Measurement of the Carbon Isotopic Ratio of Atmospheric Methane

EPA Grant Number: U914748
Title: Measurement of the Carbon Isotopic Ratio of Atmospheric Methane
Investigators: Miller, John B.
Institution: University of Colorado at Boulder
EPA Project Officer: Michaud, Jayne
Project Period: January 1, 1995 through January 1, 1996
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1995) Recipients Lists
Research Category: Academic Fellowships , Engineering and Environmental Chemistry , Fellowship - Chemistry


The main objectives of this research project are to: (1) predict future levels of methane in the atmosphere based on an accurate methane budget and an understanding of the processes that produce methane; and (2) use delta13C to make a qualitative assessment of large changes in the distribution of sources between biogenic, thermogenic, and biomass burning.

Methane is a potent greenhouse gas whose atmospheric burden has more than doubled since the onset of the industrial age. In the last 40 years, its growth rate has averaged nearly one percent per year (Cicerone and Oremland, 1998). The observed increase is due largely to human, though not exclusively industrial, activities. Although the atmospheric concentration of methane of 1,800 ppbv is only 0.5 percent of global CO2 levels, its relative rate of increase is nearly twice that of CO2, and models indicate that methane's contribution to greenhouse warming is twenty times that of CO2 on a per molecule basis (Lashof and Ahuja, 1990). Methane absorbs strongly at 7.66 µm, a region of the infrared spectrum, where neither water nor CO2 absorb strongly. It is estimated that methane accounts for approximately 20 percent of the increase in radiative forcing by trace gases since the onset of the industrial era.


Methane plays a very important role in both tropospheric and stratospheric chemistry. In the troposphere, methane helps regulate the concentration of hydroxyl radical, the primary oxidant in the troposphere, thus influencing the tropospheric concentrations of CO, SO2, NO2, and hydrochlorofluorocarbons (HCFCs). Futhermore, the products of methane oxidation are water and carbon dioxide. In the stratosphere, methane oxidation is a significant source of stratospheric water vapor. It also reacts with chlorine atoms to form hydrochloric acid, thus reducing the amount of chlorine available for reaction with ozone.

The sources of atmospheric methane are diverse and their relative magnitudes are not known with much certainty. No single source of methane comprises more than 25 percent of the total source budget, so changes in any given source are difficult to detect. Methane is produced by biogenic means through reduction of CO2 and acetate by anaerobic bacteria. It also is produced by thermogenic processes. Methane is produced by a wide variety of living systems and is a key molecule involved in the radiative forcing of climate, thus making it one of the most important molecules linking the biosphere and atmosphere. A budget for sources and sinks of methane, along with approximate 13C isotopic signatures is presented below. (Adapted from Fung, et al., 1991.)

delta13C (o/oo)
Animals (enteric fermentation)
Animal waster and Sewage
Wetlands (tropical and northern)
Rice Paddies
Natural Gas (vents and leaks)
Coal (mining and combustion)
Biomass burning
Methane Hydrates
Oceans and freshwater
Reaction with OH
Removal by soils
Removal in stratosphere
* delta13c = [(13C/12Csample)/(13C/12Creference) – 1] x 1000
** kinetic fractionation factor, a = 0.995
*** kinetic fractionation factor, a = 0.976

The current uncertainty in the methane budget hinders the explanation of methane trends such as the period of almost no growth in 1992 and 1990 (Dlugokencky, et al., 1994). In addition, without an accurate methane budget and an understanding of the processes that produce methane, prediction of future levels of the gas in the atmosphere is not possible. As global population increases, there is great potential for vastly larger atmospheric methane concentrations due to increased industrial and agricultural activity. The measurement of isotopic ratios of the stable isotopes of both carbon and hydrogen in methane along with the continued measurement of its atmospheric concentration and distribution afford an excellent means to deconvolute the location and magnitude of methane sources. Biogenically produced methane has a smaller ratio of 13C/12C sample than does methane produced thermogenically. There is a great deal of overlap in the source signatures of biogenic methane, making most biogenic sources largely indistinguishable from one another from an isotopic (13C/12C) point of view. In general, it is difficult to identify sources as being either biogenic or thermogenic based on their 13C alone. Additionally, the isotopic signature of methane produced from biomass burning is distinct, being enriched in 13C. Biogenically produced methane has delta13C in the range of 50 to 80 percent o/oo, and methane produced through biomass burning a range of -10 to -25 percent (Cicerone and Oremland, 1998). Thus, delta13C is most useful in arriving at a qualitative assessment of large changes in the distribution of sources between biogenic, thermogenic, and biomass burning.

Global monitoring of methane concentrations and delta13C, through the 40 globally distributed sites of the National Oceanic and Atmospheric Administration (NOAA)/Climate Monitoring and Diagnostics Laboratory (CMDL) cooperative air sampling network, will enable us to observe both seasonal cycles of delta13C and its inter-annual trends. Seasonality of delta13C is largely due to seasonality in the hydroxyl radical concentration.

Supplemental Keywords:

fellowship, methane, greenhouse gas, atmosphere, industrial and agricultural activity., RFA, Scientific Discipline, Air, POLLUTANTS/TOXICS, climate change, Air Pollution Effects, Chemicals, Environmental Monitoring, tropospheric ozone, Atmospheric Sciences, Atmosphere, ambient ozone data, global change, ozone formation, green house gas concentrations, methane, chemical composition, isotopic measurement technique, carbon dioxide, CO2 concentrations, greenhouse gases, atmospheric chemical cycles, atmospheric trace gases, HCFCs, ecosystem impacts, aerosol sampling, carbon isotopic ratio, global warming, air quality, climate variability

Progress and Final Reports:

  • Final