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.

Fig: main components of Photochemical Smog formation

Major Chemical Pollutants in Photochemical Smog: 
Sources and Environmental Effects

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 

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.