Strictly speaking, the term "acid rain" should only refer to rainfall, or so-called wet precipitation. However, the proper meaning of acid rain is "the deposition of acidifying substances from the atmosphere." This is because acidification is not just caused by acidic rain, but also by chemicals in snow and fog, and by inputs of gases and particulates when precipitation is not occurring.
Of the many chemicals that are deposited from the atmosphere, the most important in terms of causing acidity in soil and surface waters (such as lakes and streams) are: (1) dilute solutions of sulfuric and nitric acids (H2SO4 and HNO3, respectively) deposited as acidic rain or snow, (2) the gases sulfur dioxide (SO2) and oxides of nitrogen (NO and NO2, together called NOx), and (3) tiny particulates, such as ammonium sulfate ([NH4]2SO4) and ammonium nitrate (NH4NO3).
The depositions of these gases and particulates primarily occur when it is not raining or snowing. This type of atmospheric input is known as "dry deposition." Large regions of Europe and North America are exposed to these acidifying depositions. However, only certain types of ecosystems are vulnerable to becoming acidified by these atmospheric inputs. These usually have a thin cover of soil that contains little calcium, and sits upon a bedrock of hard minerals such as granite or quartz. There is convincing evidence that atmospheric depositions have caused an acidification of freshwater ecosystems in such areas. Many lakes, streams, and rivers have become acidic, resulting in declining or locally extirpated populations of some plants and animals. However, there is not yet conclusive evidence that terrestrial ecosystems have been degraded by acidic deposition (except for cases of severe pollution by toxic SO2).
Science EncyclopediaScience & Philosophy: 1,2-dibromoethane to AdrenergicAcid Rain - Atmospheric Deposition, Chemistry Of Precipitation, Spatial Patterns Of Acidic Precipitation, Dry Deposition Of Acidifying Substances