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

摘要

The photocatalytic effect of Arizona Test Dust (ATD) particles on SO _(2) oxidation was investigated in both the absence and the presence of atmospheric oxidants ( e.g. , NO _( x ) 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 SO _(2) 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 ( γ _(SO _(4) ~(2?) ) ) of SO _(2) corresponding to sulfate production on ATD particles. The γ _(SO _(4) ~(2?) ) value ( γ _(SO _(4) ~(2?) ,dark) ) in the presence of UV light was significantly higher than that ( γ _(SO _(4) ~(2?) ,dark) ) obtained in the dark. For example, γ _(SO _(4) ~(2?) ,light) was (1.15 ± 0.17) × 10 ~(?5) and γ _(SO _(4) ~(2?) ,dark) was (1.16 ± 0.11) × 10 ~(?6) at 20% RH. Both γ _(SO _(4) ~(2?) ,light) and γ _(SO _(4) ~(2?) ,dark) values increased exponentially as RH increased from 20% to 81%. The heterogeneous photocatalytic oxidation of SO _(2) was also considerably enhanced by both O _(3) and NO _(2) . To characterize the reaction between sulfuric acid and inorganic species of dust particles, aerosol acidity ([H ~(+) ] _(C-RUV) , μmol 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.