Abstract
Abstract. Understanding of the atmosphere/forest canopy exchange of volatile organic compounds (VOCs) requires insight into the deposition, emission, and chemical reactions of VOCs below the canopy. Between 18 July and 9 August 2009, VOCs were measured with proton-transfer-reaction mass spectrometry (PTR-MS) at six heights between 1 and 6 m beneath a 23 m high mixed-forest canopy. Measured VOCs included methanol, isoprene, acetone, methacrolein and methyl vinyl ketone (MACR + MVK), monoterpenes, and sesquiterpenes. There are pronounced differences in the behaviour of isoprene and its by-products and that of the terpenes. Non-terpene mixing ratios increase with height, suggesting predominantly downward fluxes. In contrast, the terpene mixing ratios decrease with height, suggesting upward fluxes. A 1-D canopy model was used to compare results to measurements with and without surface deposition of isoprene and MACR + MVK and emissions of monoterpenes and sesquiterpenes. Results suggest deposition velocities of 2.7 mm s−1 for isoprene and 1.2 mm s−1 for MACR + MVK and daytime surface emission rates of 63 μg m−2 h−1 for monoterpenes. The modelled isoprene surface deposition is approximately 2% of the canopy-top isoprene emissions and the modelled emissions of monoterpenes comprise approximately 15 to 27% of the canopy-top monoterpene emissions to the atmosphere. These results suggest that surface monoterpene emissions are significant for forest canopy/atmosphere exchange for this mixed-forest location and surface uptake is relatively small for all the species measured in this study.
Highlights
Biogenic volatile organic compounds (BVOCs) can play a significant role in atmospheric chemistry (Schade et al, 2010) and forests are a significant source of BVOC emissions (Lappalainen et al, 2009)
The majority of the VOCs follow the diurnal temperature trend shown in Fig. 1a, while the terpenes follow a diurnal trend similar to relative humidity (Fig. 1b)
Acetone, and monoterpenes compare well to other near-surface measurements made at pine plantations and plantations with both sweetgum and pine
Summary
Biogenic volatile organic compounds (BVOCs) can play a significant role in atmospheric chemistry (Schade et al, 2010) and forests are a significant source of BVOC emissions (Lappalainen et al, 2009). The emission of BVOCs is the largest terrestrial source of reactive carbon in the atmosphere and isoprene is the largest contributor (Guenther et al, 1995). BVOCs are involved in the formation and growth of atmospheric aerosol particles (Tunved et al, 2006). Methanol is produced in plants and is attributed to plant cell wall growth and repair (Kreuzwieser et al, 2000). Monoterpene, and methanol emissions are controlled by air temperature (Tingey et al, 1980) and light intensity (Guenther et al, 1991; Folkers et al, 2008). Direct correlations are difficult to measure, as plants acclimate to the environment following cues from previous hours, days, or even seasons (Oquist and Huner, 2003, Mäkelä et al, 2004)
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