Optical Property Measurements and Single Particle Analysis of Secondary Organic Aerosol Produced from the Aqueous-Phase Reaction of Ammonium Sulfate with Methylglyoxal

Abstract

Reactions involving the dicarbonyl species methylglyoxal (MG) have been suggested as an important pathway for the production of secondary organic aerosol (SOA) in the atmosphere. Reaction in an aqueous inorganic salt solution, such as ammonium sulfate (AS), leads to the formation of light-absorbing brown carbon (BrC) product. We report on an investigation of the optical properties of BrC aerosol generated from the aqueous-phase reaction between MG and AS as a function of aging time using calibrated cavity ring-down spectroscopy (CRDS) at a wavelength of 403 nm. The retrieved real index of refraction at 403 nm is n = 1.558 ± 0.021, with an imaginary index value of k = 0.002 ± 0.004; these values do not appear to change significantly with aging time over the course of 22 days and are similar to the AS aerosol values. The small complex index suggests that BrC aerosol formed from this pathway may not significantly impact radiative forcing. Measurements of the aerosol optical properties show significant deviat...