URBAN AIR POLLUTION

Tropospheric Ozone Ozone is a naturally occurring gas found in the troposphere and other parts of the atmosphere. Tropospheric ozone is often called "bad" ozone because it can damage living tissue and break down certain materials. Concentrations are not uniform in the troposphere. Longer exposure to ozone will increase the negative effects. The amount of ozone varies from day to day and from place to place.

Ozone occurance Ozone occurs naturally at ground- level in low concentrations. Major sources include hydrocarbons, which are released by plants and soil, and small amounts of stratospheric ozone, which occasionally migrate down to the earth's surface. Neither of these sources contributes enough ozone to be considered a threat to the health of humans or the environment.

Human Activities & ‘Bad’ Ozone With increasing populations, the more automobiles & industries, the more ozone in the lower atmosphere. Since 1900 the amount of ozone near the earth's surface has more than doubled. Unlike most other air pollutants, ozone is not directly emitted from any one source. Tropospheric ozone is formed by the interaction of sunlight, particularly ultraviolet light, with hydrocarbons and nitrogen oxides, which are emitted by automobiles, gasoline vapors, fossil fuel power plants, refineries, and certain other industries.

Pollutants When fossil fuels are burned, two of the pollutants emitted are hydrocarbons (from unburned fuel) and nitrogen monoxide (nitric oxide, NO) Nitrogen monoxide reacts with oxygen to form nitrogen dioxide (NO2), a brown gas that contributes to urban haze. Nitrogen dioxide can also absorb sunlight and break up to release oxygen atoms that combine with oxygen in the air to form ozone.

Pollutants Cycle Ozone production from NOx pollutants: Oxygen atoms freed from nitrogen dioxide by the action of sunlight attack oxygen molecules to make ozone. Nitrogen oxide can combine with ozone to reform nitrogen dioxide, and the cycle repeats.

Urban Air Pollution Processes

Impacts of Troposphere Ozone Damages forests and crops destroys nylon, rubber, and other materials injures or destroys living tissue Threat to people Affect plants Rubber, textile dyes, fibers, and certain paints may be weakened or damaged by exposure to ozone. Some elastic materials can become brittle and crack, while paints and fabric dyes may fade more quickly.

Ozone Pollution Reached High Levels Alerts the people with respiratory problems-Take precautions/ remain indoor) Smog-Damage Respiratory Tissues trough inhalation Example :Athlete's performance Worsen heart disease, bronchitis and emphysema

Photochemical Smog Can be formed over cities on warm sunny days with lots of traffic ∵ first caused problems in LA – called Los Angeles-type smog Cities suffer: Santiago, Mexico City, Beijing etc

Formation Formed when ozone, nitrogen oxides, gaseous hydrocarbons from vehicle exhaust interact with strong sunlight Complex reactions create chemicals in smog e.g. carbon monoxide, nitrogen oxides, ozone ∵ Nitrogen dioxide = important component of smog  brown haze over city These chemicals are strongly oxidizing & affect materials/ living things Photochemical smog is a mixture of about one hundred primary and secondary pollutants formed under the influence of sunlight with Ozone is the main pollutant.

Concentration Photochemical smog is at its max. in the early afternoon ∵ important smog causing reaction is a photochemical reaction, so it reaches its peak during afternoon sunshine.

Smog Occurance Factors lead to smog: climate, local topography, pop density, fossil fuel use Most often form over large cities lying in valleys Weather Normally, air over cities is relatively warm and has a tendency to rise On warm days, an even warmer layer of air on top of the warm polluted air can prevent the air rising, trapping pollution at ground level Occurs most often in warm dry climates  Weather is important in disappearing smog: rain cleans air of pollutants while winds can disperse smog

Local Topography Smog is most often formed over large cities lying in valleys. Hills or mountains surrounding these cities shelter from most of the wind and on warm, calm days severe smog can occur. (For example, Los Angeles) The frequency and severity of photochemical smogs in an area depend on local topography, climate, population density and fossil fuel use.

Thermal Inversion Thermal inversion occurs when a layer of warm air settles over a layer of cooler air that lies near the ground. The warm air holds down the cool air and prevents pollutants from rising and scattering. This makes things worse. Thermal inversions trap the smogs in valleys. (for example, Los Angeles, Santiago, Mexico City, Rio de Janeiro, Sao Paulo, Beijing) and concentrations of air pollutants can build to harmful and even lethal levels. Normally, air over cities is relatively warm and has a tendency to rise. On warm days, however an even warmer layer of air on the top of warm polluted air can prevent this air rising, trapping the pollution at ground level. This often in warm dry climates.

Climate Weather plays an important role in the disappearance of smog: rain cleans the air of pollutants while winds can disperse the smog, Under this condition, the concentration of pollutants can reach harmful or even lethal levels. Smog is not only affecting life in the city itself. Often, smog is blown out of the city by the wind and causes damage in the countryside, sometimes up to 150 km away from the city where the smog was formed.

Population Density Development of photochemical smog is typically associated with specific climatic conditions and centers of high population density.

Fossil fuel 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.

Impacts of photochemical smog Damage to Plants : Damage to humans Damage to materials and products The effects of smog on animals are similar to its effect on humans: decreased lung capacity and lung elasticity

Pollution Management Strategies Tidal Alternative Energies Wind Solar

Alternative Lifestyles Energy efficient vehicles (e.g. diesel cars) Shut down engine when car stops Public transportations