Stratospheric CH4 isotopes

This article is simply an attempt to discuss the proposed observation altitudes for MISO, a Laser Heterodyne Radiometer (LHR) satellite mission proposed to observe CH4 isotopes (\({}^{13}\)C in CH4 and \({}^{12}\)C in CH4) in the UTLS.

ADFA-UNSW are proposing 3 observation altitudes for the satellite, centred at 14, 18 and 22km, giving 4̃km vertical resolution. These altitudes are chosen from available absorption lines that provide sufficient absorption at higher altitudes without saturating at lower altitudes, as well as the available wavelength range for the detectors/lasers.

The question at the moment is whether these altitudes are sensible to include in the instrument simulator from a scientific perspective. This article will discuss some of the considerations for those choices. Some points up front:

  • We don’t know how strictly constrained the possible observation altitudes are.

  • This article therefore assumes that any observation range is possible.

  • We also do not fully know (remember?) the precision/accuracy of the proposed measurements with the LHR technique, but assume that they are insufficient to resolve the small isotopic signals in the troposphere.

  • We are concentrating only on \(\delta\)\({}^{13}\)C in CH\({}_{4}\), not on \(\delta\)D (i.e. CH\({}_{3}\)D).

  • It would be instructive if a calculation of the expected averaging kernels could be provided given the satellite observing geometry, instrument resolution, and selected lines. Have the proposed observing altitudes been selected on the basis of such calculations?

Background on methane and its isotopes in the atmosphere

Troposphere

The troposphere is well-mixed, and the CH\({}_{4}\) mole fraction is determined by the balance between its sources and sinks. Methane is a relatively long-lived gas, meaning that its tropospheric mole fractions are quite stable, with variations arising from seasonal changes in sources and sinks, and a long-term temporal trend from an imbalance between the production and loss. These signals are relatively small compared to the mole fraction.

The isotopic signatures in CH\({}_{4}\) in the troposphere are governed by the contributing processes of the sources and the sinks, including their kinetic fractionation effects. Biogenic sources of CH\({}_{4}\) have an isotopic \(\delta\)\({}^{13}\)CH\({}_{4}\) of approximately -60‰compared to nonbiogenic sources, which are relatively less depleted with \(\delta\)\({}^{13}\)CH\({}_{4}\) around -40‰. Biogenic sources include wetlands, rice paddies, termites, and ruminant animals, while nonbiogenic sources of CH\({}_{4}\) include gas venting/leakage and coal mining (Tyler 2007).

The background atmospheric \(\delta\)\({}^{13}\)CH\({}_{4}\) is measured at a subset of the background surface in situ observation sites, such as Cape Grim (Australia), Mauna Loa, South Pole, Mace Head and others. The variability is small, with values typically varying by comfortably less than 0.5‰with season, and an interhemispheric gradient of approximately 0.2‰. The detection of isotopic signatures in the troposphere therefore places high demands on measurement precision and accuracy, to levels of performance beyond current and short-term future remote sensing capabilities.

Stratosphere

In contrast, in the stratosphere the mole fraction of CH\({}_{4}\) changes rapidly with altitude. The only stratospheric methane source is transport from the troposphere. This occurs mainly through the upwelling branch of the Brewer-Dobson circulation in the tropics. In the absence of local sources, CH\({}_{4}\) mole fractions decrease rapidly with altitude as a result of removal via chemical reaction with OH, O(\({}^{1}\)D) and Cl.

CH\({}_{4}\) + OH \(\rightarrow\) CH\({}_{3}\) + H\({}_{2}\)O

CH\({}_{4}\) + O(\({}^{1}\)D) \(\rightarrow\) OH + CH\({}_{3}\)

CH\({}_{4}\) + Cl \(\rightarrow\) HCl + CH\({}_{3}\)

The first of these means that methane oxidation is a source of water vapour in the stratosphere, and therefore impacts on moistening/drying processes in the UTLS.

Concentrations decrease from approximately 1800 nmolmol\({}^{-1}\) at the tropopause via these reactions. There are differences in rate constants between \({}^{13}\)CH\({}_{4}\) and \({}^{12}\)CH\({}_{4}\) for these reactions, resulting in kinetic isotope effects (KIE). A lower zero point energy for the larger molecular mass isotope results in a lower rate constant for its reaction. The lighter isotope therefore reacts more quickly, resulting in an isotopic enrichment in the remaining stratospheric CH\({}_{4}\). Consequently, the \(\delta\)\({}^{13}\)C in CH\({}_{4}\) increases with decreasing CH\({}_{4}\) mole fraction and increasing altitude. Röckmann et al. (2011) show this with high-altitude balloon measurements of methane and its isotopic composition. Rather than the sub permille variability seen through most of the well-mixed troposphere around values of approximately -47‰, \(\delta\)\({}^{13}\)C in CH\({}_{4}\) in the stratosphere increases with height, with some values reaching higher than -20‰.