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
The emissions of biogenic volatile organic compounds (BVOCs) from
terrestrial ecosystems to the atmosphere are estimated to be around 1 Pg
(10
Temperature and photosynthetically active radiation (PAR) are the main drivers of terpene emissions, and the emissions have an exponential dependence on temperature when PAR is saturated (Niinemets et al., 2004). Hence, emissions of terpenes are modeled using algorithms based on the leaf-level response of emissions to the variations in temperature and PAR (Guenther et al., 2012; Monson et al., 2012). Common BVOC models, like the model of emissions of gases and aerosols from nature (MEGAN), estimate emissions as the product of emission activity factors and standardized emission factors representing important plant functional types (Guenther et al., 2012). Wetlands cover about 3 % of the global land surface area, and most of these wetlands are found in the boreal and tundra regions of the Northern Hemisphere (Archibold, 1995). These northern latitudes are experiencing above four times the average global warming, and it is certain (with high confidence) that northern latitudes, especially the Arctic, will continue to experience this warming (Arias et al., 2021; Post et al., 2019). Due to increased warming, these ecosystems will respond with increased terpene emissions. As an indirect effect, warming can also change the vegetation composition in these ecosystems (Valolahti et al., 2015). Furthermore, the impact of terpenes is generally more critical in high latitudes because of low anthropogenic VOC emissions (Paasonen et al., 2013).
There is an extensive assortment of studies on greenhouse gas emissions from high-latitude wetlands (Aurela et al., 2007; Rinne et al., 2007, 2018). They are well known as a sink of carbon dioxide and the largest natural methane source to the atmosphere. In contrast, their VOC emissions were investigated by relatively few studies, which have shown wetlands to be high isoprene emitters (Janson and De Serves, 1998; Haapanala et al., 2006; Hellén et al., 2006; Ekberg et al., 2009). Most of these studies were conducted using chamber/enclosure measurements and have not investigated the emission of other terpenes. Recent enclosure studies have shown a substantial increase in isoprene emissions in Arctic tundra heath and subarctic wetlands in response to warming (Kramshøj et al., 2016; Lindwall et al., 2016). Disadvantages of such chamber measurements are the rise in temperature and humidity inside the enclosure (Ortega and Helmig, 2008), the acclimation and stress issues for the vegetation inside the enclosure (Niinemets et al., 2011), and the potential loss of less volatile or highly reactive vapors to the enclosure walls (Ortega and Helmig, 2008). Eddy covariance (EC) is an ecosystem-scale flux measurement technique widely used to measure greenhouse gas fluxes (Aubinet et al., 2012). This micrometeorological technique overcomes most disadvantages of chamber measurements and directly assesses ecosystem-level fluxes compared to up-scaling fluxes measured from small enclosures. Very few ecosystem-scale BVOC flux measurement studies have been conducted in wetland ecosystems (Seco et al., 2020; Holst et al., 2010). They found isoprene emissions to have a steeper response to temperature than that used in standard BVOC emission models (Guenther et al., 1993; Monson et al., 2012), indicating a particularly high temperature sensitivity of arctic vegetation due to their acclimatization to colder temperatures. However, none of those studies had the analytical capability to measure fluxes of terpenes larger than MTs.
Isoprene is the most emitted BVOC globally, and its oxidation chemistry is well studied and is shown to contribute to SOA mass (Henze and Seinfeld, 2006). MTs and SQTs oxidation products are well known to partition into the aerosol particle phase. Highly reactive SQT could significantly contribute to SOA mass despite generally lower emissions than MTs (Barreira et al., 2021; Hellén et al., 2018). Measurements of SQT and DT are particularly challenging because they are emitted in low concentrations and sampling loss issues are aggravated by their lower volatility and generally higher reactivity than the smaller terpenes (Helmig et al., 2004). However, a recent chamber study measured SQT from a wetland ecosystem and observed their emissions exceeding the MTs emissions (Hellén et al., 2020). Since diterpenes have very low volatility, they were not thought to be emitted by terrestrial vegetation (Guenther, 2002). Matsunaga et al. (2012) were the first to report DT emissions from vegetation in enclosure measurements. Recent developments in mass spectrometric techniques have enabled measurements of DTs in ambient air (Li et al., 2020). Only one study has reported ecosystem-scale DT emission flux (Fischer et al., 2021) for a boreal forest, but they could not characterize the emissions in terms of temperature and PAR. Due to their high reactivity, DTs could play an important role in atmospheric chemistry. The high molecular weight of their oxidation products makes them relevant for SOA mass formation and NPF. Clearly, more emission studies on the larger terpene families are warranted, given their chemical diversities and important roles in NPF and SOA formation (Luo et al., 2021). Rose et al. (2018) indicated the importance of clustering of pure biogenic molecules to contribute during nighttime in the boreal environment, and recently Junninen et al. (2022) have found that high monoterpene emissions and their subsequent oxidation will cause atmospheric clustering and new particle formation (NPF) in the wetland environment of Siikaneva, southern Finland.
We focus here on the characterization of isoprene, MTs, SQTs, and the rarely reported DTs. Together these four groups are collectively referred to as terpenes throughout this paper. We report ecosystem-scale fluxes of terpenes measured by the eddy covariance (EC) technique from a boreal fen dominated by sedges using a recently developed Vocus proton transfer reaction mass spectrometer (Vocus-PTR) (Krechmer et al., 2018) from the start to the peak of the growing season in spring and early summer. This instrument has very high sensitivity compared to previous-generation PTR-MS; hence, we could measure the first DT emission fluxes from this ecosystem. The terpene emissions are parameterized by temperature, PAR, and leaf area index (LAI). We calculate the temperature dependence of the emissions and hypothesize it to be higher than other ecosystems since boreal wetlands are not acclimatized to high temperatures. Finally, we calculate the standardized emission factor (EF) of the terpenes for this ecosystem and compare the measured terpene emissions with the MEGANv2.1 emission model and their temperature dependence within the model.
The eddy covariance measurements were conducted at the Siikaneva 1 site of
the Station for Measuring Ecosystem-Atmosphere Relations II (SMEAR II).
Siikaneva is an oligotrophic fen located 5 km west of the Hyytiälä
Forestry Field Station in southern Finland (61
The EC measurements took place in the growing season from late spring to
early summer (from 19 May to 28 June 2021), 2.4 m above the fen. Figure S1
shows a satellite view of the sampling site, overlaid with the average
flux footprint calculated using a two-dimensional model (Kljun et al.,
2015). Over 90 % of the campaign-average flux footprint is within the
wetland area and
The Vocus-PTR was placed on a wooden platform at the Siikaneva 1 site. We
adapted the inlet design from Fischer et al. (2021), who used a similar
setup and essentially the same sampling strategy for EC flux measurements. A
sonic anemometer (METEK USA-1) for measuring the vertical and horizontal
components of the wind vector was mounted 0.4 m above the main inlet to
minimize the disturbance from the high inlet flow. A horizontal core
sub-sampling (Teflon tubing, 10 mm i.d.) of 5 L min
The details about the Vocus-PTR calibrations, the data pre-processing, and the EC flux calculations with the innFLUX code package (Striednig et al., 2020) are provided in Supplement Sect. S1.1 to S1.5.
BVOC measurements at a high time resolution of 10 Hz were performed using a Vocus-PTR, described in detail in Krechmer et al. (2018). This novel instrument uses PTR as an ionization technique but differs from earlier-generation PTR in many ways. The key differences are described briefly.
A low-pressure (1.5 mbar) focusing ion molecular reactor (FIMR) uses a
radiofrequency (RF) field to focus the ions on the central axis. This
drastically improves the detection efficiency of product ions. Furthermore,
the sensitivity of Vocus-PTR is independent of ambient humidity due to the
high water mixing ratio in the FIMR (up to 15 %
Fluxes of terpenes
We compared the average BVOC concentration from Vocus-PTR measurements with
offline GC–MS samples collected on 28 June 2021, the last day of the
campaign. Ambient air was sampled through three cleaned stainless steel
tubes (Markes Intl., UK) filled with adsorbents (Tenax TA and Carbograph 5,
100 mg of each, mesh
Global BVOC emission models, like the model for emissions of gases and
aerosols from nature (MEGAN), parametrize the emission rate of a terpene,
To calculate the EF from our dataset, we used G93 for isoprene and MT and a
simplified version of MEGANv2.1, hereafter referred to as G2012, without
considering any canopy environment model and assuming all leaves to be
sunlit for isoprene, MT, and SQT. Briefly, the emission rate of a terpene,
The light-dependent terms of
We performed the eddy covariance flux measurements in Siikaneva during the
growing season, from 19 May to 28 June 2021. The minimum air temperature
during the campaign was
Isoprene showed the highest ambient mixing ratio among all the terpenes. We
observed a relatively high ambient isoprene mixing ratio with a mean of 0.62
Figure S6 compares the terpene mixing ratios measured by the GC–MS at different locations and the terpene mixing ratios measured using the Vocus-PTR at 12:00 on 28 June 2021. We observed a steep concentration gradient in isoprene concentration with vertical height, indicating very high nearby emissions. Near the wooden platform and close to the wetland, the isoprene mixing ratio was about 4–5 ppbv (parts per billion by volume) and was reduced to half on top of the Vocus (2 m above the wetland). The mixing ratios for isoprene obtained from the adsorbent tube on top of the Vocus (40 cm below the high flow inlet) were comparable to those of the Vocus-PTR (1.7 vs. 1.4 ppbv).
We detected
As part of the EC flux analysis with the innFLUX code (Striednig et al. 2020), the data quality was investigated and grouped into nine quality
classes (see Sect. S1.3). For the following flux analysis, we have only
used data that fulfills the criteria for quality classes 1–3 (Table S3) and
with a minimum friction velocity of 0.1 m s
We observed vertical fluxes for more than 250 out of 1072 peaks in our mass
spectrometric dataset. Besides terpenes, we also detected oxygenated VOCs,
including smaller ones such as methyl ethyl ketone (C
Several studies have shown that isoprene is the main BVOC emitted by boreal
wetlands (Klinger et al., 1994; Janson and De Serves, 1998). In this
study, the highest emission fluxes were observed for isoprene: more than a
factor of six times higher than for MTs (Fig. 2b), on average. Overall, we
observed a similar emission pattern for all terpenes. We also report the
first-ever emission of DTs from wetlands (Fig. 2d). We observed relatively
low emissions for all the terpenes from the start of the campaign till the
end of May. During that period, the average temperature was below 10
Overall, the terpene emissions increased substantially over the course of
the campaign due to the increase in temperature and the progression of the
growing season of the vegetation in the fen. LAI varied from 0.2 to 0.54,
increasing continuously throughout the campaign from the beginning to the
peak of the growing season (Fig. 2). The maximum flux for isoprene of 19.5 nmol m
Previous studies at the same site have measured isoprene along with other
small hydrocarbons, namely C
Measured fluxes of
In our study, SQTs emitted from the boreal fen were almost of the same
magnitude as the emitted MTs. In contrast, a recent study in a subarctic fen
in northern Finland (Hellén et al., 2020) reported that SQT
emissions were significantly higher than MT emissions. Their highest SQT
emission was 0.012 nmol m
Only a few ecosystem-scale EC/disjunct EC studies have been conducted in
similar high latitude environments, three in Scandinavia (Seco et al.,
2022, 2020; Holst et al., 2010) and one in an Alaskan tundra
ecosystem (Potosnak et al., 2013). Seco et al. (2022) reports
only isoprene emission from Abisko, Sweden (68
Several chamber studies report isoprene fluxes from boreal fens, starting
with Janson and De Serves (1998) in central Sweden (Stormossen,
60
No other study has ever reported ecosystem-scale DT emissions, except for
Fischer et al. (2021). They used a similar setup as ours in
Hyytiälä, above a boreal forest canopy close to our site. They found
an average emission of 0.15 pmol m
We used Eqs. (2)–(10) and calculated the light (
If we neglect LAI (i.e., LAI
We also split our dataset into two subsets to fit EF for each of them
separately instead of a single EF: one for
As described in the methods section, the G93 algorithm only uses temperature
to predict monoterpene emissions and has no emission algorithm for
sesquiterpenes. The G2012 algorithm includes temperature and light-dependent
emissions for MT and SQT (Sect. 2.5). As for isoprene (Fig. S8 right), we
used G2012 to estimate the emission factors of MTs and SQTs in standard
conditions (EF
Emission factor (EF) at standardized conditions for
terpenes using the G93 algorithm (LAI
The red color-coded data points in Figs. S8, S9, 3, and 4 were measured
after 22 June when the highest air temperatures above 31
For the following analysis, we chose the data points saturated by light (PAR
Our fitted
A closer look at the isoprene behavior shown in Figs. 2 and 3 suggests that
there may be a threshold after which higher emissions “turn on.” The
threshold could be a specific temperature but more likely a period of
elevated temperature days. The red points in Fig. 3, which we think are
after hitting the threshold, have a lower decrease in isoprene emissions
when the temperature drops from 30 to 22
It is clear from Table 1 and Fig. 3 that terpenes have higher emissions
than in previous studies, and they do not follow the standard temperature
dependence as used in the G93 algorithm. Hence, we compare our emission
measurements with model predictions of the MEGANv2.1 emission model for a
single measurement site using its EF for the Arctic C
We also compared the temperature response (
Measured vs. MEGANv2.1 emission for
For crudely estimating the potential global-scale impact of our findings, we
upscale the emission from boreal wetlands based on the average emissions
measured in our study (in Sect. 3.2) and assuming it to be similar throughout
summer (100 d). Using the wetland cover in the boreal environment
(latitude above 50
In this study, we present ecosystem-scale eddy covariance (EC) fluxes of
isoprene, monoterpenes (MTs), sesquiterpenes (SQTs), and diterpenes (DTs),
which globally make up over 70 % of all BVOCs emitted from terrestrial
vegetation. Together these four groups are collectively referred to as
terpenes throughout this paper. The measurements were conducted at the
Siikaneva boreal fen from the start to the peak of the growing season of
2021 (mid-May/late spring to the end of June/mid-summer). These are the
first EC fluxes of terpenes to be reported based on measurements using the
novel state-of-the-art Vocus-PTR mass spectrometry technique and the
first-ever fluxes for DTs from a wetland. Using this instrument's improved
sensitivity, compared to classic PTR mass spectrometers, and a new inlet to
reduce vapor–surface interactions during sampling, we detected fluxes for
about 250 compounds from C
As found by previous studies, isoprene was the dominant terpene emitted by
the wetland, followed by MTs, SQTs, and DTs. We calculated the emission
factor in standard conditions (
We also note that the treatment of LAI is an important aspect when
predicting terpene emissions. The satellite-based LAI values for June
(Copernicus 300 m LAI) for Siikaneva were around 1.7
(
Even though the terpene emissions showed an exponential temperature
dependence, the G93 algorithm did not adequately represent the
high-temperature stress period since this parametrization is based only on
instantaneous light and temperature. On the other hand, the G2012 algorithm
also considers the history of light and temperature and their instantaneous
counterparts and estimates the emissions better. However, the terpene
emissions during the high-temperature stress period are still
underestimated also by the G2012 algorithm. To further understand the
sensitivity of the terpene emissions to temperature, we calculated the
We compared our measured emission of terpenes with the MEGANv2.1 emission
model, specifically using EF for the “Arctic C
Future studies may gain valuable insights by using our observations of
terpene emissions from wetland ecosystems. Our results illustrate that
applying the default EF (Arctic C
In light of our results, we find that there may be too few studies of BVOC (terpene) emissions from these high latitude ecosystems. In particular, longer-term measurements of BVOCs are lacking for all ecosystems. Our study demonstrates how current, highly sensitive mass spectrometers, like Vocus-PTR, can be used in conjunction with EC calculations to measure ecosystem-scale VOC fluxes accurately, even for larger terpenes and without disturbing or stressing the environment. Such measurements appear particularly important with our finding that terpene emissions from boreal wetlands are very sensitive to temperature change. Conceivably, anthropogenic global warming can induce much higher BVOC emissions in the future. More studies will be necessary to understand the steep temperature dependence of terpene emissions in wetland ecosystems, including potential differences between specific wetland types and the underlying physiological mechanisms. Also, the data presented here contain only the period until the peak of the growing season. Longer-term studies covering multiple seasons, or better yet years, may be warranted. It should also be kept in mind that longer-term ecosystem responses to climatic changes will affect BVOC emissions.
The terpene flux data and other parameters used in this article are
available for download at
The supplement related to this article is available online at:
SieS, LV, and PM conceived and designed the study. LV carried out this work as part of his PhD thesis under the supervision of SieS. PR, TP, and MK provided necessary research infrastructure, including the Vocus-PTR. LV performed Vocus-PTR measurements, collected GC adsorbent tubes, carried out all analyses, interpreted the data, and wrote the paper. AB and PR helped in troubleshooting the Vocus-PTR. PR and SimS helped with calibration analysis. RS and ABG helped in interpreting the results and comparing the measured data with MEGAN. HY analyzed the GC adsorbent tubes. SieS revised the paper and supervised all experimental and analysis aspects. EM and EST provided the LAI and information about vegetation at Siikaneva. All authors contributed to the final draft.
The contact author has declared that none of the authors has any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We thank Heidi Hellén, Pontus Roldin, Robin Wollesen de Jonge, Lukas Fischer, Michael Boy, and Petri Clusius for valuable discussions. We highly appreciate the help and support provided by the SMEAR II station team, especially Matti Salminen and Lauri Ahonen, in setting up and maintaining our measurements in Siikaneva.
Lejish Vettikkat was financially supported by the UEF EPHB doctoral degree program. This work was financially supported by the Academy of Finland Flagship program (grant nos. 337550 and 337549) and Academy of Finland project no. 310682. Simon Schallhart was supported by Academy of Finland project no. 323255. Tuukka Petäjä was supported by Academy of Finland project nos. 1325681 and 1328616. Elisa Männistö and Eeva-Stiina Tuittila were supported by Academy of Finland project no. 330840. Roger Seco was supported by a Ramón y Cajal grant (RYC2020-029216-I) funded by MCIN/AEI/10.13039/501100011033 and by “ESF Investing in your future”, as well as project PID2021-122892NA-I00 funded by MCIN/AEI and by “ERDF A way of making Europe”. IDAEA-CSIC is a Severo Ochoa Centre of Research Excellence (MCIN/AEI, Project CEX2018-000794-S). Alex B. Guenther was supported by the US National Science Foundation award ANS-2041250.
This paper was edited by Tao Wang and reviewed by two anonymous referees.