Determination of Volatile Hydrocarbons in Urban Air
In the complex hydrocarbon mixture in urban air, the alkenes and arenes are of particular interest with respect to impact on health and environment. Knowledge of hydrocarbon concentrations in urban air is essential for the assessment and prevention of health hazards and photochemical oxidant formation.
Hydrocarbons in urban air were sampled on specially made triple-layer adsorbent cartridges. Easily carried equipment was used for the sampling. After thermal desorption, the hydrocarbons were analyzed by gas chromatography. An aluminium oxide column effected the separation of C2-C6 alkenes. Detection was generally by flame ionization but optional photoionization detection was found to be advantageous for the selective detection of alkenes and alkadienes in the C3-C5 region. A recently introduced technique based on differential optical absorption spectroscopy (DOAS) was compared with the gas chromatographic method for the determination of benzene and toluene in air.
Proportions and exposure levels of hazardous hydrocarbons were determined in urban air. Ethene, propene, 1,3-butadiene and benzene made up approximately 5, 2, 0.5 and 6 per cent by weight of non-methane hydrocarbons. The composition was different for cold-start and high-speed emissions. Among the four isomeric butenes, the 2-butenes were prominent in petrol vapour and methylpropene in exhaust. The composition of pentenes was similar in petrol vapour and petrol exhaust. Concentrations of butadiene, reflecting outdoor urban exposure, were in the range of 0.5-5 mg/m3. The exposure of commuters to 22 volatile aromatic hydrocarbons of petrol exhaust was 5-10 times higher in cars than in trains. Generally, urban roadside concentrations of hydrocarbons decreased more than 10-fold with increased distance from motor traffic.
photochemical oxidant formation