Stable Carbon Isotopic Compositions of Nonmethane Hydrocarbons Collected during ACE-Asia Aircraft Campaign

Canister sampling of NMHC for the measurements of stable carbon isotopic compositions (δ ¹³C) was conducted aboard NCAR C-130 aircraft over East Asian region as part of ACE-Asia experiment during April and May 2001. Mixing ratios of ethane, acetylene, propane, n-butane, and n-pentane showed a decrease from the surface to 6 km in altitude. δ ¹³C values of ethane become heavier from around -26.5 ‰ in the marine boundary layer to around -24.5 ‰ at 6 km in altitude. Vertical trend of isotopic composition for acetylene is somewhat similar to that for ethane, but its vertical gradient is more pronounced than that of ethane. Particularly, very high isotopic ratios of acetylene up to +20 ‰ were observed in the free troposphere, being consistent with the previously published results on the largest kinetic isotope effect of acetylene and its highly ¹³C-enriched emission sources. On the contrary, more reactive NMHC, such as propane, n-butane, and n-pentane, did not show a significant vertical change. In general, changes in δ ¹³C values of less reactive NMHC due to the mixing of air masses are larger than those of more reactive NMHC, because the accumulation of the former NMHC should be relatively more enhanced in the atmosphere. Thus, the weaker isotopic gradient of more reactive NMHC with altitude suggest that the photochemical removal of NMHC during the atmospheric transport plays a minor role in determining vertical profile of the mixing ratios. Alternatively, the strong isotopic gradients found for less reactive NMHC (such as ethane and acetylene) suggest that the systematic decreses of the mixing ratios with altitude are predominantly caused by the vertical mixing (i.e. dilution) of air masses with different photochemical ages