Open Access

Petra Hausmann

• Mitigation of climate change demands a high priority and requires a sustained reduction of anthropogenic greenhouse gas emissions. In the Earth's atmosphere, greenhouse gases substantially absorb infrared radiation by molecular transitions to higher-energy vibration and rotation states. If atmospheric concentrations of these trace gases increase, the amount of radiation energy lost to space is diminished, resulting in global warming. To develop effective strategies for emission reduction, it is essential to gain an accurate understanding of greenhouse gas budgets, trends, and atmospheric transport processes. In this context, the present dissertation contributes to improving our knowledge with regard to two major greenhouse gases – water vapor and methane. Investigations performed focus on the longterm monitoring of these trace gases by means of solar absorption spectrometry at the high-altitude observatory Zugspitze (47.42° N, 10.98° E; 2964 m ü. NN). The global concentration ofMitigation of climate change demands a high priority and requires a sustained reduction of anthropogenic greenhouse gas emissions. In the Earth's atmosphere, greenhouse gases substantially absorb infrared radiation by molecular transitions to higher-energy vibration and rotation states. If atmospheric concentrations of these trace gases increase, the amount of radiation energy lost to space is diminished, resulting in global warming. To develop effective strategies for emission reduction, it is essential to gain an accurate understanding of greenhouse gas budgets, trends, and atmospheric transport processes. In this context, the present dissertation contributes to improving our knowledge with regard to two major greenhouse gases – water vapor and methane. Investigations performed focus on the longterm monitoring of these trace gases by means of solar absorption spectrometry at the high-altitude observatory Zugspitze (47.42° N, 10.98° E; 2964 m ü. NN). The global concentration of methane – the second most important anthropogenic greenhouse gas – has more than doubled since preindustrial times. After a period of near-zero growth (1999–2006), a renewed methane increase has been observed since 2007. The source attribution of these trends is highly complex due to the large variety of methane sources, which have either natural (e.g., wetlands, geologic seepage, forest fires) or anthropogenic origins (e.g., fossil fuel production, livestock farming, rice cultivation). Recent studies provide evidence that the renewed methane increase is primarily driven by increasing emissions from natural wetlands and from the production of fossil fuels. However, the relative contribution of these two drivers has not yet been accurately quantified. In the present dissertation, an innovative approach is employed to estimate the contribution of oil and natural gas emissions to the renewed methane increase. For this purpose, long-term trends of methane are assessed in relation to time series of ethane – an indicator for fossil-fuel methane emissions. Methane trend analysis requires highly precise measurements due to the low variability of column-averaged methane. In this context, a novel correction method is applied to eliminate errors that are caused by inaccuracies in the alignment of the spectrometer to the solar disk. The resulting methane and ethane trends at Mt. Zugspitze and at a reference site in the Southern Hemisphere are simulated with the help of an atmospheric two-box model developed within the scope of this dissertation. This model enables the assignment of observed trends to optimized emission scenarios. Simulations performed reveal that oil and natural gas emissions have significantly contributed by at least 39% to the renewed methane increase since 2007. In addition to methane, water vapor plays a key role in the Earth's energy budget. Water vapor is not only the dominant natural greenhouse gas, but also enhances anthropogenic climate forcing due to the positive feedback between water vapor concentration and air temperature. Under changing climate conditions, patterns of atmospheric circulation are expected to be modified as well. Valuable information on the underlying transport processes can be obtained from observations of water vapor and its isotopic composition. In that context, the present dissertation investigates to what extent water vapor isotope measurements at Mt. Zugspitze provide further insight into transport processes to Central Europe. Based on extensive backward trajectory simulations, different moisture transport patterns to the Central European free troposphere are identified for distinct water vapor isotopic compositions. In addition to the transport of water vapor, long-range transport processes are responsible for the import of various trace species (e.g., ozone, aerosols) to Central Europe and, therefore, affect regional climate, air quality and human health. Significantly different signatures in the distribution of water vapor isotope measurements are found for the long-range transport categories considered: stratospheric intrusions, intercontinental transport from North America, and transport from Northern Africa. For the successful validation of this trajectory-based result, lidar and in situ measurements are analyzed, which reveal transport events reaching the Northern Alps. These findings highlight the potential of water vapor isotope measurements for a systematic recording and investigation of atmospheric transport processes. This is of particular importance to enhance our understanding of climate-induced changes in the global water cycle, as well as to control international treaties on emission regulations. Consequently, results achieved within the scope of this dissertation can contribute to define specific steps for the implementation of the Paris Climate Agreement, which was ratified in 2016.…