A radical approach to air pollution

A radical new way to fight air pollution was uncovered quite by accident by former Engineering Experiment Station Scientist Fred Eisele. In the early 1980s, Eisele was developing a sensitive atmospheric ion-measurement technique. In the process, he discovered the technique could be adapted to measure the elusive hydroxyl radical.

Considered the single most important cleansing agent in the atmosphere, the hydroxyl radical acts as an oxidizing agent. It removes several greenhouse gases and other pollutants from the atmosphere. Tiny, even by molecular standards, it is continuously replenished during the day. Highly reactive the molecule lasts only a fraction of a second before it combines with other chemicals. The hydroxyl radical defied in situ measurement for 20 years until Eisele came along.

In early 1989, Eisele modified his ion sampling apparatus to support the physical chemistry needed to detect the hydroxyl radical. His measurement technique combines the highly reactive nature of the compound with the extreme sensitivity offered by a mass spectrometer.

Air drawn through a sampling tube is subjected to a rapid succession of chemical reactions initiated by the addition of isotopically labeled sulfur dioxide, which converts all of the naturally occurring hydroxyl into isotopically labeled sulfuric acid. To prevent the formation of new hydroxyl by certain elements in the air sample, Eisele injects propane shortly after the initial hydroxyl is titrated away.

The acid is ionized to form an isotopically labeled bisulfate ion, which is then measured with a selected ion chemical ionization mass spectrometer. Because the hydroxyl was converted into the acid in a one-to-one ratio, the amount of sulfuric acid reveals the ambient hydroxyl concentration.