Soluble aerosols found with significant concentrations in the air over two Portuguese cities in the year of 1990 are considered in this work, owing to their role in the formation and stability of local fogs. Due to the proximity of the Atlantic Ocean, NaCl is the dominant natural soluble aerosol. On the other hand, (NH_4)_2SO_4, (NH_4)NO_3 and (NH_4)Cl are anthropogenic soluble aerosols of some importance. As all these aerosols act as cloud condensation nuclei (CCN), the knowledge of the energy of formation of droplets is very important for the analysis of their contribution to fog formation. Also the values of the stability barrier, that is the difference between the maximum and the minimum of the energy of formation, allow some conclusions on droplet stability.
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