The distinctive isotopic signature of plant-derived chloromethane: possible application in constraining the atmospheric chloromethane budget

Chloromethane (CH3Cl) is the most abundant halocarbon in the atmosphere. Although largely of natural origin it is responsible for around 17% of chlorine-catalysed ozone destruction. Sources identified to date include biomass burning, oceanic emissions, wood-rotting fungi, higher plants and most recently tropical ferns. Current estimates reveal a shortfall of around 2 million t y−1 in sources versus sinks for the halocarbon. It is possible that emissions from green plants have been substantially underestimated. A potentially valuable tool for validating emission flux estimates is comparison of the δ13C value of atmospheric CH3Cl with those of CH3Cl from the various sources. Here we report δ13C values for CH3Cl released by two species of tropical ferns and show that the isotopic signature of CH3Cl from pteridophytes like that of CH3Cl from higher plants is quite different from that of CH3Cl produced by biomass burning, fungi and industry. δ13C values for CH3Cl produced by Cyathea smithii and Angiopteris evecta were respectively −72.7‰ and −69.3‰ representing depletions relative to plant biomass of 42.3‰ and 43.4‰. The characteristic isotopic signature of CH3Cl released by green plants should help constrain their contribution to the atmospheric burden when reliable δ13C values for all other major sources of CH3Cl are obtained and a globally averaged δ13C value for atmospheric CH3Cl is available.