Sulfate formation on synthetic deposits under haze conditions

Particles and droplets containing liquid water represent very important media for chemical reactions. The presented investigation was focused on the formation of acidic product (sulfate) during SO2 oxidation in the presence of manganese under haze conditions, i.e. at relative humidity below 100%. Since the reaction kinetics of the transformation of atmospheric trace gases depend on the concentration and composition of the aqueous phase, the influence of these parameters was studied. Synthetic deposits of varying composition containing mainly NaCl and NaNO3 were exposed to SO2/air gas mixture at concentrations typical for heavily polluted atmosphere. Experiments were carried out in a specially designed reaction chamber under controlled conditions. Formation of sulfate was negligible below the deliquescent point on both matrices in the presence and absence of Mn(II). However, above this threshold relative humidity, the production of SO42- was observed only in the presence of Mn(II). The reaction rate increases exponentially with relative humidity. A considerable increase was noticed at relative humidity above 85%, where the amount of condensed water becomes substantial. For example, the reaction rate at SO2 concentration of SO42-/(g(salt) h), if the RH is increased from 3 ppmv on MnCl2/NaCl deposits at T = 10degreesC increased from 3.2 to 16.6 mg 80% to 90%. Our results also showed that the process Of SO2 conversion on deposits is self-limited due to a decrease of pH of condensed water. (C) 2003 Elsevier Ltd. All rights reserved.