An international team headed by scientists from the National Oceanic and Atmospheric Administration (NOAA),USA, and the Max Planck Institute for Chemistry, Germany , has found that the concentration of hydroxyl radicals in the atmosphere varies by just a few percent from year to year. Some researchers believed that it varies by up to 25 %.

Hydroxyl radicals clean the air by breaking down organic substances such as climate-damaging methane, and make them water-soluble so that they can be removed from the atmosphere by rainfall. It is the most common oxidant in the troposphere, the lowest portion of the atmosphere. Understanding OH variability is important to evaluating human impacts on the atmosphere and climate. Measuring global OH distribution and variability, however, has been difficult, in part because the molecule has a lifetime of less than one second.

To measure the amount of hydroxyl, the scientists analyze methyl chloroform in the atmosphere which reacts with hydroxyl radicals but is much more stable and easier to measure. It has long been used as a solvent in paints, adhesives and cleaning products and is degraded in the atmosphere primarily by reaction with hydroxyl, so the amount of methyl chloroform in the air correlates with the amount of hydroxyl.

The findings will make predictions about the climate and global air quality more reliable, because as the reseachers say, they can now better describe the composition of the atmosphere with the aid of computer models.

• Getting a Better Estimate of an Atmospheric Radical,
  I. S. A. Isaksen, S. B. Dalsøren,
  Science 2011, 331(6013), 38-39.
  DOI: 10.1126/science.1199773