WASHINGTON The atmosphere's ability to cleanse itself naturally has weakened over the past decade, possibly because of a change in the mix of pollutants emanating from industrialized nations, Reuters reported on Thursday.

Atmospheric levels of the atmosphere's main cleansing agent, hydroxyl radical (OH) -- which scrubs the air of carbon monoxide, sulfur dioxide and nitrogen dioxide -- rose during the 1980s but fell by even larger amounts during the 1990s, according to their study.

The findings were published in the journal ***Science*** released on Thursday.

An international team of researchers led by Massachusetts Institute of Technology atmospheric scientist Ronald Prinn studied OH levels since 1978. The chemical promotes the destruction of air pollutants and many gases involved in ozone depletion and the greenhouse effect.

From 1978 to 2000, the concentration of OH in the southern hemisphere was up to a third higher than that of the northern hemisphere, the study showed. Globally, it increased from 1978 until around 1988 and then declined.

The cause of the fluctuation is unclear, the researchers said. But because the decrease in the global concentration of OH is driven by changes in the northern hemisphere -- where most of the world's industrialization and emission of human-made gases takes place -- the findings likely stem from man-made rather than natural causes, they said.

The chemical exists only fleetingly in the lower atmosphere and cannot be measured directly. But its existence can be inferred from long-term global measurements of a man-made gas 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.