Comment on acp-2022-588

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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 <span class="inline-formula"><i>Q</i>₁₀</span> values, the factor by which terpene emissions increases for every 10 <span class="inline-formula">^∘</span>C 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 <span class="inline-formula">^∘</span>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 (EF<span class="inline-formula">_iso</span>) was 11.1 <span class="inline-formula">±</span>  0.3 nmol m<span class="inline-formula">^−2</span> s<span class="inline-formula">^−1</span>, 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 EF<span class="inline-formula">_MT</span> of 2.4 <span class="inline-formula">±</span>  0.1 nmol m<span class="inline-formula">^−2</span> s<span class="inline-formula">^−1</span>, EF<span class="inline-formula">_SQT</span> of 1.3 <span class="inline-formula">±</span> 0.03 nmol m<span class="inline-formula">^−2</span> s<span class="inline-formula">^−1</span>, higher than typical for needle leaf and broadleaf tree functional types, and EF<span class="inline-formula">_DT</span> of 0.011 <span class="inline-formula">±</span> 0.001 nmol m<span class="inline-formula">^−2</span> s<span class="inline-formula">^−1</span>. 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<span id="page2684"/> large source of terpenes to the atmosphere, and anthropogenic global warming could induce much higher BVOC emissions from wetlands in the future.