Acid rain in the U.S. is becoming a major environmental issue. This paper reviews the known information regarding pollution sources, impact on the environment and the role of the automobile in acid rain. Although natural sources of sulfur and nitrogen pollutants are equal to or greater than man-made sources on a global scale, many scientists believe man's activities are the major cause of high levels of acidity.
Attempts to relate specific sources of SO2 to specific acid rain events in the northeastern U.S. have been unsuccessful. The roles of tall stacks, long range transport and dry vs. wet deposition are incompletely understood. Temporal and geographic trends in acidity are not well defined except for increased acidity in the southeast.
About 30% of the acidity in rain in the northeast is due to HNO3. In the process of utilizing nitrates as a nutrient, plants partly neutralize the affect of HNO3 in the rain. Pollutants, mainly NOx emitted by passenger cars make a small contribution, less than 6%, to acid rain in the northeast. In California the percent is estimated to be less than 19%. These estimates would be smaller if allowance were made for the contribution of natural sources of NOx and SOx