(8d) Aqueous-Phase Organic-Inorganic Aerosol Chemistry: Model Simulations

We have developed GAMMA (Gas-Aerosol Model for Mechanism Analysis), a photochemical box model  with coupled gas-phase chemistry and detailed aqueous aerosol chemistry. The aqueous-phase mechanism includes organosulfate formation via recently proposed epoxide and sulfate radical pathways.  The model also simulates changes in aerosol absorption caused by the dark formation of light-absorbing organics from methylglyoxal, glyoxal, and acetaldehyde (e.g. Schwier et al. 2010, Noziere et al. 2007).

We find that under low-NOx conditions, organosulfate and SOA formation from isoprene is dominated by the isoprene-derived epoxydiol pathways proposed by Paulot  et al. (2009) and Surratt et al. (2009). >80% of SOA mass after 12 hours is composed of 2-methyltetrol, and 13% is 2-methyltetrol sulfate ester. 2-methyltetrol sulfate ester comprises over 99% of organosulfate mass under these conditions.  Under high-NOx conditions, organosulfate species make a negligible contribution to isoprene-derived SOA mass.

Overnight, the formation of light-absorbing compounds from methylglyoxal and glyoxal is responsible for <10% of SOA mass. However, sufficient concentrations of light-absorbing products are achieved to significantly affect aerosol optical properties. GAMMA output is used to predict aerosol single scattering albedo (SSA) and absorption Angstrom exponent based on laboratory data, Kramers-Kronig, and Mie theory.