Encyclopedia of Environmental Science and Engineering, Volume I and II

CHEMISTRY OF ATMOSPHERIC ACID FORMATION

Acid deposition has long been recognized to be a serious

problem in Scandinavian countries, and throughout Europe,

much of the United States, and Canada. Most of the concerns

about acid deposition are related to the presence of strong

inorganic acids, nitric acid (HNO 3 ) and sulfuric acid (H 2 SO 4 ),

in the atmosphere. Sulfur dioxide (SO 2 ) and nitrogen oxides

(NO x ) are emitted from numerous stationary and mobile

combustion sources scattered throughout the industrialized

nations of the world. As this polluted air is transported over

large distances, 500 km and more, the sulfur and nitrogen

oxides can be further oxidized, ultimately to the correspond-

ing acids. The 1990 Clean Air Act Amendments require sig-

nificant reductions in SO 2 from power plants in the eastern

portion of the United States. Less significant reductions of

NO x emissions are also required.

As was suggested earlier, one of the primary goals of

air-pollution research is to take information about emissions,

topography, meteorology, and chemistry and develop a

mathematical model to predict acid deposition in the model

area. The type of model used to do this is known as a long-

range transport (LRT) model, where the dimensions are on

the order of 1000 km or more. The acid deposition that is

observed is produced by the chemical processes occurring in

the atmosphere during the transport. Prediction of the effects

of any reduction in emissions of sulfur and nitrogen oxides

requires a detailed understanding of the atmospheric reac-

tions involved in the oxidations.

Pollutant emissions are transported by the winds for hun-

dreds of kilometers within the boundary or “mixing” layer

of the atmosphere. This layer is approximately 1000 m deep

and well mixed, allowing pollutants to be dispersed both hor-

izontally and vertically throughout this layer. In the boundary

layer, a variety of chemical and physical processes affect the

concentrations of the pollutants. To form the acids, the sulfur

and nitrogen oxides must react with some oxidants present

in the atmosphere. The most important gas-phase oxidants

were discussed above. These oxidation processes may occur

in the gas phase, or they may occur as aqueous phase reac-

tions in clouds. The gas-phase oxidations of sulfur and nitro-

gen oxides are better quantified than are the aqueous-phase

oxidations.

Gas-Phase Processes

There are three potentially important gas-phase oxidation

processes for producing nitric acid. These processes were

identified earlier: the reaction of hydroxyl radicals with NO 2

(21), hydrogen abstraction reactions from organics by NO 3

(31), and the reaction of N 2 O 5 with water (32). During the

day, the dominant process leading to the formation of HNO 3

is reaction (21). At night, the N 2 O 5 reaction with water vapor

(32) is important. The hydrogen atom abstraction reaction

of NO 3 with organics is expected to be of relatively minor

importance. The 24-hour averaged rate of NO 2 conversion

to HNO 3 during the summer at 50% relative humidity is

expected to be between 15%/hour and 20%/hour.

FIGURE 7 Partitioning of the products of the ozone reaction with α-pinene between the gas and particulate phases,

assuming a total organic aerosol loading of 50 μg/m^3. From Seinfeld (2002). With permission.

ATMOSPHERIC CHEMISTRY 129

C001_008_r03.indd 129C001_008_r03.indd 129 11/18/2005 10:15:22 AM11/18/2005 10:15:22 AM