Reactive trace gas exchanges at a mixed forest site

2023-2024
On a global scale, forests are the largest source of volatile organic compounds (VOCs). How the exchange of these compounds between ecosystems and the atmosphere will adapt in a world prone to more extreme weather is not well known.

In order to obtain a more holistic view of the bi-directional exchange between the forest (and its components) and the atmosphere, 3 years of extensive VOC measurements in and above a forest in the Belgian Ardennes have been performed.

Scientific context

Volatile organic compounds are key constituents of the atmosphere with direct and indirect impacts on both climate and air quality. 

Even though land-based ecosystems are the largest source of VOCs on a global scale (90%), exchanges between the atmosphere and the forest and its components are not fully understood. Insights into these interactions are increasingly relevant to estimate how they will change in a disrupted climate. 

To respond to the need of long-term datasets of ecosystem-level exchanges of reactive trace gases, BIRA-IASB, in collaboration with the University of Liège, has performed three campaigns characterizing the exchanges and their seasonal evolution at a mixed forest site located in the Belgian Ardennes from 2022 to 2024.

Measurement site

The Vielsalm class-2 ecosystem site within the Integrated Carbon Observation System (ICOS) network was equipped with a proton-transfer-reaction time-of-flight mass-spectrometry instrument (PTR-ToF-MS) to characterize VOCs in the atmosphere. 

Air was sequentially sampled at the top, at the base, and at 5 intermediate sampling heights along a 51 m high flux tower. 

Concentrations were measured at high frequency (10 Hz) and co-located sonic anemometers at the top and base of the tower allowed for VOC flux determination using the eddy-covariance method. Currently, a database containing VOC concentrations and fluxes for all three years is being finalized.

First results

Daily analysis of the PTR-ToF-MS spectra have resulted in the automatic detection of around 350 to 1400 mass peaks. The determination of stable mass peaks occurring consistently in the spectra throughout the campaigns resulted in a final list of about 80 VOC-related ion masses for further analysis. 

From preliminary calculations, we found that 18 ion masses accounted for 90% of the upward fluxes (forest to atmosphere; emissions). In contrast, 27 ion masses were needed to represent 90% of the downward fluxes (deposition) towards the forest (Fig. 1). The 13 most exchanged masses with their most likely associated chemical compounds are shown in Fig. 2. 

Seasonal VOC fluxes show high emissions of well-known biogenic VOCs (isoprene and monoterpenes) sharply increasing in spring with a peak during the summer months. Bi-directional exchanges of low molecular mass oxygenated VOCs were seen to be linked with relative humidity. Emission bursts were observed for a selection of compounds after bud break and a hailstorm. 

The data is expected to provide insight into how physico-chemical processes occurring in the soil-canopy-atmosphere continuum affect the observed fluxes.

References

  • C. Dumont, B. Verreyken, N. Schoon, C. Amelynck, and B. Heinesch. "From forest to atmosphere: towards a more comprehensive assessment of BVOC exchanges in a mixed temperate forest." Presentation presented at ICOS Science Conference 2024, Versailles, France, 10 September 2024. url: https://hdl.handle.net/2268/323526
     
  • B. Verreyken, N. Schoon, C. Dumont, B. Heinesch, and C. Amelynck. "Volatile Organic Compounds: Concentrations and Fluxes at a Belgian Mixed Forest Site." Poster session presented at iCACGP-IGAC 2024, Kuala Lumpur, Malaysia, 10 September 2024. url: https://hdl.handle.net/2268/322091
Mixed forest ecosystem with the tower infrastructure above the canopy
The mixed forest ecosystem in Vielsalm with the tower infrastructure reaching above the canopy. Photo by Jean-Louis Wertz, JLWERTZ.BE
Unaccounted VOC exchanges in function of ion masses
Figure 1. Unaccounted VOC exchanges [% of flux in µg m-2 s-1] emitted from (green), or deposited to (blue), the mixed forest ecosystem located in Vielsalm in function of number of ion masses considered. When taking only 10 masses into account, we are able to represent about 85% (80%) of the emission (deposition) observed with the measurement system put in place. Figure from C. Dumont et al., 2024.
Monthly median diel flux profiles of the 13 most exchanged VOCs
Figure 2. Monthly median diel flux profiles of the 13 most exchanged VOCs (data from 2022 and 2023). Figure adapted from B. Verreyken et al., 2024.