|dc.contributor.author||Burger, Johanna Wilhelmina||
|dc.description||Thesis (Ph.D. (Chemistry))--North-West University, Potchefstroom Campus, 2006.||
|dc.description.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 manmade 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.||
|dc.title||Identification and comparison of the volatile organic compound concentrations in ambient air in the Cape Town metropolis and the Vaal Triangle||en