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Photochemical Smog
Aug 18, 2015

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. 

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. 

What are the major sources of photochemical smog?

While nitrogen oxides and VOCs are produced biogenically (in nature), there are also major anthropogenic (man-made) emissions of both. Natural emissions tend to be spread over large areas, reducing their effects, but man-made emissions tend to be concentrated close to their source, such as a city. 

Biogenic sources In nature, bushfires, lightning and the microbial processes that occur in soil generate nitrogen oxides. VOCs are produced from the evaporation of naturally-occurring compounds, such as terpenes, which are the hydrocarbons in oils that make them burn. Eucalypts have also been found to release significant amounts of these compounds.

Anthropogenic sources Nitrogen oxides are produced mainly from the combustion of fossil fuels, particularly in power stations and motor vehicles. VOCs are formed from the incomplete combustion of fossil fuels, from the evaporation of solvents and fuels, and from burning plant matter—such as backyard burning and wood-burning stoves.

main components of Photochemical Smog formation

Fig: main components of Photochemical Smog formation

Major Chemical Pollutants in Photochemical Smog: 
Sources and Environmental Effects

 

Toxic Chemical

Sources

Environmental Effects

Additional Notes

Nitrogen Oxides 

(NO and NO2)

- 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 
- 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



 

 

How location and weather can have an effect?

Topography The topography of the area surrounding a city can vastly influence the formation of photochemical smog. Because of the restriction of air movement, a city in a valley can experience problems that a city on an open plain may not. 

Meteorology Normally the layer of air closest to the earth’s surface is warmer than the air higher in the atmosphere because the heat of the sun is re-radiated (warmed by the earth’s surface). The higher level cool air sinks and is then warmed and displaced upwards in a convection cycle 

Convection cycle

Fig: Convection cycle

This condition is called ‘unstable’ and helps to carry pollutants upwards, where they are dispersed and diluted. This cycle is usually assisted by higher wind speeds. However, when the opposite occurs—a temperature inversion—cities can experience prolonged periods of photochemical smog. An inversion is formed when a ceiling of warmer air traps the cooler layer of air, which contains the pollutants, near the ground’s surface. This hinders the ability of the pollutants to rise to the atmosphere and be dispersed. After an inversion has formed, it keeps any smog that is present close to the ground, maximising its detrimental effect. There are two major processes that enable an inversion to happen and both are usually accompanied by low wind speeds. The first, advection, is when an upper layer of warmer air is blown in, trapping the layer of cool air below it. This ‘stable’ condition may last for several days. A variation of this is when a cooler layer of air, such as a sea breeze, is blown in underneath a warmer layer, creating the same effect. The second process, radiation inversion, usually occurs overnight. The ground cools and in turn cools the air layer closest to it, resulting in the lower air layer being cooler than air above it, forming an inversion.

How to reduce the occurrence of photochemical smog? 

The most effective way of reducing the amount of secondary pollutants created in the air is to reduce emissions of both primary pollutants.

Reduction of nitrogen oxide The main method of lowering the levels of nitrogen oxides is by a process called ‘catalytic reduction’, which is used in industry and in motor vehicles. For example, a catalytic converter fitted to a car’s exhaust system will convert much of the nitric oxide from the engine exhaust gases to nitrogen and oxygen. Nitrogen is not in the actual fuels used in motor vehicles or power stations; it is introduced from the air when combustion occurs. Using less air in combustion can reduce emissions of nitrogen oxides. Temperature also has an effect on emissions—the lower the temperature of combustion, the lower the production of nitrogen oxides. Temperatures can be lowered by using processes such as two stage combustion and flue gas recirculation, water injection, or by modifying the design of the burner.

Reduction of VOCs There are various ways to reduce VOC emissions from motor vehicles. These include the use of liquefied petroleum gas (LPG) or compressed natural gas (CNG) rather than petrol, decreasing distances vehicles travel by using other modes of transport, such as buses and bikes, and implementing various engine and emission controls now being developed by manufacturers.


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