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Acid rain is any form of precipitation with high levels of nitric and sulfuric acids. Sulfur dioxide (SO2) and nitrogen oxides (NOx) are released into the atmosphere when fossil fuel are burnt. These chemical gases react with water, oxygen, and other substances to form mild solutions of sulfuric and nitric acid. Winds may spread these acidic solutions across the atmosphere and over hundreds of miles. When acid rain reaches Earth, it flows across the surface in runoff water, enters water systems, and sinks into the soil.
Effects of Acid rain on plants -

Acid rain damages forests, especially those at higher elevations. It robs the soil of essential nutrients and releases aluminum in the soil, which makes it hard for trees to take up water. Leaves are also harmed by acids. The effects of acid rain, combined with other environmental stressors, leave trees and plants less able to withstand cold temperatures, insects, and disease. They also decrease the productivity of the plants.

Sulphur dioxide and sulphur trioxide are produced by oxidation of sulphur in the fossil fuels. These gases react with water and form sulphuric acid or sulphurous acids. These acids when precipitated as rain or snow create acid rain or acid precipitation. The pH of acid rains is less than 5-6 as could be as low as 4. Acid rain adversely affect plant vegetation by causing chlorosis and necrosis.