Abstract
Abstract. Ammonia-driven carbonyl-to-imine conversion is an important formation pathway to the nitrogen-containing organic compounds (NOCs) in secondary organic aerosols (SOAs). Previous studies have mainly focused on the dicarbonyl compounds as the precursors of light-absorbing NOCs. In this work, we investigated whether acrolein could also act as an NOC precursor. Acrolein is the simplest α, β-unsaturated mono-carbonyl compound, and it is ubiquitous in the atmosphere. Experiments probing multiphase reactions of acrolein as well as bulk aqueous-phase experiments were carried out to study the reactivity of acrolein towards ammonia and ammonium ions. Molecular characterization of the products based on gas chromatography mass spectrometry, high-resolution mass spectrometry, surface-enhanced Raman spectrometry and ultraviolet/visible spectrophotometry was used to propose possible reaction mechanisms. We observed 3-methylpyridine (commonly known as 3-picoline) in the gas phase in Tedlar bags filled with gaseous acrolein and ammonia or ammonium aerosols. In the ammonium-containing aqueous phase, oligomeric compounds with formulas (C3H4O)m(C3H5N)n and pyridinium compounds like (C3H4O)2C6H8N+ were observed as the products. The pathway to 3-methylpyridine was proposed to be the intramolecular carbon–carbon addition of the hemiaminal, which resulted from sequential carbonyl-to-imine conversions of acrolein molecules. The 3-methylpyridine was formed in the aqueous phase, but some of the 3-methylpyridine could revolatilize to the gas phase, explaining the observation of gaseous 3-methylpyridine in the bags. The (C3H4O)2C6H8N+ was a carbonyl-to-hemiaminal product from acrolein dimer and 3-methylpyridine, while the oligomeric products of (C3H4O)m(C3H5N)n were polymers of acroleins and propylene imines formed via carbonyl-to-imine conversion and condensation reactions. The pH value effect on the liquid products was also studied in the bulk aqueous-phase experiments. While the oligomeric compounds were forming in both acidic and alkaline conditions, the pyridinium products favored moderately acidic conditions. Both the oligomeric products and the pyridinium salts are light-absorbing materials. This work suggests that acrolein may serve as a precursor of light-absorbing heterocyclic NOCs in SOA. Therefore, secondary reactions of α, β-unsaturated aldehydes with reduced nitrogen should be taken into account as a source of light-absorbing NOCs in SOA.
Highlights
Carbonyl compounds are ubiquitous in the gaseous, liquid and particulate phases in the atmosphere (Carlier et al, 1986; Dai et al, 2012; Grosjean, 1982)
The results suggest that acrolein + ammonia/ammonium reactions may lead to lowvolatility nitrogen-containing organic compounds (NOCs) in secondary organic aerosol (SOA)
The dimer is commonly found in acrolein-containing solutions, and it is easier to quantify than acrolein (Bauer et al, 2010, 2012)
Summary
Carbonyl compounds are ubiquitous in the gaseous, liquid and particulate phases in the atmosphere (Carlier et al, 1986; Dai et al, 2012; Grosjean, 1982). After the uptake to the particulate phase, volatile carbonyls may undergo photooxidation (Renard et al, 2015), aldol condensation (Sareen et al, 2010; Yasmeen et al, 2010), oligomerization (De Haan et al, 2011; Renard et al, 2016; Shen et al, 2016), imine (R = NH compound) or hemiaminal (R(OH)-NR2 compound) formation A number of laboratory and field studies have highlighted the importance of carbonyl-to-imine reaction for the formation of light-absorbing nitrogen-containing organic compounds (NOCs) from dicarbonyls (De Haan et al, 2011; Hawkins et al, 2018; Lin et al, 2015). The extended conjugation in the resulting NOC compounds leads to the light absorptivity of the SOA particles containing these compounds (Lee et al, 2013)
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