Abstract
Methane is an potent greenhouse gas, second only to carbon dioxide of the anthropogenic
greenhouse gases in its influence on Earth’s radiative budget. Although less abundant in the
atmosphere, methane’s global warming potential is about twentyeight times that of carbon
dioxide. Sources of methane are numerous and diverse. Drivers of changes in the atmospheric
methane budget, as well as its current state, are therefore poorly understood. A promising
new tool to restrict the atmospheric methane budget is the concept of “clumped isotopes”,
i.e. a molecule including two or more rare (heavy) isotopes. Since a heavy atom vibrates
slower than a light atom, the substitution to heavier isotopes in a molecular bond leads to
lower Zero-Point Energy (ZPE) and thus amore stable bond. Fromstatistical thermodynamics
we know that the influence of ZPE is largest at low temperatures, therefore the clumping
of isotopes depends on the formation (or equilibration) temperature. The research field of
clumped isotope geochemistry has grown substantially over the last decade and the recent
development of new techniques has enabled measurement of clumping in methane. The
different methane producing processes occur at significantly different temperatures, therefore,
the clumped isotope signatures of methane can be used to identify the process by which
the gas was formed. Clumped isotopes can thus be a helpful tool in refining the budget
of atmospheric methane. However, the isotopic composition of the atmospheric methane
pool is altered by sink mechanisms. As opposed to methane’s sources, which are numerous
and diverse, is methane primarily removed through oxidation by atmospheric radicals; the
hydroxyl radical accounts for 92% of the total sinks and the chlorine radical for 4%. The rest is
removed through uptake of soils. As is proven in the current research project, the clumped
isotopes are removed by oxidation mechanisms at a slower rate. The residual methane pool is
therefore enriched in clumped isotopes compared to the methane from the sources. In order
to construct a top-down budget of methane, the clumped kinetic effect of the sinkmechanisms
must be taken into account. The clumped kinetic effect in atmospheric oxidation of methane
has been studied experimentally and theoretically in the three current papers: In Paper I the
effect of oxidation by the chlorine radical at roomtemperature (25 ±C) was studied, in Paper
II the effect of oxidation by the hydroxyl radical over a range of temperatures (5 ±C–40 ±C)
was studied, and in Paper III the effect of both the chlorine and the hydroxyl radical at room
temperature was studied.
All the experiments were conducted in the smog chamber of the Department of Chemistry,
University of Copenhagen. In Papers I and II, isotopically-labeled methane was used and the
reactions were studied using Fourier Transform Infrared spectroscopy (FTIR). In Paper III;
natural abundance methane was used and only the reaction yield was measured with FTIR
spectroscopy. Meanwhile, the isotopic compositions were measured from smog-chamber
gas samples by Tunable Infrared Differential Laser Absorption Spectroscopy (TILDAS) at
the Department of Earth Atmospheric and Planetary Sciences, Massachusetts Institute of
Technology, USA. Transition state theory is used study these effects theoretically. In order to
evaluate the clumping effect by a reaction, the apparent clumpiness is defined as the deviation
of the Kinetic Isotope Effect (KIE) of the reaction with the clumped isotope (13CH3D) from
the combination of KIEs of reactions with the single substituted isotopologues (13CH4 and
12CH3D). If the KIE of the reaction with 13CH3D is identical to the multiplication of the KIEs
of the reactions with the singly substituted isotopologues 13CH4 and 12CH3D, the apparent
clumpiness is equal to unity (1) and no clumping effect occurs. The reaction does not then
alter the clumped isotope signature in the residual methane pool and the clumped isotope
signature will reflect the clumped isotope signatures from the sources.
For the oxidation of methane by the hydroxyl radical, the TILDAS measurements yield an
apparent clumpiness of 0.9997§0.0012 —not significantly different from unity. This value is
supported by both the theoretical calculations and by the FTIR measurements of 0.98§0.02.
For the oxidation of methane by the chlorine radical, the TILDAS measurements yield an
apparent clumpiness of 0.9965§0.0007 (significantly different from unity). Oxidation by the
chlorine radical does, however, represent only a minor channel for tropospheric methane
and the global effect is expected to be low. The theoretical calculations result in an apparent
clumpiness of 0.9996, indicating either that the experimental value is underestimated or that
there is some effect not properly captured in the theory (e.g. inadequate tunneling correction).
The FTIR measurements yield an apparent clumpiness of 1.02§0.03; this range includes both
the TILDASmeasurements and the theoretical calculation.
At most, there is only a small clumping effect associated with atmospheric removal of methane,
indicating that an interpolation of the sources’ (relative) clumped isotope enrichments will
give a reliable quantitative estimate of the atmospheric (relative) clumped isotope enrichment.
greenhouse gases in its influence on Earth’s radiative budget. Although less abundant in the
atmosphere, methane’s global warming potential is about twentyeight times that of carbon
dioxide. Sources of methane are numerous and diverse. Drivers of changes in the atmospheric
methane budget, as well as its current state, are therefore poorly understood. A promising
new tool to restrict the atmospheric methane budget is the concept of “clumped isotopes”,
i.e. a molecule including two or more rare (heavy) isotopes. Since a heavy atom vibrates
slower than a light atom, the substitution to heavier isotopes in a molecular bond leads to
lower Zero-Point Energy (ZPE) and thus amore stable bond. Fromstatistical thermodynamics
we know that the influence of ZPE is largest at low temperatures, therefore the clumping
of isotopes depends on the formation (or equilibration) temperature. The research field of
clumped isotope geochemistry has grown substantially over the last decade and the recent
development of new techniques has enabled measurement of clumping in methane. The
different methane producing processes occur at significantly different temperatures, therefore,
the clumped isotope signatures of methane can be used to identify the process by which
the gas was formed. Clumped isotopes can thus be a helpful tool in refining the budget
of atmospheric methane. However, the isotopic composition of the atmospheric methane
pool is altered by sink mechanisms. As opposed to methane’s sources, which are numerous
and diverse, is methane primarily removed through oxidation by atmospheric radicals; the
hydroxyl radical accounts for 92% of the total sinks and the chlorine radical for 4%. The rest is
removed through uptake of soils. As is proven in the current research project, the clumped
isotopes are removed by oxidation mechanisms at a slower rate. The residual methane pool is
therefore enriched in clumped isotopes compared to the methane from the sources. In order
to construct a top-down budget of methane, the clumped kinetic effect of the sinkmechanisms
must be taken into account. The clumped kinetic effect in atmospheric oxidation of methane
has been studied experimentally and theoretically in the three current papers: In Paper I the
effect of oxidation by the chlorine radical at roomtemperature (25 ±C) was studied, in Paper
II the effect of oxidation by the hydroxyl radical over a range of temperatures (5 ±C–40 ±C)
was studied, and in Paper III the effect of both the chlorine and the hydroxyl radical at room
temperature was studied.
All the experiments were conducted in the smog chamber of the Department of Chemistry,
University of Copenhagen. In Papers I and II, isotopically-labeled methane was used and the
reactions were studied using Fourier Transform Infrared spectroscopy (FTIR). In Paper III;
natural abundance methane was used and only the reaction yield was measured with FTIR
spectroscopy. Meanwhile, the isotopic compositions were measured from smog-chamber
gas samples by Tunable Infrared Differential Laser Absorption Spectroscopy (TILDAS) at
the Department of Earth Atmospheric and Planetary Sciences, Massachusetts Institute of
Technology, USA. Transition state theory is used study these effects theoretically. In order to
evaluate the clumping effect by a reaction, the apparent clumpiness is defined as the deviation
of the Kinetic Isotope Effect (KIE) of the reaction with the clumped isotope (13CH3D) from
the combination of KIEs of reactions with the single substituted isotopologues (13CH4 and
12CH3D). If the KIE of the reaction with 13CH3D is identical to the multiplication of the KIEs
of the reactions with the singly substituted isotopologues 13CH4 and 12CH3D, the apparent
clumpiness is equal to unity (1) and no clumping effect occurs. The reaction does not then
alter the clumped isotope signature in the residual methane pool and the clumped isotope
signature will reflect the clumped isotope signatures from the sources.
For the oxidation of methane by the hydroxyl radical, the TILDAS measurements yield an
apparent clumpiness of 0.9997§0.0012 —not significantly different from unity. This value is
supported by both the theoretical calculations and by the FTIR measurements of 0.98§0.02.
For the oxidation of methane by the chlorine radical, the TILDAS measurements yield an
apparent clumpiness of 0.9965§0.0007 (significantly different from unity). Oxidation by the
chlorine radical does, however, represent only a minor channel for tropospheric methane
and the global effect is expected to be low. The theoretical calculations result in an apparent
clumpiness of 0.9996, indicating either that the experimental value is underestimated or that
there is some effect not properly captured in the theory (e.g. inadequate tunneling correction).
The FTIR measurements yield an apparent clumpiness of 1.02§0.03; this range includes both
the TILDASmeasurements and the theoretical calculation.
At most, there is only a small clumping effect associated with atmospheric removal of methane,
indicating that an interpolation of the sources’ (relative) clumped isotope enrichments will
give a reliable quantitative estimate of the atmospheric (relative) clumped isotope enrichment.
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Publication status | Published - 2016 |