Abstract
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 CH4 due to stratospheric dynamics obscures fluctuations and trends driven by tropospheric transport and local surface fluxes that are critical for understanding CH4 sources and sinks. We reduce the contribution of stratospheric variability from the total column average by subtracting an estimate of the stratospheric CH4 derived from simultaneous measurements of hydrogen fluoride (HF). HF provides a proxy for stratospheric CH4 because it is strongly correlated to CH4 in the stratosphere, has an accurately known tropospheric abundance (of zero), 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 satellite data. We also explicitly take into account the CH4 column averaging kernel to estimate the contribution of stratospheric CH4 to the total column. The resulting tropospheric CH4 columns are consistent with in situ aircraft measurements and augment existing observations in the troposphere.
Highlights
The most abundant hydrocarbon in the atmosphere, methane (CH4) is a driver of background tropospheric chemistry and a significant radiative forcing gas
To determine the stratospheric CH4 component of the Fourier transform spectrometers (FTS)-retrieved total column, we propose to use its relationship with hydrogen fluoride (HF), which is measured at almost all Total Carbon Column Observing Network (TCCON) sites
Vertical profiles of CH4 and HF mole fractions were developed from level 2, version 3.0 and 3.5 retrievals from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument on the Canadian SCISAT1 satellite
Summary
The most abundant hydrocarbon in the atmosphere, methane (CH4) is a driver of background tropospheric chemistry and a significant radiative forcing gas. Sepúlveda et al (2012) use the retrieval algorithm PROFFIT to infer vertical CH4 profiles directly from the absorption line shapes of the midinfrared (MIR) FTS spectra measured within NDACC, comparing the resulting tropospheric columns with those calculated with a HF proxy method. Quantifying the variability of stratospheric CH4 via a chemical tracer is, not without challenge, as this method is sensitive to errors in the representation of the relationship between that tracer and CH4 in the stratosphere and knowledge of their respective averaging kernels This method provides no information about vertical structure within the troposphere. Where cCtroHp4 represents the pressure-weighted DMF averaged over the tropospheric column, u is a unity vector the length of the number of vertical levels in the total column retrieval integration, and β is the time-dependent CH4–HF slope in the stratosphere. Equation (6) can be applied to determine tropospheric DMFs of trace gases other than CH4 that are correlated with HF in the stratosphere because it does not require assumptions about the relationship between the averaging kernels of the respective gases and is a more general approach than that of Washenfelder et al (2003)
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