Photochemical histories of nonmethane hydrocarbons inferred from their stable carbon isotope ratio measurements over east Asia

Abstract

The first airborne measurements of stable carbon isotope ratios (δ13C) of nonmethane hydrocarbons (NMHCs) were made over east Asia and its downwind regions as part of the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia). The measured δ13C values for ethane, n‐butane, and n‐pentane varied from approximately −30‰ to −20‰. In contrast, acetylene showed much higher δ13C with a wide variation (−10‰ to +20‰). These are consistent with the high δ13C values of combustion‐derived acetylene and a significant isotopic fractionation due to photochemical removal process. Vertical profiles of δ13C‐derived photochemical ages of NMHCs differed from one NMHC to another: less reactive ethane and acetylene showed linear increases in age with altitude (∼8 days below 1 km to ∼20 days at about 6 km altitude), whereas more reactive n‐butane and n‐pentane (∼4 days) had no age gradient. This suggests that less reactive NMHCs in high‐altitude air are transported from upwind source regions and mixed with fresh emissions from east Asia, while reactive NMHCs, even in the free troposphere, have recently been emitted. Thus vertical profiles are caused by the mixing of fresh emissions with aged air masses containing reactivity‐determined amounts of photochemically aged NMHCs. This mixing causes the difference in the photochemical ages calculated by two methods (the “hydrocarbon clock” method using n‐butane/ethane ratios and the “isotopic hydrocarbon clock” method using δ13C values of ethane).