1. When “smoke” and “fog” combine
Smog

2. The Birth of Smog
First began appearing inn the early 19th century in London due to the conditions of coal smoke and fog experienced by the city
-This smog is known as industrial, or reducing, smog
-Caused by the combustion of sulfur found in the coal used for heating, manufacturing, and the production of electricity
-Consists of water droplets, sulfur dioxide and trioxide, soot, ash, and sulfuric acid

3. Industrial Smog e.g. Chicago or Beijing
Found in coal burning cities which experience cold, wet winters, and thermal inversion which traps cool, stagnant air close to the earth’s surface
However, due to increased pollution controls, bans on coal use as a domestic fuel, and the use of alkaline scrubbers (which remove Sox, severe occurrences of industrial smog are diminishing

4. Photochemical Smog First observed in the 1940’s in Los Angeles
Unlike industrial smog, the paramount cause of photochemical smog is a combination of automobile pollution and ultraviolet radiation from the sun
Comprised of hundreds of substances
Each substance is formed by free radical reactions facilitated by ultraviolet radiation
- A free radical is a highly reactive entity with one or more unpaired electrons

5. Factors contributing to photochemical smog
-Existence of volatile organic chemicals (VOC’s)
-Existence of nitrogen oxides (NOx)
(Particularly nitrogen monoxide (NO) produced by automobiles and power plant furnaces)
-Existence of ultraviolet radiation (from the sun)
-Temperature, stagnant air (lack of wind), and bowl-shaped cities worsen photochemical smog

6. Thermal Inversion
-Normally, the air closest to the earth is warmer than the air above it.
-This warm air will rise, and the cool, denser air above it will take its place.
-That air will then be warmed by the sun’s radiation off the earth, and the process continues
-A thermal inversion takes place when the air closest to the earth is cool, and is beneath a layer of warm air
-The cool air close to the earth becomes essentially “trapped” by the blanket of warm air above

7. Thermal inversion and photochemical smog
When pollutants work their way into the layer of cool air trapped by the warm air above it, the pollutants are not dispersed
Under normal conditions, the pollutants would be dispersed with the rising warm air, but under the conditions of a thermal inversion, they are trapped
This is ideal for the formation of photochemical smog for the ultraviolet radiation from the sun can easily reach the free radical pollutants causing many chemical reactions
The products of these reactions create the visible “brown cloud” we associate with smog

8. Formation of secondary pollutants in photochemical smog
1. Primary pollutants are released in the form of nitrogen oxides (particularly NO) and volatile organic chemicals (VOC’s)
2. With an increase in concentration, the NO is oxidized to brown NO2 mainly by ozone:
NO(g) + O3(g)  NO2(g) + O2(g)
3. Photo-dissociation of the nitrogen dioxide produces oxygen atoms which are highly reactive free radicals:
NO2(g) + UV light  NO(g) + O-(g)
4. The oxygen atoms cause numerous reactions which produce secondary pollutants like ozone and hydroxyl free radicals which are photochemical oxidants:
O- + O2  O3
O- + H2O  2OH-
5. These photochemical oxidants oxidize volatile organic chemicals (VOC’s) producing secondary pollutants such as aldehydes (RCHO) and peroxyacyl nitrates (PAN’s) which cause eyes to water.
RH + OH-  R- + H2O
R- + O2  ROO-
ROO- + NO2-  ROONO2