| dc.description.abstract | Various gaseous and aerosol species that are emitted directly from anthropogenic and biogenic
sources, as well as secondary formed species, are present and mixed in the giant reactor of the
atmosphere, where multiple complex chemical and physical interactions occur. The focus of this
thesis was on volatile organic compounds (VOCs) – these compounds are ubiquitous, ranging
from strong-smelling monoterpenes and sesquiterpenes emitted from vegetation to various
anthropogenic VOCs that have been associated with toxicological effects on human health, e.g.
benzene. It has been estimated that the total VOC emissions globally are approximately 1 300
Tg C yr-1. Most of these emissions are from terrestrial ecosystems (~1 000 Tg C yr-1), of which
approximately 50 % consist of isoprene and 15 % of monoterpenes. It is estimated that biogenic
VOC (BVOC) emissions exceed anthropogenic VOC emissions by eight times. However, in
highly-industrialised regions, which include parts of South Africa, anthropogenic VOCs (e.g.
benzene, toluene, ethylbenzene and xylene, combined abbreviated as BTEX) might dominate.
Once VOCs are emitted, their lifetimes depend on removal processes, such as dispersion,
transformation, photolysis, wet and dry deposition (including deposition on aerosol particles) or
oxidation. The chemistry of the atmosphere is strongly influenced by VOCs due to their ability to
scavenge oxidants such as ozone (O3), hydroxyl radicals (•OH, referred to from here on as OH)
and nitrate radicals (NO3
•, referred to from here on as NO3). VOCs contribute to net tropospheric
production and the destruction of O3 through catalytic reactions between oxidised VOC
derivatives (peroxy radicals) and NO. The oxidation of VOCs produces structurally different
organic oxygenates, which possess a wide range of properties (e.g. reactivity, volatility and
aqueous solubility) and different susceptibilities to undergo gas-to-particle conversion. The
vapour pressures of these new species tend to be lower than their precursor compounds, which
enables them to condense onto already existing atmospheric particles and thereby contributing
to secondary organic aerosol (SOA) formation and particle growth processes. Therefore, VOCs
have an indirect regional influence on cloud condensation nucleus (CCN) budget and on the
properties of the clouds. In addition to the climatic effects, VOCs and their reaction products are
increasingly regarded as posing unacceptable risks to human health, as well as to biological
and physical environments. VOCs also have a secondary impact on human health through their
participation in the formation of photochemical smog, which is characterised by high
concentrations of O3 and SOA.
Despite VOCs playing a significant role in many different atmospheric processes, very few
papers have been published in the peer-reviewed literature on VOC measurements in South Africa. In an effort to at least partially address this knowledge gap, measurements of
anthropogenic and biogenic VOCs were conducted at the Welgegund measurement station in
South Africa, which is situated on a commercial farm in an area regarded as a grazed
savannah-grassland-agriculture landscape. Welgegund is considered to be a regionally
representative background site with few local sources, which is impacted by the major source
regions in the interior of South Africa, i.e. the Bushveld Igneous Complex, the Johannesburg-
Pretoria conurbation, the Mpumalanga Highveld and the Vaal Triangle. The site is also
frequently affected by air masses passing over a relatively clean western sector. VOC samples
were collected with an automated sampler on Tenax-TA and Carbopack-B adsorbent tubes with
a heated inlet to remove O3. Samples were collected twice a week for two hours during daytime
(11:00 to 13:00 local time, LT) and two hours during night-time (23:00 to 1:00 LT) on Tuesdays
and Saturdays for a period of more than two years, i.e. through a 13-month sampling campaign
from February 2011 to February 2012 and a 15-month sampling campaign from December 2013
to February 2015. Individual VOCs were identified and quantified using a thermal desorption
instrument, connected to a gas chromatograph and a mass selective detector.
In this thesis, three research articles are presented, each focusing on a different aspect related
to the topic. The first article focused on anthropogenic aromatic VOCs, the second paper on
BVOCs, while the third paper presented a receptor modelling and risk assessment study
conducted on all the VOCs measured at Welgegund.
In article one, results indicated that the monthly median (mean) total aromatic hydrocarbon
concentrations ranged between 0.01 (0.011) and 3.1 (3.2) ppb. Benzene levels did not exceed
the local air quality standard limit, i.e. annual mean of 1.6 ppb. Toluene was the most abundant
compound, with an annual median (mean) concentration of 0.63 (0.89) ppb. No statistically
significant differences in the concentrations measured during daytime and night-time were
found, and no distinct seasonal patterns were observed. Air mass back trajectory analysis
indicated that the lack of seasonal cycles could be attributed to patterns determining the origin
of the air masses sampled. Aromatic hydrocarbon concentrations were in general significantly
higher in air masses that passed over anthropogenically impacted regions. Inter-compound
correlations and ratios gave some indications of the possible sources of the different aromatic
hydrocarbons in the source regions defined in the paper. The highest contribution of aromatic
hydrocarbon concentrations to ozone formation potential was also observed in plumes passing
over anthropogenically impacted regions.
In article two, the annual median concentrations of isoprene, 2-methyl-3-butene-2-ol (MBO),
monoterpenes and sesquiterpenes (SQT) during the first campaign were 14, 7, 120 and 8 pptv,
respectively and during the second campaign, 14, 4, 83 and 4 pptv, respectively. The sum of
the concentration of the monoterpenes, with α-pinene being the most abundant species, was at least an order of magnitude higher than the concentrations of other BVOC species during both
sampling campaigns, which was also similar to atmospheric monoterpene levels in other
environments. The highest BVOC concentrations were observed during the wet season, with
elevated soil moisture also associated with increased BVOC concentrations. However,
comparisons with measurements conducted at other landscapes in southern Africa and the rest
of the world that have more woody vegetation indicated that BVOC concentrations were, in
general, significantly lower. Furthermore, the total BVOC concentrations were an order of
magnitude lower compared to total aromatic concentrations measured at Welgegund. An
analysis of concentrations by wind direction indicated that isoprene concentrations were
relatively higher from the western direction, while wind direction did not indicate any significant
differences in the concentrations of the other BVOC species. Statistical analysis indicated that
soil moisture had the most significant impact on atmospheric levels of MBO, monoterpenes and
SQT concentrations, while temperature had the greatest influence on isoprene levels. The
combined O3 formation potentials of all the BVOCs measured calculated with MIR coefficients
during the first and second campaign were 1 162 and 1 022 pptv, respectively. α-Pinene and
limonene had the highest reaction rates with O3, while isoprene exhibited relatively small
contributions to the O3 depletion. Limonene, α-pinene and terpinolene had the largest
contributions to the OH-reactivity of BVOCs measured for all of the months during both
sampling campaigns.
In manuscript three, positive matrix factorisation (PMF) analysis was performed on VOC data
collected at a regional background atmospheric monitoring station affected by the major
sources in the interior of South Africa in order to conduct a source apportionment study. In
addition, a risk assessment study was also performed in view of the major source regions
affecting Welgegund in order to quantify the impacts of anthropogenic VOCs measured at
Welgegund on human health. PMF analysis revealed ten meaningful factor solutions, of which
five factors were associated with biogenic emissions and five with anthropogenic sources. Three
of the biogenic factors were characterised by a specific biogenic species, i.e. isoprene,
limonene and 2-methyl-3-buten-2-ol (MBO), while the other two biogenic factors comprised
mixtures of biogenic species with different tracer species. The temporal factor contribution for
the isoprene, limonene and MBO factors correlated relatively well with the seasonal wet pattern.
Wind roses indicated that Welgegund was affected by biogenic species from all wind directions
in the surrounding environment. Two anthropogenic factors were associated with emissions
from a densely populated anthropogenic source region to the east of Welgegund
(Johannesburg-Pretoria conurbation and Mpumalanga Highveld) with a large number of
industrial activities. An anthropogenic factor was also identified that reflected the influence of
solvents on atmospheric VOC concentrations, while two anthropogenic factors were determined
that indicated the influence of farming activities in close proximity to Welgegund. A non-cancer (hazard ratios) and cancer-risk (lifetime cancer risks) assessment study conducted for VOCs
measured at Welgegund in relation to three source regions identified, indicated that the noncancerous
influence of VOCs measured in the source regions is significantly lower compared to
the cancerous influence of these species on human health, which poses a significant cancer
risk. An assessment of the OH reactivity of anthropogenic VOCs indicated that OH reactivity
was higher for VOCs in air masses passing over a highly industrialised source region, while the
highest OH reactivity was determined for species for which high ozone formation potential was
determined in previous studies | en_US |
