Adding to acid fog.

Water droplets in fogs that blanket parts of southern California often carry a heavy load of chemical acids, according to research conducted over the last few years at the California Institute of Technology in Pasadena. Frequently, the acidity of these fogs may be 100 times that typically found in acid rain. In one extreme case, a relatively light fog at Corona del Mar reached a pH of 1.7-- equivalent to the acidity of some toilet bowl cleaners.

The principal culprit is sulfur dioxide, usually generated by the burning of crude oil at local oil fields. When this gas dissolves in water drops, it is rapidly converted to sulfuric acid. But the chemistry in a fog or cloud water droplet is actually more complicated than this.

In the Jan. 17 SCIENCE, Caltech's Michael R. Hoffmann and his colleagues report that the hydroxymethanesulfonate (HMSA) ion (CH.sub.2.O;HSO.sub.3-) probably also plays an important role. Measurements at Bakerfield, Calif., confirm the presence of HMSA, which appears to form when the concentration of sulfur dioxide is high, as it would be near oil fields. Formation of this ion may account for the subsequent observation of traces of formaldehyde (CH.sup.2.O) in fogs and clouds.