From systematic analysis of electrical conductivity and pH value of aerosol leaching solutions, the amount of free acid in the water soluble portion of the aerosol is derived. In continental European aerosols, Aitken particles have a higher percentage of free acid than the particles >0.1 μm radius. In the marine atmosphere we observe an increasing percentage of free acid with decreasing radius. In winter, episodes of high free acidity are observed. Free acid in precipitation usually is low when the pH value is high. With decreasing pH the relative amount of free acid rises to 30--40% of the total amount of inorganic ions dissolved in rain. During evaporation of precipitation samples (pH>4.2) or of leaching solutions of aerosols, the electrical conductivity changes according to the decrease in volume. The H+ ion concentration, however, increases less, most probably owing to a buffer mechanism. For precipitation samples with pH0.1 μm causes chemical reactions that either decrease or increase the amount of free H+ ions. A yearly cycle was observed indicating neutralization in summer and production of free acid in winter. This effect, however, is very small.