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.
Highlights
Oranges, Citrus sinensis L., are among the most economically important and widely cultivated crops in areas with Mediterranean climates, such as California, Italy, Spain, Morocco, and Israel, and areas of cultivation are often close to densely populated areas
This is the case of acetonitrile (m/z 42), the unknown m/z 111, 113, 139 and 151, here reported only for comparison with previous works where these masses have been observed in relevant amount (Holzinger et al, 2006), and methylchavicol (m/z 149), another compound emitted in large amount from pines and palms, but relevant here (Bouvier-Brawn et al, 2009; Misztal et al, 2010)
We show vertical gradients averaged over the diurnal cycle by interpolation of mean mixing ratios at the measurement heights of 1 m, 3.76 m, 4.85 m and 9.18 m (Figs. 2, 3) during three representative periods in the year: flowering (DOY 130–140), summer (DOY 172–274), winter (DOY 37–80), and the fluxes measured over the full year (Fig. 4)
Summary
Citrus sinensis L., are among the most economically important and widely cultivated crops in areas with Mediterranean climates, such as California, Italy, Spain, Morocco, and Israel, and areas of cultivation are often close to densely populated areas. All vascular plants including Citrus species emit biogenic volatile organic compounds (BVOC), with a global estimate of BVOC emitted from plants in the range of 1–1.5 Pg C yr−1 (Guenther et al, 1995). In the presence of sunlight and nitrogen oxides (NOx), the oxidation of BVOC can lead to formation of tropospheric ozone (Chameides et al, 1988; Papiez et al, 2009), a greenhouse gas with detrimental effects on plant health, crop yields, and human health (for a reference list, see EPA 2011). S. Fares et al.: Seasonal cycles of biogenic VOC fluxes and concentrations precursors to atmospheric aerosol formation (Henze and Seinfeld, 2006), accounting for a significant fraction of secondary organic aerosol (SOA) produced in the atmosphere (Goldstein and Galbally, 2007)
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