Conductance and bulk vertical detachment energy of hydrated sulphate and oxalate dianions: a theoretical study

Sulfur radical formation from the tropospheric irradiation of aqueous sulfate aerosols

It was found that shining natural or artificial sunlight on concentrated solutions of sulfate ions mixed with organics, a mixture commonly found in atmospheric aerosol particles, can generate sulfur-containing radicals under a variety of conditions. This reaction has not previously been characterized in atmospheric chemistry. These reactive radicals can subsequently degrade organic compounds in atmospheric particles, forming a variety of products that stay in the particle water and small molecules that are volatile enough to partition to the gas phase. In particular, this source of sulfur radicals can produce surface-active organosulfates and organic acids.

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 source of SO₄^•–, from the irradiation of the liquid water of sulfate-containing organic aerosol particles under natural sunlight and laboratory UV 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 identity is of low importance. 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 reaction may impact our understanding of atmospheric sulfur reactions, aerosol properties, and organic aerosol lifetimes when inserted into aqueous chemistry model mechanisms.