Air Pollution GEOL 1350: Introduction To Meteorology 1
Overview Types and Sources of Air Pollutants Factors That Affect Air Pollution Air Pollution and the Urban Environment 2
Air pollutants are airborne substances (either solids, liquids, or gases) that occur in concentrations high enough to threaten the health of people and animals, to harm vegetation and structures, or to toxify a given environment. Air pollutants come from both natural sources and human activities. 3
Smoke from massive wild fires across southern California out over the Pacific Ocean Air pollutant natural sources include: Wind picking up dust and soot from the earth s surface Volcanoes belching tons of ash and dust into the atmosphere 4 Smoke produced from forest fires
Human-induced pollution enters the atmosphere from Fixed sources: Industrial complexes, power plants, homes, office buildings. Mobile sources: Motor vehicles, ships, jet aircraft. Certain pollutants are called primary air pollutants: They enter the atmosphere directly -- from smokestacks and tail pipes. Other pollutants, known as secondary air pollutants, form only when a chemical reaction occurs between a primary pollutant and some other component of air (e.g. water vapor or another pollutant). 5
Carbon monoxide (CO), Sulfur Oxides (SOx), nitrogen oxides (NOx), volatile organic compounds (VOC), and particulate matter (PM) are the most prevalent primary pollutants. Transportation and fuel combustion in stationary sources are the primary sources for these pollutants. 6
Sources of Primary Air Pollutants 7
Particulate matter (PM) represents a group of solid particles and liquid droplets that are small enough to remain suspended in the air. PM includes particles such as soot, dust, smoke, and pollen, etc. It also includes tiny liquid droplets of sulfuric acid, oil, and various pesticides. PM often dramatically reduces visibility in urban environment. PM collected in cities includes iron, copper, nickel, and lead. This type of pollution can immediately influence the human respiration system. 8
Denver, Colorado, on a clear day and on a day when PM and other pollutants greatly reduce visibility. Particulates are classified as having diameters < 10 µm (PM-10) and < 2.5 µm (PM-2.5). PM-10 pollutants settle out of the atmosphere relatively quickly compared with PM-2.5. PM-10 and PM-2.5 poses greatest health risks. They are small enough to penetrate the lung s natural defense 9 mechanisms. They also reduce visibility.
Cumulus clouds and a thunderstorm rise above the thick layer of haze that frequently covers the eastern half of the United States on humid summer days. Many suspended particles are hygroscopic, as water vapor readily condenses onto them. As a thin film of water forms on the particles, they grow in size. When they reach a diameter between 0.1 and 1.0 µm, these wet haze particles effectively scatter incoming sunlight to give the sky a milky white appearance. 10
Cumulus clouds and a thunderstorm rise above the thick layer of haze that frequently covers the eastern half of the United States on humid summer days. The particles are usually sulfate or nitrate particulate matter (PM) from combustion processes, such as those produced by diesel engines and power plants. The hazy air mass may become quite thick, and on humid summer days it often becomes cumulus clouds as shown in the figure. 11
Carbon monoxide (CO), a major pollutant of city air, is a colorless, odorless, poisonous gas. It is formed during the incomplete combustion of carbon-containing fuels. It is most plentiful of the primary pollutants. Environmental Protection Agency (EPA) estimate that over 60 million metric tons of CO enter the air annually over the United States alone -- more than half is from highway vehicles. CO is removed from the atmosphere by microorganisms in the soil. 12
Sulfur dioxide (SO2) is a colorless gas that comes primarily from the burning of sulfur-containing fossil fuels (such as coal and oil). Its primary source includes power plants, heating devices, smelters, petroleum refineries, and paper mills. It can also enter atmosphere naturally during volcano eruptions and as sulfate particles from ocean spray. SO2 can be oxidized to form the secondary pollutants sulfur trioxide (SO3) and in moist air high corrosive sulfuric acid (H2SO4). SO2 can cause respiratory problems. 13
Volatile organic compounds (VOCs) represent a class of organic compounds that are mainly hydrocarbons -- individual organic compounds composed of hydrogen and carbon. Methane (CH4) is the most abundant hydrocarbons. Other hydrocarbons include benzene, formaldehyde, and some chlorofluorocarbons. EPA estimates that over 18 million metric tons of VOCs are emitted into the air over the USA each year, with about 34% from vehicles used for transportation and 50% from industrial processes. 14
Nitrogen oxides are gases that form when some of the nitrogen in the air reacts with oxygen during high temperature combustion of fuel. Two primary nitrogen pollutants are nitrogen dioxide (NO2) and nitrogen oxide (NO), which together are referred as NOx or oxides of nitrogen. Both NO and NO2 can be produced by natural bacterial action. Primary sources of NO are motor vehicles, power plants, and waste disposal systems. In moist air, NO2 reacts with water vapor to form corrosive nitric acid (HNO3), a substance that adds to the problem of acid rain. 15
Ozone (O3) is a noxious substance with an unpleasant odor that irritates eyes and mucous membranes of the respiratory system. Ozone forms naturally in the stratosphere through combining of molecular oxygen and atomic oxygen. Stratospheric ozone provides a protective shield against the sun s harmful ultraviolet rays. Near the surface, in polluted air, ozone often referred as tropospheric ozone. It is a secondary pollutant. It is formed from a complex series of chemical reactions involving other pollutants. 16
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Air quality in the USA may have been at its worst in the 1970s, but programs implemented by the Clean Air Act have helped the USA 18 move toward primary ambient air quality standards.
To indicate the air quality in a particular region, the EPA developed the air quality index (AQI). AQI includes the pollutants carbon monoxide, sufur dioxide, nitrogen dioxide, particulate matter, and ozone. When the pollutant s value is the same as the air quality standard, the pollutant is assigned an AQI number of 100. A pollutant is considered unhealthful if AQI > 100. 19
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Wind Stability and Inversion Topography 21
Role of Wind: Wind speed plays a role in diluting pollution. Wind speed determines how quickly the pollutants mix with the surrounding air. 22
Smoke puffs are closer together and more concentrated when wind speed is low. Smoke puffs are farther apart and more diluted when wind speed is greater. 23
Role of Stability and Inversions Atmospheric stability determines the extent to which air will rise. Unstable atmosphere favors vertical air currents Stable atmosphere strongly resists upward vertical motions 24
Radiation temperature inversions, lasting only a few hours at morning with warm air above cold, creates a stable atmosphere and traps pollutants at the surface. 25
Subsidence inversions may last for several days, which can create major pollution threats by reducing the mixing depth and layer, forcing a build-up of unwanted pollutants in the urban environment. 26
Role of Topography Shape of landscape plays an important part in trapping pollutants. At night, cold air tends to bring pollutants into the valleys and trapped in there. Pollution problem in several large cities are partly due to topography. For example, Los Angeles is surrounded on three sides by hills and mountains. Cool air settles in the basin, trapping pollutants from industry and millions of autos. 27
Air movement from higher hills to valleys can strengthen preexisting surface inversions, as well as carry pollutants downhill, particularly in the colder months. 28
Air Pollution and the Urban Environment Cities are generally warmer than surrounding rural areas. This region of city warmth is known as urban heat island. In cities, where less vegetation and exposed soil exists, the majority of the sun s energy is absorbed by urban structures. During daylight, surface temperature at cities rise higher than in rural areas. During night, the release of heat energy is retarded by the tall vertical city walls that do not allow infrared radiation to escape. The slow release of heat tends to keep night time city temperature higher than that in rural areas. 29
Air Pollution and the Urban Environment Cities are generally warmer than surrounding rural areas. This region of city warmth is known as urban heat island. [Streutker, 2002; Remote Sensing of Environment] 30
Air Pollution and the Urban Environment Products of combustion, such as oxides of sulfur and nitrogen can slowly settle to the ground in dry form (dry deposition) or they can be removed from the air during the formation of cloud particles and then carried to the ground in rain, called Acid rain (wet deposition). Acid deposition encompasses both dry and wet acidic substances. 31
Atmospheric concentrations of CO2 make rainfall slightly acidic (H2CO3) with PH within 5.0-5.6. Power plant emissions of SO2 and car exhaust of NOx form acid rain (H2SO4, HNO3) with PH within 4-4.5. Air Pollution 32
Air Pollution Acid precipitation is not confined to the Northeast; the acidity of precipitation has increased rapidly during the past 30 years in the southeastern states too. 33
Acidic particles causes serious deficiencies in certain elements necessary for the tree s growth. Trees are thus weakened and become susceptible to insects and drought. Air Pollution 34
SUMMARY 1. Primary air pollutants enter the atmosphere directly. Secondary pollutants form by chemical reactions that involve other pollutants. 2. Most air pollution episodes occur when the winds are light, skies are clear, the mixing layer is shallow, the atmosphere is stable, and a strong inversion exists. These conditions usually prevail when a high pressure area stalls over a region. 3. Urban environments tend to be warmer and more polluted than the rural areas that surround them. 35