Raindrops keep fallin’ on my head

Ever wonder what happens to all the stuff that we put in the atmosphere—gases like ammonia, particles of soot, and other materials, some natural and some as pollutants? These are deposited from the atmosphere, usually downwind of the source, by either wet-deposition—a fancy word for rainfall—or dry deposition. Dry deposition includes the gravitational settling of large particles as well as the reaction of some gases, like ammonia, with plant leaves and other materials on the Earth’s surface.

The deposition of materials added to the atmosphere was noted after the first atmospheric tests of atomic weapons in the 1950s, when radioactive particles were deposited half-way around (or more) from the source.  Since that time, we have recognized that many materials are carried for long distances in the atmosphere, reacting with surface materials and with rainfall all along the way.  During the years that we used leaded gasoline in the United States, lead particles were deposited in the Greenland snows and accumulated in the ice pack there at levels much higher than in the layers deposited before and after that era.

Some of the most reactive gases, like nitric oxide (NOx) and ammonia don’t persist long in the atmosphere. They are absorbed by plant leaves or converted to other forms, often particles, in the atmosphere. The average lifetime of a nitric oxide molecule in the atmosphere is about 1 day.  Thus, the deposition of nitric oxide in rainfall is a good measure of local emissions; only a portion of the atmospheric burden of nitric oxide is carried across state boundaries, where it contributes to acid rain downwind of sources.

On the other hand, mercury persists in the atmosphere for up to a year, which means that some emissions of mercury from power plants can potentially circulate globally.  At least some of the mercury deposited in the eastern U.S., is probably derived from emissions in China.  It seems clear that some of the ozone found in the atmosphere over California also derives from Asia.

Maps of the concentration of various substances in rainfall across the U.S., available from the National Atmospheric Deposition Network (http://nadp.sws.uicu.edu), give a good picture of the sources of chemicals in the air.  It is not surprising that sodium and chloride are found in high concentrations in coastal rain—they are derived from sea spray from the ocean.  Over the desert Southwest, high concentrations of calcium derive from desert dusts.  Concentrations of ammonia are greatest in the Mid-west, where they are derived from the application of nitrogen fertilizer and from the maintenance of large populations of cattle and hogs.  (The spot of high ammonia deposition in eastern North Carolina is probably derived from the hog and poultry operations in that state).  On a positive note, the high deposition of acidity and sulfate found in the eastern U.S., has diminished in recent years as a result of regulations to reduce the emissions of sulfur dioxide (SO2) from power plants.  SO2 has a mean lifetime of about 3 days in the atmosphere.

These observations of rainfall chemistry should reinforce the notion that the atmosphere is not a global commons where stuff can be deposed willy-nilly.   What goes up must come down, and when it comes down it can have unexpected and undesirable effects in places far removed from sources of emission.



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