Articles | Volume 18, issue 2
Research article
23 Jan 2018
Research article |  | 23 Jan 2018

Temporal variation of VOC fluxes measured with PTR-TOF above a boreal forest

Simon Schallhart, Pekka Rantala, Maija K. Kajos, Juho Aalto, Ivan Mammarella, Taina M. Ruuskanen, and Markku Kulmala

Abstract. Between April and June 2013 fluxes of volatile organic compounds (VOCs) were measured in a Scots pine and Norway spruce forest using the eddy covariance (EC) method with a proton transfer reaction time-of-flight (PTR-TOF) mass spectrometer. The observations were performed above a boreal forest at the SMEAR II site in southern Finland.

We found a total of 25 different compounds with exchange and investigated their seasonal variations from spring to summer. The majority of the net VOC flux was comprised of methanol, monoterpenes, acetone and butene + butanol. The butene + butanol emissions were concluded to not originate from the forest and, therefore, be anthropogenic. The VOC exchange followed a seasonal trend and the emissions increased from spring to summer. Only three compounds were emitted during the snowmelt while in summer emissions of some 19 VOCs were observed. During the measurement period in April, the emissions were dominated by butene + butanol, while during the start of the growing season and in summer, methanol was the most emitted compound. The main source of methanol was likely the growth of new biomass. During a 21-day period in June, the net VOC flux was 2.1 nmol m−2 s−1. This is on the lower end of PTR-TOF flux measurements from other ecosystems, which range from 2 to 10 nmol m−2 s−1. The EC flux results were compared with surface layer profile measurements, using a proton transfer reaction quadrupole mass spectrometer, which is permanently installed at the SMEAR II site. For the major compounds, the fluxes measured with the two different methods agreed well.

Short summary
Emissions of volatile organic compounds (VOCs) have impact to air quality, human health and climate. We investigated the development of VOC exchange in a boreal forest between April and June 2013. VOC exchange and diversity increased towards summer, but over 75 % of the biogenic net exchange was driven by methanol, monoterpenes and acetone only. The boreal forest emitted less than 0.2 % carbon in form of VOCs in relation to the carbon uptake.
Final-revised paper