The gentle, peaceful fog of poet Carl Sandburg may once have come ''on little cat feet,'' but in California those feet now have claws.
Southern California's fog contains nitrogen oxides and sulfur dioxide, which oxidize to form ''acid fog,'' according to California Institute of Technology researcher Michael Hoffmann.
Dr. Hoffmann, who has published his findings in the latest issue of Science magazine, says he has found acid fog with pH readings a hundred times stronger than the acid rains that trouble Europe, Canada, and the Northeastern United States.
Chemists measure acidity and alkalinity on a ''pH'' scale from 0 to 14, with 7 representing neutral. The lower the number, the higher the acidity. Normal atmospheric moisture has a pH value of 5.6, but Hoffmann says he has measured fog with acidity as high as 2.2 pH. The average for acid fog in the Los Angeles basin is closer to 2.8 pH, he says. Further north in Bakersfield, the fog measures 3 pH, while San Francisco's famous fog rates much lower with 3.8 to 4 pH.
Hoffmann says the acid comes from emissions of automobiles, power plants, and oil production facilities. These sources produce hundreds of tons of nitrogen oxides and sulfur dioxide daily. The pollutants are picked up by atmospheric moisture that condenses into fog. The nitrogen and sulfur compounds then oxidize into nitric acid and sulfuric acid.
California has long been thought free of acid rain because of strict laws controlling industry and automobile emissions. However, recent increases in the acidity levels of the state's Sierra Nevada lakes have led researchers to intensify acid investigations.
The chemical composition of acid fog differs from location to location, reflecting special circumstances in each site, according to Dr. Douglas Lawson, air pollution researcher for the California Air Resources Board, which commissioned Dr. Hoffmann's $75,000 study. For example, Los Angeles fog contains more nitric oxides than sulfur dioxide due to the large number of automobiles. But in Bakersfield, at the southern end of the oil-rich San Joaquin Valley, oil extraction techniques emit higher concentrations of sulfur dioxide into the air.
The effects of acid fog on people have not been studied in the US, Hoffmann says. But in December 1952 in London, for instance, about 4,000 deaths over the norm were recorded in the five worst days of a fog that settled over the city. Over the three-month period, some 12,000 deaths were blamed on the fog.
Hoffmann says California fogs do not have the same devastating effect. A London fog can remain in one locality for days or weeks, while fog in California generally burns off in six to eight hours, he explains.