Acid Rain Molecule Tells All

Aug. 18, 2008 -- A mysterious chemical reaction that transforms acid rain into other pollutants has given up its secret to scientists, and an unusual molecule is the culprit.

Nitric acid is a major contributor to acid rain, but it can also react in the atmosphere with an extremely reactive molecule, the hydroxyl radical, to form nitrogen oxides, which react further to make the pollutant ozone.

However, researchers have observed that unlike most chemical reactions, which proceed faster at higher temperatures, the nitric acid reaction accelerates under colder conditions.

"For about 40 years people have been trying to understand how nitric acid is removed by the hydroxyl radical," according to study author Joseph Francisco of Purdue University in West Lafayette, Ind. "It doesn't really follow what you would normally expect. People have been puzzled by that and tried to figure it out."

To explain this unusual observation, many had proposed that the reaction proceeds by creating another short-lived molecule -- a so-called "reactive intermediate" -- but nobody had succeeded in capturing the intermediate to uncover its identity.

Francisco and colleague Marcia Lester of the University of Pennsylvania in Philadelphia managed to isolate this intermediate and determine its structure using specialized lasers in Lester's lab and supercomputers at Purdue.

"It's a really exciting finding, but actually there was a little more to the finding," Francisco said. "What we found had some unusual structural and bonding properties."

The molecule has a ring shape incorporating both the nitric acid molecule and the hydroxyl radical. It also has two strong hydrogen bonds -- the same forces that act between water molecules and give water its unusual properties, such as expanding upon freezing, Francisco said.