Seasonal variation in vertical volatile compounds air concentrations within a remote hemiboreal mixed forest

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, 3 -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.

Evident elevation of atmospheric monoterpenes due to degradation-induced species changes in a semi-arid grassland

“…Isoprene concentrations we observed in Inner Mongolia were similar to reported values ranging from 0.1 to 7.5 μg m −3 in boreal forest (Lappalainen et al, 2009;Noe et al, 2012). Based on the regression analysis, sedge was the main contributor, although only small sedge patches existed in our study areas.…”

“…In A. frigida-dominated areas with biomass ≥ 1000 g m −2 , the concentration of monoterpenes ranged from 94.5 to 186.1 μg m −3 in hot and sunny days (Temperature ≥ 25°C and PAR ≥ 1000 μmol s −1 m −2 ). Such high concentrations were at least an order of magnitude higher than reported maximum values in most other studies in forest ecosystems (Clement et al, 1990;Hakola et al, 2012;Hakola et al, 2003;Harrison et al, 2001;Noe et al, 2012), although the biomass in grassland is much lower than tree biomass in forest. The possible reason for such high concentration is that A. frigida contains high amounts of essential oil (1.5% w/w dry basis), in which monoterpenes and oxygenated monoterpenes accounted for 10.4 and 75.6%, respectively (Lopes-Lutz et al, 2008).…”

“…Based on the diurnal pattern of monoterpene emission, the emission from A. frigida is lightdependent (Fig. 3b), similar to some species such as oak and beech (Dindorf et al, 2006;Staudt and Seufert, 1995) but different from most coniferous trees (Hakola et al, 2000;Juuti et al, 1990;Noe et al, 2012;Tarvainen et al, 2005).Thus, strong sun light (1125 μmol s −1 m −2 in average) during our measurements in the grassland might be another reason for the observed high concentrations. Therefore, the increase of A. frigida resulting from over-grazing and climate change in grassland may raise ambient BVOCs concentration by more than 100 times, and also possibly results in increasingly detected odor if environmental stresses continue to increase (Peñuelas, 2008;Peñuelas and Staudt, 2010).…”

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).…”

Exchange of volatile organic compounds in the boreal forest floor

“…Ecosystem-scale VOC fluxes in forests are dominated by emissions from the foliage (Guenther et al 2006, Tarvainen et al 2007, Greenberg et al 2012, Noe et al 2012. In addition to the forest floor, stems and branches of trees also emit VOCs (Figure 9).…”