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Catechol Oxidation by Ozone and Hydroxyl Radicals at the Air–Water Interface
Version 2 2021-05-18, 12:36
Version 1 2015-12-17, 06:22
journal contribution
posted on 2015-12-17, 06:22 authored by Elizabeth
A. Pillar, Robert C. Camm, Marcelo I. GuzmanAnthropogenic emissions of aromatic
hydrocarbons promptly react
with hydroxyl radicals undergoing oxidation to form phenols and polyphenols
(e.g., catechol) typically identified in the complex mixture of humic-like
substances (HULIS). Because further processing of polyphenols in secondary
organic aerosols (SOA) can continue mediated by a mechanism of ozonolysis
at interfaces, a better understanding about how these reactions proceed
at the air–water interface is needed. This work shows how catechol,
a molecular probe of the oxygenated aromatic hydrocarbons present
in SOA, can contribute interfacial reactive species that enhance the
production of HULIS under atmospheric conditions. Reactive semiquinone
radicals are quickly produced upon the encounter of 40 ppbv–6.0
ppmv O3(g) with microdroplets containing [catechol] = 1–150
μM. While the previous pathway results in the instantaneous
formation of mono- and polyhydroxylated aromatic rings (PHA) and chromophoric
mono- and polyhydroxylated quinones (PHQ), a different channel produces
oxo- and dicarboxylic acids of low molecular weight (LMW). The cleavage
of catechol occurs at the 1,2 carbon–carbon bond at the air–water
interface through the formation of (1) an ozonide intermediate, (2)
a hydroperoxide, and (3) cis,cis-muconic acid. However, variable [catechol] and [O3(g)]
can affect the ratio of the primary products (cis,cis-muconic acid and trihydroxybenzenes) and higher
order products observed (PHA, PHQ, and LMW oxo- and dicarboxylic acids).
Secondary processing is confirmed by mass spectrometry, showing the
production of crotonic, maleinaldehydic, maleic, glyoxylic, and oxalic
acids. The proposed pathway can contribute precursors to aqueous SOA
(AqSOA) formation, converting aromatic hydrocarbons into polyfunctional
species widely found in tropospheric aerosols with light-absorbing
brown carbon.