Laboratory studies of heterogeneous reactions of SO2

The rates of removal of gaseous SO₂ over solids commonly found in urban aerosols were measured in the laboratory. A tubular flow reactor, in which the walls of an inner, concentric cylinder were coated with the solid of interest, was used in these studies. Analysis of the data, using models that specifically accounted for mass transport in the laboratory system, yielded collision efficiencies or the fraction of gas-solid collisions that are effective in removing SO₂. Experimentally measured collision efficiencies for fresh solid coatings range from < 10⁻⁶ to 10⁻³. Wet chemical and X-ray photoelectron spectroscopic results indicate that, to within an experimental error of a factor of 2, gaseous SO₂ is converted to adsorbed sulfate on most of the solids examined.As the time of SO₂ exposure increased we found that the rates of SO₂ removal gradually diminished until, with prolonged exposure, the solids completely lost their ability to remove this species from the gas phase. The relative humidity of the reaction mixture was found to be important in determining the total amount, but not the initial rate, of SO₂ uptake, with SO₂ uptake increasing at higher humidities. Overall, selected solids removed up to several tenths of a gram of SO₂ per gram solid from humidified reaction mixtures. Further reaction could be induced by exposure to small amounts of ammonia.The saturation type of behavior observed on prolonged exposure to SO₂ led to the suspicion that fly ash materials examined in this study, as received, may already have undergone substantial reaction with SO₂, before or during collection. Further experiments on these materials, involving washing those as received materials with distilled water to remove soluble sulfates, supported this contention. Of the six fly ash materials examined, initial collision efficiencies for four of these materials were increased by factors ranging from 2 to > 300 by the water pretreatment. The other two materials exhibited high initial collision efficiencies (~10⁻⁴) that were unaffected, to within experimental error, by the water washing.Atmospheric projection of results from this study suggests that freshly emitted aerosols can be quite effective in converting gaseous SO₂ to particulate sulfate. The capacity limited nature of the reactions suggests that these processes will be most important at or near emission sources, although further, non-source interactions can be induced by atmospheric ammonia.