The accumulation of an acidic chemical from the atmosphere to the surface of the earth, or to plants and structures at the surface. Acids have high concentrations of hydrogen ions when dissolved in water, indicated by a pH less than 7. Acids can corrode metals, dissolve some types of rocks such as limestone, injure plants, and exacerbate some conditions in humans and animals. Acid deposition can occur in two forms: 1) wet deposition including acid rain, acid snow, acid hail, acid dew, acid frost, and acid fog; and 2) dry deposition including fallout of heavy particles, gravitational settling of lighter particles, and interception by and reaction with plant surfaces. Sometimes all forms of acid deposition are loosely called acid rain, although literally acid rain refers only to the liquid form. Ambient carbon dioxide, always present in the air, dissolves in cloud drops and raindrops creating carbonic acid with pH ≈ 5. 6. Because this is a normal occurrence in the atmosphere, rain is defined to be acid rain only when it has pH < 5. 6. However, even in remote areas, there are sufficient sulfate, nitrate, ammonia, or soil cations (calcium or magnesium that are typically associated with carbonates) to cause “clean” atmospheric water to have pH in the range of 4. 5–5. 5. Polluted regions typically have pH in the range of 3–4, with values as low as 2–3. The chemicals that cause the greatest acid-deposition problems are oxides of sulfur (abbreviated as SOx) and oxides of nitrogen (NOx), which can react in the presence of atmospheric oxidants and water (e.g., clouds, fog and precipitation) to become sulfuric acid and nitric acid, respectively. These strong acids have an affinity for water, allowing droplets to grow hygroscopically in the atmosphere to produce haze or smog, even at relative humidities as low as 60% to 70%.