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 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, 26, 6, 13 and 3 % of species tentatively identified were oxygenated species, hydrocarbons, halogenated compounds, N-containing 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 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.