Carbon isotopic compositions of C2‐C5 hydrocarbons and methyl chloride in urban, coastal, and maritime atmospheres over the western North Pacific

Abstract

C2‐C5 non‐methane hydrocarbons (NMHCs) and methyl chloride in the remote maritime atmosphere over the western North Pacific are analyzed in regard to their variation of mixing ratio and 13C/12C ratio (δ 13C), together with those in polluted urban (Nagoya and Yokohama) and coastal (Tokyo Bay) atmospheres in Japan. NMHCs show large atmospheric mixing ratio differences between urban (coastal) and maritime atmospheres probably due to emission from urban areas and degradation within the maritime atmosphere. Reflecting isotopic fractionation during the degradation within the maritime atmosphere, ethane shows large and systematic δ13C variation between urban (around −27‰ Peedee belemnite(PDB)) and maritime atmospheres (up to −22‰ PDB). Except for ethane, however, alkanes show small isotopic variation around δ13C = −27±2‰ PDB (1σ) without systematic isotopic differences between urban and maritime atmospheres, suggesting both small δ13C variation within major emission sources and also little isotopic fractionation during atmospheric degradation for alkanes other than ethane. Alkenes show large δ13C variation from −37 to −12‰ PDB for ethylene and from −27 to −14‰ PDB for propylene. Combination of both large δ13C differences between major sources (especially between land and maritime sources) and large isotopic fractionation effect during atmospheric degradation can be suggested for alkenes. Methyl chloride also shows large isotopic variation from −44 to −30‰ PDB in spite of their similar atmospheric mixing ratios from 580 to 710 parts per trillion by volume (pptv), probably due to the contribution of highly 13C‐depleted, anthropogenic methyl chloride especially to urban atmospheres. The general δ13C pattern of NMHCs and methyl chloride in polluted urban city air agrees strongly with those of biomass (C‐3 plant) burning plumes, suggesting that thermal breakdown of C‐3 plant (or related organic matter) is one of the representative sources of these hydrocarbons in urban atmospheres. Further investigations of the isotopic signature of source materials as well as laboratory studies of isotopic fractionation processes resulting from atmospheric degradation will improve our understanding of the sources, sinks, and atmospheric distributions of NMHCs and methyl chloride.