New Insights into the Brown Carbon Chromophores and Formation Pathways for Aqueous Reactions of α-Dicarbonyls with Amines and Ammonium

Comprehensiveoptical and structural characterizations reveala large fraction of new BrC chromophores in aqueous reactions of & alpha;-dicarbonylswith amines and ammonium; the formation and transformation of thesechromophores depend strongly on the precursors and reaction pH. Aqueous-phasereactions of & alpha;-dicarbonylswithammonium or amines have been identified as important sources of secondarybrown carbon (BrC). However, the identities of most chromophores inthese reactions and the effects of pH remain largely unknown. In thisstudy, the chemical structures, formation pathways, and optical propertiesof individual BrC chromophores formed through aqueous reactions of & alpha;-dicarbonyls (glyoxal and methylglyoxal) withammonium, amino acids, or methylamine at different pH's werecharacterized in detail by liquid chromatography-photodiode array-highresolution tandem mass spectrometry. In total, 180 chromophores areidentified, accounting for 29-79% of the light absorption ofbulk BrC for different reactions. Thereinto, 155 newly identifiedchromophores, including 76 imidazoles, 57 pyrroles, 10 pyrazines,9 pyridines, and 3 imidazole-pyrroles, explain additionally 9-69%of the light absorption, and these chromophores mainly involve fourformation pathways, including previously unrecognized reactions ofammonia or methylamine with the methylglyoxal dimer for the formationof pyrroles. The pH in these reactions also shows remarkable effectson the formation and transformation of BrC chromophores; e.g., withthe increase of pH from 5.0 to 7.0, the light absorption contributionsof imidazoles in identified chromophores decrease from 72% to 65%,while the light absorption contributions of pyrazines increase from5% to 13% for the methylglyoxal + ammonium reaction; meanwhile, moresmall nitrogen heterocycles transformed into oligomers (e.g., C-9 and C-12 pyrroles) via reaction with methylglyoxal.These newly identified chromophores and proposed formation pathwaysare instructive for future field studies of the formation and transformationof aqueous-phase BrC.