Heterogeneous photooxidation of sulfur dioxide in the presence of airborne mineral dust particles

The photocatalytic effect of Arizona Test Dust (ATD) particles on SO2 oxidation was investigated in both the absence and the presence of atmospheric oxidants (e.g., NOx and O-3) under varying relative humidity (RH) using a 2 m(3) indoor photo-irradiation chamber. To the best of our knowledge, this is the first time that the kinetic rate constant has been determined for heterogeneous photooxidation of SO2 in the presence of airborne mineral dust particles. Chamber-generated sulfate data, measured using a particle into liquid sampler coupled with ion chromatography, were used to estimate the kinetic uptake coefficient (gamma SO42-) of SO2 corresponding to sulfate production on ATD particles. The gamma SO42- value (gamma SO42-,(dark)) in the presence of UV light was significantly higher than that (gamma SO42-,(dark)) obtained in the dark. For example, gamma SO42-,(light) was (1.15 +/- 0.17) x 10(-5) and gamma SO42-,(dark) was (1.16 +/- 0.11) x 10(-6) at 20% RH. Both gamma SO42-,(light) and gamma SO42-,(dark) values increased exponentially as RH increased from 20% to 81%. The heterogeneous photocatalytic oxidation of SO2 was also considerably enhanced by both O-3 and NO2. To characterize the reaction between sulfuric acid and inorganic species of dust particles, aerosol acidity ([H+](C-RUV), mu mol L-1 by aerosol volume) was measured using colorimetry integrated with a reflectance UV-Visible spectrometer. Under the experimental conditions employed (sulfate : dust particle = 1 : 20 in mass), the value determined for [H+](C-RUV) was much lower than the value of [ H+] determined from the concentrations of sulfate and ammonium ions measured using ion chromatography, suggesting that most sulfuric acid reacted with alkaline components of ATD particles.