Emissions and ambient air concentrations of isoprene, monoterpenes and sesquiterpenes at a Northern wetland

Wetlands cover an area of about 2% of the total land surface area of the world and are most common in the boreal and tundra zones. Northern wetlands are important sinks for carbon dioxide and sources of methane, but knowledge on their VOC emissions is very limited. Currently, we know that northern wetlands are high isoprene emitters (e.g. Holst et al., 2010), but very little is known on the emissions of other VOCs.

We have studied VOC emissions and their ambient concentrations at a sub-Arctic wetland (Lompolojänkkä) in Northern Finland, using an in situ TD-GC-MS. For the emission measurements, a dynamic flow-through FEP chamber was used.

Earlier studies have shown that isoprene is emitted from wetlands and it turned out to be the most abundant compound in the current study also. Monoterpene (MT) emissions were generally less than 10 % of the isoprene emissions, but sesquiterpenes (SQT) emissions were surprisingly high, exceeding MT emissions at all times. Both MT and SQT emissions were dependent on temperature.

Even with the higher emissions from the wetland, ambient air concentrations of isoprene were clearly lower than MT concentrations. This indicates that wetland was not the only source affecting atmospheric concentrations at the site, but surrounding coniferous forests, which are high MT emitters, contribute as well. In May concentrations of SQTs and MTs at Lompolojänkkä were higher than in earlier boreal forest measurements in southern Finland (Hellén et al., 2018). At that time, the snow cover on the ground was melting and soil thawing and VOCs produced under the snow cover, e.g. by microbes and decaying litter, can be released to the air. Daily mean MT concentrations were very highly negatively correlated with daily mean ozone concentrations indicating that vegetation emissions can be a significant chemical sink of ozone at this sub-Arctic area.

References

Hellén, H.et al. 2018, Atmos. Chem. Phys., 18, 13839-13863, https://doi.org/10.5194/acp-18-13839-2018.

Holst, T., et al. 2010, Atmos. Chem. Phys., 10, 1617-1634, https://doi.org/10.5194/acp-10-1617-2010.