Abstract
Abstract. The vertical distribution of ambient biogenic volatile organic compounds (BVOC) concentrations within a hemiboreal forest canopy was investigated over a period of one year. Variability in temporal and spatial isoprene concentrations, ranging from 0.1 to 7.5 μg m−3, can be mainly explained by biogenic emissions from deciduous trees. Monoterpene concentrations exceeded isoprene largely and ranged from 0.01 to 140 μg m−3 and during winter time anthropogenic contributions are likely. Variation in monoterpene concentrations were found to be largest right above the ground and the vertical profiles suggest a weak mixing leading to terpene accumulation in the lower canopy. Exceptionally high values were recorded during a heat wave in July 2010 with very high midday temperatures above 30 °C for several weeks. During summer months, monoterpene exceeded isoprene concentrations 6-fold and during winter 12-fold. During summer months, dominance of α-pinene in the lower and of limonene in the upper part of the canopy was observed, both accounting for up to 70% of the total monoterpene concentration. During wintertime, Δ3-carene was the dominant species, accounting for 60% of total monoterpene concentration in January. Possible biogenic monoterpene sources beside the foliage are the leaf litter, the soil and also resins exuding from stems. In comparison, the hemiboreal mixed forest canopy showed similar isoprene but higher monoterpene concentrations than the boreal forest and lower isoprene but substantially higher monoterpene concentrations than the temperate mixed forest canopies. These results have major implications for simulating air chemistry and secondary organic aerosol formation within and above hemiboreal forest canopies. Possible effects of in-cartridge oxidation reactions are discussed as our measurement technique did not include oxidant scavenging. A comparison between measurements with and without scavenging oxidants is presented.
Highlights
Emissions of biogenic hydrocarbons from forest ecosystems are a dominant source of reduced organic gases to the atmosphere
The vertical distribution of ambient biogenic volatile organic compounds (BVOC) concentrations within a hemiboreal forest canopy was investigated over a period of one year
While near to the forest floor, α-pinene was the most abundant species, we found limonene dominating on top of the canopy for most times of the year, indicative of different sources contributing to individual monoterpene species. 3-carene showed the smallest fractional contribution to the total monoterpene concentrations at 0 m for the most times measured and its contribution was growing to top of the canopy
Summary
Emissions of biogenic hydrocarbons from forest ecosystems are a dominant source of reduced organic gases to the atmosphere. Nitrogen oxides (NOx) and hydroxyl radical (OH) on the vertical distribution of BVOCs or vice-versa have been assessed by means of 1-D canopy chemistry models including atmospheric transport terms (Forkel et al, 2006; Fuentes et al, 2007; Karl et al, 2004; Stroud et al, 2005; Strong et al, 2004) These studies mostly conclude that the discrepancy between upscaled leaf level BVOC emission fluxes
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