Effects of Acid Rain on Humans

Acid rain looks, feels, and tastes just like clean rain. The harm to people from acid rain is not direct. Walking in acid rain, or even swimming in an acid lake, is no more dangerous than walking or swimming in clean water. The air pollution that causes acid rain is more damaging to human health. Sulfur dioxide and nitrogen oxides, the major sources of acid rain, can irritate or even damage our lungs.

The pollutants that cause acid rain can also reduce visibility, limiting how far into the distance we can see. The primary pollutants associated with acid rain and poor visibility are human-made sulfur dioxide emissions. These emissions form small sulfate particles, or aerosols, in the atmosphere. These aerosols reduce visibility by scattering light. Sulfate aerosols are the main cause of poor visibility in the eastern United States.

Nitrogen oxide emissions are also associated with the acid rain problem. They, too, can form aerosols in the atmosphere that significantly reduce visibility. Nitrate aerosols are often the main cause for poor visibility in the western United States where sulfur dioxide emissions and humidity are lower than in the east.

Making Acid in the Air

The final concentration of acids (mainly sulfuric acid, H₂SO₄ and nitric/nitrous acid, HNO₃/HNO₂) in precipition (acidity) is dependent on emissions of SO₂ and NO_x from sources, weather conditions andother chemicals present in the atmoshpere. The formation of sulfuric and nitric/nitrous acid in the atmosphere is a complex process involving several chemical reactions involving both solution and gas phase reactions.

SO₂

Gas phase	Solution phase
2SO2 + O2® 2SO3 very slow in the absence of a catalyst, therefore this is not a significant contributor	1) SO2 (aq)® H+ + HSO3- 2) HSO3- (aq)® H+ + SO32- The equilibrium concentraions for the above reactions are dependent upon such things as pH, droplet size, the "sticking coefficient" for SO2 on water, etc.
2SO3 + H2O® H2SO4 occurs quickly, therefore any formation of SO3 in the moist atmosphere is assumed to lead to the formation of sulfuric acid	1) HSO3- + H2O2® A- + H20 2) A- + H+® H2SO4 The dominant oxidation process occurs by hydrogen peroxide (formed in the gas phase from free radicals). The reaction involves the formation of an intermediate (A-), possibly a peroxymonosulfurous acid ion
1) HSO3- + H2O2® A- + H20 2) A- + H+® H2SO4 The dominant oxidation process occurs by hydrogen peroxide (formed in the gas phase from free radicals). The reaction involves the formation of an intermediate (A-), possibly a peroxymonosulfurous acid ion

NO_x

Gas Phase	Aqueous Phase
HO + NO2 (+M)® HONO2(+M) the principal contributor to the formation of nitric acid is the reaction with hydroxy radicals. These radicals are highly reactive and abundant in the atmosphere.	1. 2NO2(g) + H2O(l)® 2H+ + NO3- + NO2- 2. NO(g) + NO2(g) + H2O(l)® 2H+ + 2NO2- 3. 3NO2(g) + H2O(l)® 2H+ + 2NO3- + NO(g) Equilibria of three reactions define the aqueous oxidation of Nox and depend on the partial pressures of NOx present in the atmosphere, temperature, humidity and the low solubility of NOx.