001109-1 10.1038/nsu001109-1 3 November 2000 environment Fast-acting atmospheric detergent PhilipBall Last yearŐs solar eclipse demonstrated how quickly the atmosphere adjusts to nightfall, reports Philip Ball.

Last year's eclipse gave new insights into the troposphere
It's not every day that you get to experiment with the entire sky. But 11 August 1999 was no ordinary day. Across a narrow band of the northern hemisphere from the North Atlantic to the Indian Ocean, the sun vanished for a breathtaking two minutes, eclipsed by the moon. For many, this was an enchantingly rare astronomical event. For Michael Pilling and colleagues at the University of Leeds, UK, it was the kind of experiment that only nature could conduct.

In Geophysical Research Letters1, the researchers report how the 1999 eclipse enabled them to make direct measurements of how the composition of the lower atmosphere (the 'troposphere') changes when the light is shut out. Differences between the daytime and night-time atmosphere are well known; but normally they happen gradually. The eclipse was an opportunity to examine just how rapidly the air can be transformed by light -- or its absence.

Most of the atmosphere is insensitive to the difference between day and night: it remains a mixture of about one fifth oxygen and four fifths nitrogen. But the air is also spiced with a delicate melange of other gases, many in proportions so tiny that only the most sensitive instruments can detect them. Yet these 'trace gases' are central to the 'health' of the atmosphere.

Amongst them, for example, are pollutants such as nitrogen oxides and ozone, which together cause urban smog and the associated human health problems. Sulphur and nitrogen oxide trace gases in the troposphere also cause acid rain.

One of the most important chemical components of the troposphere is the 'hydroxyl radical': a highly reactive substance composed of an oxygen and a hydrogen atom, denoted 'OH'. Hydroxyl radicals react with just about any other trace gas they encounter, removing them from the atmosphere.

For example, OH combines with toxic carbon monoxide to generate carbon dioxide. In this sense, OH 'cleans up' the troposphere, and is sometimes called the detergent of the atmosphere.

Yet OH is present only in tiny amounts in daytime air -- typically only four of every hundred trillion molecules. And because of its reactivity, it doesn't stick around more than seconds at a time.

OH is regenerated only if there is sunlight present. When ultraviolet rays from the sun split apart ozone in a process called photolysis, this creates fragments that react with water vapour to make OH. So the OH concentration in the atmosphere rises and falls steadily as sunlight waxes and wanes. Because these changes in natural daylight are slow, it is difficult to measure how quickly OH levels respond to light.

The solar eclipse gave Pilling and colleagues a substitute for instant nightfall. They measured the OH content of air at Silwood Park, west of London, where the eclipse was just 97 per cent at its height at 10.19 in the morning. This was enough for hydroxyl concentration to fall virtually to zero. By about an hour later, with the eclipse fully over, OH levels were back to normal.

The researchers also tracked the changes in ozone photolysis during this time, and found that it matched the changes in OH perfectly, reinforcing our understanding that OH is fed by light. This rapid response of the atmosphere's chemistry was predicted by laboratory studies of the reactions involved; but it provides a dramatic real-world demonstration that night touches the air as fast as the sun sets.

Abram, J. P., Creasey, D. J., Heard, D. E., Lee, J. D. & Pilling, M. J. Hydroxyl radical and ozone measurements in England during the solar eclipse of 11 August 1999. Geophysical Research Letters 27, 34373440 2000.