The “SO2–NH3–Solution Droplets” System in an Urban Atmosphere

Abstract

The transformation of airborne SO₂ into SO₄^— taking place in a polluted urban atmosphere has been investigated in terms of the equilibrium of the system composed of SO₂, liquid water and NH₃. The procedure used for selecting, compiling and assembling the field data set is described. Ten classes of atmospheric liquid water content were set up on the basis of average spectra of airborne particulate matter and local visibilities; 174 suitable selected days were available for investigation. The behavior of the above–mentioned system resulting from our computations fits satisfactorily the Junge and Ryan laboratory model. The observed seasonal trend in the yield of the system in transforming atmospheric SO₂ can be explained in terms of a seasonal change in both the catalytic efficiency of the airborne material (different sources of pollutants) and the efficiency Of the scavenging mechanisms which include airborne particles in the droplets. The pH of the liquid water droplets computed on the basis of the SO₂ concentration agrees fairly well with the pH observed in actual urban hazes and fogs.