Factors influencing the stable carbon isotopic signature of methane from combustion and biomass burning

Factors controlling the delta(13)C of methane released by combustion include the combustion efficiency of the fire and the delta(13)C of the fuel. Smoldering fires produced C-13-depleted methane relative to hot, flaming fires in controlled forest and grassland burns and within a wood stove. Pine forest burns in the southeastern United States produced methane which ranged from -21 to -30 parts per thousand, while African grassland burns varied from -17 to -26 parts per thousand, depending upon combustion phase. African woodland burns produced methane at -30 parts per thousand. In forest burns in the southeastern United States, the delta(13)C Of methane released with smoldering was significantly C-13 depleted relative to methane released under hot flaming conditions. Methane released with smoldering was depleted by 2-3 parts per thousand relative to the fuel delta(13)C, but this difference was not significant. The delta(13)C Of methane produced in a variety of wood stove conditions varied from -9 to -25 parts per thousand and also depended upon combustion efficiency. Similar results were found for methane produced by gasoline automobile engines, where the delta(13)C Of methane varied from -9 to -22 parts per thousand. For combustion occurring within the confining chamber of a wood stove or engine the delta(13)C of methane was clearly C-13 enriched relative to the delta(13)C of the fuel, possibly because of preferential combustion of (CH4)-C-12 in the gas phase. Significant quantities of ethylene (up to 25 to 50% of methane concentrations) were produced in southeastern U.S. forest fires, which may have consequences for physiological and reproductive responses of plants in the ecosystem. Methane production in these fires varied from 0.2 to 8.5% of the carbon dioxide production.