Matthias J. Zeeman

Summertime elemental mercury exchange of temperate grasslands on an ecosystem-scale

In order to estimate the air-surface mercury exchange of grasslands in temperate climate regions, fluxes of gaseous elemental mercury (GEM) were measured at two sites in Switzerland and one in Austria during summer 2006. Two classic micrometeorological methods (aerodynamic and modified Bowen ratio) have been applied to estimate net GEM exchange rates and to determine the response of the GEM flux to changes in environmental conditions (e.g. heavy rain, summer ozone) on an ecosystem-scale. Both methods proved to be appropriate to estimate fluxes on time scales of a few hours and longer. Average dry deposition rates up to 4.3 ng m-2 h-1 and mean deposition velocities up to 0.10 cm s-1 were measured, which indicates that during the active vegetation period temperate grasslands are a small net sink for atmospheric mercury. With increasing ozone concentrations depletion of GEM was observed, but could not be quantified from the flux signal. Night-time deposition fluxes of GEM were measured and seem to be the result of mercury co-deposition with condensing water. Effects of grass cuts could also be observed, but were of minor magnitude.

Continuous field measurements of δ13C–CO2 and trace gases by FTIR spectroscopy

Continuous analysis of the 13C/12C ratio of atmospheric CO2 (δ13C–CO2) is a powerful tool to quantify CO2 flux strengths of the two major ecosystem processes assimilation and respiration. Traditional laboratory techniques such as isotope ratio mass spectrometry (IRMS) in combination with flask sampling are subject to technical limitations that do not allow to fully characterising variations of atmospheric δ13C–CO2 at all relevant timescales. In our study, we demonstrate the strength of Fourier transform infrared (FTIR) spectroscopy in combination with a PLS-based calibration strategy for online analysis of δ13C–CO2 in ambient air. The ability of the instrument to measure δ13C–CO2 was tested on a grassland field-site and compared with standard laboratory-based IRMS measurements made on field-collected flask samples. Both methods were in excellent agreement, with an average difference of 0.4‰ (n=81). Simultaneously, other important trace gases such as CO, N2O and CH4 were analysed by FTIR spectroscopy.

Concurrency of Coherent Structures and Conditionally Sampled Daytime Sub-canopy Respiration

We investigated an alternative means for quantifying daytime ecosystem respiration from eddy-covariance data in three forests with different canopy architecture. Our hypothesis was that the turbulent transport by coherent structures is the main pathway for carrying detectable sub-canopy respiration signals through the canopy. The study extends previously published work by incorporating state-of-the-art wavelet decomposition techniques for the detection of coherent structures. Further, we investigated spatial and temporal variability of the respiration signal and coherent exchange at multiple heights, for three mature forest sites with varying canopy and terrain properties for one summer month. A connection between the coherent structures and identified sub-canopy respiration signal was clearly determined. Although not always visible in signals collected above the canopy, certain cases showed a clear link between conditionally sampled respiration events and coherent structures. The dominant time scales of the coherent structure ejection phase (20–30 s), relative timing of maximum coincidence between respiration events and the coherent structure ejection phase (at approximately −10 s from detection) and vertical transport upward through the canopy were shown to be consistent in time, across measurement heights and across the different forest sites. Best results were observed for an open canopy pine site. We conclude that the presented method is likely to be applicable at more open rather than dense (closed) canopies. The results provided a confirmation of the connection between below- and above-canopy scalar time series, and may help the development or refinement of direct methods for the determination of component fluxes from observations above the canopy.

Optimization of automated gas sample collection and isotope ratio mass spectrometric analysis of δ13C of CO2 in air

The application of (13)C/(12)C in ecosystem-scale tracer models for CO(2) in air requires accurate measurements of the mixing ratios and stable isotope ratios of CO(2). To increase measurement reliability and data intercomparability, as well as to shorten analysis times, we have improved an existing field sampling setup with portable air sampling units and developed a laboratory setup for the analysis of the delta(13)C of CO(2) in air by isotope ratio mass spectrometry (IRMS). The changes consist of (a) optimization of sample and standard gas flow paths, (b) additional software configuration, and (c) automation of liquid nitrogen refilling for the cryogenic trap. We achieved a precision better than 0.1 per thousand and an accuracy of 0.11 +/- 0.04 per thousand for the measurement of delta(13)C of CO(2) in air and unattended operation of measurement sequences up to 12 h.

Montane ecosystem productivity responds more to global circulation patterns than climatic trends

Regional ecosystem productivity is highly sensitive to inter-annual climate variability, both within and outside the primary carbon uptake period. However, Earth system models lack sufficient spatial scales and ecosystem processes to resolve how these processes may change in a warming climate. Here, we show, how for the European Alps, mid-latitude Atlantic ocean winter circulation anomalies drive high-altitude summer forest and grassland productivity, through feedbacks among orographic wind circulation patterns, snowfall, winter and spring temperatures, and vegetation activity. Therefore, to understand future global climate change influence to regional ecosystem productivity, Earth systems models need to focus on improvements towards topographic downscaling of changes in regional atmospheric circulation patterns and to lagged responses in vegetation dynamics to non-growing season climate anomalies.