Volatile organic compound speciation above and within a Douglas fir forest

Highlights

• Concentrations and fluxes measured using PTR-MS with virtual disjunct eddy covariance.

• First non-terpenoid species fluxes and mixing ratios for Douglas fir canopy.

• Above-canopy emissions of monoterpenes comparable to previous studies of Pseudotsuga menziesii.

• Fluxes of several non-terpenoid VOCs were significant.

• Acetaldehyde, acetone & MTs elevated near bottom of canopy, MBO & estragole at top.

Abstract

Mixing ratios and fluxes of volatile organic compounds (VOCs) were measured by PTR-MS (and GC–MS) and virtual disjunct eddy covariance during a three-week field campaign in summer 2009 within and above a Douglas fir (Pseudotsuga menziesii) forest in Speulderbos, the Netherlands. Measurements included the first non-terpenoid species fluxes and mixing ratios for Douglas fir canopy. Above-canopy emissions of monoterpenes were comparable to previous studies of P. menziesii, with estimated standard emission factors for the first and second halves of the campaign of 0.8 ± 0.4 and 0.8 ± 0.3 μg g_dw⁻¹ h⁻¹, and temperature coefficients of 0.19 ± 0.06 and 0.08 ± 0.05 °C⁻¹, respectively. Estimated isoprene standard emission factors for the two halves of the campaign were 0.09 ± 0.12 and 0.16 ± 0.18 μg g_dw⁻¹ h⁻¹. Fluxes of several non-terpenoid VOCs were significant, with maximum fluxes greater than has been measured for other coniferous species. α-Pinene was the dominant monoterpene within and above the canopy. Within-canopy mixing ratios of individual species were generally greatest in early evening consistent with reduced vertical mixing and continued temperature-dependent emissions. Acetaldehyde, acetone and monoterpenes had elevated mixing ratios toward the bottom of the canopy (5–10 m) with assumed contribution from the large quantities of forest-floor leaf litter. MBO (2-methyl-3-buten-2-ol) and estragole had peak mixing ratios at the top of the canopy and are known to have coniferous sources. MVK + MACR (methyl vinyl ketone and methacrolein) also had highest mixing ratios at the top of the canopy consistent with formation from in-canopy oxidation of isoprene. The work highlights the importance of quantifying a wider variety of VOCs from biogenic sources than isoprene and monoterpenes.

Introduction

Emissions of volatile organic compounds (VOC) from vegetation are estimated as about 10 times greater globally than VOC emissions from anthropogenic sources (Guenther et al., 1995, Steiner and Goldstein, 2007). The dominant BVOC is isoprene (Guenther et al., 2006), estimated to contribute 44% of global BVOC emissions, with monoterpenes contributing 11%, and other VOCs comprising the remainder. The high atmospheric reactivity of many BVOC means such emissions have important regional impacts on atmospheric oxidising capacity, and on tropospheric ozone and secondary organic aerosol formation.

Coniferous forests are normally associated with monoterpene emissions and low (or zero) isoprene emissions (Geron et al., 2000, Karl et al., 2009). Douglas fir is reported to be within the top 10 monoterpene-emitting tree species in the USA (Geron et al., 2000) but previous studies on Douglas fir have investigated emissions at the leaf and branch scale. Arey et al. (1995) measured a standardised monoterpene emission rate (at 30 °C and 1000 μmol m⁻² s⁻¹) of 1.1 μg g_dw⁻¹ h⁻¹ for bigcone Douglas fir (Pseudotsuga macrocarpa), with α-pinene and limonene as major components, and β-pinene and 3-carene also detected. An emission rate of 0.44 μg g_dw⁻¹ h⁻¹ was reported for coastal Douglas fir (Pseudotsuga menziesii) (Pressley et al., 2004). The highest reported standardised monoterpene emission rate is 2.60 μg g_dw⁻¹ h⁻¹ from P. menziesii (Drewitt et al., 1998). An isoprene emission rate of 1.72 μg g_dw⁻¹ h⁻¹ was also reported. A study which investigated a wider range of VOC emissions from saplings also detected sesquiterpenes, oxygenated terpene products (including 2-methyl-3-buten-2-ol (MBO)), methyl salicylate and a homoterpene (C₁₁H₁₈) (Joó et al., 2011).

Quantification of VOC emissions from Douglas fir warrants further research due to paucity of data at canopy scale and for non-terpenoid compounds, and the variability in previously reported standard emissions. In this work, fluxes and mixing ratios of VOCs were measured above and within a Douglas fir canopy at Speulderbos, the Netherlands. A previous study at this site measured forest floor and above-canopy mixing ratios of α-pinene, β-pinene, 3-carene and limonene (Dorsey et al., 2004). Monoterpene mixing ratios above the forest floor were greater than those measured above-canopy, which was attributed to leaf litter emissions, and were also greatest at night because of the lower atmospheric mixing. These findings broadly corroborated those of an earlier study (Peters et al., 1994). However, neither of these studies measured fluxes or other VOC species.