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Isotopic Composition of Non-Methane Hydrocarbons in Emissions from Biomass Burning
Authors:K v Czapiewski  E Czuba  L Huang  D Ernst  AL Norman  R Koppmann  J Rudolph
Institution:(1) Centre for Atmospheric Chemistry, Rm. 006 Steacie Science Building, York University, 4700 Keele Street, Toronto, Ontario, M3J 1P3, Canada;(2) Institut für Atmosphärische Chemie, Forschungszentrum Jülich, Jülich, Germany;(3) Environment Canada, Meteorological Services of Canada, Toronto, Ontario, M3H 5T4, Canada;(4) University of Calgary, Calgary, Alberta, T2N 1N4, Canada
Abstract:The stable carbon isotope ratios of nonmethane hydrocarbons (NMHC) and methyl chloride emitted from biomass burning were determined by analyzing seven whole air samples collected during different phases of the burning process as part of a laboratory study of wood burning. The average of the stable carbon isotope ratios of emitted alkanes, alkenes and aromatic compounds is identical to that of the burnt fuel; more than 50% of the values are within a range of ±1.5permil of thecomposition of the burnt fuel wood. Thus for the majority of NMHC emitted from biomass burning stable carbon isotope ratio of the burnt fuel a good first order approximation for the isotopic composition of the emissions. Of the more than twenty compounds we studied, only methyl chloride and ethyne differed in stable carbon isotope ratios by more than a few per mil from the composition of the fuel. Ethyne is enriched in 13C by approximately 20–30permil, and most of the variability can beexplained by a dependence on flame temperature. The delta 13C values decreaseby 0.019 permil/K (±0.0053permil/K) with increasing temperature. Methyl chloride is highly depleted in 13C, on average by25permil. However the results cover a wide range of nearly 30permil. Specifically, in two measurements with wood from Eucalyptus (Eucalyptus delegatensis) as fuel we observed the emission of extremely light methyl chloride (–68.5permiland–65.5permil). This coincides with higher than average emission ratiosfor methyl chloride (15.5 × 10–5 and 18 ×10–5 mol CH3Cl/mol CO2). These high emission ratios are consistent with the highchlorine content of the burnt fuel, although, due to the limited number of measurements, it would be premature to generalize these findings. The limited number of observations also prevents any conclusion on a systematic dependence between chlorine content of the fuel, emission ratios and stable carbon isotope ratio of methyl chloride emissions. However, our results show that a detailed understanding of the emissions of methyl chloride from chloride rich fuels is important for understanding its global budget. It is also evident that the usefulness of stable carbon isotope ratios to constrain the global budget of methyl chloride will be complicated by the very large variability of the stable carbon isotope ratio of biomass burning emissions. Nevertheless, ultimately the large fractionation may provide additional constraints for the contribution of biomass burning emissions to the atmospheric budget of methyl chloride.
Keywords:stable carbon isotope ratios  biomass burning  atmospheric emissions  nonmethane hydrocarbons  methyl chloride
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