Aqueous-Phase Secondary Organic Aerosol and Organosulfate Formation in Atmospheric Aerosols: A Modeling Study

We have examined aqueous-phase secondary organic aerosol (SOA) and organosulfate (OS) formation in atmospheric aerosols using a photochemical box model with coupled gas-phase chemistry and detailed aqueous aerosol chemistry. SOA formation in deliquesced ammonium sulfate aerosol is highest under low-NOx conditions, with acidic aerosol (pH = 1) and low ambient relative humidity (40%). Under these conditions, with an initial sulfate loading of 4.0 ?g m?3, 0.9 ?g m?3 SOA is predicted after 12 h. Low-NOx aqueous-aerosol SOA (aaSOA) and OS formation is dominated by isoprene-derived epoxydiol (IEPOX) pathways; 69% or more of aaSOA is composed of IEPOX, 2-methyltetrol, and 2-methyltetrol sulfate ester. 2-Methyltetrol sulfate ester comprises >99% of OS mass (66 ng m?3 at 40% RH and pH 1). In urban (high-NOx) environments, aaSOA is primarily formed via reversible glyoxal uptake, with 0.12 ?g m?3 formed after 12 h at 80% RH, with 20 ?g m?3 initial sulfate. OS formation under all conditions studied is maximum at low pH and lower relative humidities (<60% RH), i.e., when the aerosol is more concentrated. Therefore, OS species are expected to be good tracer compounds for aqueous aerosol-phase chemistry (vs cloudwater processing).