Eddy covariance measurements reveal high emissions of terpenes from a boreal fen

Wetlands are well-known for their high emissions of methane to the atmosphere, but emissions of volatile organic compounds (VOCs) are also reported from wetlands. Wetlands cover about 2 % of the total land surface area and most of these wetlands are found in the boreal and tundra regions. A class of compounds called terpenes that include isoprene, monoterpenes, sesquiterpenes, and diterpenes make up 80% of the global biogenic volatile organic compound (BVOC) emissions. These compounds are highly reactive towards oxidants like ozone (O₃), hydroxyl radicals (OH), and nitrate radicals (NO₃) and form secondary organic aerosols in the atmosphere. Hence, quantifying the BVOC emissions accurately is crucial in determining the organic aerosol budget and constraining their contribution to climate-relevant processes such as new-particle formation and cloud formation.

In this study we performed ecosystem scale eddy covariance (EC) measurements of BVOCs and their oxidation products at Siikaneva, a southern Finnish boreal wetland (61^o48' N, 24^o09' E, 160 m a.s.l.), from 19th May 2021 to 28th June 2021 using a Vocus-proton transfer reaction mass spectrometer (Vocus-PTR) co-located with a sonic anemometer (METEK USA-1) at 10 Hz.  BVOCs were sampled from a platform, 2.5 m above the wetland using a high flow main inlet (5000 lpm), with core sampling of 5 lpm into the Vocus-PTR, which substantially reduced the wall losses of less volatile compounds such as sesquiterpenes, diterpenes, and oxygenated VOCs. The EC data were analyzed following standard correction procedures such as lag correction, coordinate rotation, and uncertainty analysis using the InnFLUX tool by Striednig et al. (2020). The high frequency attenuations of the fluxes were corrected using transfer functions estimated using the sensible heat flux cospectra.

We observed high emissions of isoprene, monoterpenes, sesquiterpenes and the first-ever emission fluxes of diterpenes from a wetland. The average normalized standard emission factor (EF) at standard photosynthetically active radiation of 1000 μmols m^-2 s^-1 and standard temperature of 30 ^oC for isoprene using the emission algorithm by Guenther et al. (2012) was determined as 1200 μmols m^-2 day^-1. For comparison, a relaxed eddy accumulation (REA) flux measurement study at the same site by Haapanala et al. (2006) had reported much lower EF of 240 μmols m^-2 day^-1. We observed sesquiterpene emissions reaching up to 50% of monoterpene emissions on average and occasionally even higher than monoterpenes emissions. For diterpenes, we found mean emissions of 0.4 μmols m^-2 day^-1.

During the campaign, the temperature peaked at 32 ^oC which is abnormally high for boreal environments and all the terpenoid emissions showed an exponential temperature dependence. The derived exponential temperature coefficient (Q10) value for isoprene was 4 times higher than the values used in the widely used MEGAN model. Our study reveals that VOC emissions from boreal environment are very sensitive to temperature change and since temperature is one of the main drivers of BVOC emission, anthropogenic global warming can induce much higher BVOC emissions in the future.