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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 17, issue 16 | Copyright
Atmos. Chem. Phys., 17, 10001-10017, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 25 Aug 2017

Research article | 25 Aug 2017

Modeling atmospheric mineral aerosol chemistry to predict heterogeneous photooxidation of SO2

Zechen Yu, Myoseon Jang, and Jiyeon Park Zechen Yu et al.
  • Department of Environmental Engineering Sciences, Engineering School of Sustainable Infrastructure and Environment, University of Florida, P.O. Box 116450 Gainesville, FL 32611, USA

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 NOx (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 NOx level was attributed to 55% of total sulfate (56ppb SO2, 290µgm−3 ATD, and NOx less than 5ppb). At high NOx ( > 50ppb of NOx 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).

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Mineral dust is an important sink for sulfur dioxide that can be oxidized and further influence the acidification of particles as well as cloud formation. In this study, a model was developed to capture the importance of mineral dust on sulfate formation in various environments. The results suggest that the oxidation of sulfur dioxide is greatly promoted in the presence of dust particles. Our model is helpful to enhance the accuracy of sulfate formation predicted.
Mineral dust is an important sink for sulfur dioxide that can be oxidized and further influence...