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Measurement of the Carbon Isotopic Ratio of Atmospheric Methane
EPA Grant Number: U914748Title: Measurement of the Carbon Isotopic Ratio of Atmospheric Methane
Investigators: Miller, John B.
Institution: University of Colorado at Boulder
EPA Project Officer: Broadway, Virginia
Project Period: January 1, 1995 through January 1, 1996
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1995)
Research Category: Academic Fellowships , Engineering and Environmental Chemistry , Fellowship - Chemistry
Description:
Objective: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 
13C
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.
Approach: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.)
| SOURCES | LIKELY (Tg CHL/yr.) |
RANGE |
13C (o/oo) |
| Animals (enteric fermentation) | 80 |
30-80 |
-60 |
| Animal waster and Sewage | 50 |
35-110 |
-60 |
| Wetlands (tropical and northern) | 125 |
55-155 |
-61 |
| Rice Paddies | 60 |
20-100 |
-62 |
| Landfills | 40 |
20-70 |
-51 |
| Natural Gas (vents and leaks) | 40 |
30-80 |
-41 |
| Coal (mining and combustion) | 45 |
20-85 |
-41 |
| Biomass burning | 50 |
20-80 |
-25 |
| Termites | 20 |
10-50 |
-60 |
| Methane Hydrates | 5 |
0-5 |
-65 |
| Oceans and freshwater | 20 |
5-75 |
-40 |
| TOTAL SOURCES | 535 |
245-890 |
-53 |
| SINKS | |||
| Reaction with OH | 450 |
330-560 |
** |
| Removal by soils | 20 |
15-45 |
*** |
| Removal in stratosphere | 30 |
25-55 |
|
| TOTAL SINKS | 500 |
-47 |
|
| ATMOSPHERIC INCREASE | |||
| * 13c = [(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 13C 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, 13C 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 13C, 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 13C and its inter-annual
trends. Seasonality of 13C is largely due to seasonality in the hydroxyl
radical concentration.
fellowship, methane, greenhouse gas, atmosphere, industrial and agricultural activity. , POLLUTANTS/TOXICS, Air, Scientific Discipline, RFA, climate change, Air Pollution Effects, Atmosphere, Chemicals, Atmospheric Sciences, Environmental Monitoring, tropospheric ozone, CO2 concentrations, HCFCs, carbon dioxide, Global Climate Change, air quality, chemical composition, ozone formation, isotopic measurement technique, aerosol sampling, methane, ecosystem impacts, carbon isotopic ratio, climate variability, global change, ambient ozone data, atmospheric chemical cycles, atmospheric trace gases, green house gas concentrations, global warming