Acid deposition- commonly called acid rain- is caused by
emissions of sulphur dioxide and nitrogen oxides. Although natural
sources of sulphur oxides and nitrogen oxides do exist, more
than 90% of the sulfur and 95% of the nitrogen emissions occuring
in eastern North America are of human origin. These primary air
pollutants arise from the use of coal in the production of electricitiy,
from base-metal smelting, and from fuel combustion in vehicles.
Once released into the atmosphere, they can be converted chemically
into such secondary pollutants as nitric acid and sulfuric acid,
both of which dissolve easily in water. The resulting acidic
water droplets can be carried long distances by prevailing winds,
returning to Earth as acid rain, snow, or fog.