Nitrophenols are a major component of light-absorbing
atmospheric
organic aerosols, commonly referred to as brown carbon (BrC). Most
nitrophenol formation pathways involve reactions of phenolic compounds
with OH, NO3, and NO2 in the gas phase. In this
study, an aqueous nitrophenol production pathway is investigated that
can proceed in the dark without apparent OH radical formation. Using
high-performance liquid chromatography–mass spectrometry, we
demonstrate that catechol reacts in acidic solutions with dissolved
nitrite to form nitrocatechol. The rate of nitration increases significantly
from pH 4.4 to pH 3.4 such that nitrocatechol is susceptible to second-generation
reactions under the most acidic conditions producing chromophores
that absorb in the visible region (peak at 425 nm). Increases in the
N:C ratio of the reaction solution, as detected by aerosol mass spectrometry,
and enhanced absorption from 300 to 500 nm of wood and peat smoke
aqueous extracts exposed to nitrite suggest that this is a general
nitration pathway. The atmospheric conditions under which this BrC
formation process may occur are discussed.
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