Reduction of airmass-dependent biases in TCCON XCH₄ retrievals during polar vortex conditions

Abstract. Trace gas measurements from the Total Carbon Column Observing Network (TCCON) are important for monitoring the global climate system and for validating satellite measurements. In the Arctic, ground-based data coverage is relatively limited due the inherent challenges of conducting measurements in this region (e.g., remoteness, harsh weather) and the polar nights, which prevent solar absorption measurements for half of the year. TCCON measurements from the Arctic sites are of significant value for the validation of satellite data products in this region, as these measurements can extend the spatio-temporal coverage in the Arctic. In this study, we investigate the TCCON methane (CH₄) retrieval under polar vortex conditions. The CH₄ profile exhibits a distinct shape inside the vortex, which is related to the descent of stratospheric air inside the vortex. We show that the standard TCCON CH₄ prior does not sufficiently reproduce this profile shape, leading to airmass dependencies (AMDs), increased spectral residuals and less sensitive averaging kernels. These effects can be explained by the fact that TCCON uses a profile scaling retrieval (PSR) where the prior shape is fixed and only a scaling factor is retrieved. We further show that changes in the prior can improve the retrieval within the polar vortex. This leads to mean differences between 1 and 2 ppb in XCH₄ compared to the standard retrieval, and maximum differences up to roughly 17 ppb. This manuscript highlights the importance of understanding the limitations of retrieval methods to avoid misinterpretation of data. Furthermore, it emphasizes the need to investigate the shape of trace gas profiles inside the polar vortex to improve PSR in the Arctic, which could include in situ data campaigns focusing on inside-vortex air.