Probing the Hygroscopicity, Phase, and Chemical Composition of Methylglyoxal-Glycine Aqueous Brown Carbon Aerosol: A Multi-Instrument Approach

Methylglyoxal (MeGX) and glyoxal are commonly occurring α-dicarbonyl compounds that react with nitrogen-containing species to produce aqueous brown carbon (aqBrC), including imidazoles and oligomeric products. The composition and physical properties of aerosol particles that contain these species will influence their effects in the environment via their role in cloud formation, light scattering, and further chemistry. Here, we produce aqBrC from the reaction of MeGX and glycine (Gly) in bulk solution at three possible molar mixing ratios (1:1, 1:2, and 2:1) aged for up to 11 weeks. Individual particles generated from both fresh and aged solutions were levitated in a linear quadrupole electrodynamic balance to measure the hygroscopic growth, phase state, and chemical composition of single particles as a function of bulk aging time and relative humidity (RH). Complementary hygroscopic growth and phase state measurements were conducted using a humidified-tandem differential mobility analyzer and quartz crystal microbalance. Hygroscopic growth measurements reveal a slight decrease in growth with increasing aging time, and a corresponding increase in the refractive index (RI). At low RH, the phase state is shown to be a strong function of the starting composition and drying rate, with samples containing equal quantities of reactants consistently observed to form amorphous phase states, with a corresponding slowing of water diffusion, indicating an increase in viscosity. Compared with bulk solution aging, levitated droplets show much faster product formation. Compounds detected after >3 days in bulk solution appear within <10 min in the droplets. Conversely, other products identified after >3 days of bulk aging were not observed in droplets over the measured time scales. These results show that bulk aqueous aging alone is not sufficient to predict aqBrC aerosol properties. The particle composition, phase state, and water uptake are shaped by the coupled effects of precursor ratio, RH history, drying rate, and accelerated chemistry during and after water loss.