pH-Dependent Aqueous-Phase Brown Carbon Formation: Rate Constants and Implications for Solar Absorption and Atmospheric Photochemistry

• DOI: 10.1021/acs.est.3c07631
• Publication Date: 2024-01-03T14:47:26Z
• Authors: Lu Yang; Ru-Jin Huang; Wei Yuan; Dan Dan Huang; Cheng Huang
• Keywords: Glyoxal

Aqueous-phase reactions of α-dicarbonyls with amines or ammonium have been identified as important sources secondary brown carbon (BrC). However, the kinetics BrC formation and effects pH are still not very clear. In this study, by aqueous (glyoxal methylglyoxal) ammonium, amino acids, alkylamines in bulk solution at different values investigated. Our results reveal pH-parameterized production rate constants, kBrCII (m–1 [M]−2 s–1), based on light absorption between 300 500 nm: log10(kBrCII) = (1.0 ± 0.1) × – (7.4 1.0) for glyoxal (6.3 0.9) methylglyoxal. The linear slopes closing to 1.0 indicate that is governed nitrogen nucleophilic addition pathway. Consequently, absorptivities produced increase exponentially pH. from methylglyoxal higher (≥6.5) exhibits optical properties comparable biomass burning coal combustion, categorized "weakly" absorbing BrC, while lower (<6.0) (pH 5.0–7.0) falls into "very weakly" BrC. pH-dependent feature significantly affects solar ability thus atmospheric photochemical processes, e.g., 7.0 absorbs 14–16 times more power compared 5.0, which turn could lead a decrease 1 order magnitude photolysis constants O3 NO2.