Abstract :
2-Methyltetrols produced by the oxidation of isoprene have been recently found to contribute toward the formation of atmospheric secondary organic aerosol (SOA). However, the oxidation mechanism relevant to the formation of these polyols has not been completely understood. In this study, the photooxidation of four conjugated dienes (isoprene, 1,3-butadiene, 2,3-dimethyl-1,3-butadiene and 2,4-hexadiene) in the presence of 0.2–1 ppmv NO was examined by a series of laboratory experiments, and the polyols, organic acids and nitric acid in an aqueous solution of the resulting SOA were analysed by using ion-exclusion liquid chromatography/mass spectrometry (LC–MS). In the experiments performed using isoprene, 2-methyltetrols (comprising 0.5–2% of aerosol mass), methylnitrooxybutanetriols (comprising 1–7% of aerosol mass), methyldinitrooxybutanediols (comprising 0.3–8% of aerosol mass) and nitric acid (comprising 4–9% of aerosol mass) were found in the aqueous solution of the SOA samples. Three days after the extraction, the concentrations of nitrooxypolyols (i.e. methylnitrooxybutanetriols and methyldinitrooxybutanediols) decreased, whereas the concentrations of polyols and nitric acid increased. Similar results were obtained for all the four dienes. Nitrooxypolyols, which are produced by the gas-phase oxidation of dienes in the presence of NOx, contribute toward the SOA formation, and these compounds can decompose to polyols and nitric acid in an aqueous solution. The polyols and the nitric acid present in the aqueous solution are hydrolysis products, and not real constituents of aerosol. The direct gas-phase formation of polyols from the diene oxidation is suppressed in the presence of NOx.
