The stability of tropospheric OH during ice ages, inter-glacial epochs and modern times

Hydroxyl (OH) radicals remove many man-made and natural gases from the atmosphere and therefore play a key role in global tropospheric chemistry. Recent increases in CH₄ and CO have caused concern that the levels of OH may decrease, thus reducing the capacity of the atmosphere to remove and control man-made pollutants. We have calculated OH concentrations over a wide range of climatic conditions to examine its long term stability and to determine the major factors that may cause changes in its levels. We used a one-dimensional photochemical model, the concentrations of CH₄ and N₂O from polar ice cores and the current understanding of the sources and sinks of CO, NO_y and other gases involved in OH chemistry. We find that mean OH concentrations are stabilized against changes even though the climatic conditions and atmospheric trace gas composition change considerably between ice-ages, inter-glacial periods and the present. In these transitions, the more rapid destruction of OH from increased CH₄ and CO is compensated by increases in the production processes. Our calculations indicate that only a small part of the 5-fold increase of methane between the present and the peak of the last ice age, is due to changes in OH levels.