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13C/12C and D/H Isotopic Analysis of Atmospheric MethaneEPA Grant Number: U915835
Title: 13C/12C and D/H Isotopic Analysis of Atmospheric Methane
Investigators: Trudeau, Michael E.
Institution: University of Colorado at Boulder
EPA Project Officer: Jones, Brandon
Project Period: August 1, 2000 through August 1, 2003
Project Amount: $79,280
RFA: STAR Graduate Fellowships (2000) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Air Quality and Air Toxics , Fellowship - Atmospheric Chemistry
This project proposes to estimate various source contributions to the global methane budget by making atmospheric observations of the mean methane isotopic signature and relating these isotopic signals to sources using mass balance.
An infrared absorption spectrometer will be developed that will have sufficient sensitivity to resolve spatial and temporal trends in both 13C/12C and D/H stable isotopic ratios. To date, measurements of atmospheric methane 13C/12C and D/H values have been performed almost exclusively by isotope ratio mass spectrometry. However, because 13CH4 and 12CH3D have the same isotopic mass, their measurement requires extensive procedures to convert the CH4 to CO2 and H2. Infrared absorption spectroscopy offers a simple, nondestructive way to simultaneously measure the methane, carbon, and hydrogen isotopic compositions. Samples will be obtained from a global air-sampling network to provide a good spatial distribution of sites from which to collect methane variability data and deduce source and sink information. The spectrometer employs periodically poled lithium niobate (PPLN) to generate different frequency radiation from two near infrared diode lasers (one at 811 nm and the other at 1066 nm). This technique yields ~10 microwatts of single mode, tunable radiation that probes the v3 rotational-vibrational absorption band at 3.4 microns. The radiation is passed through a multipass absorption cell with a path length of 36 meters and a volume of 0.3 liter. Second harmonic wavelength modulation is utilized to reduce spectrometer noise. The current detection limit is 2 ppb of methane. Because the tropospheric mixing ratio of 12CH3D is ~1 ppb and is comparable to the instrument detection limit, a procedure to concentrate methane samples is required before isotopic analysis. Methane is extracted from 25 L of air using a cryogenic chromatographic column and is injected into the multipass cell with a helium carrier gas to give a final pressure of 25 torr. This procedure enriches the methane mole fraction in the absorption cell by ~1000 times. The abundance of the rare isotopic species (13CH4 and 12CH3D) are measured relative to 12CH4 by comparing the absorbance of an intrinsically strong spectral line of a rare species with a weak line of 12CH4.
The magnitudes of the sources and sinks of greenhouse gases like methane need to be better understood to accurately measure changes in emissions and ensure international compliance with global environmental policies such as the Kyoto Protocol.