Fluxes of BOVOC between mountain grassland ecosystems and the atmosphere

Volatile organic compounds (VOC), which have a decisive influence on the chemical and physical properties of the atmosphere and thus climate, are emitted through anthropogenic activities and the biosphere, the main source, however, is terrestrial vegetation. Historically, hydrocarbon compounds, such as methane and isoprenoids, are most intensively studied, disregarding the group of biogenic oxygenated VOCs (BOVOCs) which have been measured in surprising abundance throughout the remote troposphere. These BOVOCs include three compounds, methanol, acetaldehyde and acetone, whose combined biogenic source strength is estimated to amount to about half of the biogenic source strength of isoprene, which dominates the global biogenic VOC emissions. Nevertheless, global budgets of methanol, acetaldehyde and acetone, in particular biotic sources and sinks, are highly uncertain. The ultimate goal of the proposed project is to improve our understanding of methanol -, acetaldehyde - and acetone fluxes fromo vegetation. To this end a combined experimental and modelling study is carried out at a temperate mountain grassland site in Stubai Valley (Austria). The detailed objectives of the proposed project are: (i) To quantify season long ecosystem scale fluxes of methanol, acetaldehyde and acetone. With a few exceptions, VOC flux measurements are often confined to short intensive campaigns or at most to a single vegetation period. Such strategies cut off low frequency variability of VOC fluxes and impair our understanding of the magnitude and causes of the inter annual variability. To this end we propose to conduct VOC flux measurements for two more vegetation periods (i.e. in addition to our previous measurements during 2008 and 2009) in order to be able to start analysing inter annual variability. (ii) To quantify and analyse the contribution of ecosystem components, that is fromo the leaves of the various plant species and the soil, to the ecosystem scale BOVOC fluxes. To this end BOVOC flux measurements will be made on plant organs of the dominant plant species and, employing a novel non destructive method, fromo the soil, both in the field and in the laboratory. (iii) To scale up ecosystem component BOVOC flux measurements (ii) to the ecosystem level and to compare these with the ecosystem scale flux measurements (i) using a process oriented model of ecosystem scale BOVOC fluxes. This exercise is thought to represent the ultimate test of our process understanding regarding the biosphere- atmosphere exchange of these three important BOVOCs.

Land ecosystems represent a significant source of biogenic volatile organic compounds (BVOC) that by far exceeds human VOC sources. VOCs play a pivotal role in atmospheric chemistry and land ecosystems thus have the ability to modulate regional air chemistry and global climate. In a previous project we quantified which BVOC are exchanged between mountain grasslands and the atmosphere. Capitalising on these data, within this follow-up project we focussed on the major BVOC exchanged by grasslands, which are the so-called oxygenated biogenic volatile organic compounds (BoVOC). To this end, the objectives of the project were threefold: (1) To continue the measurements of BoVOC exchange at a managed temperate mountain grassland in order to be able to start addressing the magnitude of inter-annual variability and its drivers. (2) To disentangle the flux contributions by above- and below-ground ecosystem components to the total ecosystem-scale BoVOC exchange. (3) To integrate all these data and measurements into models which allow up-scaling this information. Ecosystem-scale BoVOC flux measurements were carried out during two full vegetation periods (2011 and 2012) using a proton-transfer-reaction mass spectrometer (PTR-MS) and the virtual disjunct eddy covariance method. Methanol was identified as being the BoVOC exhibiting the largest exchange flux at our study site, a managed mountain grassland in the Stubai Valley (Austria). The site was a source of methanol on an annual basis and the sub-diurnal, seasonal and inter-annual variability of the methanol exchange was well explained by stomatal conductance and changes in plant growth and environmental factors influencing these. The exchange of acetaldehyde and acetone was lower by a factor of 10 compared to methanol and in addition exhibited a much more bidirectional character. While the study site was a source for acetaldehyde on an annual basis in all four study years, it was a source for acetone in one year and a sink in the other three years. In contrast to methanol, modelling the exchange of acetaldehyde and acetone turned out to be much more difficult. Soil BVOC concentrations and exchange was quantified using a combination of buried membrane tubes, a time-of-flight PTR-MS and 222Rn diffusivity measurements in 2014. Soil measurements suggest an uptake of methanol and acetone, while no clear results were found for acetaldehyde. The results of this project will help to better understand the role of mountain grasslands in modulating atmospheric chemistry and thus regional air quality.