Remote sensing measurements of methane in the Arctic

Methane (CH₄) is the second-largest contributor to global warming, and plays an important role in the global carbon cycle. Atmospheric CH₄ concentrations have more than doubled since industrialization, and more than half of the global CH₄ emissions originate from human-made sources such as agriculture, waste processing, and the fossil fuel industry. The Arctic has been warming faster than the rest of the Earth, leading to increased concern about potential CH₄ emissions from the Arctic's large permafrost regions, which store enough carbon to more than double the amount currently present in the atmosphere. Monitoring of atmospheric CH₄ is possible using remote sensing systems. These systems allow indirect quantification of atmospheric CH₄ by measuring (reflected) sunlight. In this thesis, ground-based remote sensing measurements from TCCON and space-based remote sensing measurements made by the TROPOMI onboard Sentinel-5P are examined, with a focus on the Arctic region. Both datasets were investigated regarding a range of potential Arctic-specific issues. For the WFMD data, issues with the cloud filter, especially over the Arctic Ocean, were identified, and it was shown that the use of outdated or inaccurate digital elevation model data in the WFMD retrieval led to significant biases. It was furthermore shown that airmass-dependent biases are present in TCCON XCH₄ during polar vortex conditions, and that these biases can be reduced by improving the CH₄ prior profiles. Both datasets were then compared at the four Arctic TCCON sites, with overall good agreement. Following the assessment of data quality, both WFMD and TCCON data were used to calculate growth rates. To achieve this, an approach based on dynamic linear models was developed that can handle the inhomogeneous data coverage in the Arctic. First, CH₄ growth rates were calculated for global data and for zonal bands using WFMD data. Subsequently, growth rates were also derived for four Arctic TCCON sites and compared to satellite-derived growth rates. High-latitude growth rates did not differ significantly from those in the mid-latitudes.