Levy published in 1971 that the global troposphere has a self-cleansing property based on OH radials. Namely, O3 + hv (< 320 nm) gives O2 + O(1D) followed by O(1D) + H2O yielding 2 OH. OH is responsible for the removal of nearly all reduced gases from the troposphere, e.g. OH + CO gives CO2 + H. Ever since then, atmospheric chemists are struggling to find out how much of this - natural bleach - is actually present, and how it is distributed.
From summer to winter, day to night, from the high latitudes to the tropics, OH changes by orders of magnitude. Its direct link to the availability of short wave sun light, of ozone, and of water vapor, but also its link with an entire family of radicals, including NO, contribute to a complex variable distribution that peaks in the tropics. Add to this that the maximum concentration of OH is merely 1 million radicals/cm3, and that its lifetime is only 1 second, it is clear that direct measurements alone cannot reconstruct the OH distribution.
Thus how can atmospheric chemists be sure that the 3 D distribution fields we reconstruct using highly complex chemistry transport models are correct ? Is there a test possibility for this elusive OH ? It there a chemical tracer that reacts with OH, of which we know the source ?
An independent possibility to nail down the oxidative capacity of the troposphere is to use 14CO as was proposed by Weinstock already in 1969. 14CO is abundant at 5 to 20 molecules per cm3, and is for ¾ due to direct production of 14C from 14N(n,p)14C. 14C is produced throughout the troposphere, and basically an ideally mixed tracer compared to industrial chemicals. 14CO reacts with OH relatively fast, which allows 14CO measurements to provide a detailed picture.
We asked an accomplished atmospheric chemist in the year 2069. His answer was that it had been a hell of a good project to weekly measure 14CO at 6 sites over the period of one solar cycle. The measurements have provided among other a benchmark for OH and can be continued in the next centuries for direct comparison. He mentioned that the atmospheric 14CO signal is not preserved in firn air or ice cores, and praised the wise and enthusiastic insight and participation of AMS scientists who had made it possible to obtain time series of 14CO.
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