UV/Vis and IRMPD Spectroscopic Analysis of the Absorption Properties of Methylglyoxal Brown Carbon

Brown carbon (BrC) organic molecules absorb solar radiation in the visible range and thus can influence the optical properties of atmospheric aerosol particles and cloud droplets. The absorption properties for different types of BrC can change depending on the chemical environment of the chromophores. Here, the absorption cross sections and the photo-decay rates for BrC molecules formed from methylglyoxal (MG) mixed with ammonium sulfate (MG BrC) are investigated as a function of pH. In solutions acidified to pH similar to 2, MG BrC shows an similar to 16 nm blueshift in peak absorbance compared to neutral solutions (pH similar to 8). To probe the origins of this shift, the gas-phase infrared ion spectra of protonated and neutral imidazole carbonyl (IC), one of the main components of MG BrC, have been measured to provide insights into their structures. Excited-state energy calculations based on these structures suggest that the blueshift occurs for IC in part because protonation of the basic nitrogen in the imidazole ring inhibits conjugation with the carbonyl group attached to the ring. When the photolysis rate of a MG BrC solution that contains IC is extrapolated to atmospheric conditions, the observed blueshift at low pH increases the lifetime of the BrC mixture by similar to 42%, with an estimated atmospheric lifetime of similar to 82 min. These results highlight the importance of understanding how BrC photolysis rates vary across different environmental conditions.