Influence of ammonia on sulfate formation under haze conditions

In the presence of hygroscopic material, stable aqueous droplets form at relative humidity below 100%. In this work, we examined the role and importance of NH3 in SO2 oxidation under haze conditions. Synthetic deposits composed mainly of NaCl and NaNO3 were exposed to SO2/NH3/air gas mixture under controlled conditions, typical for heavily polluted atmosphere. In contrast to catalytic reaction by Mn(II) in the absence of NH3, which is self-limited, the production of sulfate in the presence of NH3 at RH 80% increases considerably and linearly with time. The catalytic effect of Mn(II) was observed only for NaNO3 deposits. Non-catalytic SO2 oxidation in the presence of NH3 is negligible below the deliquescent relative humidity, i.e. RH<75%. Above this value a considerable increase of sulfate formation in a narrow range of relative humidity (75–80%) was noticed. We found out that the reaction rate is not proportional to the volume of condensed water; we suggest that available surface play a role in limiting the reaction. The rate of conversion for both pure salts at RH 80% was found to be 1.1% h−1 ([SO2]=2 ppm, [NH3]=1 ppm, T=10°C), which is more than an order of magnitude higher in comparison with our previous results on MnCl2/NaCl and MnCl2/NaNO3 at RH=80% in the absence of NH3.