Size-resolved characterisation of organic compounds in atmospheric aerosols collected at Welgegund, South Africa

Abstract

The organic fraction of atmospheric aerosols is 20 to 90 % of which only a small percentage has been chemically characterised. Two-dimensional gas chromatography with a time-of-flight mass spectrometer (GCxGC-TOFMS) is a powerful instrument used to chemically characterise organic compounds. Size-resolved characterisation and semi-quantification of ambient organic aerosol compounds were performed with a GCxGC-TOFMS for the first time in South Africa. Twenty-four-hour samples were collected for 1 year for three different size ranges. A combined total of 1056 different organic compounds could be tentatively characterised. The largest number of organic compounds tentatively identified was PM 2.5-1 (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, 26, 6, 13 and 3 % of species tentatively identified were oxygenated species, hydrocarbons, halogenated compounds, Ncontaining compounds and S-containing compounds, respectively. Oxygenated compounds were the most abundant species. Alkane and mono-aromatic 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 PM 2.5-1 size fraction. The largest number of N-containing J Atmos Chem species tentatively characterised with the highest ∑NRFs were amines. A small number of Scontaining 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.

Plume characterization of a typical South African braai

“…A significant smaller number of compounds were pre-identified for the braai experiment, i.e. 50, compared to the 1056 organic compounds pre-identified by Booyens et al 15 This can be attributed to the difference in sampling periods, i.e. Booyens et al 15 collected 24-hour samples, while aerosol samples were only collected for~2 hours.…”

“…50, compared to the 1056 organic compounds pre-identified by Booyens et al 15 This can be attributed to the difference in sampling periods, i.e. Booyens et al 15 collected 24-hour samples, while aerosol samples were only collected for~2 hours. In addition, the number of organic compounds presented by Booyens et al 15 was the combined number of organic compounds pre-identified for the entire one-year sampling campaign.…”

“…These filters were used to characterize and semi-quantify organic compounds. Each filter was extracted with a 1:1 mixture of methanol and acetone by utilizing dynamic ultrasonic assisted extraction for 40 min as described by Booyens et al 15 The analysis was performed with comprehensive twodimensional gas chromatography coupled with a time-of-flight mass spectrometer (GCxGC-TOFMS). A LECO Pegasus 4D GCxGC-TOFMS system equipped with an Agilent 7890 A GC and an Agilent 7683 B auto sampler was used.…”

“…In this study, 1-1'binaphthyl was used as an internal standard and the relative concentrations of the organic compounds were expressed as the sum of the normalized response factors (SNRF), which is a measure of the detector response of an analyte compared to the internal standard. The calculation of these SNRF values is described in Booyens et al 15 In addition to the atmospheric measurements, a thermocouple was also used to determine the temperature in the centre of the briquettes during the different braai phases previously indicated, i.e. fire and smoke, stable, grill and post.…”

“…The results can, however, be compared to a one-year sampling campaign conducted by Booyens et al at Welgegund. 15 In Fig. 14a, the total number of compounds pre-identified in each of the size fractions and their corresponding SNRF are presented.…”