Determining the acidity and chemical composition of fog, haze and cloud droplets in Israel

The chemistry and acidity of individual droplets were analyzed in samples of fog, haze and cloud collected at Tel Aviv University on the Mediterranean seacoast and at the top of Mt Meron and of Mt Carmel in northern Israel. Acidity was measured using pH paper in a cascade impactor. The characteristics of droplets and interstitial particles 0.3-20.0 μm in size were identified using microspot and electron microscopy techniques. The results revealed the following: (1) the aerosols in the micrometer and sub-micrometer range were mostly very acidic droplets, with pH as low as 2.0, caused by the presence of sulfuric acid; (2) lower acidity was associated with an increase in droplet size, due to dilution by condensation or nucleation scavenging of mineral dust, i.e. gypsum, calcite and dolomite; (3) both acidic and alkaline droplets contained significant amounts of sulfur; and (4) the alkaline droplets contained minerals and salt solutions of Na₂SO₄, CaSO₄, or NaCl.