420 POWER PLANT ENGINEERING13.9.2 Acid Rain
The precursors of which are nitric oxide (NO) and sulfur dioxide (SO 2 ). Over hours and days the
NO and SO 2 oxidize to NO 2 and SO 3 , respectively, which subsequently form acids of nitrogen and
sulfur. Because of the relatively long time for the chemical transformations to occur, the impact of the
acid is generally felt several hundred kilometers downwind of the sources. The sulfur also forms sulfate
aerosol. Sulfate aerosol reflects sunlight and is thought to be keeping some industrialized parts of the
world cooler than they would otherwise be. That is, these industrialized parts of the world are not
receiving proper warming by the greenhouse effect.13.9.3 Global Climate Change
It also knows as the Greenhouse Effect. The major greenhouse gases are CO 2 , CH 4 , nitrous oxide
(N 2 O), and chloro-fluoro-carbon species (i.e., CFCs), though ozone and soot also play a role. The Green-
house Effect is already discussed.
13.9.4 Stratospheric Ozone Depletion
Due to gases such as CFC’s and NO. Fossil fuel combustion is a very significant cause of Urban
Air Pollution and Acid Rain, and is strongly implicated in Global Climate Change. However, land-
based combustion systems, with the exception of Fluidized Bed Coal Combustors, are not strongly
implicated in Stratospheric Ozone Depletion. This is because pollutants from land-based sources capa-
ble of destroying ozone (such as NO) do not reach the stratosphere — they are destroyed in the tropo-
sphere. Fluidized Bed Coal Combustion, on the other hand, though attractive because of its low emis-
sions of SO 2 and NO, has a high exhaust emission of N 2 O (of several hundred parts per million). Al-
though N 2 O is not toxic — it is laughing gas — it is a greenhouse gas and it has few enemies in the
troposphere. Thus, it reaches the stratosphere where it breaks down into NO, which is an ozone deplet-
ing gas. Conversion of the N2O to NO can occur as follows in the stratosphere:
N 2 O + O → NO + NO
The NO depletes the ozone, without depleting itself, as follows:
O 3 + NO → NO 2 + O 2
NO 2 + O → NO + O 2
Overall, an O 3 is lost, and an O-atom, which could have formed O 3 through the reaction below, is
lost.
O + O 2 → O 313.9.4 Acid Fog
A recently noted major acid pollutant is acid fog. Its origin is the same as acid rain or snow, i.e.,
sulfuric and nitric oxides from power plants and, to a lesser extent, motor vehicles. It forms by the
mixing of these pollutants with water vapor near the ground. The acid vapors then begin to condense
around very tiny particles of fog or smog, pick up more water vapor from the humid air, and turn into
acid fog. When the water in the fog burns off (evaporates) due to the sun or other causes, drops of nearly
pure sulfuric acid are left behind. It is these drops that make acid fog so acidic. In Los Angeles and
Bakersfield in southern California, the mists have a pH of 3.0 compared with 4 or 4.5 for acid rain. Acid
fog 100 times as acidic as acid rain has been detected. Cases have been reported where people had