Publication date: May 2018
Source:Atmospheric Environment, Volume 181
Author(s): Yayi Yi, Zhaoyu Cao, Xuehua Zhou, Likun Xue, Wenxing Wang
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 π–π* transitions of Schiff bases and the n–π* 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 (kI = 2.39 × 10−6 s−1) > MAm (kI = 1.19 × 10−6 s−1) > AS (kI = 8.33 × 10−7 s−1) at identical low AS/amine concentrations and were in the order of MAm (kI = 2.5 × 10−6 s−1) > AS (kI = 1.39 × 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.
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