High emission rates and strong temperature response make boreal wetlands a large source of terpenes

Abstract. Terpenes, a class of hydrocarbons including isoprene (C5H8), monoterpenes (MTs; C10H16), sesquiterpenes (SQTs; C15H24), and diterpenes (DTs; C20H32), are highly reactive to atmospheric oxidants and can form highly oxidized organic molecules (HOMs), leading to new particle formation (NPF), and secondary pollutants like tropospheric ozone and secondary organic aerosols (SOA). Wetlands are primarily found in the boreal and tundra regions of the Northern hemisphere and are well-known for their high methane emissions. However, their VOC emissions were investigated by relatively few studies, showing them to be high isoprene emitters. Terpene emissions have an exponential temperature dependence, while boreal wetlands are experiencing above two times the average global warming, and recent enclosure studies show a substantial increase in terpene emissions. In this study, we present ecosystem-scale eddy covariance (EC) fluxes of terpenes (up to DTs) from Siikaneva, a boreal fen in southern Finland, from the start to the peak of the growing season of 2021 (19 May 2021 to 28 June 2021). These are the first EC fluxes reported using the novel state-of-the-art Vocus- proton transfer reaction mass spectrometer (Vocus-PTR) and the first-ever fluxes reported for DTs from a wetland. Isoprene was the dominant terpene emitted by the wetland, followed by MTs, SQTs, and DTs. All terpenes exhibited distinct diurnal patterns with maxima around noon and a strong exponential dependence on temperature. The Q10 values, the factor by which terpene emissions increases for every 10 °C rise in temperature, were up to 5 times higher than those used in most BVOC models. During the campaign, the air temperature peaked above 31 °C on 21–22 June 2021, which is abnormally high for boreal environments, and the maximum flux for all terpenes coincided with this period. We observed that terpene emissions were elevated after this abnormally “high-temperature stress period,” indicating that past temperatures alter emissions significantly. The standardized emission factor (EF) of the fen for isoprene (EFiso) was 11.1 ± 0.3 nmol m-2 s-1, which is at least two times higher than in previous studies and as high as the emission factors typical for broadleaf and other forests in the lower latitudes. We observed EFMT of 2.4 ± 0.1 nmol m-2 s-1, EFSQT of 1.3 ± 0.03 nmol m-2 s-1, higher than typical for needle leaf and broadleaf tree functional types, and EFDT of 0.011 ± 0.001 nmol m-2 s-1. We also compared the landscape average emissions to the Model of Emissions of Gases and Aerosols from Nature (MEGAN) v2.1, specifically using EF for the “C3 Arctic grass” plant functional type, and found that the emissions were underestimated by over 9 times for isoprene, over 300 times for MTs, and 800 times for SQTs. Our results show that due to very high EFs and high sensitivity to increasing temperatures, these high latitude ecosystems can be a large source of terpenes to the atmosphere, and anthropogenic global warming could induce much higher BVOC emissions from wetlands in the future.