Table of Contents

(a). Introduction
(b). Development of Photochemical Smog
(c). Chemistry of Photochemical Smog
(d). Photochemical Smog and the Okanagan Valley

(a). Introduction

The industrial revolution has been the central cause for the increase in pollutants in the atmosphere over the last three centuries. Before 1950, the majority of this pollution was created from the burning of coal for energy generation, space heating, cooking, and transportation. Under the right conditions, the smoke and sulfur dioxide produced from the burning of coal can combine with fog to create industrial smog. In high concentrations, industrial smog can be extremely toxic to humans and other living organisms. London is world famous for its episodes of industrial smog. The most famous London smog event occurred in December, 1952 when five days of calm foggy weather created a toxic atmosphere that claimed about 4000 human lives. Today, the use of other fossil fuels, nuclear power, and hydroelectricity instead of coal has greatly reduced the occurrence of industrial smog. However, the burning of fossil fuels like gasoline can create another atmospheric pollution problem known as photochemical smog. Photochemical smog is a condition that develops when primary pollutants (oxides of nitrogen and volatile organic compounds created from fossil fuel combustion) interact under the influence of sunlight to produce a mixture of hundreds of different and hazardous chemicals known as secondary pollutants. The Table below describes the major toxic constituents of photochemical smog and their effects on the environment. Development of photochemical smog is typically associated with specific climatic conditions and centers of high population density. Cities like Los Angeles, New York, Sydney, and Vancouver frequently suffer episodes of photochemical smog. In recent years, scientists have also noticed that smaller communities, like Kelowna and Kamloops, can develop similar pollution problems if conditions are right.

Major Chemical Pollutants in Photochemical Smog:
Sources and Environmental Effects

Toxic Chemical


Environmental Effects

Additional Notes

Nitrogen Oxides
(NO and NO
- combustion of oil, coal, gas in both automobiles and industry
- bacterial action in soil
- forest fires
- volcanic action
- lightning
- decreased visibility due to yellowish color of NO2
- NO2 contributes to heart and lung problems
- NO2 can suppress plantgrowth
- decreased resistance to infection
- may encourage the spread of cancer
- all combustion processes account for only 5 % of NO2 in the atmosphere, most is formed from reactions involving NO
-concentrations likely to rise in the future
Volatile Organic Compounds (VOCs)
- evaporation of solvents
- evaporation of fuels
- incomplete combustion of fossil fuels
- naturally occurring compounds like terpenes from trees
- eye irritation
- respiratory irritation
- some are carcinogenic
- decreased visibility due to blue-brown haze
- the effects of VOCs are dependent on the type of chemical
- samples show over 600 different VOCs in atmosphere
- concentrations likely to continue to rise in future
Ozone (O3)
- formed from photolysis of NO2
- sometimes results from stratospheric ozone intrusions
- bronchial constriction
- coughing, wheezing
- respiratory irritation
- eye irritation
- decreased crop yields
- retards plant growth
- damages plastics
- breaks down rubber
- harsh odor
- concentrations of 0.1 parts per million can reduce photosynthesis by 50 %
- people with asthma and respiratory problems are influenced the most
- can only be formed during daylight hours
Peroxyacetyl Nitrates (PAN) - formed by the reaction of NO2 with VOCs (can be formed naturally in some environments)
- eye irritation
- high toxicity to plants
- respiratory irritation
- damaging to proteins
- was not detected until recognized in smog
- higher toxicity to plants than ozone

(b). Development of Photochemical Smog

Certain conditions are required for the formation of photochemical smog. These conditions include:

1. A source of nitrogen oxides and volatile organic compounds. High concentrations of these two substances are associated with industrialization and transportation. Industrialization and transportation create these pollutants through fossil fuel combustion.

2. The time of day is a very important factor in the amount of photochemical smog present. The following diagram illustrates the daily variation in the key chemical players. The diagram suggests:

3. Several meteorological factors can influence the formation of photochemical smog. These conditions include:

4. Topography is another important factor influencing how severe a smog event can become. Communities situated in valleys are more susceptible to photochemical smog because hills and mountains surrounding them tend to reduce the air flow, allowing for pollutant concentrations to rise. In addition, valleys are sensitive to photochemical smog because relatively strong temperature inversions can frequently develop in these areas.

(c). Chemistry of Photochemical Smog

The previous section suggested that the development of photochemical smog is primarily determined by an abundance of nitrogen oxides and volatile organic compounds in the atmosphere and the presence of particular environmental conditions. To begin the chemical process of photochemical smog development the following conditions must occur:

If the above criteria are met, several reactions will occur producing the toxic chemical constituents of photochemical smog. The following discussion outlines the processes required for the formation of two most dominant toxic components: ozone (O3) and peroxyacetyl nitrate (PAN). Note the symbol R represents a hydrocarbon (a molecule composed of carbon, hydrogen and other atoms) which is primarily created from volatile organic compounds.

Nitrogen dioxide can be formed by one of the following reactions. Notice that the nitrogen oxide (NO) acts to remove ozone (O3) from the atmosphere and this mechanism occurs naturally in an unpolluted atmosphere.

O3 + NO »»» NO2 + O2

2 »»» NO2 + other products

Sunlight can break down nitrogen dioxide (NO2) back into nitrogen oxide (NO).

NO2 + sunlight »»» NO + O

The atomic oxygen (O) formed in the above reaction then reacts with one of the abundant oxygen molecules (which makes up 20.94 % of the atmosphere) producing ozone (O3).

O + O2 »»» O3

Nitrogen dioxide (NO2) can also react with radicals produced from volatile organic compounds in a series of reactions to form toxic products such as peroxyacetyl nitrates (PAN).

NO2 + R »»» products such as PAN

It should be noted that ozone can be produced naturally in an unpolluted atmosphere. However, it is consumed by nitrogen oxide as illustrated in the first reaction. The introduction of volatile organic compounds results in an alternative pathway for the nitrogen oxide, still forming nitrogen dioxide but not consuming the ozone, and therefore ozone concentrations can be elevated to toxic levels.

(d). Photochemical Smog and the Okanagan Valley

Photochemical smog can be a significant pollution problem in the Okanagan Valley. The Okanagan meets all the requirements necessary for the production of photochemical smog, especially during the summer months. During this time period there is an abundance of sunlight, temperatures are very warm, and temperature inversions are common and can last for many days. The Okanagan Valley also has some very significant sources of nitrogen oxides and volatile organic compounds, including:

1. High emissions of nitrogen oxides and volatile organic compounds primarily from burning fossil fuels in various forms of transportation.

2. The release of large amounts of nitrogen oxides and volatile organic compounds into the atmosphere from forestry and agriculture. Forestry contributes to the creation of photochemical smog creation in two ways: the burning of slash from logging; and, the burning of woodchip wastes in wood product processing plants. Agriculture produces these chemicals through the burning of prunings and other organic wastes.

The idea that the Okanagan is immune to the big city problems of photochemical smog may simply be wishful thinking. In fact, recent monitoring of ground level ozone has shown that the values between here and the Lower Mainland are quite comparable (see Figure). In addition, research over a 4 year period (1985-1989) has shown that ozone levels can at times be higher over the Okanagan Valley than the Lower Mainland of British Columbia by almost 49 %.

Additional References Available in Public Libraries, Okanagan University College Library, or University College of the Cariboo Library

Environment Canada: Major Programs and Initiatives

Environment Canada: National Environmental Indicator Series

Environment Canada: Publication List

Other Related Home Pages On This Topic

October 17, 1996
Created by Tracy Gow and Michael Pidwirny, 1996
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