Derivation of tropospheric methane from TCCON CH4and HF total column observations

Abstract

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 atmospheric methane (CH4). Temporal variability in the total column of CH4due to stratospheric dynamics obscures fluctuations and trends driven by tropospheric transport and local surface fluxes that are critical for understanding CH4sources and sinks. We reduce the contribution of stratospheric variability from the total column average by subtracting an estimate of the stratospheric CH4derived from simultaneous measurements of hydrogen fluoride (HF). HF provides a proxy for stratospheric CH4because it is strongly correlated to CH4in the stratosphere, has an accurately known tropospheric abundance (of zero), and is measured at most TCCON stations. The stratospheric partial column of CH4is calculated as a function of the zonal and annual trends in the relationship between CH4and HF in the stratosphere, which we determine from ACE-FTS satellite data. We also explicitly take into account the CH4column averaging kernel to estimate the contribution of stratospheric CH4to the total column. The resulting tropospheric CH4columns are consistent with in situ aircraft measurements and augment existing observations in the troposphere.