1Max Planck Institute for Chemistry, P.O. Box 3060, D-55020 Mainz, Germany
2Joint Research Centre, Institute for Environment and Sustainability, TP280, I-21020 Ispra (Va), Italy
3National Oceanic and Atmospheric Administration, Climate Monitoring and Diagnostics Laboratory, 325 Broadway, Boulder, CO 80305, USA
4Utrecht University, Institute for Marine and Atmospheric Research, P.O. Box 80005, NL-3508 TA Utrecht, The Netherlands
Abstract. Billions of tons natural and anthropogenic gases are released and subsequently removed from the troposphere each year. Photochemical reactions, initiated by hydroxyl (OH) radicals, oxidise most gases to products which are more easily removed by precipitation and dry deposition at the Earth's surface. Since human-induced pollution emissions strongly affect OH formation and loss, large global changes in OH concentrations are possible. The available evidence as reviewed here, however, indicates that global mean OH has changed relatively little in the past century. Depletion of OH by reactive carbon gases has been compensated by increased OH formation by nitrogen oxides, an act of "inadvertent geo-engineering". Analyses of global mean OH for the past 2.5 decades, however, partly based on methyl chloroform measurements, are ambiguous. Especially the OH trend in the 1990s based on these analyses is at odds with present understanding. Since the discrepancy is dominated by the uncertainty in methyl chloroform emission estimates, improvements of source inventories and model analyses, combined with continued high precision methyl chloroform measurements, will help resolve this problem.