Nitrogen-containing Secondary Organic Aerosols Formation by Acrolein Reaction with Ammonia/Ammonium

Ammonia-driven carbonyl-to-imine conversion is an important formation pathway to the nitrogen-containing organic compounds (NOC) in secondary organic aerosols (SOA). Previous studies have mainly focused on the dicarbonyl compounds as the precursors of light-absorbing NOC. In this work, we investigated whether acrolein could also act as a NOC precursor. Acrolein is the simplest α,β-unsaturated mono-carbonyl compound, and it is ubiquitous in the atmosphere. Experiments probing gas-phase and surface reactions of acrolein as well as bulk liquid-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 spectrometry was used to propose possible reaction mechanisms. We observed 3-methyl pyridine (also called 3-picoline) in the gas phase in the Tedlar bag filled with gaseous acrolein and ammonia. In the liquid phase, oligomeric compounds with formulas (C3H4O)m(C3H5N)n and pyridinium compounds like (C3H4O)2C6H8N+ were observed as the products of acrolein reaction with ammonium ions. The 3-picoline could be the product of acrolein reaction with gaseous ammonia in the gas phase. The pathway to 3-picoline was proposed to be the intramolecular carbon-carbon addition of the hemiaminal which resulted from sequential carbonyl-to-imine conversions of acrolein molecules. The similar reaction of dissolved acrolein with ammonium ions leads to the formation of 3-methyl pyridinium (also called 3-picolinium) cations in the liquid phase. The (C3H4O)2C6H8N+ was a carbonyl-to-hemiaminal product from acrolein dimer and 3-picolinium cations, while the oligomeric products of (C3H4O)m(C3H5N)n were polymers of acroleins and propylene imines via carbonyl-to-imine conversion and condensation reactions. Part of the 3-picolinium could re-volatilize to the gas phase as 3-picoline, explaining the observation of gaseous 3-picoline in the bag filled with acrolein and ammonium sulfates (or chlorides) aerosols. The pH value effect on the liquid products was also studied in the bulk liquid-phase experiments. Compared to the oligomeric compounds forming in both acid and alkaline conditions, there is a tendency for the pyridinium products to be formed under moderately acidic conditions. Both the oligomeric products and the pyridinium salts are light absorbing materials. This work suggests a potential role for acrolein reaction with ammonia/ammonium as a source of light-absorbing heterocyclic NOC in SOA. Therefore, secondary reactions of α,β-unsaturated aldehydes with reduced nitrogen should be taken into account when evaluating climate and health effects of SOA.