The characterisation of secondary organic aerosol formed during the photodecomposition of 1,3-butadiene in air containing nitric oxide

The formation of secondary organic aerosol (SOA) at yields of 0.4–0.5% and having a geometric mean diameter <100 nm has been observed during indoor environmental chamber experiments on 1.0–2.2 ppmv 1,3-butadiene in the presence of 0.5–1.1 ppmv NO. The SOA was collected on glass fibre filters, some of which were acetylated using a pyridine/acetic anhydride mixture immediately after collection. Analysis of the SOA by Fourier transform infrared spectroscopy (FTIR) resulted in bands assigned to OH stretching in alcoholic and carboxylic hydroxyl groups, NO stretching in NO3 and CO stretching at 1728 cm−1, the latter indicative of formate esters rather than aldehydes or ketones. Initial NMR spectra showed a broad polymeric-like feature, which separated into peaks representative of monomeric units as the SOA hydrolysed over 3 days. Subsequent GC-MS and NMR analyses were used to identify 18 species, which represented 75–80% of the SOA mass. Some of the unidentified mass is probably composed of organic nitrates. Low vapour pressure (⩽10−7 Torr) species detected were glycerol, threitol, erythritol and isomeric forms tentatively identified as threonic and erythronic acid nitrate. Gel permeation chromatography of acetylated SOA gave a polymer molecular weight distribution range up to ∼4.0×105 g mol−1, with a peak molecular weight of 6.12×104 g mol−1. A chemical mechanism for the formation of endogenous seed aerosol directly from 1,3-butadiene is presented. It is proposed that the SOA is polymeric and composed of C1–C4 oxygenated species, including formate esters and hemiacetal formates.