Measurement of organic compounds in atmospheric aerosols collected at Welgegund, South Africa
Atmospheric aerosols or particulate matter (PM) are a complex mixture of solid and liquid particulates suspended in the atmosphere that originate from natural and anthropogenic sources. These species influence climate change and general air quality, while also having other detrimental impacts on the environment (e.g. acidification, eutrophication). These impacts of atmospheric aerosols are determined by their physical and chemical properties. Atmospheric aerosols consist of inorganic and organic chemical species, of which the organic fraction is estimated to contribute between 20 and 90 % of the total PM in the atmosphere. It is therefore important to characterise these organic species in atmospheric aerosols in order to establish the impacts of these species on the environment and human health. Atmospheric aerosols consist of thousands of organic compounds with various chemical and physical properties. At present, little is known about the actual chemical composition of atmospheric organic compounds, which necessitates the development and employment of new methods to allow for more precise speciation of organic aerosol compounds. One such method is comprehensive twodimensional gas chromatography coupled with a time-of-flight mass spectrometer (GCxGC-TOFMS), which is a powerful instrument used for the chemical characterisation of organic compounds in complex matrices. GCxGC-TOFMS has been successfully applied in characterising a wide range of organic compounds present in complex ambient atmospheric samples. Therefore, the aim of this study was to characterise and semi-quantify ambient organic aerosols collected in different size ranges at a regional background site in South Africa using GCxGC-TOFMS, which will be the first time that this technique has been used for analysis of ambient aerosols in South Africa. South Africa has the largest industrialised economy in Africa and is considered to be a significant source of atmospheric pollutants. However, the region is still considered to be understudied with regard to atmospheric measurements, especially relating to the characterisation of ambient atmospheric aerosols. Aerosol samples were collected at the Welgegund atmospheric monitoring station, which is a comprehensively equipped atmospheric measurement station located 100 km west of Johannesburg. Welgegund is considered to be a regional background site with no sources in close proximity. It is, however, impacted by major source regions in the north-eastern interior of South Africa. Size-resolved (PM1, PM2.5-1 and PM10-2.5) 24-hour aerosol samples were collected once a week for one year from 12 April 2011 to 4 April 2012. This is the most comprehensive number of size-resolved ambient atmospheric aerosol samples collected in South Africa for the characterisation of organic compounds. The collected samples were analysed using a GCxGC-TOFMS, which detected a large number of peaks (approximately 8 000 compounds). Procedures and rules were applied in order to optimise the number of compounds identified, as well as to increase the reliability of organic compounds characterised. In order to characterise more organic compounds detected using the GCxGC-TOFMS for the samples, less restrictive positive characterisation parameters were applied and the compounds were therefore considered tentatively characterised. The concentrations of the large number of organic compounds that were tentatively characterised were expressed as response factors (RF) in relation to an internal standard, i.e. 1-1'binaphthyl. A combined total of 1 056 different organic compounds could be tentatively characterised. The largest number of organic compounds tentatively identified was associated with PM2.5-1 (particles in the size range 1-2.5 μm), while this size fraction also had the highest total number of normalised response factors (?NRF). On average, 52 %, of species tentatively identified were oxygenated species, while 26 %, 6 %, 13 % and 3 % of the species tentatively characterised were hydrocarbons, halogenated compounds, N-containing compounds and S-containing compounds, respectively. Alkane and monoaromatic species were the largest number of hydrocarbons tentatively identified with the highest ?NRFs. The largest number of oxygenated species tentatively characterised were carboxylic acids and esters, while ether compounds had the highest ?NRFs. Most of the halogenated compounds tentatively identified were chlorinated species with the highest ?NRFs in two size fractions. Iodate species had a significantly higher ?NRF in the PM2.5-1 size fraction. The largest number of N-containing species tentatively characterised with the highest ?NRFs were amines. A small number of S-containing compounds with low ?NRFs were tentatively identified. The major sources of organic compounds measured at Welgegund were considered to be biomass burning and air masses moving over the anthropogenically impacted source regions. An assessment of polar organic aerosol compounds, i.e. oxygenated species (alcohols, ethers, aldehydes, ketones, carboxylic acids, esters), halogenated compounds (Cl, Br, I, F), as well as nitrogen (N)- and sulphur (S)-containing organic compounds characterised at Welgegund was conducted in order to provide a more detailed picture of organic aerosol composition for southern Africa. The influence of meteorological conditions and major sources impacting air masses measured at Welgegund on polar organic compounds measured was assessed. No distinct seasonal pattern was observed for the total number of polar organic compounds tentatively characterised and their corresponding semi-quantified concentrations (?NRFs). There was, however, a period during late winter and early spring with a significantly lower total number of polar organic compounds and corresponding ?NRFs, while it also seemed that the total numbers of polar organic compounds and the corresponding ?NRFs for the period from mid-autumn to mid-winter were relatively higher compared to the period from late spring to midautumn. The influences of source regions, meteorology and open biomass burning were investigated in order to assess the temporal variability. A relatively lower total number of polar organic compounds and the corresponding ?NRFs could be attributed to fresher plumes arriving at Welgegund from a source region relatively close to Welgegund, while a relatively higher total number of polar organic compounds and the corresponding ?NRFs were associated with aged air masses passing over another source region and the regional background. Meteorological parameters indicated that the wet removal of aerosols during the wet season contributed to a lower total number of polar organic compounds and associated ?NRFs, while increased anticyclonic recirculation and more pronounced inversion layers in winter contributed to a higher total number of polar organic compounds and corresponding ?NRFs. The large-scale influence of biomass burning on organic aerosol compounds was also indicated by fire pixel counts. The period with significantly lower total number of polar organic compounds and the corresponding ?NRFs was attributed to fresh biomass burning plumes from wild fires occurring within close proximity of Welgegund, consisting mainly of volatile organic compounds and non-polar hydrocarbons. Multiple linear regression (MLR) was performed in an effort to quantify the influence of each of these factors on the total number of polar organic compounds and their corresponding ?NRFs, which supported the hypothesis that the temporal variations were related to a combination of the influence of source regions, meteorology and the occurrence of wild fires within close proximity of Welgegund. Although atmospheric organic N compounds are considered important within the global N cycle, these species are not that well understood, which can be attributed to the lack of consistency in sampling and measurement techniques, as well as their chemical complexity. Therefore, the characteristics of organic N compounds identified and semi-quantified using GCxGC-TOFMS in aerosol samples collected at Welgegund were assessed. 135 atmospheric organic N compounds were tentatively characterised and semi-quantified, which included amines, nitriles, amides, urea, pyridine derivatives, amino acids, nitro- and nitroso compounds, imines, cyanates and isocyanates, and azo compounds. Nearly half of the semi-quantified concentrations of organic nitrogen compounds was attributed to amines (51 %), while nitriles, pyridine derivatives and amides comprised 20 %, 11 % and 8 %, respectively, of the semi-quantified concentrations. The semi-quantified concentrations of the other organic N functional groups were very low. The temporal variations of amines, nitriles, amides and pyridine derivatives were similar to that observed for all the polar organic compounds, i.e. a period between 12 April 2011 and 12 July 2011 coinciding with the dry season with elevated semi-quantified concentrations of these species. These temporal variations were attributed mainly to meteorological parameters and the influence of local open biomass burning. Anthropogenic sources in the major source regions impacting air masses measured at Welgegund, as well as regional agricultural activities, were considered the major sources of amines. The regional influence of household combustion was considered the main sources of nitriles and amides. Most of the other organic N functional groups were most likely related to the influence of local and regional agricultural activities. This is the first time that atmospheric particulate organic N species were identified and semi-quantified for southern Africa, while, in general, only a few studies have been conducted globally utilising GCxGC-TOFMS to characterise atmospheric organic N.