Stable carbon isotope composition of ambient VOC and its use in the determination of photochemical ages of air masses

Stable Carbon Isotopic Composition measurements can provide valuable information about the processing of trace gases in the atmosphere. Not only can it be used to distinguish physical processes such as dilution and mixing from photochemical ageing, but it can also be an important tool in identification of sources, in calculating the photochemical age and qualitatively and quantitatively connecting precursors with their atmospheric products.

Even though isotopic composition analysis is a valuable technique, its use is hindered by the low concentrations of compounds in the atmosphere, complexity of the samples and complex measuring instrumentation. The intention of this research project was to develop and validate sampling and instrumental analysis techniques that can be used to perform isotopic composition measurements of volatile organic compounds (VOC) and to apply these methods to analysis of ambient samples.

Since most VOC are present in the atmosphere in sub-ppbv to ppbv levels and more than 1 ng of carbon is required for isotopic analysis, collection of large volumes of air is required. A method based on sampling onto cartridges filled with an adsorbent (Carboxene-569) for VOC collection in the field has been developed. VOC are selectively collected by passing large volumes (up to 100 L) of air through the cartridges. Thermal desorption of VOC from the cartridges is followed by two step cryogenic trapping and separation by gas chromatography. Once separated, all VOC are oxidized in a combustion interface. The isotopic composition of resulting carbon dioxide is then determined on-line by isotope ratio mass spectrometry. Various validation tests were performed in order to test accuracy and precision of both the preconcentration system and sampling-desorption procedure.

The newly developed sampling and analysis techniques were applied in field studies: Border air quality study (BAQS) (2007) and Environment Canada-York University campaign (EC-YU) (2009-2010). Ambient samples were collected over various time periods and the isotopic composition of individual compounds was analyzed. Determined mixing ratios were in pptv to low ppbv ranges and isotope composition varied from -30%0 to -20%0 for most of the compounds. Analysis of mixing ratios and isotope composition, their distribution and trends indicated that sampling locations can be qualitatively classified as rural (Ridgetown), semi-rural (Harrow and Egbert) and semi-urban (Toronto) areas, with strong local vehicle emission sources. Quantitative analysis of the photochemical ages (PCA) determined using hydrocarbon and isotope hydrocarbon clocks (Egbert and Toronto samples) resulted in similar values, suggesting that both of these methods are valid and are applicable. However, while both PCA methods indicated that local sources have larger impact on the air quality in these two locations, PCA from isotope composition analysis has demonstrated that different VOC in photochemically processed air masses differ in their PCA depending on VOC reactivity.