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Earth & Environmental Sciences
EESC 1172
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Weather, Climate, and the Environment Notes III Chapter 14: Air Pollution A Brief History of Air pollution ● Strictly speaking, air pollution is not a new problem. It more than likely began when humans invented fire whose smoke choked the inhabitants of poorly ventilated caves. ● To alleviate the smoke problem in old England, King Edward I issued a proclamation in 12 73 forbidding the use of sea coal. ● As industrialization increased, the smoke problem worsened. By the 1850s, London had become notorious for its “pea-soup” fog, or smog, a thick mixture of smoke and fog that hung over the city. ○ very dangerous; responsible for thousands of deaths in London ● Finally, the Parliament passed a Clean Air Act in 1956. ● The industrial revolution brought air pollution to the United States, as homes and coal-burning industries belched smoke, soot, and other undesirable emissions into the air. ○ air pollution episodes in Los Angeles, New York, and other large American cities led to the establishment of much stronger emission standards for industry and automobiles. (e.g. The Clean Air Act of 1970 was revised in 1977 and updated in 1990 to include even stricter emission requirements) Types and Sources of Air pollutants ● Air pollutants are airborne substances (either solids, liquids, or gas) 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 ○ natural sources: wind picking up dust and soot from earth’s surface and carrying it aloft, volcanoes belching tons of ash and dust into atmosphere, and forest fires producing vast quantities of smoke ○ human-induced pollution enters the atmosphere from both fixed sources and mobile sources. ■ fixed sources: industrial complexes, power plants, homes, office buildings, etc. ■ mobile sources: motor vehicles, ships, and jet aircraft ● Primary air pollutants enter the atmosphere directly--from smokestacks and tailpipes, for example. ● Secondary air pollutants form only when a chemical reaction occurs between a primary pollutant and some other component of air, such as water vapor or another pollutant. ● Carbon monoxide is the most abundant primary air pollutant in the U.S. ● The primary source for all pollutants is transportation, with fuel combustion from stationary (fixed) sources coming in a distant second. Principal air pollutants ● particulate matter represents a group of solid particles and liquid droplets that are small enough to remain suspended in the air. ○ collectively known as aerosols ○ include solid particles that may irritate people are usually not poisonous ○ some are dangerous ○ often dramatically reduces visibility in urban environments and therefore most noticeable ○ some particulate matter collected in cities includes iron, copper, nickel, and lead, which can immediately influence human respiratory system. ○ PM-10 are finer particles with diameters smaller than 10 um; they pose a greatest health risk because they are small enough to penetrate the lung’s natural defense mechanisms and are very easily carried by winds to far distances ○ PM-2.5 are especially dangerous because they can penetrate further into longs and these tiny particles frequently consist of toxic or cancer-causing combustion products. ● Rain and snow remove many of these particles from the air because these particles act as nuclei for cloud droplets and ice crystals. Many of the suspended particles are hygroscopic, as water vapor readily condenses onto them. ● Carbon monoxide (CO) is a major pollutant of city air that is colorless, odorless, and poisonous. It is a gas that forms during the incomplete combustion of carbon-containing fuels. ○ quickly removed from the atmosphere by microorganisms in the soil ○ it can kill you without warning. Normally, your cells obtain oxygen through a blood pigment called hemoglobin, which picks up oxygen from the lungs, combines with it, and carries it throughout your body. Unfortunately, human hemoglobin prefers carbon monoxide to oxygen, so if there is too much carbon monoxide in the air you breathe, your brain will soon be starved of oxygen, and headache, fatigue, drowsiness, and even death may result. ● Sulfur dioxide (SO2) is a colorless gas that comes primarily from the burning of sulfur-containing fossil fuels such as coal and oil. ○ primary source includes power plants, heating devices, smelters ○ can also enter atmosphere naturally during volcanic eruptions and from ocean spray ○ readily oxidizes to form the secondary pollutants sulfur trioxide (SO3) and, in moist air, highly corrosive sulfuric acid (H2SO4). ○ when inhaled, high concentration can aggravate respiratory problems such as asthma and bronchitis. ● Volatile organic compounds (VOCs) represent a class of organic compounds that are mainly hydrocarbons--individual organic compounds composed of hydrogen and carbon. ○ methane is the most abundant ○ certain VOCs such as benzene (an industrial solvent) and benzo-a-pyrene (a product of burning wood, tobacco, and barbecuing), are known to be carcinogens--cancer causing agents. ● Nitrogen dioxide (NO2) and Nitric oxide (NO) are two nitrogen pollutants that are produced by natural bacterial action. However, their concentration in urban environments is between 10 and 100 times greater than in nonurban areas. ○ in moist air, NO2 reacts with water vapor to form corrosive nitric acid, a substance that adds to the problem of acid rain. ○ primary sources are motor vehicles, power plants, and waste disposal systems. Ozone in the Troposphere ● photochemical smog forms when chemical reactions take place in the presence of sunlight (photochemical reactions) ○ main component is the gas tropospheric ozone (O3), as opposed to stratospheric ozone, a noxious substance with an unpleasant odor that irritates eyes and the mucous membranes of the respiratory system ■ forms from a complex series of chemical reactions involving other pollutants, such as nitrogen oxides and volatile organic compounds (hydrocarbons). ■ because sunlight is required to produce ozone, concentrations of tropospheric ozone are normally higher during the afternoons and during the summer months ■ Sunlight dissociates nitrogen dioxide into nitric oxide and atomic oxygen. The atomic oxygen then combines with molecular oxygen to form ozone. Ozone in the Stratosphere ● the atmosphere is stable in the stratosphere, as there exists a strong temperature inversion--the air temperature increases rapidly with height. The inversion is due, in part, to the gas ozone that absorbs ultraviolet radiation at wavelengths below about 0.3 micrometers. ● Stratospheric ozone at an altitude near 25 km absorbs UV radiation. Although thin, this layer of ozone is significant, for it shields earth’s inhabitants from harmful amounts of UV solar radiation. ● ozone forms naturally in stratosphere by combining of atomic oxygen (O) with molecular oxygen (O2) in the presence of another molecule. ● human activities alter the amount of stratospheric ozone. ○ emissions of chemicals at the earth’s surface, such as nitrous oxide emitted from nitrogen fertilizers and chlorofluorocarbons (CFCs). Until the late 1970s, they were most widely used propellants in spray cans, such as deodorants and hairsprays. ○ These gases are quite safe in the troposphere but they enter the stratosphere, where it could destroy ozone. ○ a sharp drop in ozone is known as the ozone hole. Air pollution: trends and patterns ● US Clean Air Act 1970 has decreased air pollution by reducing emissions of most pollutants substantially. ● Still, large quantities of pollutants still spew into our air largely because even with stricter emission laws, increasing numbers of autos. ● EPA set two standards ○ Primary ambient air quality standards are set to protect human health ○ Secondary standards protect human welfare, as measured by the effects of air pollution on visibility, crops, and buildings. ○ Areas that do not meet air quality standards are called nonattainment areas. ● To indicate the air quality in a particular region, the EPA developed the air quality index. It includes the pollutants carbon monoxide, sulfur dioxide, nitrogen dioxide, particulate matter, and ozone. Factors that affect air pollution The role of the wind ● wind speed plays a role in diluting pollution ○ determines how quickly the pollutants mix with the surrounding air and how fast they move away from their source ● strong winds tend to lower the concentration of pollutants by spreading them apart as they move downwind ● stronger the wind, the more turbulent the air. Turbulent air produces swirling eddies that dilute the pollutants by mixing them with the cleaner surrounding air. The role of stability and inversions ● Smoke emitted into a stable atmosphere tends to spread horizontally, rather than mix vertically. ● the stability of the atmosphere is determined by the way the air temperature changes with height (the lapse rate). ○ when the measured air temperature decreases rapidly as we move up into the atmosphere, the atmosphere tends to be more unstable, and pollutants tend to be mixed vertically. ● If, however, the measured air temperature either decreases quite slowly as we ascend, or actually increases with height (inversion; extremely stable atmosphere where warm air lies above the cool air), the atmosphere is stable. ○ radiation inversion typically forms during the night and early morning hours when the sky is clear and the winds are light. ○ As the sun rises and the surface warms, the radiation inversion normally weakens and disappears before noon. By afternoon, the atmosphere is sufficiently unstable so that, with adequate winds, pollutants are able to disperse vertically. The changing atmospheric stability from stable in the early morning to conditionally unstable in the afternoon, can have a profound effect on the daily concentrations of pollution in certain regions. ○ subsidence inversions persist for several days or longer. ■ associated with major air pollution episodes ■ form as the air above a deep anticyclone slowly sinks (subsides) and warms. ■ the base of the subsidence inversion acts as a cap or lid on the pollutants below. ■ example: coast of California ● mixing layer is the region of relatively unstable (well-mixed) air that extends from the surface to the base of the inversion. The vertical extent of the mixing layer is called the mixing depth. ○ Since the atmosphere tends to be most unstable in the afternoon and most stable in the early morning, we typically find the greatest mixing depth in the afternoon and the most shallow one (if one exists at all) in the early morning. The role of Topography ● At night, cold air tends to drain downhill, where it settles into low-lying basins and valleys. the cold air can have several effects: ○ strengthen a pre-existing surface-inversion ○ carry pollutants downhill from the surrounding hillsides ● valleys prone to pollution are those that have a shallow mixing layer and are completely encased by mountains and hills. the surrounding mountains block prevailing wind. the poorly ventilated cold valley air can only slosh back and forth like a murky bowl of soup. ● air pollution concentrations in mountain valleys tend to be greatest during the colder months. During the warmer months, daytime heating can warm the sides of the valley to the point that upslope valley winds vent the pollutants upward, like a chimney. Ingredients for severe air pollution ● many sources of air pollution ● a deep high-pressure area that becomes stationary over a region ● light surface winds that are unable to disperse the pollutants ● a strong subsidence inversion produced by the sinking of air aloft ● a shallow mixing layer with poor ventilation ● a valley where the pollutants can accumulate ● clearskies so that radiational cooling at night will produce a surface inversion, which can cause an even greater buildup of pollutants near the ground ● for photochemical smog, adequate sunlight to produce secondary pollutants, such as ozone. ● atmospheric stagnation: light winds and poor vertical mixing Air pollution and the Urban environment ● cities are generally warmer than surrounding rural areas. ● this region of city warmth, known as the urban heat island, can influence the concentration of air pollution. ● How does it form? ○ due to industrial and urban development ○ because of less vegetation and exposed soil in cities, there is less evaporative cooling and the majority of the sun’s energy is absorbed by urban structures and asphalt. ○ at night, the solar energy absorbed is slowly released into city air. The release of heat energy is retarded by the tall vertical city walls that do not allow infrared radiation to escape as readily. the slow release of heat ends to keep nighttime city temperatures higher than those of the faster cooling rural areas. ● How does it affect air pollution? ○ On clear still nights when the heat island is pronounced, a small thermal low-pressure area forms over the city. ○ Sometimes a light breeze--called a country breeze--blows from the countryside into the city. ○ If there are major industrial areas along the city’s outskirts, pollutants are carried into the heart of town, where they become even more concentrated. ■ especially true if an inversion inhibits vertical mixing and dispersion Acid Deposition ● air pollution emitted from industrial areas can be carried many kilometers downwind and it either slowly settles to the ground in dry form (dry deposition) or it is removed from the air during the formation of cloud particles and then carried to the ground in rain and snow (wet deposition). ● Acid rain is used to describe wet deposition, while acid deposition encompasses both dry and wet acidic substances. ● Rain is naturally slightly acidic because of the carbon dioxide occurring naturally in the air dissolves in rain. ● Rain combines with sulfur dioxide and nitrogen oxides to create sulfuric and nitric acid. ● žAcidic deposition damages ecosystems, particularly lakes, and buildings ● žPrevailing winds transport pollutants great distances from sources ● žCoal burning plants in Ohio Valley are source, greatest damage in Adirondacks Chapter 13: The Earth’s Changing Climate ● the climate is always changing. Evidence shows that climate has changed in the past, and nothing suggests that it will not continue to change. ● Climate change, in the form of a persistent drought or a delay in the annual monsoon rains, can adversely affect the lives of millions. ● Climate change is taking place right now as the world is warming at an alarming rate. Reconstructing Past Climates ● A mere 18,000 years ago the earth was in the grip of a cold spell, with alpine glaciers extending their icy fingers down river valleys and huge ice sheets covering vast areas of N. America and Europe. ● In the warmer periods, between glacier advances, average global temperatures were slightly higher than at present. Some scientists feel that we are still in an ice age, but in the comparatively warmer part of it. ● the study of the geological evidence left behind by advancing and retreating glaciers is one factor suggesting that global climate has undergone slow but continuous changes. To reconstruct past climates, scientists must examine and then carefully piece together all the available evidence. ● Other evidence of global climate comes from core samples taken from the ocean floor sediments and ice from Greenland and Antarctica. ○ thousands of meters of ocean sediment obtained with a hollow-centered drill were analyzed. This sediment contained calcium carbonate shells of organisms that once lived near the surface. ○ oxygen-isotope ratio of these shells provided information about the sequence of glacier advances. ■ most of the oxygen in seawater has an atomic weight of 16. However, about one out of every thousand oxygen atoms contains an extra 2 neutrons, giving it an atomic weight of 18. When ocean water evaporates, the heavy oxygen 18 tends to be left behind. ■ Consequently, during periods of glacier advance, the oceans, which contain less water, have a higher concentration of oxygen 18. ■ Since the shells of marine organisms are constructed from the oxygen atoms existing in ocean water, determining the ratio of oxygen 18 to oxygen 16 within these shells yield information about how the climate may have varied in the past. ■ A high ratio of oxygen 18 to oxygen 16 in the sediment record suggests a colder climate. ● Dendrochronlogy is the study of tree rings that provide data on past temperatures and precipitation. ○ The changes in thickness and other properties of the annual rings indicate climatic changes that may have taken place from one year to the next. ○ Changes in weather, rain, temperature, soil pH, plant nutrition, CO2 concentration, etc. affect the tree rings. ● Ice cores is a core sample that is typically removed from an ice sheet, most commonly from the polar ice caps of Antarctica, Greenland or from high mountain glaciers elsewhere. ○ contain an abundance of information about climate because inclusions in the snow of each year remain in the ice, such as wind-blown dust, ash, bubbles of atmospheric gas and radioactive substances. Climate Throughout the Ages ● About 65 m.y.a., the earth was warmer than it is now and polar ice caps did not exist. ● Beginning about 55 m.y.a., the earth entered a long cooling trend and polar ice cap appeared million years later. ● About 2.5 m.y.a., continental glaciers appeared in the Northern Hemisphere, marking the beginning of the Pleistocene epoch. ○ not a period of continuous glaciation but a time when glaciers alternately advanced and retreated. ○ between the glacial advances were warmer periods called interglacial periods, which lasted for 10,000 years or more. ● The ice began to retreat about 14,000 y.a. as surface temperatures slowly rose, producing a warm spell. ● Then, about 12,700 y.a., the average temperature suddenly dropped and northeastern North America and northern Europe reverted back to glacial conditions. ● Another 1000 years later, the cold spell (Younger Dryas) ended abruptly and temperature rose rapidly in many areas. ● Beginning about 8000 y.a.., the mean temperature dropped by as much as 2 degrees Celsius over central Europe. ● The cold period ended and temperatures began to rise, and by about 6000 y.a., the continental ice sheets over North America were gone. This warm spell during the current interglacial period, or Holocene epoch, is sometimes called the mid- Holocene maximum. ● Temperature trends during the past 1000 years ○ About 1000 y.a., the Northern Hemisphere was slightly cooler than average. ○ However, certain regions in the Northern Hemisphere were warmer than others. This relatively warm, tranquil period of several hundred years over western Europe is sometimes referred to in that region as the Medieval Climatic Optimum. ○ Northern Hemisphere experienced a slight cooling during the 15th to 19th centuries (1350-1850). Over Europe, this cold period has come to be known as the Little Ice Age. ■ the graph of temperatures during the past 1000 years is known as the “Hockey Stick” graph due to the strong warming since the late 1800s. ● Temperature trend during the past 100+ years ○ In the early 1900s, the average global surface temperature began to rise. In fact, over the Northern Hemisphere, the decade of the 1990s was the warmest of the 20th century, with 1998 and 2005 being the warmest years in over 1000 years. ○ The changes in air temperature are derived from three main sources: air temperatures over land, air temperatures over ocean, and sea surface temperatures. Climate Change caused by Natural Events ● There are three “external” causes of climate change ○ changes in incoming solar radiation ○ changes in the composition of the atmosphere ○ changes in the earth’s surface ● Nature can cause climate change through all three causes while human activities can change climate through the last two causes. ● Climate Change: Feedback mechanisms ○ A positive feedback mechanism is when the initial increase in temperature is reinforced by the other processes. ■ Example: water vapor-greenhouse feedback ● Warmer temperature rapidly evaporates water from the oceans into the warmer air → increased quantity of water vapor absorbs more of the earth’s infrared energy → strengthen greenhouse effect → raise temperature even more → more water vapor into the atmosphere → more greenhouse effect → air temperature rises more ■ Example 2: snow-albedo feedback ● an increase in global surface air temperature might cause snow and ice to melt in polar latitudes → melting reduces the albedo of the surface → more solar energy reach the surface → raise the temperature ■ All feedback mechanisms work simultaneously and in both directions (e.g. positive feedback on a cooling planet as well) ○ A negative feedback mechanism weaken the interactions among the variables rather than reinforce them. ■ Example: As the surface warms, it emits more infrared radiation. This increase in radiant energy from the surface would greatly slow the rise in temperature and help to stabilize the climate → strongest negative feedback in the climate system ● Climate Change: Plate tectonics and mountain building ○ Less movement of plates means less volcanic activity which means less CO2 spewed into the atmosphere. ○ A reduction in CO2 levels weakens the greenhouse effect, which in turn, causes global temperatures to drop. ● Climate Change: Variations in the Earth’s orbit ○ another external cause ○ involves a change in the amount of solar radiation that reaches the earth ○ a theory ascribing climatic changes to variations in the earth’s orbit is the Milankovitch theory. ○ the basic premise of the theory is that, as the earth travels through space, three separate cyclic movements combine to produce variations in the amount of solar energy that falls on the earth. ○ the first cycle deals with changes in the shape (eccentricity) of the earth’s orbit as the earth revolves about the sun. ■ changes from being elliptical to being nearly circular ■ takes about 100,000 years ■ the greater the eccentricity of the orbit (that is, the more elliptical the orbit), the greater the variation in solar energy received by the earth between its closest and farthest approach to the sun. ■ the more eccentric orbit will also change the length of seasons in each hemisphere by changing the length of time between the vernal and autumnal equinoxes. ○ the second cycle takes into account the fact that, as the earth rotates on its axis, it wobbles like a spinning top. ■ this wobble, known as the precession of the earth’s axis, occurs in a cycle of about 23,000 years. ■ presently, the earth is closer to the sun in January and farther away in July. ■ Due to precession, the reverse will be true in about 11,000 years. ○ the third cycle relates to the changes in tilt (obliquity) of the earth as it orbits the sun. ■ takes about 41,000 years to complete ■ presently, the earth’s orbital tilt is 23 ½ degrees but during the 41,000-year cycle, the tilt varies from about 22 degrees to 24 ½ degrees. ■ the smaller the tilt, the less seasonal variation there is between summer and winter in middle and high latitudes; thus winters tend to be milder and summers colder. ○ Other factors may work in conjunction with the earth’s orbital changes to explain the temperature variations between glacial and interglacial periods. Some of these are: ■ the amount of dust and other aerosols in the atmosphere ■ the reflectivity of the ice sheets ■ the concentration of other GHG gases ■ the changing characteristics of clouds ■ the rebounding of land, having been depressed by ice. ○ The Milankovitch cycles, in association with other natural factors, may explain the advance and retreat of ice over periods of 10,000 to 100,000 years. But what caused the Ice Age to begin in the first place? ● Climate Change: variations in solar output ○ sun’s energy output (called brightness) may vary slightly with sunspot activity. ○ sunspots are huge magnetic storms on the sun that show up as cooler (darker) regions on the sun’s surface. ○ they occur in cycles, with the number and size reaching a maximum approximately every 11 years. ○ During periods of maximum sunspots, the sun emits more energy (about 0.1 percent more) than during periods of sunspot minimums. ○ During the 1600s, the Maunder minimum with little sunspot activity was associated with the little ice age and global cooling. ● Climate Change: atmospheric particles ○ microscopic liquid and solid particles (aerosols) that enter the atmosphere from both human-induced and natural sources can have an effect on climate. ○ particles near the surface ■ particles can enter the atmosphere in a variety of natural ways ■ wildfires can produce copious amounts of tiny smoke particles and dust storms sweep tons of fine particles into the atmosphere ■ smoldering volcanoes can release significant quantities of sulfur- rich aerosols into the lower atmosphere ■ even the oceans are a major source of natural sulfur aerosols, as phytoplankton produce a form of sulfur that slowly diffuses into the atmosphere ■ the overall effect they have is to cool the surface by reflecting solar radiation and preventing sunlight from reaching the surface. ○ volcanic eruptions ■ scientists agree that the volcanic eruptions having the greatest impact on climate are those rich in sulfur gases. ■ these gases, when ejected into the stratosphere, combine with water vapor in the presence of sunlight to produce tiny, reflective sulfuric acid particles that grow in size, forming a dense layer of haze. The haze may reside in the stratosphere for several years, absorbing and reflecting back to space a portion of the sun’s incoming energy. ● the reflection of incoming sunlight by the haze tends to cool the air at the earth’s surface, especially in the hemisphere where the eruption occurs. ■ two of the largest volcanic eruptions of the 20th century in terms of
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