Hazy memories

A recent Science Concentrate featured a new mechanism for aerosol formation (C&EN, May 30, page 7). There is another photochemical process for developing haze events.

In 1990, in Hilton Head, S.C., I presented a paper concerning carboxylic acids in acid rain at an international conference for the National Acid Precipitation Assessment Program. After listening to others, I was curious about carboxylic acids in haze. Thanks to high-performance liquid chromatography, it was determined that the main, indeed often the only, carboxylic acid present in haze is oxalic acid. Because of the belief that this acid could result from the photochemistry of hydrocarbons, water vapor, and air, a reactor was constructed. After several hours, more than 40 compounds formed, indicating a pathway that did not generate carboxylic acid. A paper concerning this research was presented at the ACS national meeting in Denver in 1993.

A friend, David Badger, jokingly suggested that oxalic acid can be formed from carbon dioxide and water. A reactor was constructed. CO₂ and water were placed inside, and the ultraviolet lamp was turned on. I went to class, returned, and observed that a haze had formed. CO₂ and water vapor in the presence of light from a low-pressure mercury vapor lamp had reacted to form oxalic acid. The reaction did not occur when a medium-pressure lamp was used.

Oxalic acid is a major component of photochemical haze and only requires CO₂ and water vapor in the presence of sunlight to form.

W. G. Sayre
Castine, Maine