LECTURE 4 CHAPTER 6 ACIDIC DEPOSITION
-Subject of the chapter: Acidic substances and their precursors.
Section I- Acid Pollutions
-This section identifies the major pollutants responsible for acid deposition, and describes
how they are formed.
-Sulfur dioxide (SO2) and nitrogen oxides (NOx) are the major precursors of acid deposition.
-With moisture these gases convert to sulphuric and nitric acids, deposited in rain, snow,
- With dry conditions, sulfur dioxide= sulphate & nitrogen oxides= nitrate.
-Half of acid deposition is dry, which is more likely to settle near emission sources.
- Other chemicals can contribute to acid deposition, CO2 in moist temp= carbonic acid.
-Acid deposition, first described in 1852.
-First half of the twentieth century damage to tress and vegetation was seen near smelters
(facilities that melts or fuses ores that contain metals in order to separate out the metals
that ores contain).
-Smelters release large quantities of sulfur dioxide from sulfur-rich metal ores.
-To protect local communities, the 1970 US Clean Air Act required power plants and
smelters to build emission stacks 1000 ft. (305 m) above ground level.
-Pollutants released at this height would become so diluted that they would cause no
-High stacks were built, but another problem started: stack emissions of sulfur dioxide and
nitrogen oxides are carried away by the wind for hundreds of miles.
-Meanwhile, gases where converted to acid aerosols.
- They rain out or settle on land, water, and on man-made materials.
-Acid deposition, also stored in winter snow until spring snow melts and falls into water
bodies, can also fall directly in water bodies.
- Water can also become acidic enough to harm or kill fish and other aquatic organisms.
- pH at 7 is neutral ---- pH decline can cause death from snails, to fish eggs, to fish species,
to eventually flies and frogs
- The US government spent half billion on National Acid Precipitation Assessment Program
- Carried by teams of scientists this study aimed to understand acid deposition and
-These factors complicated the study:
-Rain does not have a “normal” pH. Rain in the eastern US is naturally acidic
-The pH of lakes and streams varies.
- Difficult to determine whether acid rain was affecting forests, they can be affected
by other stresses
-1990, NAPAP reported that acid deposition did have adverse effects.
Section II- Adverse Effects