Oxidation of SO2 to sulfate in sea salt aerosols

Summary In laboratory-scale experiments sea salt particles are exposed to SO₂ at a temperature of 22°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 SO ₄ ^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 SO ₄ ^2– /g·min; however, at a r.h. of 60% from 0.075 to 0.04 mg SO ₄ ^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 SO ₄ ^2– /g·min.