Annual cycle of volatile organic compound exchange between a boreal pine forest and the atmosphere

Abstract

Abstract. Long-term flux measurements of volatile organic compounds (VOC) over boreal forests are rare, although the forests are known to emit considerable amounts of VOCs into the atmosphere. Thus, we measured fluxes of several VOCs and oxygenated VOCs over a Scots-pine-dominated boreal forest semi-continuously between May 2010 and December 2013. The VOC profiles were obtained with a proton transfer reaction mass spectrometry, and the fluxes were calculated using vertical concentration profiles and the surface layer profile method connected to the Monin-Obukhov similarity theory. In total fluxes that differed significantly from zero on a monthly basis were observed for 13 out of 27 measured masses. Monoterpenes had the highest net emission in all seasons and statistically significant positive fluxes were detected from March until October. Other important compounds emitted were methanol, ethanol+formic acid, acetone and isoprene+methylbutenol. Oxygenated VOCs showed also deposition fluxes that were statistically different from zero. Isoprene+methylbutenol and monoterpene fluxes followed well the traditional isoprene algorithm and the hybrid algorithm, respectively. Emission potentials of monoterpenes were largest in late spring and autumn which was possibly driven by growth processes and decaying of soil litter, respectively. Conversely, largest emission potentials of isoprene+methylbutenol were found in July. Thus, we concluded that most of the emissions of m/z 69 at the site consisted of isoprene that originated from broadleaved trees.Methanol had deposition fluxes especially before sunrise. This can be connected to water films on surfaces. Based on this assumption, we were able to build an empirical algorithm for bi-directional methanol exchange that described both emission term and deposition term. Methanol emissions were highest in May and June and deposition level increased towards autumn, probably as a result of increasing relative humidity levels leading to predominance of deposition.

Boreal forest BVOC exchange: emissions versus in-canopy sinks

“…The concentrations of 27 different masses (mass-to-charge ratio, m/z) of BVOCs were measured using the proton transfer reaction quadrupole mass spectrometer (PTR-MS, manufactured by Ionicon Analytik GmbH, Innsbruck, Austria) at the same six height levels as the air temperature (Rantala et al, 2015). The fluxes of BVOCs, based on 45 min averages of BVOC concentrations, were computed every third hour with the surface layer profile method (Rannik, 1998;Rantala et al, 2014).…”

“…The fluxes of BVOCs, based on 45 min averages of BVOC concentrations, were computed every third hour with the surface layer profile method (Rannik, 1998;Rantala et al, 2014). Then the fluxes were filtered as suggested in Rantala et al (2015), according to which the data points were removed from the time series when ζ (Obukhov stability parameter) < −2, ζ > 1 or u * (friction velocity at 23 m) < 0.2 m s −1 . Since only 1 month of data were used for comparison with the model results, we did not disregard the outliers and did not apply the gap filling.…”

“…MBO fragmented heavily on m/z 69; thus, its concentrations were not calibrated. Therefore, the m/z 69 was not only related to isoprene but also to the fragments of MBO (Rantala et al, 2015). Hence, in this study isoprene and MBO are analysed together as one group, written as isoprene + MBO.…”

“…More compounds in addition to monoterpenes, including methanol, acetaldehyde, and acetone, were measured by Rinne et al (2007) on an ecosystem scale. Recently, Rantala et al (2015) revised the fluxes of isoprene, monoterpenes, and several OVOCs over a boreal forest canopy based on long-term measurements from 2010 to 2013. In addition, the boreal forest floor also plays a significant role in BVOC exchange (e.g.…”

Field measurements of biogenic volatile organic compounds in the atmosphere using solid-phase microextraction Arrow

“…They also have an important biological role, for example as allelopathic and defense compounds against pathogens and herbivores (Kesselmeier and Staudt, 1999). Measurement of monoterpenes is usually performed by online proton transfer reaction-mass spectrometry (PTR-MS; e.g., Aalto et al, 2014;Rantala et al, 2015). This technique offers fast detection of VOCs, high sensitivity, good time resolution, and low detection limits (Graus et al, 2010).…”