Sulfate formation on synthetic deposits under haze conditions

Abstract

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 SO 2 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 NaNO 3 were exposed to SO 2 /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 SO 4 2− 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 SO 2 concentration of 3 ppmv on MnCl 2 /NaCl deposits at T =10°C increased from 3.2 to 16.6 mg SO 4 2− /(g salt h), if the RH is increased from 80% to 90%. Our results also showed that the process of SO 2 conversion on deposits is self-limited due to a decrease of pH of condensed water.