Methane is an important greenhouse gas, with a global-warming potential of 72 over 20 years (Denman et al., 2007). In the troposphere, abundances of methane have increased since the Industrial Revolution, from mixing ratios of about 700 ppb in the 1800s to over 1700 ppb by the 1990s (Etheridge et al., 1998). From 1999 to 2006, methane levels remained stable, but have begun to increase again since 2007 (Terao et al., 2011). The mixing ratio of methane in the atmosphere is controlled by its sources and sinks. All sources of methane are from surface emissions, including wetlands (Bartlett and Harriss, 1993), ruminant livestock (Lassey, 2007), fossil fuel production (Kort et al., 2014), and biomass burning (Hao and Ward, 1993). Methane is primarily consumed by the OH radical in the troposphere but may also react with Cl and singlet O (O1D) or be destroyed by photolysis higher in the atmosphere. With a global lifetime of about 9 years (Denman et al., 2007), methane is well mixed in the troposphere but decreases rapidly with altitude in the stratosphere. The distribution of methane is also affected by atmospheric circulation patterns. One major pattern is Brewer–Dobson circulation, in which equatorial air rises through the tropopause, travels poleward in the stratosphere, then descends back into the troposphere at high latitudes and returns to the equator (Remsberg, 2015).

Knowing the relative strengths of the different sources and sinks of methane is crucial for understanding its atmospheric behavior. As these sources and sinks are subject to isotopic fractionation, measurement of the common stable isotopologues of methane (12CH4, 12CH3D, and 13CH4) gives more information about the origin of methane present in the atmosphere. Abundances of heavy isotopologues are typically reported using delta notation, where (for the case of carbon-13) δ13C=13C/12C13Cstd/12Cstd-1×1000‰. In this paper, δD will refer to the isotopologue CH3D, and δ13C will refer to 13CH4.

Different isotopologues of methane will react at different rates, a phenomenon known as the kinetic isotope effect (KIE). Because of this, the isotopic signature of methane in an air mass will change over time as methane is consumed. As is true of most molecules, the heavier isotopologues of methane react more slowly than unsubstituted methane, meaning δD and δ13C will increase as methane is consumed. KIEs are commonly reported as a ratio of rate constants: kD/kH and k12/k13 for methane. The KIEs of methane with OH, O (1D), and Cl at room temperature are listed in Table 2. Since each methane sink has a different KIE, the values of δD and δ13C can give some information about which species have reacted with methane.

As with several gases in the atmosphere, there is a strong inverse correlation between the total mixing ratio of methane and δD and δ13C. This relation was first noted by Keeling (1958) in samples of CO2, so a plot of [CH4] or [CH4]-1 vs. δ is often called a “Keeling plot”. This phenomenon has more recently been demonstrated by, e.g., Röckmann et al. (2011) for methane. Plotting [CH4]-1 vs. δ of a time series of measurements results in a ellipse rather than a straight line due to seasonal variation in the sources and sinks of methane (Allan et al., 2001; Lassey et al., 2011).

Measuring methane and its isotopologues is most commonly done in the troposphere. One large ground-based sampling program is the Global Greenhouse Gas Reference Network, overseen by NOAA's Earth System Research Laboratory (Andrews et al., 2014). Sampling higher in the troposphere is frequently done by aircraft such as the CARIBIC program (Brenninkmeijer et al., 2007) or by balloon flights. In the upper stratosphere, measurements are far less common as only balloons can reach this height for sampling (Röckmann et al., 2011). An alternative to direct sampling at this altitude is satellite-based remote sensing. Some satellite instruments point toward nadir including GOSAT (Yokota et al., 2009), TES onboard the Aura satellite (Wecht et al., 2012), and IASI on MetOp (Xiong et al., 2013). Others observe the limb of the atmosphere including MIPAS (Payan et al., 2009) and HALOE (Park, 2004). A few, such as SCIAMACHY on ENVISAT (Schneising et al., 2009) and TES can look in either direction. However, these satellite measurements do not include the heavy isotopes of methane, and many of them have limited vertical sampling or only measure the total column density.

In this paper we present data on methane and its two heavy isotopologues from the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS). Additionally, we performed a model run with Whole Atmosphere Climate Community Model for comparison to the data from ACE.

The ACE-FTS is an infrared spectrometer on board the Canadian satellite SCISAT. SCISAT was launched in 2003 and is still currently active. The satellite orbits the Earth at an inclination of 74∘ and at a height of 650 km. It makes measurements of the atmosphere via solar occultation in which sunlight is used as a light source as it passes through the atmosphere at sunrise and sunset. The satellite has an orbital period of 97.6 min, allowing it to make measurements about every 49 min. The latitude of the measurement tangent point varies slowly with time (Fig. 1) but accumulates a set of global measurements in about 2 months. The geographic measurement cycle repeats in latitude annually (with a small degree of slippage), which provides a framework for determining trends as a function of time.

The analysis of ACE-FTS spectra begins by deriving altitude profiles for pressure and temperature through the analysis of CO2 lines in the spectra, using an assumed profile for the CO2 volume mixing ratio (VMR) below 60 km (Boone et al., 2013). The VMR profile for a particular molecule or isotopologue of interest is then retrieved via the analysis of a set of microwindows: a collection of relatively small (typically less than 0.5 cm-1) portions of the spectrum containing spectral features primarily from the target atmospheric constituent. Spectroscopic data employed in the forward model calculations in version 3.5 processing come mostly from the HITRAN 2008 database (Rothman et al., 2009).

Retrievals of ACE-FTS methane data were obtained using the latest version 3.5 of the ACE-FTS software (Boone et al., 2013). These retrievals are performed in the same way as those from version 3.0 (Boone et al., 2005) but include a fix of a problem with pressure and temperature information obtained from the Canadian Meteorological Center for measurements after September 2010. Briefly, the retrieval employs a nonlinear least-squares global fitting approach, where the volume mixing ratio altitude profiles for the target molecule and all significant interferers (molecules or isotopologues other than the target that feature significant absorption within the set of microwindows) are determined simultaneously. No constraints from a priori information are employed in this process, and therefore no averaging kernels are produced.

The microwindows corresponding to the ν4, 2ν4, and ν3 vibrational bands of CH4 were used for these retrievals. The fundamental ν4 and ν3 bands are used at higher altitudes while the 2ν4 overtone band is used at lower altitudes where the fundamentals are saturated. A summary of the microwindows used in the current study is given in Table 1 while the full microwindow list is given in the Supplement.

Note that the microwindow set for the main isotopologue is different from that employed in normal version 3.5 processing. Preliminary results in the current study indicated problems in the main isotopologue results that yielded sharp, latitude-independent, systematic features in the derived δD and δ13C plots. Such features were also present in recently published ACE data of δ13C-CO (Beale et al., 2015). Therefore, a research product was generated for the molecule using a new microwindow set. This new set featured more microwindows between 40 and 50 km than the version 3.5 microwindow set, as well as more microwindows at low altitudes (below 25 km). Spectroscopic parameters (positions, line widths, pressure shifts, and intensities) for CH4 lines in the 2ν4 band were adjusted to improve fitting residuals (which are poor when using existing spectroscopic data for these lines) and to improve the internal consistency between the intensities of lines in this band and lines in the other two bands employed in the retrieval. These sharp features are still present in the data presented here, but their magnitudes are greatly reduced. Some further discussion of these adjustments are given in Appendix A.

For the main isotopologue of methane, random errors from the least-squares fitting process are in the range of 2 to 6 % over the range of altitudes employed in the current study. For 13CH4, fitting errors range from 2 to 6 % below 30 km, and the errors increase for altitudes above 30 km, reaching 10 % near the upper-altitude limit of the retrieval. For CH3D, fitting errors range from 4 to 12 % below 15 km and increase for higher altitudes, approaching 30 % near the upper-altitude limit of the retrieval.

Spacing of retrieval altitudes for VMR profiles of the isotopologues of methane varies from ∼ 2 km (low altitudes) to 6 km (high altitudes) and averages around 4 km. The profiles were interpolated onto a 1 km grid using a piecewise quadratic interpolation. Then, the profiles were placed into seasonal bins and 10∘ latitude bins. Figure 2 shows the spatial distribution of the profiles by season. Most of the profiles are at higher latitudes, but there are at least 40 profiles in each equatorial bin every season. The binning process averages the results from many occultations in order to reduce the impact of the random error on the results (the random error decreases according to the square root of the number of elements included in the average).

Model calculations were performed using version 4 of the Whole Atmospheric Community Climate Model (WACCM), a component of the Community Earth System Model (Marsh et al., 2013). WACCM extends from the surface to 5 × 10-6 hPa (∼ 140 km) and includes fully interactive chemistry and circulations patterns for the whole atmosphere. WACCM can be run as a standalone model or as the atmospheric component of CESM.

Out of the box, WACCM does not support molecular isotopologues, but the two isotopologues of CH4 can be inserted as separate species with a few modifications. First, the reactions of the first step of methane oxidation are duplicated and their rate constants adjusted by the kinetic isotope effects kD/kH and k12/k13. The KIE of methane with each oxidant is given in Table 2 and the full set of modified reactions is listed in Table 3. No further reactions or molecules are modified as only the isotopic composition of methane is studied here. Next, new photolytic cross sections were added for all three isotopologues (Lee et al., 2001; Nair et al., 2005). The blue shifts of the cross sections are approximately 1 nm for CH3D and 0.04 nm for 13CH4. Finally, boundary conditions representing surface emissions were calculated for the two heavy isotopologues. Keeling plots presented by Röckmann et al. (2011) were used to derive relations between [CH4] vs. δD and δ13C: δD=1.50×104[CH4]/(ppm)-55.6‰,δ13C=1.29×104[CH4]/(ppm)-151.4‰. These relations were applied to the existing CH4 boundary conditions used by WACCM (Lamarque et al., 2010) to derive boundary conditions for CH3D and 13CH4.

WACCM was run as a standalone model with a resolution of 4 × 5∘ (latitude/longitude) and 66 vertical levels. The model was run as a perpetual year 2000 for a total of 20 years: 17 years of spin-up time followed by 3 years that were analyzed. Data from WACCM was analyzed in two ways. First, to observe general trends, the entire data set from the final 3 years was averaged monthly and placed into 10∘ latitude bins. Second, to remove sampling bias from ACE when comparing to WACCM, a smaller data set was constructed by measuring “profiles” from the whole WACCM data set at the same times and locations as each ACE profile. This data set was averaged seasonally and placed into 10∘ latitude bins to match the analysis of ACE data.

Figure 3 shows the total concentration of methane as a function of latitude and altitude as measured by ACE. In the well-mixed troposphere, the concentration of methane is nearly constant at around 1750 ppb. Above the tropopause, methane concentrations decrease steadily at higher altitudes to about 300 ppb at 20–25 km above the tropopause. Methane near the equator extends higher into the atmosphere primarily due to the higher tropopause, as well as the transport of air containing elevated levels of methane from the troposphere to the lower stratosphere in the tropics (as part of the Brewer–Dobson circulation). Some seasonal variation is visible. Pockets of methane-depleted air are present over the poles especially during the summer and fall months: December to May over the South Pole and June to November over the North Pole.

ACE data for δD as a function of latitude and altitude are plotted in Fig. 4. CH3D data are available from 5 to 30–35 km, depending on latitude. Above 12 km, values of δD steadily increase with altitude from tropospheric values around 0 ‰, then sharply increase at the highest few kilometers of the available data to between +250 and +400 ‰. This sharp increase occurs at the same altitudes where the fitting errors during retrieval are the highest. In addition, high levels of CH3D are noticeably present over the South Pole from June to November. Below 12 km, the δD data are much noisier and average around +35‰. This “step function” in the plot, with a sharp change in δD at a particular altitude that does not vary with latitude, likely indicates a problem in the retrieval below 12 km for either CH3D or the main isotopologue. An additional horizontal line is present around 20 km and is discussed with δ13C below. Finally, there is another artifact present below 80∘ S in June–August: a single altitude with a very low δD. This is due to the low number and quality of measurements taken over the poles caused by the satellite's non-polar orbit.

ACE data for δ13C are plotted in Fig. 5. These data are available from 6 to 50 km except over the poles during some seasons. Overall the data are noisier than for δD, but values of δ13C still increase with altitude. Tropospheric values average near -20 ‰ , while lower stratospheric values average near 0 ‰. Seasonal changes are also more apparent than in δD. Enrichment of 13C is strongest during the summer and fall months. Values of δ13C as high as +100 ‰ are present over both poles between 35 and 50 km. Higher δ13C values are present in two bands at 22 and 40 km. Since these bands show no variation in altitude as a function of latitude, they are also believed to be artifacts of the retrieval process.

Note that there was a very large step function in the original version 3.5 δ13C results around 22 and 40 km, a consequence of poor internal consistency between the spectroscopic data for the CH4 lines used to derived CH4 VMR at low altitudes and the CH4 lines used for the high-altitude portion of the retrieval. This also affected δD, resulting in a band at around 20 km. For the CH4 research product employed in the current study, the intensities of CH4 lines in the 2ν4 band (the lines that contribute to retrieved CH4 VMR at low altitude) were adjusted in an effort to improve the agreement with other CH4 bands employed in the retrieval (i.e., the bands that contribute to the retrieved VMR at higher altitudes). The step function in the ACE δ13C results was greatly reduced, but the “bump” in δ13C near 22 km suggests that there may remain a spectroscopic compatibility problem for CH4 lines in different bands. These features also appear in ACE observations of δ13C–CO (Beale et al., 2015) for the same reason, suggesting that they are artifacts of the retrieval.

The ACE-sampled WACCM data set is presented in Fig. 6 (total CH4), Fig. 7 (δD), and Fig. 8 (δ13C). Figures of the full WACCM data set are present in the Supplement. The model output of total methane agrees well with ACE's observations. Tropospheric methane fluctuates slightly by season but is steady around 1700 ppb. The plume of methane-rich air over the equator in the stratosphere is also present, and mixing ratios of methane decrease with higher altitudes in the stratosphere and mesosphere. Seasonal variation is noticeable here; air masses low in methane form over each pole around 50 km during the summer, then sink and become further depleted during the fall.

These seasonal trends are especially visible in δD and δ13C. The polar air masses of depleted CH4 are enriched in both CH3D and 13CH4 and become further enriched as they sink. Enrichment in the southern air mass reaches a lower altitude and lingers for a longer period, February to June, than the northern air mass which is enriched only from July to October. This difference in altitude is also shown in ACE; enrichment in the Southern Hemisphere reaches low enough to be detected by ACE, while enrichment in the Northern Hemisphere remains too high to be measurable by ACE.

The ACE data set presented in this paper greatly expands the number of observations of methane and its isotopologues in the stratosphere. The data for CH3D have been shown to be consistent with both model predictions and existing balloon-based measurements after calibrating the ACE results using tropospheric δD calculated from the WACCM model. However, the data for 13CH4 still show large discrepancies. The addition of new microwindows and adjustment of spectroscopic parameters for CH4 lines in the 2ν4 band significantly reduced the large step function observed in δ13C when using the spectroscopic parameters for this band that are currently available in the HITRAN database. However, a systematic latitude-independent bump near 22 km in the δ13C profiles derived from ACE in the current study suggests that further refinement of these spectroscopic constants will be required to improve the retrieval results for the isotopologues CH4 from ACE.