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

The first airborne measurements of stable carbon isotope ratios (delta C-13) 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 delta C-13 values for ethane, n-butane, and n-pentane varied from approximately -30 parts per thousand to -20 parts per thousand. In contrast, acetylene showed much higher delta C-13 with a wide variation (-10 parts per thousand to +20 parts per thousand). These are consistent with the high delta C-13 values of combustion-derived acetylene and a significant isotopic fractionation due to photochemical removal process. Vertical profiles of delta C-13-derived photochemical ages of NMHCs differed from one NMHC to another: less reactive ethane and acetylene showed linear increases in age with altitude (similar to 8 days below 1 km to similar to 20 days at about 6 km altitude), whereas more reactive n-butane and n-pentane (similar to 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 delta C-13 values of ethane).