Carbon isotope ratios of methyl bromide and methyl chloride emitted from a coastal salt marsh

Abstract

Methyl bromide (CH 3 Br) and methyl chloride (CH 3 Cl) play important roles in stratospheric ozone depletion, but their atmospheric budgets have large uncertainties. The analysis of stable isotope composition of methyl halides may provide useful independent information for further constraining their budgets. Here we report the first measurements of CH 3 Br and CH 3 Cl stable carbon isotope ratios emitted from a biogenic source under in situ conditions. CH 3 Br and CH 3 Cl emissions from the salt marsh plant Batis maritima showed a strong diurnal variation in δ 13 C, from −65‰ during the daytime to −12‰ at night. The minimum δ 13 C values were observed at midday, coinciding with the time of greatest emissions and ambient temperature. At night, when the emissions were much smaller, the stable carbon isotopic ratios of CH 3 Br and CH 3 Cl became enriched in 13 C. The daily mean δ 13 C of CH 3 Br and CH 3 Cl emissions, weighted by emission rate, were −43‰ and −62‰ respectively.