Isotopic composition of non-methane hydrocarbons in emissions from biomass burning

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 +/-1.5parts per thousand of the composition 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 (13)C by approximately 20-30parts per thousand, and most of the variability can be explained by a dependence on flame temperature. The delta(13)C values decrease by 0.019parts per thousand/K (+/- 0 0053parts per thousand/K) with increasing temperature. Methyl chloride is highly depleted in (13)C, on average by 25parts per thousand. However the results cover a wide range of nearly 30parts per thousand. Specifically, in two measurements with wood from Eucalyptus (Eucalyptus delegatensis) as fuel we observed the emission of extremely light methyl chloride (-68.5parts per thousand and -65.5parts per thousand). This coincides with higher than average emission ratios for methyl chloride (15.5 x 10(-5) and 18 x 10(-5) mol CH(3)Cl/ mol CO(2)). These high emission ratios are consistent with the high chlorine 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.