Question

What is the role of acid in our atmosphere

Answer 1

An "acid" is a substance that releases hydrogen ions (H+) when dissolved in water. The relative acidity of water is usually reported as the pH of the solution defined as the negative logarithm of the hydrogen ion concentration, pH = -log [H+]. Normally water has an equilibrium amount of hydrogen (H+) and hydroxide ions (OH-) that are at a molar concentration of 10-7 moles per liter. This gives rise to a pH of 7. Solutions that are lower than 7 are considered acidic, while those that have pH values higher than 7 are considered basic. Background acidity of precipitation is typically at a pH of 5.6. This is due to carbon dioxide, which is a weakly acidic gas that dissolves into the cloud water to form bicarbonate (HCO3-), carbonate (CO3-2), and acidity (H+). Other natural organic acids from the oxidation of natural hydrocarbons can lead to lower values for pH even for rains that are considered unpolluted. Typical "clean" atmospheric waters have a pH of 4.5–5.5.

Strong acids release acidity upon dissolution directly into the rain. As described below, they act to lower the pH and lead to acid deposition. The most important of these strong acids in precipitation are sulfuric (H2SO4) and nitric acid (HNO3). These strong acids are produced from the oxidation of sulfur dioxide and nitrogen dioxides emitted into the atmosphere from combustion of fossil fuels. In more polluted areas, pHs in precipitation range from 3 to 4, and in some low liquid water content clouds, pHs as low as 2–3 have been measured. The lower the pH, the higher the concentration of H+ and acidity. Since pH is a logarithmic scale, a decrease in one pH unit means a 10-fold increase in acidity or H+concentration, and a decrease of two means a 100-fold increase in acidity, etc.