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

Wetlands cover only 3 % of the global land surface area, but boreal wetlands are experiencing an unprecedented warming of four times the global average. These wetlands emit isoprene and terpenes (including monoterpenes (MT), sesquiterpenes (SQT), and diterpenes (DT)), which are climate-relevant highly reactive biogenic volatile organic compounds (BVOCs) with an exponential dependence on temperature. In this study, we present ecosystem-scale eddy covariance (EC) fluxes of isoprene, MT, SQT, and DT (hereafter referred to together as terpenes) at 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 compound emitted by the wetland, followed by MTs, SQTs, and DTs, and they all exhibited a strong exponential temperature dependence. The Q10 values, the factor by which terpene emissions increases for every 10 ? rise in temperature, were up to five times higher than those used in most BVOC models. During the campaign, the air temperature peaked above 31? 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 nmolm(-2) s(-1), EF(SQT )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 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.

Automated summary

Wetlands cover a small portion of the land surface but boreal wetlands are experiencing significant warming and emit climate-relevant volatile organic compounds. This study presents the first-ever measurements of terpene emissions from a boreal fen wetland and found that isoprene was the dominant compound emitted. The emissions showed a strong exponential dependence on temperature and were significantly higher than previous estimates.