Toward using atmospheric carbon dioxide observations to estimate the biospheric carbon flux of the Swiss Plateau

The lack of knowledge of the terrestrial carbon cycle translates into uncertain climate change projections due to the strong, yet uncertain relationship between terrestrial carbon fluxes and climate. Atmospheric carbon observations (defined here as the sum of CO2, CH4, and CO) integrate information on carbon fluxes on Earth’s surface, which can be used to assess the understanding of these processes through inverse carbon flux modeling. Due to the high spatial and temporal variability of terrestrial carbon fluxes, atmospheric observations representative of the regional scale (∼10–1000 km) are necessary to better understand these fluxes. My contributions have aided the goal of development of a comprehensive carbon observation and modeling system. This system aims to better quantify and understand natural fluxes and anthropogenic emissions over the Swiss Plateau, as part of the CarboCount project, using observations from its newly established network of four sites: Beromünster, Früebüel, Gimmiz, and Lägern-Hochwacht. The accurate measurement of atmospheric carbon is a prerequisite of inverse carbon flux modeling. Therefore, reference gas cylinders were prepared, their mixing ratios referenced to the respective international scales, and distributed to the observation sites, in order to complete site construction. The simple measurement system at Lägern-Hochwacht has, except for a single pump failure and an extended failure to store data, successfully operated since August 2012 and delivered data sets with a temporal coverage of more than 97 %. Therefore, a detailed description of its instrumentation is presented. The measurements of all four sites were repeatedly evaluated for quality with the reference gases. The uncertainty assessment of these measurements and their temporal aggregation are presented and discussed. Finally, gathered experience is offered for the future long-term, cost-effective monitoring efforts of atmospheric carbon. The value of an atmospheric carbon monitoring site to regional-scale studies depends on the information contained in its observations. The seasonal and diurnal meteorological variability of the lower troposphere play an important role in determining this information. Therefore, the characteristics relevant to forthcoming regional scale modeling studies of the established observation sites were investigated, and strengths and weaknesses of the site’s data sets as well as the employed atmospheric transport models are discussed. Due to local topography, tower characteristics, and site elevation, among others, observations differ between sites, especially during nighttime. However, these observations differ much less during daytime, owing to intense daytime vertical mixing. From the analysis of simulated surface sensitivity, it is likely that atmospheric carbon observations consistently representative of the Swiss Plateau are provided by Beromünster and LägernHochwacht. On the other hand, temporal filtering of the observations from Gimmiz and Früebüel is likely necessary to remove local influence, due likely to the model’s inability to accurately represent transport in the lower troposphere. Furthermore, due to the complex terrain directly surrounding Lägern-Hochwacht, similar filtering of its observations may be also necessary. These observations contain much information reflecting the complexity of the Swiss Plateau landscape, and will assuredly provide a challenge for future inverse modeling studies. To constrain biospheric carbon fluxes with carbon dioxide CO2 observations, it is necessary to first isolate the biospheric component of these observations. The standard procedure to accomplish this entails using an