REAC-FORTE

By

Reactivity of Forest Air and Tree Emissions

The aim of the project is to identify with hydroxyl radical (OH) and ozone (O3) total reactivity measurements of ambient air as well as of tree emissions the fraction of unknown compounds in the air of boreal and alpine forested areas. This research will help to resolve the question of whether these compounds are emitted directly by trees or whether they are oxidation products of these emissions.

Motivation

Total hydroxyl radical (OH) loss rate measurements or OH reactivity measurements were performed first at the beginning of the 21st century (Kovacs and Brune, 2001) in order to improve photochemical models, which are often overestimating OH levels in the atmosphere when compared to observations. A few years later Di Carlo et al. (2004) found unknown reactivity from a forest site, i.e. reactivity that cannot be explained with the set of measured volatile organic compounds (VOCs) at the site. Since, many observations have been done during various campaigns. Yang et al. (2016) recently published a good review of available observations and Williams and Brune (2015) advocate a broad implementation of total OH reactivity measurements alongside VOCs measurement at monitoring stations.
In the boreal forest, studies by Sinha et al. (2010) and Nölscher et al (2012) revealed a large fraction of unexplained reactivity for given periods in summer months. Therefore Dr. Hellén from the Finnish Meteorological Institute designed the project Defining unknown reactivity in the ambient air of Boreal and Arctic environments (Academy of Finland, Academy Research Fellowship 275608).

REAC-FORTE (Academy of Finland, Academy Research Fellowship 307797) extends this research further introducing total ozone reactivity measurements (Matsumoto, 2014). These are more selective than OH reactivity measurements, providing an additional tool to identify the kind of compounds that generates unknown reactivity. We will investigate ambient air and tree emissions from the boreal forest in Finland and make a comparison with higher altitude alpine forests in Switzerland.

Figure 1: Left: OH reactivity instrument based on the CRM in the FMI container in Hyytiälä, Finland (Arnaud Praplan, CC BY-SA 4.0). Centre: Branch enclosure to measure spruce emissions (Arnaud Praplan, CC BY-SA 4.0). Right: GC/MS with on-line thermodesorption unit (Heidi Hellén, CC BY-SA 4.0).

 Methods

  • Comparative Reactivity Method: Total OH reactivity measurements (Sinha et al., 2008)
    This is an indirect method to measure OH reactivity, that is more portable than the original method. It is based on the difference of signal of a tracer (pyrrole, C4H5N, not present in ambient air) exposed to OH in zero and ambient air, alternatively. A gas chromatograph is used as a detector in our implementation, instead of a proton-transfer-reaction mass spectrometer (PTR-MS).
  • Total ozone reactivity
    Few studies have used ozone reactivity so far and the development of the method is an important part of the project.
  • Compound specific analysis
    • On-line monitoring
      Gas chromatographs coupled to mass spectrometers (GC/MSes) are used to measure VOCs as close as possible from the reactivity measurements. Additional ambient concentrations of reactive trace gases are retrieved whenever possible from the monitoring of the stations where the measurements are conducted.
    • Off-line sampling and chemical analysis
      If possible, additional off-line sampling on adsoprtion tubes or derivatization cartridges followed by analysis in the laboratory (by gas and/or liquid chromatography) will be used to determine the concentrations of compounds that are not measured with on-line GC/MSes.
  • Measurement of emissions with branch enclosures (Hakola et al., 2006)
    To measure tree emissions, branches are inserted in a Teflon enclosure, which is flushed with clean air.  The mixture of clean air and emitted compounds is then analyzed chemically.
  • Modelling
    A box model based on the reaction from the Master Chemical Mechanism will be used in order to 1) better understand the chemistry in the reactivity instruments and 2) to include in the calculated reactivity from expected compounds (e.g. oxidation products) that have/could not been measured in situ.

Sites

  • Hyytiälä, Finland
    The second Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II) is an important hub for research on atmosphere-biosphere interactions. It is a very well equipped and characterized site in the Finnish boreal forest.
  • Pallas, Finland
    This station in the Finnish Lapland is maintained by FMI. Measurements will be conducted at the forested Kenttärova site.
  • Rigi Beromünster, Switzerland
    This monitoring site is part of the Swiss monitoring network NABEL and is a (low altitude) rural background site. (Note: Originally, measurements were planned on Rigi, but EMPA moved the monitoring of VOCs to Beromünster.)
  • Davos, Switzerland
    This alpine site (also part of NABEL) is located in a higher altitude forest (roughly 1640m a.s.l.).
Figure 2: Measuring stations. From left to right: SMEAR II, Hyytiälä, Finland (Arnaud Praplan, CC BY-SA 4.0); Kenttärova, Finland (Annalea Lohila, CC BY-SA 4.0); Beromünster, Switzerland (Krol:k, CC BY-SA 3-0); Davos, Switzerland (Lino Schmid, CC BY-SA 4.0).

Collaborations

  • Waseda University, Tokyo, Japan
  • University of Colorado, Boulder CO, U.S.A.
  • Max Planck Institute for Chemistry, Mainz, Germany 
  • EMPA, Dübendorf, Switzerland

References

  • Di Carlo, P.; Brune, W. H.; Martinez, M.; Harder, H.; Lesher, R.; Ren, X.; Thornberry, T.; Carroll, M. A.; Young, V.; Shepson, P. B.; Riemer, D.; Apel, E. & Campbell, C. Missing OH Reactivity in a Forest: Evidence for Unknown Reactive Biogenic VOCs, Science, 2004, 304, 722-725, doi: 10.1126/science.1094392.
  • Hakola, H.; Tarvainen, V.; Bäck, J.; Ranta, H.; Bonn, B.; Rinne, J. & Kulmala, M. Seasonal variation of mono- and sesquiterpene emission rates of Scots pine, Biogeosciences, 2006, 3, 93-101, doi: 10.5194/bg-3-93-2006.
  • Kovacs, T. A. & Brune, W. H. Total OH Loss Rate Measurement, J. Atmos. Chem., 2001, 39, 105-122, doi: 10.1023/A:1010614113786.
  • Matsumoto, J. Measuring Biogenic Volatile Organic Compounds (BVOCs) from Vegetation in Terms of Ozone Reactivity, Aerosol Air Qual. Res., 2014, 14, 197-206, 10.4209/aaqr.2012.10.0275.
  • Nölscher, A. C.; Williams, J.; Sinha, V.; Custer, T.; Song, W.; Johnson, A. M.; Axinte, R.; Bozem, H.; Fischer, H.; Pouvesle, N.; Phillips, G.; Crowley, J. N.; Rantala, P.; Rinne, J.; Kulmala, M.; Gonzales, D.; Valverde-Canossa, J.; Vogel, A.; Hoffmann, T.; Ouwersloot, H. G.; Vilà-Guerau de Arellano, J. & Lelieveld, J. Summertime total OH reactivity measurements from boreal forest during HUMPPA-COPEC 2010, Atmos. Chem. Phys., 2012, 12, 8257-8270, doi: 10.5194/acp-12-8257-2012.
  • Sinha, V.; Williams, J.; Crowley, J. N. & Lelieveld, J. The Comparative Reactivity Method -- a new tool to measure total OH Reactivity in ambient air, Atmos. Chem. Phys., 2008, 8, 2213-2227, doi: 10.5194/acp-8-2213-2008.
  • Sinha, V.; Williams, J.; Lelieveld, J.; Ruuskanen, T.; Kajos, M.; Patokoski, J.; Hellen, H.; Hakola, H.; Mogensen, D.; Boy, M.; Rinne, J. & Kulmala, M. OH Reactivity Measurements within a Boreal Forest: Evidence for Unknown Reactive Emissions, Environ. Sci. Technol., 2010, 44, 6614-6620, doi: 10.1021/es101780b.
  • Williams, J. & Brune, W. A roadmap for OH reactivity research, Atmos. Environ., 2015, 106, 371-372, doi: 10.1016/j.atmosenv.2015.02.017.
  • Yang, Y.; Shao, M.; Wang, X.; Nölscher, A. C.; Kessel, S.; Guenther, A. & Williams, J. Towards a quantitative understanding of total OH reactivity: A review, Atmos. Environ., 2016, 134, 147-161, doi: 10.1016/j.atmosenv.2016.03.010.