Heterogeneous Reaction of Gaseous Mercuric Chloride with Atmospherically Relevant Organic Films

Uptake by atmospheric aerosols is an important step in the deposition and photoreduction of gaseous oxidized mercury (GOM). The aerosols contain a broad range of inorganic and organic constituents whose reactivity toward GOM is still inadequately quantified. Recently, we investigated the uptake of GOM by inorganic surfaces, finding a significant dependence of the reactivity on the surface composition. Here, we report the uptake of GOM on films made of chemicals mimicking common organic and carbonaceous aerosols. Primary combustion aerosols were represented by soot, levoglucosan, and polycyclic aromatic hydrocarbons (perylene and pyrene). Secondary aerosols were represented by organic acids (succinic, pimelic, and citric acids) and dioctyl sebacate. Gas-surface uptake was studied in a fast-flow reactor coupled to an ion drift-chemical ionization mass spectrometer, using mercuric chloride (HgCl2) as a model GOM. Kinetic curves showed a combination of reversible and irreversible uptakes, where the irreversible uptake was most pronounced for soot and dioctyl sebacate. Overall, the reactivity varied significantly between the different chemicals, with the initial uptake coefficient decreasing from 2.6 x 10(-2) to less than 1.0 x 10(-5) in the series: dioctyl sebacate > levoglucosan > perylene > pyrene > pimelic acid > succinic acid > soot >> citric acid. The uptake on organic acids showed a strong pH dependence, increasing significantly when the acids dissociated at a higher pH. We conclude that carbonaceous aerosols can represent an important sink for GOM, where the contribution of organic acids is highly dependent on the particle acidity.