Daniel Rosenfield and his colleagues at the Hebrew University of Jerusalem studied satellite data of air masses over the Indian Ocean, which contain large numbers of air pollution particles blown off the surrounding continents. Over land, the small size of these aerosol particles tends to suppress rainfall because the water droplets that condense on them are light enough to remain aloft. But according to the report, water droplets started by salt that reaches the atmosphere grow by attracting these smaller, pollution-initiated drops. Because they are larger to begin with, the salty drops soon grow so heavy that they fall out of the atmosphere as precipitation, taking much of the dirt down with them. "If the oceans were not salty," the authors write, "air pollution would remain much longer in the lower troposphere and spread to much greater areas of the oceans."
The salty spray emanating from the ocean acts as an atmospheric cleaner, new research suggests. The idea that large salt particles help clouds form over the ocean was proposed decades ago, but studies conducted in the early 1970s suggested that they did not have a major effect on precipitation levels. Now a report published online by the journal Science indicates that salt particles do indeed encourage rain that can rid the air above of dust and other pollutants.