Elsevier

Atmospheric Environment

Volume 60, December 2012, Pages 327-335
Atmospheric Environment

Volatile organic compound emissions from Miscanthus and short rotation coppice willow bioenergy crops

https://doi.org/10.1016/j.atmosenv.2012.06.065Get rights and content

Abstract

Miscanthus × giganteus and short rotation coppice (SRC) willow (Salix spp.) are increasingly important bioenergy crops. Above-canopy fluxes and mixing ratios of volatile organic compounds (VOCs) were measured in summer for the two crops at a site near Lincoln, UK, by proton transfer reaction mass spectrometry (PTR-MS) and virtual disjunct eddy covariance. The isoprene emission rate above willow peaked around midday at ∼1 mg m−2 h−1, equivalent to 20 μg gdw−1 h−1 normalised to 30 °C and 1000 μmol m−2 s−1 PAR, much greater than for conventional arable crops. Average midday peak isoprene mixing ratio was ∼1.4 ppbv. Acetone and acetic acid also showed small positive daytime fluxes. No measurable fluxes of VOCs were detected above the Miscanthus canopy. Differing isoprene emission rates between different bioenergy crops, and the crops or vegetation cover they may replace, means the impact on regional air quality should be taken into consideration in bioenergy crop selection.

Highlights

Miscanthus and coppice willow are increasingly important bioenergy crops. ► Above-canopy fluxes were measured using PTR-MS and virtual disjunct eddy covariance. ► Willow isoprene emission peaked at ∼1 mg m−2 h−1, ≡20 μg gdw−1 h−1 standardised. ► Bioenergy crop species choice should consider their impact on regional air quality.

Introduction

Bioenergy crops are those grown specifically for energy production rather than food, as a means of mitigating two problems associated with the use of traditional fossil fuels: anthropogenic climate forcing and energy security (McKay, 2006). Such crops contribute to carbon neutrality since CO2 produced during the combustion of the crop is offset by the CO2 sequestered during growth. There is also potential for long-term storage of carbon via uptake by soil through plant roots (Grogan and Matthews, 2002). Consequently, cultivation of bioenergy crops is increasing rapidly. For example, power generators in the UK are required to increase to 15.4% by 2015/16 the energy derived from renewable sources (DTI, 2005), with biomass being acknowledged as a key resource in achieving this target.

Although bioenergy crops are perceived to be carbon neutral, full life-cycle analysis needs also to take account of changes in emissions of other potent greenhouse gases such as CH4 or N2O. Also, few studies have investigated volatile organic compound (VOC) emissions from bioenergy crops. Biogenic VOC emissions from vegetation (Steiner and Goldstein, 2007) are estimated as about 10 times greater globally than VOC emissions from anthropogenic sources (Guenther et al., 1995). The dominant BVOC is isoprene (Guenther et al., 2006), but other important compounds include oxygenated VOCs and terpenoids.

Emissions of VOCs are important for several reasons. Their rapid oxidation chemistry, particularly in the presence of NOx, affects the oxidative capacity of the atmosphere, the generation of tropospheric ozone (Atkinson, 2000), of concern for human and plant health (Ashmore, 2005) and as a radiative forcing gas, and on formation of secondary organic particles, which likewise affect human health (Dockery et al., 1993) and radiative forcing (Kulmala et al., 2004).

The potential for BVOC emissions from crops to have a significant impact on atmospheric composition has been demonstrated in the tropics (Hewitt et al., 2009). The aim of this study was to determine fluxes of BVOCs for two bioenergy crops grown in the UK and elsewhere: short rotation coppice (SRC) willow (Salix spp.), a woody crop grown in dense plantations of multi-stemmed plants and harvested every 3 years; and Miscanthus × giganteus, a perennial grass native to Asia, of the same taxonomic group as sugarcane, sorghum and maize (Naidu et al., 2003) but more resilient to lower temperature whilst maintaining high CO2 assimilation and biomass conversion efficiency. The crop grows up to 3.5 m per year (Rowe et al., 2009), and is harvested annually between January and March. The chipped and dried biomass of both crops is used to fuel biomass burners or to co-fire existing coal-fired power stations.

Fluxes from this work are compared with those for conventional UK arable crops to assess the potential impact of this land-use change on atmospheric chemistry.

Section snippets

Sampling site

The field measurements were carried out from mid July to mid August 2010 near Lincoln, UK (53° 19′ N, 0° 35′ W). Fig. 1 shows the layout of the site, which consisted of several fields of Miscanthus, willow and wheat, located within an area of predominantly flat arable fields separated by hedgerows and isolated areas of mixed deciduous woodland. Mean annual rainfall at the site was 600 mm and the soil was a fine loam, overlying Charnmouth mudstone. The nearest settlement (population: 113), which

Miscanthus

The time series of VOC fluxes above Miscanthus are shown in Fig. 2 along with u* and sensible heat flux. Two periods of missing data 21st–22nd and 25th–27th July were due to failure of the sampling pump. Data in the first few days were relatively noisy, showing no particular diurnal trend up to 20th July. This was likely due to elevated O2+ impurities during transport of the instrument resulting in less reliable primary ion counts or higher LOD. Additionally, episodes of rainfall on 17th, 18th,

Discussion

In the context of SRC willow as a bioenergy crop, the significant isoprene emission factor could potentially impact local and regional air quality by affecting tropospheric ozone production and SOA formation. Conventional agricultural crops are regarded as being low emitting species. For example, wheat and oats are estimated as having isoprene emission factors in the range 0–0.5 μg gdw−1 h−1 (Karl et al., 2009; Konig et al., 1995), while those for rapeseed, rye and barley are zero (Karl et al.,

Conclusions

Measurements of above-canopy fluxes and mixing ratios of VOCs revealed significant emissions of isoprene from short rotation coppice willow, with a standard emission factor of 20 μg gdw−1 h−1. No significant emissions were measured from Miscanthus. This is the first field study of bioenergy crops in the UK and shows that a change in land use from conventional to bioenergy crops could result in increased isoprene emissions. Bioenergy crop species choice should therefore include consideration of

Acknowledgements

N. Copeland acknowledges PhD studentship funding from EaStChem School of Chemistry and CEH Edinburgh. The authors thank Jonathan Wright and Frank Wilson for site access, Julia Drewer, Jon Finch and Eilidh Morrison for assistance with fieldwork set up, and Kirsti Ashworth and Catherine Hardacre for help with standard emission factors. We are also grateful to the anonymous reviewers of this paper for their helpful comments and suggestions.

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