Volatile organic compound measurements at a grazed savannah grassland in South Africa

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