posted on 2024-01-03, 15:04authored byLu Yang, Ru-Jin Huang, Wei Yuan, Dan Dan Huang, Cheng Huang
Aqueous-phase reactions of α-dicarbonyls with amines
or ammonium
have been identified as important sources of secondary brown carbon
(BrC). However, the kinetics of BrC formation and the effects of pH
are still not very clear. In this study, the kinetics of BrC formation
by aqueous reactions of α-dicarbonyls (glyoxal and methylglyoxal)
with ammonium, amino acids, or alkylamines in bulk solution at different
pH values are investigated. Our results reveal pH-parameterized BrC
production rate constants, kBrCII (m–1 [M]−2 s–1), based on the light absorption
between 300 and 500 nm: log10(kBrCII) = (1.0 ±
0.1) × pH – (7.4 ± 1.0) for reactions with glyoxal
and log10(kBrCII) = (1.0 ± 0.1) × pH –
(6.3 ± 0.9) for reactions with methylglyoxal. The linear slopes
closing to 1.0 indicate that BrC formation is governed by the nitrogen
nucleophilic addition pathway. Consequently, the absorptivities of
the produced BrC increase exponentially with the increase of pH. BrC
from reactions with methylglyoxal at higher pH (≥6.5) exhibits
optical properties comparable to BrC from biomass burning or coal
combustion, categorized as the “weakly” absorbing BrC,
while BrC from reactions with methylglyoxal at lower pH (<6.0)
or reactions with glyoxal (pH 5.0–7.0) falls into the “very
weakly” absorbing BrC. The pH-dependent BrC feature significantly
affects the solar absorption ability of the produced BrC and thus
the atmospheric photochemical processes, e.g., BrC produced at pH
7.0 absorbs 14–16 times more solar power compared to that at
pH 5.0, which in turn could lead to a decrease of 1 order of magnitude
in the photolysis rate constants of O3 and NO2.