Cosmic rays produce carbon-14, which enters Earth's carbon cycle after being oxidized. It is of great service to atmospheric chemists in providing a way of tracking the degree to which the atmosphere keeps itself clean.
As Martin Manning and colleagues report on page 1001 of this issue1, changes in the amount of hydroxyl (OH) radicals in Earth's atmosphere can be tracked by analysing time-series measurements of naturally produced carbon monoxide containing radiocarbon (14CO). This is no mean feat and is of con-siderable significance — OH is the chief oxidant in Earth's atmosphere, and as such acts as a natural bleaching agent. Atmospheric chemists have been struggling to estimate how much OH there is, and how much it varies in concentration in space and time: Manning and colleagues' approach constitutes a big step forward.
The self-cleansing capacity of Earth's atmosphere is remarkable. Every year, roughly half-a-billion tonnes of methane (CH4) and 2.5 billion tonnes of CO are removed from the troposphere by chemical reaction. (The troposphere is the lowermost layer of the atmosphere; it is the site of the machinery that creates weather, and extends 10–15 km above Earth's surface.) This miracle of self-cleansing occurs even though CH4, CO and several other reduced gases do not react at any significant rate with the atmosphere's major oxidant, molecular oxygen (O2), or the rarer but more powerful ozone (O3).
The true cause was not discovered until 1971, when it was recognized2 that even in remote regions, far away from photochemical smog, active atmospheric chemistry occurs. The breakdown of O3 by ultraviolet sunlight produces excited oxygen radicals. Some re-form into O3. But others retain enough energy to split water molecules and create OH radicals. These are stable but highly reactive, and constitute the troposphere's bleaching agent. Thanks to OH, the lifetime of CH4 (a greenhouse gas) is kept below ten years, whereas on average a CO molecule perishes in a matter of months in the reaction CO + OH → CO2 + H.
It is this last reaction, but using 14CO, that Manning et al.1 have exploited to estimate levels of OH. The lifetime of OH is merely one second, making direct measurements technically demanding. Its extreme reactivity not only implies low abundances, at an average level of 1 million radicals per cm3 (below part-per-trillion levels), but also great variability in its concentration — from night to day, from cloudy sky to clear sky, from summer to winter, and depending on latitude.
But 14CO, which originates from 14C produced by cosmic rays (Fig. 1), is an excellent natural tracer for tracking OH. The principle of this indirect approach was first outlined by Bernard Weinstock3: when the oxidative capacity of the atmosphere falls, with fewer OH radicals present, 14CO levels can rise because the rate of removal of 14CO — via oxidation by OH to 14CO2 — is lower. Rates of production and destruction are assumed to be in equilibrium. The product 14CO2 is, of course, well known in the environmental sciences: following its uptake by plants and subsequent entry into the food chain, its radioactive decay provides the basis for radiocarbon dating.
Manning and colleagues have analysed 13 years of 14CO measurements at Baring Head, New Zealand, together with similar data from Antarctica and ship cruises. After correcting the time series for the large modulation of 14C production caused by the 11-year solar cycle, residual variations in 14CO remain. As the authors argue, two instances of higher 14CO can only have been caused by short-term reductions in OH, and the coincidence with known atmospheric changes confirms their hypothesis. Their work clearly shows one of the advantages of using 14CO for tracking OH. Because of its short lifetime, 14CO is sensitive to rapid atmospheric changes such as those that occur after major volcanic eruptions or large-scale episodes of biomass burning related to El Niño climatic events. It is also notable that the short lifetime of 14CO enabled the authors to consider the remote Southern Hemisphere as a fairly self-contained atmospheric ‘laboratory’ for testing its use.
Computer models of atmospheric transport and chemistry can generate a fairly detailed picture of OH distribution. Typically, the maximum values occur in the tropics, as might be expected: it is here that atmospheric chemistry is at its most active because of the intense solar radiation and high amounts of water vapour. Verifying the model picture is another matter, and a previous approach that has been repeatedly applied is based on methyl chloroform, which also offers an indirect way of estimating OH levels. Careful measurement of this chlorinated industrial chemical at several locations, and calculation of emissions from manufacturers' data, have shown that OH has apparently undergone surprisingly large changes over the past decades4. Yet this and related findings have been controversial5,6 because of uncertainties about the actual rates of emissions. Moreover, the production of methyl chloroform has been phased out, and — thanks to OH — it is disappearing from the atmosphere. So this is not a tracer that can be used in the long term.
By contrast, 14CO is produced naturally and largely independently of human activity. It should become the principal diagnostic tool for monitoring the oxidative capacity of the atmosphere now and in decades to come. This tracer is a cosmic dowry for atmospheric chemists — Manning et al. have made a strong case for them to accept it with gratitude.
Manning, M. R., Lowe, D. C., Moss, R. C., Bodeker, G. E. & Allan, W. Nature 436, 1001–1004 (2005).
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