The photochemistry of tropospheric trace species: The central role of radicals and their possible variability
In the presence of sunlight, reduced species emitted into the atmosphere from the earth's surface are oxidized, converted to soluble forms, and then returned to the biosphere-lithosphere-hydrosphere. The free radical OH, predicted to be present in the atmosphere at levels of about 10 5-10 6 molecular cm -3, is believed to be the primary species in initiating the oxidation of most of these reduced atmospheric compounds. The concentration of the OH radical in the remote troposphere is primarily controlled by the atmospheric levels of H 2O, O 3, CO, CH 4 and NO x. However, it appears that anthropogenic activities can significantly affect the atmospheric levels of O 3, CO, CH 4, and NO x and thus may have conceivably caused a perturbation in OH levels on a global scale. A recent analysis of CH 4 in ice cores which indicated that CH 4 has increased significantly over the past 400 years suggests that OH levels have in fact undergone a secular variation in the recent past. © 1986 Pontificia Academia Scientiarum - Città Del Vaticano.
