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

Aqueous-phase
reactions of α-dicarbonyls
with
ammonium or amines have been identified as important sources of secondary
brown carbon (BrC). However, the identities of most chromophores in
these reactions and the effects of pH remain largely unknown. In this
study, the chemical structures, formation pathways, and optical properties
of individual BrC chromophores formed through aqueous reactions of
α-dicarbonyls (glyoxal and methylglyoxal) with
ammonium, amino acids, or methylamine at different pH’s were
characterized in detail by liquid chromatography-photodiode array-high
resolution tandem mass spectrometry. In total, 180 chromophores are
identified, accounting for 29–79% of the light absorption of
bulk BrC for different reactions. Thereinto, 155 newly identified
chromophores, 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 four
formation pathways, including previously unrecognized reactions of
ammonia or methylamine with the methylglyoxal dimer for the formation
of pyrroles. The pH in these reactions also shows remarkable effects
on the formation and transformation of BrC chromophores; e.g., with
the increase of pH from 5.0 to 7.0, the light absorption contributions
of imidazoles in identified chromophores decrease from 72% to 65%,
while the light absorption contributions of pyrazines increase from
5% to 13% for the methylglyoxal + ammonium reaction; meanwhile, more
small nitrogen heterocycles transformed into oligomers (e.g., C9 and C12 pyrroles) via reaction with methylglyoxal.
These newly identified chromophores and proposed formation pathways
are instructive for future field studies of the formation and transformation
of aqueous-phase BrC.