DETERMINATION OF C2-C5 HYDROCARBONS IN THE ATMOSPHERE AT LOW PARTS PER 109 TO HIGH PARTS PER 1012 LEVELS

By far the most abundant hydrocarbon in unpolluted air is methane (mixing ratio cu. 1.6 ppm). The mixing ratios of other hydrocarbons are typically in the low parts per lo9 (ppb) and parts per 10” (ppt) ranges. Although methane is several orders of magnitude more abundant in clean air, it is conceivable that other hydrocarbons are still of considerable importance to clean air photochemistry, because their reaction with hydroxyl radicals proceeds much faster than that of methane. Owing to this high reactivity of many of the light non-methane hydrocarbons (NMHC), mixing ratios of NMHC as low as a few ppb or several ppt can have a considerable influence on the photochemistry of unpolluted air. For this reason a gas chromatographic method has been developed that permits the determination of several C2-C, hydrocarbons with detection limits of a few ppt from grab samples of 0.5-Z dm3 (STP). The samples are collected in evacuated 2-1 stainless-steel containers with metal bellows-sealed stainless-steel valves. These sample collection and storage cans are specially pre-treated and cleaned to avoid changes in sample composition during transport of the samples to the laboratory. In the laboratory the samples are analysed by enrichment of the hydrocarbons on a packed pre-column at sub-ambient temperatures (L’LI. - 35°C) and subsequent separation on a 7 m x 0.8 mm I.D. packed column (Spherosil XOB 075). A flame-ionization detector is used. This method allowed survey measurements on a global scale of C,-C, hydrocarbons. which gave an estimate of the contributions of light hydrocarbons to atmospheric photochemical reactions.