Ground-based high-resolution remote sensing of sulphur hexafluoride (SF) over Hefei, China: characterization, optical misalignment, influence, and variability.

It is a challenge to retrieve atmospheric sulphur hexafluoride (SF) with high resolution solar spectra because it has only one single retrieval micro window and has interfered with many factors in the retrieval. Optical misalignment is one of the key factors that affect the accuracy of SF retrieval. In this study, we first present a long term time series of the SF total column over Hefei, China, between January 2017 and December 2020, retrieved by mid-infrared (MIR) solar spectra recorded by ground-based high-resolution Fourier transform infrared spectroscopy (FTIR). The sensitivities of the total column, root mean square of fitting residual (RMS), total error budgets, degrees of freedom for signal (DOFs), and vertical mixing ratio (VMR) profile with respect to different levels of optical misalignment for SF retrieval were assessed. The SF total column is sensitive to optical misalignment. In order to avoid inconsistencies in the total column due to optical misalignment, we use the true instrumental line shape (ILS) derived from regular low-pressure HBr cell measurements to retrieve the time series of SF. The total column of SF over Hefei presents strong seasonal dependent features. The maximum monthly average value of (3.57 ± 0.21) × 10 moleculescm in summer is (7.60 ± 3.50) × 10 moleculescm (21.29 ± 9.80) % higher than the minimum monthly average value of (2.81 ± 0.14) × 10 moleculescm in winter. The annual average SF total columns in 2017-2020 are (3.02 ± 0.17), (3.50 ± 0.18), (3.25 ± 0.18), and (3.08 ± 0.16) × 10 moleculescm, respectively, which are close to each other. It indicates that SF total column over Hefei is stable in the past four years. Our study can improve the current understanding for ground-based high-resolution remote sensing of SF and also contribute to generate new reliable remote sensing data in this sparsely monitored region for investigations of climate change, global warming, and air pollution.