THE air above our heads is becoming cleaner. A breath of fresh air has been running right round the planet for the past three years, according to a new study published in the journal Science, as the planet apparently purges itself of pollution.
Estimates of the death toll from urban smogs have been steadily rising, so the new cleaner trend could have significant consequences for life expectancy in cities as well for the planet itself. The sudden and unexpected reversal of several decades of worsening pollution extends from the air in the city streets to the remotest mid-Pacific Ocean and Antarctica.
"It seems as if the planet's own cleansing service has suddenly got a new lease of life," says Paul Novelli of the University of Colorado in the US, co-author of the report. "Suddenly there are a lot of changes going on up there."
Among the pollutants that have begun to disappear from the atmosphere are carbon monoxide, from car exhausts and burning rain forests, and methane, which comes from the guts of cattle, paddy fields and gas fields. Even carbon dioxide, the main gas behind global warming, fell slightly in 1992, says Novelli.
It could be that local clean-ups, such as laws to cut acid rain and urban smogs in Europe, are starting to have a global impact. The turn around in carbon monoxide is the most startling. Concentrations have doubled since pre-industrial times, and were rising by one to two percent a year in the 1980s. But since 1991, they have fallen by a staggering six to seven percent each year. The same trend, published by Novelli, shows up at 27 monitoring stations around the globe.
There are two theories about why the pollution is disappearing. First, that there is less pollution to start with. Second, that the planet may be becoming more efficient at cleaning up.
The main planetary clean-up agent is a chemical called hydroxyl. It is present throughout the atmosphere in tiny quantities and removes most pollutants from the air by oxidising them.
From the 1950s to the 1980s, the "cleansing service" seemed badly overstretched, as amounts of hydroxyl in the air fell by about a quarter. As a result, all the main pollutants were spending longer times in the air. Estimates of how long methane lasted, for instance, have risen from eight to 11 years.
Now hydroxyl may be reviving, and the big question is why. One theory is that the atmosphere is producing more hydroxyl. Ironically, that could be because of the ozone hole, which allows more ultraviolet radiation into the lower atmosphere, where it manufactures hydroxyl. Volcanic eruptions would speed up the process as the particles released help destroy the ozone layer. The eruption of Mount Pinatubo in the Philippines in June 1991 released millions of tons of dust into the upper atmosphere, where it stayed for about two years - coinciding with the greatest reduction in carbon monoxide.
Another theory, backed by Novelli, is that the driving force is recent controls on vehicle exhausts in Europe and America. These may have cut the global carbon monoxide emissions enough to take the pressure off hydroxyl's clean-up duties, allowing more spare hydroxyl to clean up other pollutants.
Novelli hopes that whatever the precise cause, the clean-up could turn into a long-term phenomenon. He admits that "three years is not long for a trend", but adds: "There are a lot of reasons to believe that there is a real reduction ... that should persist."
Don't expect a sudden drastic clean-up of city air as the planetary cleansing service goes into over-drive. Calculations show that, in England and Wales, for instance, about 10,000 a year die from effects of air pollution. None the less, a few percentage points off average levels of pollutants such as carbon monoxide and sulphates could, if it persists, keep thousands more alive in Europe's cities.
(Article has a picture of an erupting volcano: Mount Alaid in the Kurile Islands: volcanic eruptions speed up the production of hydroxyl, the main planetary clean-up)
Source: "The European" 10 - 16 June 1994 (A weekly newspaper)