions assuming that the grass remains green from April to October, starts gett

Compounds (A) Elephant grass, emwhen harvesting (kg h

Growing Harv

m/z 33 (Methanol) 19 (10) 0.7 (m/z 45 (Acetaldehyde) 27 (14) 3.6 (m/z 59 (Acetone) 34 (14) 3 (m/z 69 (Isoprene/1-penten-3-ol) 1.3 (0.4) 0.3 (

m/z 137 (Monoterpenes)P

hexenals (m/z 99 þ m/z 81) 0.9 (1.15) 0.5 (Hexanal þ P

hexenols (m/z 57 þ m/z 83 þ m/z 101) 3.2 (0.6) 0.6 (

isoprene (13%), for the sum of hexenals (25%), acetaldehyde (23%),acetone (30%), hexanal plus hexenols (20%), while for methanoltherewas a 20% increase. Since the uncertainties of the values are inthe same order these changes are not significant, but point ina direction of reduced total emission.

of elephant grass; one week of post-harvest emissions are assumed and yields ofwhole cycle (48 weeks of emissions during growing) and is cut green; (B) elephanting dry in mid-October, and is harvested yellow in mid-February.

issions assuming greena�1 y�1)

(B) Elephant grass, emissions assuming greengrass ApreOct, yellow grass OcteFeb, dry whenharvesting (kg ha�1 y�1)

esting Total Growing Harvesting Total

0.4) 20 (10) 24 (10) 0.35 (0.1) 24 (10)0.7) 30 (15) 23 (13) 0.25 (0.10) 23 (13)1) 37 (15) 26 (13) 0.10 (0.02) 26 (13)0.1) 1.6 (0.5) 1.3 (0.3) 0.03 (0.01) 1.4 (0.3)

0.1) 1.4 (1.3) 1.1 (0.9) 0.037 (0.006) 1.1 (0.9)0.3) 3.8 (0.9) 3.3 (0.5) 0.052 (0.007) 3.3 (0.5)

E. Crespo et al. / Atmospheric Environment 65 (2013) 61e68 67

During harvesting, when local air quality is influenced the mostdue to the emissions of highly reactive wound compounds, thedifference between green and yellow grass is most significant. Herethe emissions for all compounds are larger for green grass, whichhas been traditionally used for wounding experiments. In the caseof methanol, emissions during drying are twice as high whenstarting with green grass as compared to yellow grass. For all theother measured compounds including wound compounds, greengrass emissions are at least a factor of 10 higher than yellow grass.Therefore, actual emissions for grasses should more likely becalculated using the yellowgrass emission rates, dependent on typeof crop and the harvesting procedure.

This study provides a first estimate of VOC emissions fromelephant grass and bamboo, and points out the importance ofsimulating natural conditions when measuring plant emissions.Many laboratory studies do not account for the seasonal fluctua-tions in sunlight and temperature, which may lead to over-estimation of emissions. Moreover, as we have shown in this study,perennial grass emissions after harvesting strongly depend on theseasonal stage. Therefore, post-harvesting emissions should bemeasured at the appropriate stage for better emission estimates.Our VOC emission data, combined with the data for switchgrass(Eller et al., 2011), also suggests that the use of perennial grasses forextensive growing for bioethanol production have lower emissionsthan woody species, which might be important for regional airquality.

In this study we have only looked at plant enclosure measure-ments of emissions. We have tried to simulate natural conditions asbest as possible, but clearly eddy covariance flux measurements atcanopy or aircraft scale are needed to better address representa-tiveness of these measurements on a regional level. For example,effects like bi-directional exchange of VOCs with plants at highambient mixing ratios cannot be taken into account and thereforethe estimates of fluxes presented here might be an upper limit ofemissions (Karl et al., 2005a).

Acknowledgments

This work was funded by the USDA grant (2009-35112-05217),the EU-FP6-Infrastructures program (FP6-026183) and the ClimateChange and Air Quality programs of NOAA supported some of thelaboratory work.

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