Stable carbon isotope analysis of atmospheric nonmethane hydrocarbons using continuous-flow isotope ratio mass spectrometry

In order to create effective control strategies in reducing air pollution problems, it is essential to have both a better understanding of the complex atmospheric chemical processes and the sources of nonmethane hydrocarbons (NMHCs). From the past successful studies of other atmospheric trace gases such as CH4 and CO2, we expected that the stable carbon isotope ratio (13C/12C or delta13C value) of each NMHC species reflects the delta13C value of its source material and/or provides information on chemical loss processes that fractionate carbon isotopes. Therefore, this dissertation explored the applications of stable carbon ratio analysis on NMHC collected in urban atmospheric samples.; Our delta13C analysis method development for atmospheric NMHCs was adapted from the continuous flow gas chromatography combustion isotope ratio mass spectrometry (cf-GC/C/IRMS) design developed by Rudolph et al. [1997] in their pioneering measurements of delta 13C-NMHCs, and also from the NMHC preconcentration system designed for the earlier NMHC concentration measurements developed by Greenberg et al. [1994]. For the method validation, pure or diluted standard CO2 reference gases of known delta13C values were used to check for possible isotope fractionation due to the physical nature of the system. Then, pure or diluted propane (C8H8 ) reference gases of known delta13C were used to check for any fractionation problems associated with the chemical nature of the system. A final verification study was performed with urban whole air samples collected at UC Irvine in 2003. An average precision of +/-0.5‰ (1sigma) was measured for delta13C values of resolved NMHCs.; Results of the cf-GC/C/IRMS delta13C analyses of ambient NMHCs conducted using the urban air samples collected in three different cities of Los Angeles County, California, USA, during the summer of 2003 and winter of 2004 are discussed. The observed delta13C values for C2-C5 n-alkanes (-27.4 +/- 2.4‰), i-alkanes (-26.6 +/- 2.3‰), alkenes (-20.9 +/- 2.9‰) and ethyne (-10.8 +/- 2.9‰) in Los Angeles were very similar to the values reported for three other urban air studies (Rudolph et al., 1997 and 2002, Tsunogai et al., 1999).