Investigation of natural halogenated volatile organic compounds in the Amazon rainforest
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Abstract
Halogenated volatile organic compounds (XVOCs) form a compound class that impacts our atmosphere on many different scales. From a global perspective, the most important aspects of XVOCs are their ability to deplete stratospheric ozone and their global warming potential. While the most dangerous compounds in this respect, chlorofluorocarbons (CFCs), are exclusively of anthropogenic origin (i.e. produced by human activities) and their production has now been banned, the focus of research has shifted to naturally formed XVOCs.
The Amazon rainforest is of great importance for natural XVOCs in several respects: first, tropical plants are considered the largest source of some XVOCs, including chloromethane (CH3Cl), the most important naturally formed XVOC. Secondly, the XVOCs emitted in the tropics are particularly important as they can reach the upper troposphere and subsequently the stratosphere relatively quickly due to frequent deep convection. This makes them relatively more harmful for stratospheric ozone depletion than if emitted at higher latitudes. Thirdly, the fact that local anthropogenic influences and emissions can be excluded when measuring XVOCs in a remote location such as the Amazon rainforest is added to the reasons already mentioned. On one hand, this ensures that detected changes in XVOC abundances can be traced back to natural emissions or sinks. On the other hand, it enables monitoring of the atmospheric background of long-lived anthropogenic XVOCs such as CFCs and thus complements existing global monitoring networks such as those of the Advanced Global Atmospheric Gases Experiment (AGAGE) or the National Oceanic and Atmospheric Administration (NOAA).
In this dissertation, XVOCs in the Amazon rainforest ecosystem are considered from different perspectives. A first study investigated the triple-element stable isotope composition (²H, ¹³C, ³⁷Cl) of chloromethane formed and degraded by plants. Experiments with the royal fern (Osmunda regalis) provided the first complete isotopic fingerprint of chloromethane emissions of this kind. At the same time, isotopic analysis of chloromethane precursors relevant for formation by plants indicated minimal changes in hydrogen and chlorine isotopic ratios during formation, simplifying future isotopic predictions. This is an important finding as determining the isotopic composition of an XVOC offers the potential to differentiate between different sources and sinks and to quantify their respective strengths. In the case of chloromethane, there are large uncertainties in its global budget with reported sinks outweighing the sources, which indicates that the formation and degradation processes are not fully understood yet. This is supported by the fact that degradation experiments with a club moss (Selaginella kraussiana) revealed substantial isotopic fractionation across all three elements, suggesting a previously unrecognized biotic degradation mechanism. The findings from this study offer valuable insights for isotope-based models aimed at improving the accuracy of the global chloromethane budget.
The second study presented in this dissertation investigated two halogenated very short-lived substances (VSLSs), chloroform and bromoform, at the Amazon Tall Tower Observatory (ATTO) research site in central Amazonia. The underlying method was ambient air sampling on adsorbent-filled tubes and subsequent thermodesorption and analysis with a GC-ToF-MS setup. Chloroform abundances within the canopy showed comparable medians throughout three different seasons, with significant spikes in abundance observed during the transitional and wet seasons. Elevated tower measurements (80 m and 320 m) indicated that emissions follow a diel pattern, peaking at midday and decreasing overnight and that these emissions originate from ground level. Soil flux measurements confirmed the soil to be a chloroform source throughout all seasons. Bromoform levels displayed weaker local source indications compared to chloroform. Nonetheless, slight diel cycles and occasional spikes point to potential local bromoform sources. In summary, this study hypothesizes chloroform levels of the ambient air in the Amazon rainforest to be primarily influenced by local sources, while bromoform levels are more affected by long-range transport. The detected seasonal variations emphasize the need for further investigations on the impact of El Niño-Southern Oscillation (ENSO) anomalies on local emissions.
The third part of this dissertation describes the method development and resulting setup of a cryogen-free air preconcentration unit coupled to a GC-MS system at ATTO. This is the first attempt to implement such a setup in the middle of a tropical rainforest, and it serves several objectives. First, it enables the acquisition of long-term in situ XVOC data in the Amazon rainforest. Secondly, the abundance of a whole range of XVOCs at different heights of the ATTO tower can be measured, from ground level up to 320 m above ground. It therefore provides the potential to identify local sources and sinks, to determine diel and seasonal fluctuations, and to estimate the net fluxes of the Amazon rainforest ecosystem. Thirdly, the setup aims to complement campaign-based measurements of single XVOC sources and sinks. First results are presented as a proof of concept and method validation, potential sources of error are discussed in detail, and future modifications and applications are suggested.