Low on self defence

the atmosphere can no more cleanse itself naturally as well as it used to. Its ability to do so has weakened over the past decade, possibly because of a change in the mix of pollutants emanating from industrialised nations, report us -based researchers.

Atmospheric levels of the atmosphere's main cleansing agent, hydroxyl radical ( oh ), which scrubs the air of carbon monoxide, sulphur dioxide and nitrogen dioxide, rose during the 1980s but fell by a larger amount during the 1990s, according to the recently published study in the journal Science . A team of researchers led by Ronald Prinn, atmospheric scientist at the Massachusetts Institute of Technology, usa , studied the changes in oh levels since 1978 and many gases involved in ozone depletion and the greenhouse effect. From 1978 to 2000, the concentration of oh in the southern hemisphere about one third time higher in than that in the northern hemisphere, the study showed. World over, it increased from 1978 until 1988 and then declined.

The cause of the fluctuation is unclear say researchers. But as the decrease in the global concentration of oh is driven by changes in the northern hemisphere -- where most of the world's industrialisation and emission of human-made gases takes place -- the depletion could be caused by human activity, they add.

The chemical exists only in the lower atmosphere and cannot be measured directly. But its existence can be inferred from long-term global measurements of human-induced gas emissions that it obliterates, the study found.

The researchers said factors may include the changing mix of air pollutants in the northern hemisphere such as nitrogen oxides and carbon monoxide, along with tiny particulates known as aerosols that may be removing hydroxyl radical and its related molecules from the atmosphere. The aerosols also may be reflecting and absorbing the sun's ultraviolet radiation, which could lower oh production, the researchers said.