Stable hydrogen-isotope analysis of methyl chloride emitted from heated halophytic plants

Stable isotope techniques are increasingly applied to study atmospheric budgets of methyl halides. Here we use compound specific thermal conversion isotope ratio mass spectrometry to measure the stable hydrogen isotope values of methyl chloride ( δ 2 H CH 3 Cl ) released thermally from dried leaves of halophyte plants collected from different geographical locations. We developed an automated purification and pre-concentration unit to enable measurement of δ 2 H CH 3 Cl from samples (100 ml) when mixing ratios were as low as 1 ppmv. Even though this is considerably higher than normal atmospheric mixing ratios this unit enabled measurement of the δ2H values of CH3Cl released during heating of samples at temperatures ranging from 30 to 300 °C. No substantial changes of δ 2 H CH 3 Cl values were observed over this temperature range. However, the δ 2 H CH 3 Cl values of all plants examined were strongly depleted (−178 ± 34‰) relative to the δ2H values of their modelled meteoric water, but differ only by 1–50‰ from those observed for their methoxyl groups. Our results indicate that plant methoxyl groups are an important precursor of the methyl group for CH3Cl thermally emitted from lyophilised and homogenized dry halophyte leaves. Furthermore, the thermal reaction producing CH3Cl did not show a substantial kinetic hydrogen isotope fractionation between CH3Cl and precursor methoxyl groups over the temperature range investigated. Moreover, we found a moderate linear correlation (R2 = 0.37) for the relationship between δ 2 H CH 3 Cl values released from halophytes and the δ2H values of modelled precipitation at their geographical locations. As CH3Cl emissions from terrestrial vegetation, including senescent and dry plants and biomass burning, are considered to provide a substantial fraction of the global emissions our findings are important when establishing a global hydrogen isotope model for atmospheric CH3Cl.