Derivation of tropospheric methane from TCCON CH<sub>4</sub> and HF total column observations

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 sources and sinks. We remove 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 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 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.

Intercomparability of X<sub>CO<sub>2</sub></sub> and X<sub>CH<sub>4</sub></sub> from the United States TCCON sites

“…The subscript s represents the surface. These assumptions are better for X CO 2 than for X CH 4 as changes in tropopause height can also make the a priori methane profile significantly different from the true profile (Saad et al, 2014). Over this time at Caltech, X HF averaged ∼ 50 ppt and γ HF averaged ∼ 0.87, suggesting an a priori tropopause height that is too low.…”

“…Over this time at Caltech, X HF averaged ∼ 50 ppt and γ HF averaged ∼ 0.87, suggesting an a priori tropopause height that is too low. Using the β value from Saad et al (2014) we estimate a 13 % difference in γ HF due to tropopause height would cause about a 0.24 % change in γ CH 4 (∼ 4 ppb), which is large enough that Eq. (2) is not valid for X CH 4 .…”

Seasonal Variability of Stratospheric Methane: Implications for Constraining Tropospheric Methane Budgets Using Total Column Observations

“…Fluctuations in stratospheric dynamics, including the height of the tropopause, change the contribution of the stratosphere to the total column. CH 4 profiles with similar tropospheric values can thus have significant differences in X CH 4 (Saad et al, 2014;Washenfelder et al, 2003;Wang et al, 2014).…”

Assessment of errors and biases in retrievals of X<sub>CO<sub>2</sub></sub>, X<sub>CH<sub>4</sub></sub>, X<sub>CO</sub>, and X<sub>N<sub>2</sub>O</sub> from a 0.5 cm<sup>–1</sup> resolution solar-viewing spectrometer

“…Here atmospheric profiles are generated from the NCEP/NCAR 2.5 • reanalysis product (Kalnay et al, 1996) by interpolating to the correct location at local noon of the desired day. These profiles also include the tropopause height which is used to vertically shift a priori profiles, as tropopause height can significantly affect column DMFs such as X CH 4 and X HF (Saad et al, 2014). Selecting a profile for an incorrect location or day could lead to errors.…”