Aqueous-phase brown carbon formation from α-dicarbonyls with amines and ammonium: kinetics, chromophores and formation mechanisms

Abstract

Brown carbon (BrC) aerosols play an important role in affecting radiative forcing and atmospheric photochemistry. Laboratory simulation studies show that aqueous-phase reactions of water-soluble carbonyls with ammonium or amines are one efficient BrC formation pathway. However, the identities of most chromophores, the mechanisms affecting the BrC formation kinetics and the role of pH in these reactions remain largely unknown. In this study, we investigated the BrC formation kinetics, chemical structures and formation pathways of previously unidentified chromophores. The results reveal pH-parameterized BrC production rate constants, kII BrC(m-1 [M]-2 s-1), based on the light absorption between 300 and 500 nm: log10(kII BrC) = (1.0 ± 0.1)×pH－(7.4 ± 1.0) for reactions with glyoxal and log10(kII BrC) = (1.0 ± 0.1)×pH－(6.3 ± 0.9) for reactions with methylglyoxal. The chromophores are mainly composed of nitrogen heterocycle compounds initiated by nitrogen nucleophilic addition reaction. Specifically, 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. These results will improve our understanding of BrC chemistry, the links with optical properties, and model parametrization of secondary BrC for simulating climate effects of BrC.