Aqueous-phase dark
reactions during the co-oxidation
of glyoxal
and S(IV) were recently identified as a potential source of brown
carbon (BrC). Here, we explore the effects of sunlight and oxidants
on aqueous solutions of glyoxal and S(IV), and on aqueous aerosol
exposed to glyoxal and SO2. We find that BrC is able to
form in sunlit, bulk-phase, sulfite-containing solutions, albeit more
slowly than in the dark. In more atmospherically relevant chamber
experiments where suspended aqueous aerosol particles are exposed
to gas-phase glyoxal and SO2, the formation of detectable
amounts of BrC requires an OH radical source and occurs most rapidly
after a cloud event. From these observations we infer that this photobrowning
is caused by radical-initiated reactions as evaporation concentrates
aqueous-phase reactants and aerosol viscosity increases. Positive-mode
electrospray ionization mass spectrometric analysis of aerosol-phase
products reveals a large number of CxHyOzoligomers
that are reduced rather than oxidized (relative to glyoxal), with
the degree of reduction increasing in the presence of OH radicals.
This again suggests a radical-initiated redox mechanism where photolytically
produced aqueous radical species trigger S(IV)–O2auto-oxidation chain reactions, and glyoxal-S(IV) redox reactions
especially if aerosol-phase O2is depleted. This process
may contribute to daytime BrC production and aqueous-phase sulfur
oxidation in the atmosphere. The BrC produced, however, is about an
order of magnitude less light-absorbing than wood smoke BrC at 365
nm.