Modeling atmospheric mineral aerosol chemistry to predict heterogeneous photooxidation of SO₂

Abstract. The photocatalytic ability of airborne mineral dust particles is known to heterogeneously promote SO₂ oxidation, but prediction of this phenomenon is not fully taken into account by current models. In this study, the Atmospheric Mineral Aerosol Reaction (AMAR) model was developed to capture the influence of air-suspended mineral dust particles on sulfate formation in various environments. In the model, SO₂ oxidation proceeds in three phases including the gas phase, the inorganic-salted aqueous phase (non-dust phase), and the dust phase. Dust chemistry is described as the absorption–desorption kinetics of SO₂ and NO_x (partitioning between the gas phase and the multilayer coated dust). The reaction of absorbed SO₂ on dust particles occurs via two major paths: autoxidation of SO₂ in open air and photocatalytic mechanisms under UV light. The kinetic mechanism of autoxidation was first leveraged using controlled indoor chamber data in the presence of Arizona Test Dust (ATD) particles without UV light, and then extended to photochemistry. With UV light, SO₂ photooxidation was promoted by surface oxidants (OH radicals) that are generated via the photocatalysis of semiconducting metal oxides (electron–hole theory) of ATD particles. This photocatalytic rate constant was derived from the integration of the combinational product of the dust absorbance spectrum and wave-dependent actinic flux for the full range of wavelengths of the light source. The predicted concentrations of sulfate and nitrate using the AMAR model agreed well with outdoor chamber data that were produced under natural sunlight. For seven consecutive hours of photooxidation of SO₂ in an outdoor chamber, dust chemistry at the low NO_x level was attributed to 55 % of total sulfate (56 ppb SO₂, 290 µg m⁻³ ATD, and NO_x less than 5 ppb). At high NO_x ( >  50 ppb of NO_x with low hydrocarbons), sulfate formation was also greatly promoted by dust chemistry, but it was suppressed by the competition between NO₂ and SO₂, which both consume the dust-surface oxidants (OH radicals or ozone).