Spectroscopic and Photochemical Properties of Secondary Brown Carbon from Aqueous Reactions of Methylglyoxal

Abstract

Chromophoric organic compounds in the atmosphere termed brown carbon (BrC) play an important role in tropospheric photochemistry and radiative forcing due to their absorption in the near-UV region of the solar spectrum (300–400 nm). BrC chromophores can form in secondary reactions of carbonyls with ammonium and amine precursors commonly found in aqueous aerosols and cloud water, although there is still uncertainty regarding the contribution of these reactions to atmospheric BrC. Herein, experimental results are presented characterizing the spectroscopic properties of BrC generated from aqueous aerosol/cloud water mimics containing methylglyoxal and ammonium sulfate (MG + AS) or methylamine chloride (MG + MA). Spectroscopic results include mass absorption coefficients, fluorescence quantum yields, and excitation–emission matrix (EEM) spectra, including parallel factor (PARAFAC) analysis. This work also provides a detailed comparison to previous laboratory and field measurements on the classification of aerosols and environmental water with EEM spectroscopy. In addition to spectroscopic characterization, the chemical composition of these samples is investigated using electrospray ionization mass spectrometry and shows the formation of oligomers of methylglyoxal- and nitrogen-containing heterocycles as contributors to absorption in the near-UV consistent with the previous work. The BrC samples were also irradiated using a solar simulator to study the changes in the optical properties and estimate the atmospheric photolytic lifetimes, which have not been previously reported for MG + MA BrC. These results reveal that BrC derived from MG + MA is more stable under solar irradiation than MG + AS BrC with measured atmospheric photolytic half-lives of 39 ± 8 and 12 ± 3 min, respectively.