Derivation of Tropospheric Methane from TCCON CH4 and HF Total Column Observations

The Total Carbon Column Observing Network (TCCON) is a global ground-based network of Fourier transform spectrometers that produce precise measurements of column averaged dry-air mole fractions of methane (CH₄). In addition to dynamical transport and local sources and sinks, temporal variability in the total column of CH₄ is caused by the shift in tropopause height. We remove the variability caused by changes in tropopause height from the total column average by subtracting an estimate of the stratospheric CH₄, derived from simultaneous measurements of hydrogen fluoride (HF). HF provides a robust proxy for stratospheric CH₄ because it resides solely in the stratosphere, has a nearly linear inverse relationship with stratospheric CH₄, and is measured at most TCCON stations. The stratospheric partial column of CH₄ is calculated as a function of the zonal and annual trends in the relationship between CH₄ and HF in the stratosphere, which we determine from ACE-FTS data. We also explicitly take into account the CH₄ column averaging kernel and the a priori profiles of HF and CH₄. Using this technique, tropospheric methane mole fractions can be used to assess seasonal and long-term trends and verify chemical transport models.