20-3 Photochemical and Industrial Smog

What Is Photochemical Smog? Brown-Air Smog

The formation of photochemical smog involves a complex series of chemical reactions. It begins inside automobile engines and in the boilers of coal-burning power and industrial plants. At the high temperatures found there, nitrogen and oxygen in air react to produce colourless nitric oxide (N2 + O2 → 2 NO). In the atmosphere, some of the NO is converted to nitrogen dioxide (NO2), a yellowish-brown gas with a choking odour (Table 20-2). NO2 is the cause of the brownish haze that hangs over many cities during the afternoon of sunny days, explaining why photochemical smog sometimes is called brown-air smog.

Hotter days lead to higher levels of ozone and other components of smog. As traffic increases on a sunny day, smog builds up to peak levels by early afternoon, irritating people’s eyes and respiratory tracts. All modern cities have some photochemical smog, but it is much more common in cities with sunny, warm, dry climates and lots of motor vehicles.

Smog is likely to become more widespread and more serious globally as more people in developing countries begin to drive gasoline-powered vehicles.

How Can Trees Contribute to Photochemical Smog? Hydrocarbon Emitters

Trees have many environmental benefits. They emit oxygen, absorb CO2, provide shade (which reduces energy needed for air conditioning), and help absorb and remove various pollutants from the air.

Unless forests of such trees are close to urban areas, most of their VOC emissions occur in nonurban areas and disperse into the atmosphere. Thus they do not make a significant contribution to the formation of photochemical smog except in forested areas near urban areas with large sources of NOx and plenty of sunlight. This is in contrast to cars, which along with refineries and other sources emit most of their VOCs, NOx, and other pollutants into urban air. Because of trees’ ecological and aesthetic benefits, environmentalists support their widespread planting in urban areas. But they say the emphasis should be on tree species that emit low levels of VOCs.

How Does Industrial Smog Form, and How Big a Problem Is It? Grey-Air Smog Is a Danger in Some Developing Countries

However, industrial smog is a problem in industrialized urban areas of China, India, Ukraine, and some eastern European countries (especially the “black triangle” region of Slovakia, Poland, Hungary, and the Czech Republic), where large quantities of coal are burned with inadequate pollution controls.

In addition to providing electricity and running industries, coal is burned for heating homes and cooking by millions of poor families. As a result, China has some of the world’s most polluted indoor and outdoor air. This explains why many residents of Chinese cities develop serious respiratory problems and some die prematurely from the coal-generated air pollution. After only a few days in this region visitors often suffer from coughs and bronchial irritation.

What Factors Influence the Formation of Photochemical and Industrial Smog? Rain, Wind, Buildings, Mountains, and Temperature

The frequency and severity of smog in an area depend on local climate and topography, population density, the amount of industry, and the fuels used in industry, heating, and transportation. Three natural factors help reduce outdoor air pollution.

• One is rain and snow, which help cleanse the air of pollutants. This helps explain why cities with dry climates are more prone to photochemical smog than cities with wet climates.

• A second factor is salty sea spray from the oceans, which can wash out particulates and other water-soluble pollutants from air that flows from land onto the oceans.

• A third factor is winds, which can help sweep pollutants away, dilute them by mixing them with cleaner air, and bring in fresh air. However, these pollutants are blown somewhere else or are deposited from the sky onto surface waters, soil, and buildings.

• Four other factors can increase outdoor air pollution. One is urban buildings, which can slow wind speed and reduce dilution and removal of pollutants. Another is hills and mountains. They can reduce the flow of air in valleys below them, allowing pollutant levels to build up at ground level. In addition, high temperatures found in most urban areas promote the chemical reactions leading to photochemical smog formation. A fourth factor is called the grasshopper effect based on atmospheric distillation, which transfers volatile air pollutants from tropical and temperate areas to the Earth’s poles.

How Can Temperature Inversions Increase Outdoor Air Pollution? Trapping Pollutants Near the Ground

During daylight, the sun warms the air near the Earth’s surface. Normally, this warm air and most of the pollutants it contains rise to mix with the cooler air above it.

Under certain atmospheric conditions, however, a layer of warm air can lie atop a layer of cooler air nearer the ground, a situation known as a temperature inversion. Because the cooler air is denser than the warmer air above it, the air near the surface does not rise and mix with the air above it. Pollutants can concentrate in this stagnant layer of cool air near the ground.

Areas with two types of topography and weather conditions are especially susceptible to prolonged temperature inversions:

• One such area is a town or city located in a valley surrounded by mountains that experiences cloudy and cold weather during part of the year

• The second type of area typically is a city with several million people and motor vehicles in an area with a sunny climate, light winds, mountains on three sides, and the ocean on the other. Here, the conditions are ideal for photochemical smog worsened by frequent thermal inversions

Case Study: South Asia’s Massive Brown Cloud—Choking in China and India

A number of studies have warned of the harmful effects of a huge blanket of industrial smog plus smoke from sources such cooking fires and burning crop waste—called the Asian brown cloud or the atmospheric brown cloud.

This cloud is caused by huge emissions of ash, smoke, dust, and acidic compounds produced by people burning coal in industries and homes and clearing and burning forests for planting crops, along with dust blowing off deserts in western Asia.

There are reports that black carbon (soot) from the Asian brown cloud and other sources is contributing to the melting of snow and ice in the Arctic; the dark particles of soot have a warming effect on the atmosphere and darken sea ice, reducing its ability to reflect sunlight.