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. The acids can be transformed chemically into sulphur dioxide gas or into sulphur and nitrogen salts. In this form they are deposited 'dry', causing the same damage as when they land dissolved in rain or snow. In this form they can also do internal damage to plants as they are taken up from the soil.
Water solutions vary in their degree of acidity.
If there were no pollution at all, normal rainwater would fall on the acid side of this scale, not the alkaline side. Normal rainwater is less acidic than tomato juice, but more acidic than milk. What pollution does is cause the acidity of rain to increase. In some areas of Canada, rain can be as acidic as vinegar or lemon juice. However, if precipitation is more highly acidic, then acid-buffering chemicals can eventually become depleted.