Oceans may help clean our air

By Randolph E. Schmid
Associated Press

WASHINGTON — That refreshing breath of sea air may do more than raise your spirits. The world's oceans could be helping clean the atmosphere, according to a study that says the salty sea spray encourages rain that washes out dust and other pollutants.

"We have discovered a process by which nature apparently cleanses the ... air pollution quite effectively when it spills from land over to the oceans," said Daniel Rosenfeld of Hebrew University in Jerusalem.

Practically all the pollutants are removed at the lower layer of the atmosphere, Rosenfeld said. That's one reason why the air in Hawaii is always so pristine, except during volcanic eruptions, he said.

It's the salt that does the trick, he added: "The conclusion stands that the air that we breathe near the surface remains clean because of the fact that the oceans are salty."

Rosenfeld and his colleagues used satellite data to study the air over the Indian Ocean, where massive amounts of particles from burning, urban air pollution and desert dust are blown from southern Asia. He stressed that such effects "are not unique to that part of the world."

Nearly three-fourths of the Earth's surface is covered by water and the winds are constantly moving air from land onto the oceans and from the oceans onto land worldwide.

Rosenfeld's findings are reported in today's issue of Science.

"The paper presents an interesting point, which has not been discussed for some time," said John N. Porter of the Hawaii Institute for Geophysics and Planetology. Porter said the idea of coarse sea salt initiating rainfall was proposed as early as the 1950s, but studies in the 1970s seemed to show it was not an important factor. Rosenfeld's work seems to support the earlier research, he said.

Porter, who was not on Rosenfeld's team, said his own research indicated salt helped to increase rainfall in relatively clean air but was less effective in more polluted conditions.

In his paper, Rosenfeld explained that tiny specks of air pollution can suppress rain by serving as nuclei on which moisture can condense. They form such tiny droplets that they can remain suspended in the air without falling.

But the salty sea spray attracts water into larger droplets, which can merge with the tiny ones, growing big enough to form rain and wash the pollutants out of the air.

Graham Feingold of the National Oceanic and Atmospheric Administration's Environmental Technology Laboratory in Boulder, Colo., was cautious about the report.

"We must bear in mind that only about one in every 100,000 particles is a giant particle capable of forming a small raindrop. Our current methods for measuring these particles ... will have to be improved significantly before we can quantify the role of giant salt particles," Feingold said.