Abstract
Abstract. Three groups of aliphatic carbonyl compounds, the n-alkanals (C8–C20), n-alkan-2-ones (C8–C26), and n-alkan-3-ones (C8–C19), 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.
Highlights
The most abundant atmospheric carbonyls are methanal and ethanal, and many studies have described their emission sources and chemical formation in urban and rural samples (Duan et al, 2016)
The first campaign used two sampling sites, one located on the roof of a building (15 m above ground) of the Regent’s University (51°31′N, -0°9′W), hereafter referred to as RU, sampled from 23 January 2017 to 19 February 2017, the other located on the roof (20 m above ground) of a building which belongs to the University of Westminster on the southern side of Marylebone Road, sampled from 24 January 2017 to 20 February 2017
The third sampling site was located at ground level at Eltham (51°27′N, 0°4′E), hereafter referred to as EL, sampled from 23 February 2017 to 21 March 2017, which is located in suburban south London, and the fourth sampling site was located at ground level on the southern side of Marylebone Road
Summary
The most abundant atmospheric carbonyls are methanal (formaldehyde) and ethanal (acetaldehyde), and many studies have described their emission sources and chemical formation in urban and rural samples (Duan et al, 2016). Long-chain aliphatic carbonyl compounds have been identified in PM and reported in few published papers (Gogou et al, 1996; Andreou and Rapsomanikis, 2009), and these compounds are considered to be formed from atmospheric oxidation processes affecting biogenic emissions of alkanes. It was believed that the n-alkanals with carbon atoms numbering less than 20 indicate oxidation of alkanes, whereas the higher compounds were usually considered to be of direct biogenic origin (Rogge et al, 1998). The homologues and isomers of n-alkanals and n-alkanones have been identified as OH oxidation products of n-alkanes in many chamber and flow tube studies (Zhang et al, 2015; Schilling Fahnestock et al, 2015; Ruehl et al, 2013). The chamber studies of dodecane oxidation have identified 1-undecanal, hexan-3-one, octan-3-one, heptan-2-one, nonan-2-one and decan-2-one as OH oxidation products (Schilling Fahnestock et al., 2015; Yee et al, 2012)
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