Acid Rain

The acidity of an aqueous solution is measured as its concentration of hydrogen ions (H⁺). The pH scale expresses this concentration in logarithmic units to the base 10, ranging from very acidic solutions of pH 0, through the neutral value of pH 7, to very alkaline (or basic) solutions of pH 14. It is important to recognize that a one-unit difference in pH (for example, from pH 3 to pH 4) implies a 10-fold difference in the concentration of hydrogen ions. The pHs of some common solutions include: lemon juice, pH 2; table vinegar, pH 3; milk, pH 6.6; milk of magnesia, pH 10.5.

As just noted, an acidic solution, strictly speaking, has a pH less than 7.0. However, in environmental science the operational definition of acidic precipitation is a pH less than 5.65. This is the pH associated with the weak solution of carbonic acid (H₂CO₃) that forms when water droplets in clouds are in chemical equilibrium with carbon dioxide (CO₂), an atmospheric gas with a concentration of about 360 ppm (parts per million; this is a unit of concentration).

Water in precipitation contains a mixture of positively charged ions (or cations) and negatively charged ions (or anions). The most abundant cations are usually hydrogen (H⁺), ammonium (NH₄⁺), calcium (Ca²⁺), magnesium (Mg²⁺), and sodium (Na⁺), while the major anions are sulfate (SO₄^2-), chloride (Cl^-), and nitrate (NO₃^-). The principle of conservation of electrochemical neutrality of aqueous solutions states that the total number of cation charges must equal that of anions, so the net electrical charge is zero. Following from this principle, the quantity of H⁺ in an aqueous solution is related to the difference in concentration of the sum of all anions, and the sum of all cations other than H⁺.

Data for the chemistry of precipitation in a region experiencing severe acid rain are available from Hubbard Brook, New Hampshire, where one of the world's best long-term studies of this phenomenon has been undertaken. The average pH of precipitation at Hubbard Brook is 4.2, and H⁺ accounts for 71% of the total amount of cations, and SO₄^2- and NO₃^- for 87% of the anions. Therefore, most of the acidity of precipitation at Hubbard Brook occurs as dilute sulfuric and nitric acids. The SO₄^2- is believed to originate from SO₂ emitted from power plants and industries, and oxidized by photochemical reactions in the atmosphere to SO₄^2-. The NO₃^- originates with emissions of NO_x (i.e., NO and NO₂) gases from these sources and automobiles. Not surprisingly, air masses that pass over the large emission sources of Boston and New York produce storms with the highest concentrations of H⁺, SO₄^2-, and NO₃^- at Hubbard Brook.

Regions differ greatly in their precipitation chemistry. This can be demonstrated using data for precipitation chemistry monitored during a study in eastern Canada. The village of Dorset in southern Ontario is close to large sources of emission of SO₂ and NO_x. On average, the precipitation at Dorset is highly acidic at pH 4.1, and the large concentrations of SO₄^2- and NO₃^- suggest that the acidity is caused by dilute sulfuric and nitric acids. In comparison, the Experimental Lakes Area (ELA) is in a remote landscape in northwestern Ontario that is infrequently affected by polluted air masses. The ELA site has a less acidic precipitation (average pH 4.7) and smaller concentrations of SO₄^2- and NO₃^- than at Dorset. Another site near the Atlantic Ocean in Nova Scotia receives air masses that pass over large sources of emissions in New England and southeastern Canada. However, by the time Nova Scotia is reached much of the acidic SO₄^2- and NO₃^- have been removed by prior rain-out, and the precipitation is only moderately acidic (pH 4.6). Also, because Nova Scotia is influenced by the ocean, its precipitation chemistry is characterized by high concentrations of Na⁺ and Cl^-. Finally, Lethbridge in southern Alberta is in a prairie landscape, and its precipitation is not acidic (average pH 6.0) because of the influence of calcium-rich, acid-neutralizing dusts blown into the atmosphere from agricultural fields.

In some places, fog moisture can be especially acidic. For example, fogwater at coastal locations in New England can be as acidic as pH 3.0-3.5. At high-elevation locations where fog is frequent there can be large depositions of cloudwater and acidity. At a site in New Hampshire where fog occurs 40% of the time, cloudwater deposition to a conifer forest is equivalent to 47% of the water input by rain and snow, and because of its large concentrations of some chemicals, fog deposition accounted for 62% of the total inputs of H⁺, and 81% of those of SO₄^2- and NO₃^-.