Radical-Initiated Brown Carbon Formation in Sunlit Carbonyl–Amine–Ammonium Sulfate Mixtures and Aqueous Aerosol Particles

Abstract

Brown carbon (BrC) formed from glyoxal+ammonium sulfate (AS) and methylglyoxal+AS reactions photobleaches quickly, leading to the assumption that BrC formed overnight by Maillard reactions will be rapidly destroyed at sunrise. Here, we tested this assumption by reacting glyoxal, methylglyoxal, glycolaldehyde, or hydroxyacetone in aqueous mixtures with reduced nitrogen species at pH 4–5 in the dark and in sunlight (>350 nm) for at least 10 h. The absorption of fresh carbonyl+AS mixtures decreased when exposed to sunlight, and no BrC formed, as expected from previous work. However, the addition of amines (either methylamine or glycine) allowed BrC to form in sunlight at comparable rates as in the dark. Hydroxyacetone+amine+AS aqueous mixtures generally browned faster in sunlight than in the dark, especially in the presence of HOOH, indicating a radical-initiated BrC formation mechanism is involved. In experiments with airborne aqueous aerosol containing AS, methylamine, and glyoxal or methylglyoxal, browning was further enhanced, especially in sunlight (>300 nm), forming aerosol with optical properties similar to “very weak” atmospheric BrC. Liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) analysis of aerosol filter extracts indicates that exposure of methylglyoxal+AS aqueous aerosol to methylamine gas, sunlight, and cloud processing increases incorporation of ammonia, methylamine, and photolytic species (e.g., acetyl radicals) into conjugated oligomer products. These results suggest that when amines are present, photolysis of first-generation, “dark reaction” BrC (imines and imidazoles) initiates faster, radical-initiated browning processes that may successfully compete with photobleaching, are enhanced in aqueous aerosol particles relative to bulk liquid solutions, and can produce BrC consistent with atmospheric observations.

Kinetics, Products, and Brown Carbon Formation by Aqueous-Phase Reactions of Glycolaldehyde with Atmospheric Amines and Ammonium Sulfate

“…Unlike in experiment 1, cloud processing (even in simulated sunlight) did not reduce the imaginary part of the index of refraction measured in dried aerosol at 450 nm. These observations indicate that BrC produced in multiphase GAld + AS + methylamine reactions is resistant to drying and also to hydrolysis and photobleaching, as was recently observed in a study conducted on bulk liquid water solutions …”

“…Imidazole derivatives are typical thermodynamic end points for Maillard dark chemistry. 41 In unprocessed GAld + AS seed aerosol (experiment 3), the average number of N atoms/molecule (weighted by peak area) 39 was 2.0, which supports the dominance of diimine and especially imidazole products in terms of concentrations, electrospray ionization efficiencies, or both.…”

“…Setting f Ald = 1.0 instead of 0.053 results in a factor of ∼20 acceleration of reaction rates in aerosol particles relative to bulk liquid, which is quite modest compared to that observed for glyoxal–AS or glyoxal–amine reactions. 18 , 38 Furthermore, fast photolytic radical-initiated oligomerization reactions between aldehyde and amine species in suspended aqueous aerosol particles have also been reported 39 but are not included in these estimates. We explore GAld reactions in aqueous aerosol in the laboratory measurements described in the next section.…”

“…These observations indicate that BrC produced in multiphase GAld + AS + methylamine reactions is resistant to drying and also to hydrolysis and photobleaching, as was recently observed in a study conducted on bulk liquid water solutions. 39 3.3. Aerosol-Phase Reaction Products.…”

Reaction of Glyoxal and Ammonium as a Potential Contributor to Protein-like Fluorescence in Atmospheric Measurements

“…Although, the photolysis lifetime in cloud water is >1 h suggesting that the near-UV absorption of GL + AS BrC can persist for longer periods of time in this phase and potentially play a more important role in radiative processes. It is important to note that in addition to reaction in bulk-phase liquid as studied here, glyoxal and ammonium can also participate in surface reactions in droplets and particles, which can lead to differences in the predicted photochemistry of the corresponding BrC. , …”