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Characterization of secondary organic aerosol from photooxidation of isoprene and alpha-pinene with mass spectrometric approaches

Final Activity Report Summary - SOAMASS (Characterization of secondary organic aerosol from photooxidation of isoprene and alpha-pinene with mass spectrometric approaches)

The SOAMASS project dealt with the chemical characterisation of secondary organic aerosol (SOA) from volatile organic compounds that are oxygenated compounds which are formed through photooxidation in the atmosphere, partition to the particle phase owing to their low-volatility or are retained in the particle phase owing to heterogeneous reactions.

Emphasis was given to the structural characterisation of unknown SOA tracers from the photooxidation of alpha-pinene which was emitted in large amounts by conifers. In particular, an unknown compound with molecular weight of 204 was structurally elucidated as the C8-tricarboxylic acid, 3-methyl-1,2,3-butanetricarboxylic acid. This compound was a higher-generation photooxidation product of alpha-pinene and was present at significant concentrations in ambient fine aerosol from forested sites during summertime when alpha-pinene emissions were high. To this aim, reference compounds were synthesised and their chromatographic and mass spectrometric characteristics were compared to those of the unknown compound.

A similar strategy was followed for the characterisation of a second unknown alpha-pinene SOA tracer with molecular weight of 172. For this compound evidence was obtained that it was related to 3-methyl-1,2,3-butanetricarboxylic acid, nevertheless further efforts were required to firmly elucidate its structure.

Emphasis was also given to the structural characterisation of unknown SOA tracers from the photooxidation of isoprene which was emitted in large amounts from broadleaf trees, in particular to the characterisation of organosulfates and nitrooxy organosulfates. The latter SOA tracers had only been recently discovered and served as ambient tracers for SOA formation under acidic conditions. Organosulfates had a mixed biogenic and anthropogenic origin considering that the sulfate part in the molecules was mainly from anthropogenic origin. A number of organosulfates related to isoprene SOA were characterised in ambient fine aerosol from K-puszta, Hungary, i.e. sulfate derivatives of 2-methyltetrols, 2-methylglyceric acid, 2-methyltetrol mono-nitrate derivatives, glyoxal and methylglyoxal. In addition, unknown polar organosulfates were structurally characterised as derivatives of photooxidation products of unsaturated fatty acids. These compounds were of climatic relevance because they might contribute to the hydrophilic properties of fine ambient aerosol.

Furthermore, a mechanistic study was undertaken in order to gain more insights into the formation of the 2-methyltetrols. This laboratory study showed that the yield of 2-methyltetrols was significantly enhanced in the presence of sulfuric acid. A formation mechanism involving acid-catalysed degradation of gas-phase oxidation products could be formulated. Unlike alpha-pinene SOA that was formed through gas-to-particle partitioning of low-volatility gas-phase products, isoprene SOA also required a reaction in the particle phase by which polar SOA was generated from gas-phase oxidation products and could be retained in that phase.