Sulfate radical from irradiated aqueous sulfate solutions

The sulfate anion radical (SO4• –) is known to be formed in the
autoxidation chain of sulfur dioxide, and from minor reactions when
sulfate or bisulfate ions are activated by OH radicals,
NO3 radicals, or iron. Here we report a new source of SO4• –,
from the irradiation of the liquid water of sulfate-containing organic
aerosol particles under natural sunlight and laboratory ultraviolet
radiation. Irradiation of aqueous sulfate mixed with a variety of
atmospherically relevant organic compounds degrades the organics well
within the typical lifetime of aerosols in the atmosphere. Products of the
SO4• – + organic reaction include surface-active organosulfates
and small organic acids, alongside other products. Scavenging and
deoxygenated experiments indicate that SO4• – radicals, instead
of OH, drive the reaction. Ion substitution experiments confirm that
sulfate ions are necessary for organic reactivity, while the cation is
nearly irrelevant. The reaction proceeds at pH 1-6, implicating both
bisulfate and sulfate in the formation of photoinduced SO4• –. Certain
aromatic species may further accelerate the reaction through synergy. This
new reaction may impact our understanding of atmospheric sulfur reactions,
aerosol properties, and organic aerosol lifetimes when inserted into
aqueous chemistry model mechanisms.