On the role of hydroxyl radicals in the self-cleansing capacity of the troposphere

[摘要] Thousands of megatons natural and anthropogenic gases are released andsubsequently removed from the troposphere each year. Photochemicalreactions, initiated by hydroxyl (OH) radicals, oxidise most gases toproducts which are more easily removed by precipitation and dry depositionat the earth's surface. Since human-induced pollution emissions stronglyaffect OH formation and loss, large global changes in OH concentrations arepossible. Global models and observations of trace gas distributions fromglobal networks have been used to study geographical and temporal changes intropospheric OH. Here we present a synopsis of recent studies, indicatingthat global mean OH has changed remarkably little in the past century, eventhough regional changes have probably been substantial. Globally, depletionof OH by reactive carbon gases has been compensated by increased OHformation by nitrogen oxides, an act of "inadvertent geo-engineering".However, OH analyses for the past 1-2 decades, partly based on methylchloroform measurements, are inconclusive. Some work, assuming that methylchloroform emissions have largely ceased, suggests a very strong downwardglobal OH trend in the 1990s, inconsistent with modelling studies. Thediscrepancy could be much reduced by assuming continued small emissions ofmethyl chloroform. We recommend the continuation of high precisionmonitoring of this compound and improved analyses based on detailedmeteorological-chemical models.

[学科分类] 大气科学