Seasonal cycles of biogenic volatile organic compound fluxes and concentrations in a California citrus orchard

Abstract

Abstract. Orange trees are widely cultivated in Mediterranean climatic regions where they are an important agricultural crop. Citrus have been characterized as emitters of volatile organic compounds (VOC) in chamber studies under controlled environmental conditions, but an extensive characterization at field scale has never been performed using modern measurement methods, and is particularly needed considering the complex interactions between the orchards and the polluted atmosphere in which Citrus is often cultivated. For one year, in a Valencia orange orchard in Exeter, California, we measured fluxes using PTRMS (Proton Transfer Reaction Mass Spectrometer) and eddy covariance for the most abundant VOC typically emitted from citrus vegetation: methanol, acetone, and isoprenoids. Concentration gradients of additional oxygenated and aromatic compounds from the ground level to above the canopy were also measured. In order to characterize concentrations of speciated biogenic VOC (BVOC) in leaves, we analyzed leaf content by GC-MS (Gas Chromatography -Mass Spectrometery) regularly throughout the year. We also characterized in more detail concentrations of speciated BVOC in the air above the orchard by in-situ GC-MS during a few weeks in spring flowering and summer periods. Here we report concentrations and fluxes of the main VOC species emitted by the orchard, discuss how fluxes measured in the field relate to previous studies made with plant enclosures, and describe how VOC content in leaves and emissions change during the year in response to phenological and environmental parameters. The orchard was a source of monoterpenes and oxygenated VOC. The highest emissions were observed during the springtime flowering period, with mid-day fluxes above 2 nmol m −2 s −1 for methanol and up to 1 nmol m −2 s −1 for acetone and monoterpenes. During hot summer days emissions were not as high as we expected considering the known dependence of biogenic emissions on temperature. We provide evidence that thickening of leaf cuticle wax content limited gaseous emissions during the summer.

Eddy covariance emission and deposition flux measurements using proton transfer reaction – time of flight – mass spectrometry (PTR-TOF-MS): comparison with PTR-MS measured vertical gradients and fluxes

“…Both Tani et al (2003) and Misztal et al (2012) showed different fragmentation patterns for several specific monoterpenes when changing the instrumental conditions. Our GC data at this site in August showed d-limonene contributed 89 % to the sum of speciated monoterpene mixing ratios (Fares et al, 2012a). Therefore, m/z 81.070 fluxes measured by our PTR-TOF-MS should be higher than m/z 137.131 and m/z 95.086, consistent with our results (Fig.…”

“…During the measurement period, the averaged sensitivities for PTR-TOF-MS ranged between 8 and 36 ncps ppbv −1 , similar to that reported by Ruuskanen et al (2011). For PTR-MS, the sensitivity to each measured compound has been reported by Fares et al (2012a). Calculation of VOC volume mixing ratios using transmission factors and reaction rate constants was done according to the method described in Holzinger et al (2010b).…”

“…Flux calculation for the PTR-MS was based on the continuous flow disjunct eddy covariance method following Davison et al (2009) and described elsewhere in detail (Fares et al, 2012a). For PTR-TOF-MS, we applied the following method to calculate EC fluxes for all identified mass peaks:…”

“…2). These inlets, within (1.0 m and 3.76 m) and above (4.85 m and 9.18 m) the canopy, were used to sample vertical gradients of 21 species (m/z) including the masses selected for the flux measurements (Fares et al, 2012a) height level were averaged to form hourly values. The PTR-MS instrument was maintained at an E/N ratio of ∼ 128 Td (drift tube temperature: 45 • C, voltage: 600 V, pressure: 2.0-2.2 hPa).…”

Seasonal trends of biogenic terpene emissions

“…Recent studies have highlighted uncertainty in the ability to model monoterpene (MT) and sesquiterpene (SQT) emission rates over a seasonal cycle (Guenther, 1997;Staudt et al, 2000;Holzinger et al, 2006;Barkley et al, 2009); which to a large extend is due to the fact that only few experimental studies describe MT and SQT BVOC emissions over the course of a complete growing season and that findings vary considerably between plant species that were studied (i.e. Hakola et al, 2001Hakola et al, , 2012Staudt et al, 2002;Holzke et al, 2006;Fares et al, 2012;Noe et al, 2012;Matsunaga et al, 2013). Despite the call for more phenologically based descriptors to accompany BVOC emission rate data sets, most emission data currently available are expressed solely by empirical light and temperature dependence algorithms (Duhl et al, 2008).…”