Disjunct eddy covariance measurements of oxygenated volatile organic compounds fluxes from an alfalfa field before and after cutting

Abstract

Disjunct eddy covariance measurements of oxygenated volatile organic compounds fluxes from an alfalfa field before and after cutting [1] There is interest in and significant uncertainty about the emissions of oxygenated volatile organic compounds (oxVOCs) from vegetation to the atmosphere. Here, we measured the fluxes of selected oxVOCs from an alfalfa field, before, during, and after cutting, using a combination of disjunct eddy covariance and proton-transfer-reaction mass spectrometry. Over the course of 1 day a significant methanol flux of 4 mg m À2 h À1 was observed from undisturbed alfalfa with a maximum at 0800 LT, possibly caused by the evaporation of dew. A smaller release of hexenals during this day (0.04 mg m À2 h À1 ) demonstrated the sensitivity of the method. Other results suggested that acetaldehyde and acetone were released in the afternoon but were lost by dry deposition in the evening and morning; deposition velocities were estimated to be 0.2 cm s À1 (acetaldehyde) and 0.09 cm s À1 (acetone). After the alfalfa was cut the emissions of methanol, acetaldehyde, acetone, and hexenals were significantly enhanced and remained high for three days during which the alfalfa was drying. After a rainstorm the oxVOC emissions from the cut, wet alfalfa increased even more. Nighttime measurements yielded low oxVOC fluxes in general, but the high variability of the concentrations during the night and the high degree of correlation between different oxVOCs suggest that the nighttime releases of oxVOCs from alfalfa were nonzero. This work suggests that the global source of oxVOCs due to the production of hay is of minor importance. The emission flux of methanol from vegetation during the growing season may be very large on a global basis.

A new plant chamber facility, PLUS, coupled to the atmosphere simulation chamber SAPHIR

“…Normalized isoprene and monoterpene emissions revealed similar responses to the diel course of light intensity in all temperature regimes. Methanol, however, showed emission pulses after the onset of illumination, as has been observed previously for different plant species both in field and laboratory experiments under controlled conditions (Folkers et al, 2008;Warneke et al, 2002;Nemecek-Marshall et al, 1995). Morning pulses of methanol emissions were explained as a consequence of accumulation in extracellular water of the plant's apoplast when stomata are closed during nighttime (Niinemets and Reichstein, 2003).…”

First space-based derivation of the global atmospheric methanol emission fluxes

“…The lower emission rates for tropical broadleaf forests are based on recent measurements indicating lower emissions for these landscapes (Karl et al, 2007;Langford et al, 2010). The few measurements reported for croplands include methanol emission fluxes of alfalfa (Warneke et al, 2002) that are much higher than the forest values, and emission fluxes from ryegrass (Schade and Custer, 2004), that are much lower. Since the MEGAN landcover data do not distinguish between high and low emission crops, the value used for forests is currently used for all crops.…”

“…These emission factors are based on whole ecosystem net methanol flux measurements reported by 17 studies that characterized various ecosystems including tropical forest (Geron et al, 2002;Karl et al, 2004Karl et al, , 2007Langford et al, 2010), warm conifer forest (Karl et al, 2005), cool temperate conifer forest (Schade and Goldstein, 2001;Baker et al, 2001;Karl et al, 2002), temperate broadleaf forest and plantation (Spirig et al, 2005;Karl et al, 2003a;Jardine et al, 2008), boreal forest (Rinne et al, 2007), croplands (Warneke et al, 2002;Schade and Custer, 2004) and grassland (Kirstine et al, 1998;Fukui and Doskey, 1998;Ruuskanen et al, 2010). Among these studies, Kirstine et al (1998) and Fukui and Doskey (1998) have used whole ecosystem enclosure techniques with gas chromatography analysis to quantify emissions from grasslands, Schade and Goldstein (2001), Baker et al (2001) and Geron et al (2002) used above canopy relaxed eddy accumulation with gas chromatography analysis to measure methanol fluxes above forests, whereas all the other studies used protontransfer reaction mass spectroscopy (PTR-MS) and the eddy covariance, or disjunct eddy covariance, approach (see Karl et al, 2002).…”

Methanol exchange between grassland and the atmosphere

“…Grasslands cover one quarter of the earth's land surface (Graedel and Crutzen, 1993). Apart from studies concerning the methanol emissions due to harvesting (De Gouw et al, 1999;Karl et al, 2001;Warneke et al, 2002) only few long-term flux studies exist for grassland (Kirstine et al, 1998;Fukui and Doskey, 1998). These are based on chamber measurements characterised by a low time resolution.…”

“…In addition, senescing, injuring (e.g. herbivore attacks, cutting) and drying of plant leaves as well as biomass burning are known to be sources of methanol (de Gouw et al, 1999;Warneke et al, 2002;Loreto et al, 2006;Holzinger et al, 1999 and. The major removal processes for methanol are oxidation by OH radicals (in the gas and the aqueous phase; Monod et al, 2000) as well as dry and wet deposition (Heikes et al, 2002;Galbally and Kirstine, 2002;Jacob et al, 2005).…”