Formation of In-Cloud Aqueous-Phase Secondary Organic Matter and Related Characteristic Molecules

Atmospheric particles are largely represented by secondary organic aerosols (SOAs) produced by either aqueous- or gas-phase reactions. Recently, the contribution of the former to SOA formation has been shown to substantially increase and even reach that of the latter, which necessitates in-depth mechanistic investigations. For a deeper understanding of aqueous-phase SOA generation, we herein studied the production of these aerosols in the dark from glycolaldehyde (GAld) and ammonium sulfate (AS)/amines (methylamine (MAm) and glycine (Gly)). UV-vis spectroscopy showed that reaction mixtures featured two main absorption bands (at 209-230 and 280-330 nm) that were attributed to the pi-pi* transitions of Schiff bases and the n-pi* transitions of oligomers produced in the above reactions, respectively. Further studies revealed that irrespective of reactant concentration and pH, all the investigated reactions were well fitted by first-order kinetics and were accelerated by increasing AS/MAm concentrations and solution pH under acidic conditions. The reaction rate constants (determined from changes of absorption at 300 nm) followed the order of Gly (k(I) = 2.39 x 10(-6) s(-1)) > MAm (k(I) = 1.19 x 10(-6) s(-1)) > AS (k(I) = 8.33 x 10(-7) s(-1)) at identical low AS/amine concentrations and were in the order of MAm (k(I) = 2.5 x 10(-6) s(-1)) > AS (k(I) = 1.39 x 10(-6) s(-1)) at high AS/MAm concentrations. The main reaction pathways corresponded to the aldol self-condensation of GAld and the nucleophilic attack of AS/amines on GAld followed by dehydration, which afforded imines as the major products. The stronger light absorption of (GAld + Gly) mixtures than that of (glyoxal/methylglyoxal + Gly) mixtures was ascribed to the increased amount of imine- and carbonyl group-containing products produced in the former case.