OXIDATION OF SO2 TO SULFATE IN SEA SALT AEROSOLS

In laboratory-scale experiments sea salt particles are exposed to SO2 at a temperature of 22-degrees-C and relative humidities of 40, 60 and 80%; the SO2 gas concentration is fixed to 0.2, 0.5 and 1.0 ppm (v), respectively. In further test series NO2 is added to the gas phase. As kinetic data the capacity values of the sea salt particles (mg formed sulfate/g dry aerosol) are determined as function of time and from this the reaction rates (mg formed sulfate/g dry aerosol and minute) are calculated in dependence of the yield. The relative humidity (r.h.) has proved to be a decisive reaction parameter. For example, the rate (at a reaction time of one hour) increases at a SO2 concentration of 0.5 ppm (v) from 0.01 to approx. 0.1 mg SO4(2-)/g.min, if the r.h. will increase from 40 to 80%. However, the gas concentration has only an importance at high humidities (where the reaction takes place in droplets) for the sulfate formation in sea salt aerosols. If the SO2 concentration is reduced from 1.0 to 0.2 ppm (v) at a r.h. of 80%, the rate will be decreased from 0.2 to about 0.07 mg SO4(2-)/g. min; however, at a r.h. of 60% from 0.075 to 0.04 mg SO4(2-)/g. min. As an increased sulfate formation but no nitrate formation can be detected when NO2 is added to the gas phase, it can be assumed that SO2 is oxidized in the electrolyte layer around the sea salt particles whereas NO2 is reduced. If NO2 (SO2:NO2 = 1:1) is added to the gas phase, the rate - for example at a r.h. of 40% - will be increased from 0.01 to 0.24 mg SO4(2-)/g. min.