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No north/south divide in global hydroxyl levels

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10 September 2014

The self-cleansing power of the atmosphere does not differ substantially between the northern and southern hemisphere according to surprising new findings, based on observational data and modelling, published in Nature today.

Up to now, model simulations generally showed higher levels of hydroxyl (the dominant 'bleaching agent' of the atmosphere that oxidizes many pollutants) in the northern hemisphere.  This seemed logical in view of the massive amounts of nitrogen oxides emitted by traffic, domestic heating and industry in the northern hemisphere.  These nitrogen oxides act as catalysts that promote the formation of ozone (ozone smog) and subsequently hydroxyl.

Using measurements from aircraft and remote observing stations, Dr Prabir Patra of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and colleagues including Professor Simon O'Doherty of the University of Bristol, modelled the global distribution of the hydroxyl radical (OH – the highly reactive form of the hydroxide ion responsible for keeping the Earth atmosphere clean).  OH controls the removal and formation of air pollution species and greenhouse gases in the troposphere at altitudes of around 10-15km.

The study estimated that the northern/southern hemisphere ratio of OH concentration is close to 1.

OH is very hard to measure across the globe given its very short lifetime (around 1 second), its very low concentration and its high variability. This has prevented a direct validation of model-simulated OH distributions, and indirect methods to gauge OH are called for.

The new study uses measurements of methyl chloroform (CH3CCl3), an entirely man-made chemical but with emissions now limited by the Montreal Protocol, that is chiefly removed from the atmosphere by OH.

Methyl chloroform measurements from two ground-based observation networks operated by the international Advanced Global Atmospheric Gases Experiment (AGAGE) and the U.S. National Oceanic and Atmospheric Administration (NOAA), as well as from the High-performance Instrumented Airborne Platform for Environmental Research Pole-to-Pole Observations (HIPPO) campaign were used in combination with JAMSTEC’s atmospheric chemistry-transport model (ACTM).

Although state-of-the-art chemistry-transport models predict that the OH concentration in the northern hemisphere is around 28 per cent (13–42 per cent) higher than in the southern hemisphere, the present detailed study derives a northern/southern hemisphere ratio of 0.97±0.12.

Uncertainties about the relative abundance of OH in the two hemispheres have persisted since the early 1990s due to uncertainties about how much CH3CCl3 was actually released into the atmosphere and also due to imperfections in interhemispheric transport in models used to estimate OH concentrations.

The study suggests that top-down emission estimates of reactive species, such as methane and carbon monoxide, in key emitting countries in the northern hemisphere that are based on a northern/southern hemisphere OH ratio larger than 1 may have been overestimated.  For example, inverse modelling studies with relatively high OH in the northern hemisphere require unrealistically large CH4 emissions to counterbalance OH removal.  

The authors conclude: "Chemical reactions simulated by the models may be incomplete and further research is needed to investigate how global/hemispheric OH responds to the wide range of human influences and to improve our ability to predict Earth’s environmental change."


'Observational evidence for interhemispheric hydroxyl parity' by Patra et al in Nature

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