Throughout the atmosphere in tiny quantities and removes most pollutants from the air by oxidizing them. The amount of hydroxyl in the air had fallen by a quarter in 1980s. Now, it may be reviving for two reasons: ironically, because the ozone hole has expanded, letting in more ultraviolet radiation into the lower atmosphere, where it manufactures hydroxyl. Then the stricter controls on vehicles exhausts in America and Europe may have cut global carbon monoxide emissions, thereby allowing more hydroxyl to clean up other pollutants.