Abstract
Three groups of aliphatic carbonyl compounds, the n-alkanals (C-8-C-20), n-alkan-2-ones (C-8-C-26), and n-alkan-3-ones (C-8-C-19), were measured in both particulate and vapour phases in air samples collected in London from January to April 2017. Four sites were sampled including two rooftop background sites, one ground-level urban background site, and a street canyon location on Marylebone Road in central London. The n-alkanals showed the highest concentrations, followed by the n-alkan-2-ones and the n-alkan-3-ones, the latter having appreciably lower concentrations. It seems likely that all compound groups have both primary and secondary sources and these are considered in light of published laboratory work on the oxidation products of high-molecular-weight n-alkanes. All compound groups show a relatively low correlation with black carbon and NOx in the background air of London, but in street canyon air heavily impacted by vehicle emissions, stronger correlations emerge, especially for the n-alkanals. It appears that vehicle exhaust is likely to be a major contributor for concentrations of the n-alkanals, whereas it is a much smaller contributor to the n-alkan-2-ones and n-alkan-3-ones. Other primary sources such as cooking or wood burning may be contributors for the ketones but were not directly evaluated. It seems likely that there is also a significant contribution from the photo-oxidation of n-alkanes and this would be consistent with the much higher abundance of n-alkan-2-ones relative to n-alkan-3-ones if the formation mechanism were through the oxidation of condensed-phase alkanes. Vapour-particle partitioning fitted the Pankow model well for the n-alkan-2-ones but less well for the other compound groups, although somewhat stronger relationships were seen at the Marylebone Road site than at the background sites. The former observation gives support to the n-alkane-2-ones being a predominantly secondary product, whereas primary sources of the other groups are more prominent.