Sulfur radical formation from the tropospheric irradiation of aqueous sulfate aerosols

Abstract

The sulfate anion radical (SO 4 •– ) 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, NO 3 radicals, or iron. Here, we report a source of SO 4 •– , 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 SO 4 •– + organic reaction include surface-active organosulfates and small organic acids, alongside other products. Scavenging and deoxygenated experiments indicate that SO 4 •– 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 SO 4 •– . 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.

Significant Conversion of Organic Sulfur from Hydroxymethanesulfonate to Inorganic Sulfate and Peroxydisulfate Ions upon Heterogeneous OH Oxidation

“…HMS is known to be secondary in nature and can be formed from the nucleophilic addition of bisulfite (HSO 3 – ) or sulfite (SO 3 2– ) ion to formaldehyde (HCHO) in the aqueous phase. ,− While significant progress had been made regarding the understanding of abundance and formation mechanisms of HMS, its subsequent transformation kinetics and mechanisms remain unclear. HMS exhibits a surface affinity and tends to migrate to the air–liquid interface. ,, It adopts an arrangement with the methylene pointing out of the bulk (i.e., the bulk aerosol solution phase), the hydroxyl group pointing into the bulk, and the C–S bond pointing slightly out of the bulk. , This makes it more likely to be oxidized by gas-phase oxidants (e.g., hydroxyl radical (OH) and ozone (O 3 )) near the gas-aerosol interface (termed heterogeneous oxidation). − In addition, OH oxidation of sulfonates (e.g., MSA) is known to produce inorganic sulfur species (e.g., sulfate (SO 4 2– ) ion). − Given that sulfur in its inorganic and organic forms exhibit very different physicochemical properties including surface activity, water uptake, and cloud condensation nuclei activity, it is desirable to understand the conversion rates and mechanisms between the organic sulfur and inorganic sulfur forms upon oxidation.…”

Second-Order Kinetic Rate Coefficients for the Aqueous-Phase Sulfate Radical (SO4•–) Oxidation of Some Atmospherically Relevant Organic Compounds

“… 18 , 19 Although the atmospheric concentration of the SO 4 •– radical has not been directly measured, it has been estimated between 9.1 × 10 –13 and 5.5 × 10 –17 M, assuming known sources. 4 New mechanisms of reactive sulfur species relevant to sulfate aerosols have been recently reported, including SO 4 •– formation from irradiated sulfate aerosols, 20 OH-initiated oxidation of organosulfates, 21 and autocatalysis of the SO 4 2– anion in the presence of phenols. 22 The formation of reactive sulfur was also reported from the interfacial redox of ammonium sulfate aerosol particles.…”

“…•− formation from irradiated sulfate aerosols, 20 OH-initiated oxidation of organosulfates, 21 and autocatalysis of the SO 4 2− anion in the presence of phenols. 22 The formation of reactive sulfur was also reported from the interfacial redox of ammonium sulfate aerosol particles.…”

“…SO 4 •– radicals have been recognized as a potentially important source of surface-active organosulfates (OSs) in the environment, − which can modify aerosol cloud interactions. , A large variety of OSs have been identified in both field and laboratory studies, several of which are proposed to only be formed from SO 4 •– radicals. ,− The SO 4 •– radical is thought to be formed primarily in the autoxidation of SO 2 ; however, it can also be formed from the activation of sulfate and bisulfate anions with OH, NO 3 , and to a small extent ferric ion complexes. , Although the atmospheric concentration of the SO 4 •– radical has not been directly measured, it has been estimated between 9.1 × 10 –13 and 5.5 × 10 –17 M, assuming known sources . New mechanisms of reactive sulfur species relevant to sulfate aerosols have been recently reported, including SO 4 •– formation from irradiated sulfate aerosols, OH-initiated oxidation of organosulfates, and autocatalysis of the SO 4 2– anion in the presence of phenols . The formation of reactive sulfur was also reported from the interfacial redox of ammonium sulfate aerosol particles .…”

Kinetics Study of Acid-Catalyzed Sulfate Esterification Reactions for Atmospherically Relevant Polyols

“…There are a number of potential organosulfate-forming mechanisms in the atmosphere, including ring-opening nucleophilic addition of sulfate to epoxides, − nucleophilic substitution of organonitrates by sulfate, , and sulfate radical addition to alkenes. − In the pioneering atmospheric chamber experiments by Surratt et al, because organosulfate formation was positively correlated with SOA acidity, it was suggested that organosulfates form by sulfate esterification of alcohols . In this reaction, an alcohol group in the reactant is replaced by a sulfate group under strong acid catalysis conditions.…”