Identification and comparison of the volatile organic compound concentrations in ambient air in the Cape Town metropolis and the Vaal Triangle

Abstract

The growing concern for environmental problems underlines the importance of correctly predicting the fate of pollutants released into the environment. In the case of VOCs, this is a complex task due to the large number of VOCs with different reactivity's present in ambient air (Atkinson, 1990). In Cape Town and the Vaal Triangle brown haze layers develops in ambient air during windless days in the wintertime. This leads to the build-up of pollutants emitted into the atmosphere. The haze is usually most intense in the early mornings, gradually dispersing during the day. The aim of the study was the identification, quantification and comparison of VOCs in Cape Town and the Vaal Triangle. Different sampling techniques have been used during intensive field campaigns in Cape Town and the Vaal Triangle. Three different sampling techniques were used, namely: 6 litre TO canisters, CarbotrapTM 300 tubes and 75 mm Carboxen-PDMS SPME fibres. Samples were also taken at different altitudes in the lower troposphere, because the pollution layers are formed at different altitudes. Background corrections were also carried out. A Supelco (Cat no: 41900-U) calibration standard, was used as external standard. Samples were analysed by a Hewlett Packard Agilent 6890 gas chromatograph (GC) and Micromass Autospec-TOF mass spectrometer (MS) according to the EPA TO-14a compendium method. The samples were concentrated on a Perkin-Elmer Turbo matrix thermal desorber. A temperature program was used and VOCs not present in the Supelco standard were identified using the MS data system library (NIST). SMPE was only used as a qualitative comparison to the other techniques. A large number of VOCs were identified and quantified at ground level and at different altitudes in ambient air in both Cape Town and in the Vaal Triangle region. The aim was identifying and quantifying man made emissions. The total VOC profile may differ from these since oxygenated species have not been focussed on. In the Cape Town study more unsaturated VOCs and longer chain HCs were detected during the night than during the day. The number of ketones present also seemed to be higher during the day. In the city centre and Khayelitsha a wide range of halogenated hydrocarbons was detected at ground level. Chlorinated HCs do not take part in photochemical reactions and the concentrations of these VOCs did not to change very much in the day and night samples. It appeared that the concentration of the VOCs at different altitudes in some cases differ significantly. This correlated with the brown haze that forms visible layers and it seemed that the concentration of VOCs in layers differ. The VOCs found at ground level were in most cases related to petroleum products while the VOCs detected at higher altitudes are compounds that remained in the atmosphere and can be transferred from their source over great distances, or photochemical products. In the Vaal Triangle study a very wide variety of VOCs that included a large range of halogenated VOCs were detected. The north-east wind prevailing on the day of sampling diluted the VOCs sampled in the Vaal Triangle. The comparison of the two study regions showed that in both regions the toluene had the highest concentrations of all the measured VOCs. The reported daytime benzene concentrations at Goodwood, Table View and the city centre and the nighttime levels in Khayelitsha exceeded 1.6 ppb (5 pg.m-3). The low benzene concentration levels in the Vaal Triangle are mainly due to the wind diluting pollution at the time of sampling. A wider variety of VOCs were detected in the Vaal Triangle than in Cape Town. Pollutants detected in the Vaal Triangle had very low concentrations, mostly even below the detection limits. This was due to the strong wind that is typical for August in the Vaal Triangle. BVOCs were detected in both regions. In both areas the influence of photochemical processes is evident and secondary products of photochemical reactions were found. A large range of halogenated VOCs was found in the ground level samples in the Vaal Triangle and at higher altitudes in the Cape region. Halogenated VOCs were also detected in the city centre in Cape Town and in Khayelitsha. In both regions a large range of complex benzene derivates were found. The comparison of the values obtained using canisters and the CarbotrapTM 300 tubes showed differences that cannot be explained unambiguously. VOCs sampled with SPME correlated with the above-mentioned techniques but the identification of the unknown compounds was much easier in samples taken with the SPME than with the other techniques used. SPME proved to be a handy "screening" tool for the identification of VOCs. A comparison of the two different regions investigated gave insight into the concentrations and the fate of VOCs on a regional and global scale in South Africa. It followed from the results reported in this study that VOC emissions in Cape Town and in the Vaal Triangle would most definitely play a significant role in the formation of photochemical smog.