Preview Environmental Chemistry, 5th Edition by Colin Baird, Michael Cann (2012)

Preview Environmental Chemistry, 5th Edition by Colin Baird, Michael Cann (2012) Preview Environmental Chemistry, 5th Edition by Colin Baird, Michael Cann (2012) Preview Environmental Chemistry, 5th Edition by Colin Baird, Michael Cann (2012) Preview Environmental Chemistry, 5th Edition by Colin Baird, Michael Cann (2012) Preview Environmental Chemistry, 5th Edition by Colin Baird, Michael Cann (2012)

This page intentionally left blank ENVIRONMENTAL CHEMISTRY baird_fm.indd i 22/02/12 9:45 AM this page left intentionally blank baird_fm.indd ii 22/02/12 9:45 AM ENVIRONMENTAL CHEMISTRY Fifth Edition Colin Baird University of Western Ontario Michael Cann University of Scranton W H Freeman and Company • New York baird_fm.indd iii 22/02/12 9:45 AM Executive Editor: Jessica Fiorillo Development Editor: Brittany Murphy Marketing Manager: Alicia Brady Media and Supplements Editor: Dave Quinn Senior Media Producer: Keri Fowler Editorial Assistant: Nicholas Ciani Senior Project Editor: Vivien Weiss Photo Editor: Ted Szczepanski Photo Researcher: Cecilia Varas Art Director: Diana Blume Illustrations: Macmillan Publishing Solutions Senior Illustration Coordinator: Bill Page Production Coordinator: Susan Wein Composition: MPS Ltd Printing and Binding: RR Donnelley Library of Congress Control Number: 2011945363 ISBN-13: 978-1-4292-7704-4 ISBN-10: 1-4292-7704-1 © 2012, 2008, 2005, 1999 by W H Freeman and Company All rights reserved Printed in the United States of America First printing W H Freeman and Company 41 Madison Avenue New York, NY 10010 Houndmills, Basingstoke RG21 6XS, England www.whfreeman.com baird_fm.indd iv 22/02/12 9:45 AM Contents Preface xii Introduction to Environmental Problems, Sustainability, and Green Chemistry xix PART I Atmospheric Chemistry and Air Pollution Chapter Stratospheric Chemistry: The Ozone Layer Introduction The Physics, Chemistry, and Biology of UV Activity 11 Stratospheric Chemistry: The Ozone Layer 13 Catalytic Processes of Ozone Destruction 20 Box 1-1 The Rates of Free-Radical Reactions 22 Box 1-2 Calculating the Rates of Reaction Steps 24 Box 1-3 The Steady-State Analysis of Atmospheric Reactions Review Questions 33 Additional Problems 34 Chapter The Ozone Holes 30 37 Introduction 37 The Ozone Hole and Mid-Latitude Ozone Depletion 37 The Chemistry of Ozone Depletion 40 Polar Ozone Holes 49 Activity 49 Box 2-1 The Chemistry Behind Mid-Latitude Decreases in Stratospheric Ozone 52 The Chemicals That Cause Ozone Destruction 54 Green Chemistry: The Replacement of CFC and Hydrocarbon Blowing Agents with Carbon Dioxide in Producing Foam Polystyrene 57 Green Chemistry: Harpin Technology—Eliciting Nature’s Own Defenses Against Diseases 64 Review Questions 65 Green Chemistry Questions 66 Additional Problems 66 Chapter The Chemistry of Ground-Level Air Pollution Introduction 69 Box 3-1 The Interconversion of Gas Concentrations Urban Ozone: The Photochemical Smog Process 76 Activity 81 69 71 v baird_fm.indd v 23/02/12 7:10 PM vi Contents Improving Air Quality: Photochemical Smog 87 Green Chemistry: Strategies to Reduce VOCs Emanating from Organic Solvents 101 Green Chemistry: A Nonvolatile, Reactive Coalescent for the Reduction of VOCs in Latex Paints 101 Green Chemistry: The Replacement of Organic Solvents with Supercritical and Liquid Carbon Dioxide; Development of Surfactants for This Compound 103 Box 3-2 Supercritical Carbon Dioxide 104 Green Chemistry: Using Ionic Liquids to Replace Organic Solvents: Cellulose, a Naturally Occurring Polymer Replacement for Petroleum-Derived Polymers 105 Improving Air Quality: Sulfur-Based Emissions 109 Particulates in Air Pollution 118 Air Quality Indices and Size Characteristics for Particulate Matter 126 Box 3-3 The Distribution of Particle Sizes in an Urban Air Sample 129 Review Questions 131 Green Chemistry Questions 131 Additional Problems 132 Chapter The Environmental and Health Consequences of Polluted Air—Outdoors and Indoors 135 Introduction 135 Acid Rain 137 Activity 143 The Human Health Effects of Outdoor Air Pollutants Indoor Air Pollution 152 Review Questions 161 Additional Problems 162 145 PART II 163 Chapter Energy and Climate Change The Greenhouse Effect 165 Introduction 165 The Mechanism of the Greenhouse Effect 166 Activity 169 Box 5-1 A Simple Model of the Greenhouse Effect 173 Molecular Vibrations: Energy Absorption by Greenhouse Gases 175 The Major Greenhouse Gases 177 Other Greenhouse Gases 187 Box 5-2 Determining the Emissions of “Old Carbon” Sources of Methane 190 The Climate-Modifying Effects of Aerosols 197 Box 5-3 Cooling over China from Haze 202 Global Warming to Date 202 baird_fm.indd vi 22/02/12 9:45 AM Contents vii Geoengineering Earth’s Climate to Combat Global Warming 210 Atmospheric Residence Time Analysis 216 Review Questions 219 Additional Problems 220 Chapter Energy Use, Fossil Fuels, CO2 Emissions, and Global Climate Change 223 Introduction 223 Global Energy Usage 224 Fossil Fuels 230 Box 6-1 Shale Gas 233 Box 6-2 Petroleum Refining: Fractional Distillation 237 Box 6-3 The Deepwater Horizon Oil Spill Disaster 242 Green Chemistry: Polylactic Acid—The Production of Biodegradable Polymers from Renewable Resources; Reducing the Need for Petroleum and the Impact on the Environment 249 Sequestration of CO2 252 The Storage of Carbon Dioxide 257 Activity 264 Other Schemes to Reduce Greenhouse Gases 264 Box 6-4 Removing CO2 from the Atmosphere: Direct Air Capture 265 Carbon Dioxide Emissions in the Future 267 Activity 268 The Extent and Potential Consequences of Future Global Warming 276 Review Questions 288 Green Chemistry Questions 289 Additional Problems 290 Chapter Biofuels and Other Alternative Fuels 291 Introduction 291 Biomass and Biofuels: Issues 292 Ethanol 295 Biodiesel from Plant Oils and from Algae 303 Activity 310 Green Chemistry: Bio-based Liquid Fuels and Chemicals 310 Green Chemistry: Recycling Carbon Dioxide—A Feedstock for the Production of Chemicals and Liquid Fuels 311 Thermochemical Production of Fuels, Including Methanol 313 Hydrogen—Fuel of the Future? 320 Review Questions 334 Green Chemistry Questions 335 Additional Problems 336 baird_fm.indd vii 22/02/12 9:45 AM viii Contents Chapter Renewable Energy Technologies: Hydroelectric, Wind, Solar, Geothermal, and Marine Energy and Their Storage 337 Introduction 337 Hydroelectric Power 338 Wind Energy 340 Marine Energy: Wave and Tidal Power 348 Geothermal Energy 349 Direct Solar Energy 354 The Storage of Renewable Energy—Electricity and Heat Activity 371 Review Questions 371 Additional Problems 372 Chapter 369 Radioactivity, Radon, and Nuclear Energy 373 Introduction 373 Radioactivity and Radon Gas 374 Box 9-1 Steady-State Analysis of the Radioactive Decay Series 379 Nuclear Energy 383 Environmental Problems of Uranium Fuel 390 Box 9-2 Radioactive Contamination by Plutonium Production Accidents and the Future of Nuclear Power 398 Nuclear Fusion 402 Review Questions 405 Additional Problems 406 PART III Water Chemistry and Water Pollution Chapter 10 The Chemistry of Natural Waters 395 407 409 Introduction 409 Oxidation–Reduction Chemistry in Natural Waters 413 Green Chemistry: Enzymatic Preparation of Cotton Textiles 418 Acid–Base and Solubility Chemistry in Natural Waters: The Carbonate System 430 Box 10-1 Derivation of the Equations for Species Diagram Curves 432 The CO2–Carbonate System 432 Box 10-2 Solubility of CaCO3 in Buffered Solutions 437 Ion Concentrations in Natural Waters and Drinking Water 442 Activity 445 Review Questions 451 Green Chemistry Questions 452 Additional Problems 452 baird_fm.indd viii 22/02/12 9:45 AM 120 Chapter The Chemistry of Ground-Level Air Pollution FIGURE 3-19 Sizes of common airborne fine and coarse particulates [Source: Adapted from J G Henry and G W Heinke, Environmental Science and Engineering (Upper Saddle River, NJ: Prentice Hall, 1989).] Coarse particles Rain Fine particles Mist/Fog/Clouds Smog Tobacco smoke Coal dust Atmospheric dust Foundry dust Agriculture sprays Oil smoke Pollen Bacteria 100,000 10,000 1,000 100 1,000,000 (1 mm) (1 m) 10 Viruses 10–1 10–2 10–3 10–4 (1 nm) Particle diameter in micrometers ( µ m) airborne for days or weeks, whereas coarse particulates settle out fairly rapidly In addition to this sedimentation process, particles also are commonly removed naturally from tropospheric air by their incorporation into falling raindrops, usually within a week or two 3.23 Sources and Composition of Coarse Particles The primary-versus-secondary distinction made between atmospheric gaseous pollutants (Section 3.3) is also applied to suspended particles Most coarse particles are primary, although they often begin their existence as even coarser matter, since they originate chiefly from the disintegration of larger pieces of matter Minerals constitute one important type of the coarse particulates in air Because many of the large particles in atmospheric dust, particularly in rural areas, originate as soil or rock, their elemental composition is similar to that of the Earth’s crust, namely high concentrations of Al, Ca, Si, and O in the form of aluminum silicates, some of which also contain the calcium ion (Chapter 16) baird_ch03.indd 120 23/02/12 8:07 AM Particulates in Air Pollution 121 • Wind storms in deserts sweep large amounts of fine sand into the air Dust storms in Asia, whose effects reach as far away as North America, are increasing due to the continuing transformation of fertile land into desert as a consequence of global warming, deforestation, and overgrazing • The wind generates coarse particles by the mechanical disintegration of leaf litter • Pollen released from plants also consists of coarse, primary particles • Wildfires and volcanic eruptions generate both fine and coarse particulate matter • Near and above oceans, the concentration of solid NaCl is very high, since sea spray leaves sodium chloride particles airborne when the water evaporates Indeed, sea-salt aerosols are by far the largest mass of primary particles in air, followed by soil dusts and debris from natural fires Although most coarse particulates originate with natural sources, human activities such as stone crushing in quarries and land cultivation result in particles of rock and topsoil being picked up by the wind Coarse particles in many areas are basic, reflecting the calcium carbonate and other such salts in soils 3.24 Sources and Composition of Fine Particles Primary fine particles of anthropogenic origin include ones generated by the wearing of tires and vehicles brakes, and the dust from metal smelting The incomplete combustion of carbon-based fuels such as coal, oil, gasoline, and diesel fuel produces many fine soot particles, which are mainly crystallites (miniature crystals) of carbon Consequently, one of main sources of carbonbased primary atmospheric particulates, both fine and coarse, is the exhaust from vehicles, especially those having diesel engines About half the organic content from heavy-duty diesel vehicles is elemental carbon (EC); this soot is commonly seen as the black smoke that emanates from such equipment Most carbon-containing emissions from gasoline-powered engines are composed of organic compounds (OC) rather than elemental carbon Whereas coarse particles result mainly from the breakup of larger ones, fine particles are formed mainly by chemical reactions between gases and by the coagulation of even smaller species, including molecules in the vapor state; they are mainly secondary particles in nature Although most of the mass of atmospheric fine particles arises from natural sources, that over urban areas often has mainly an anthropogenic origin The average organic content of fine particles is generally greater than that of coarse ones In areas subject to photochemical smog, a substantial fraction of the organic compounds in the particulate phase are formed from the reaction of VOCs and nitrogen oxides in the photochemical smog reaction, and correspond to partially oxidized hydrocarbons that have baird_ch03.indd 121 The main final products from photochemical smog are gaseous ozone and fine suspended particulates containing OC 2/2/12 11:40 AM 122 In Iceland itself, fluoride emitted from the volcano was a worse problem, since it proved fatal to crops, livestock, and people Chapter The Chemistry of Ground-Level Air Pollution incorporated oxygen to form carboxylic acids, etc and nitrogen to form nitro groups, etc Aromatic hydrocarbons with at least seven carbon atoms (e.g., toluene) that enter the air of cities as VOCs from the evaporation of gasoline also form aerosols Hydrocarbons having fewer than seven carbons give oxidation products with substantial vapor pressures that remain in the gas phase Research performed following the 2010 Deepwater Horizon oil spill discovered that, although VOC hydrocarbons in the surface oil formed organic aerosols relatively quickly, longer-chain semivolatile and intermediate volatility organic compounds reacted to form the bulk of the organic aerosol, even though they were slower to vaporize The other important fine particles suspended in the atmosphere consist predominantly of inorganic compounds of sulfur and of nitrogen Much of the natural sulfur in air originates as dimethyl sulfide, (CH3)2S, emitted from the oceans A by-product of its oxidation in air is carbonyl sulfide, COS, a longlived trace atmospheric component that also results from the atmospheric oxidation of carbon disulfide, CS2, and from direct emissions from oceans and biomass Some of the COS makes its way into the stratosphere, where it is oxidized and produces the natural sulfate aerosol found at those altitudes Both dimethyl sulfide and hydrogen sulfide are oxidized in air mainly to SO2 Sulfur dioxide gas is also emitted directly in large quantities both by natural sources such as volcanoes and as pollution from power plants and smelters It becomes oxidized over a period of hours or days to sulfuric acid and sulfates in the air Sulfuric acid itself travels in air not as a gas but as an aerosol of fine droplets, since it has such a great affinity for water molecules A huge volcanic eruption in Iceland in 1783 produced enough sulfuric acid particles to blanket Europe in a “great dry fog” for the entire summer, killing many people Another natural source of atmospheric particles has been discovered Alkyl iodine compounds such as CH2I2 are emitted by seaweed into the air above coastal regions Absorption of the ultraviolet component of light is sufficient to detach iodine atoms from such gaseous molecules In subsequent reactions analogous to those of chlorine in the stratosphere, the iodine atoms react with ozone to form iodine monoxide, IO, which in turn dimerizes to form I2O2 The dimer and other iodine-oxygen compounds condense to form fine particles PROBLEM 3-16 By analogy with the reactions of atomic chlorine discussed in Chapter 2, write balanced equations for the reaction of atomic iodine with ozone and for ● the dimerization of IO 3.25 The Neutralization of Acids in Air Fine particles in many areas are acidic, due to their content of sulfuric and nitric acids The nitric acid is the end-product of the oxidation of baird_ch03.indd 122 2/2/12 11:40 AM Particulates in Air Pollution 123 nitrogen-containing atmospheric gases such as NH3, NO, and NO2, and the sulfuric acid comes from the oxidation of SO2 Because HNO3 has a much higher vapor pressure than does H2SO4, there is less condensation of nitric acid onto preexisting particles than occurs with H2SO4 Both sulfuric and nitric acids in tropospheric air often eventually encounter ammonia gas that is released as a result of biological decay processes occurring at ground level The atmospheric acids undergo an acid– base reaction with the ammonia, which transforms them into the soluble salts ammonium sulfate, (NH4)2SO4, and ammonium nitrate, NH4NO3 Since sulfuric acid contains two hydrogen ions, the neutralization reaction occurs in two-stages, the first producing ammonium bisulfate, NH4HSO4: H2SO4(aq) ϩ NH3(g) 9: NH4HSO4(aq) NH4HSO4(aq) ϩ NH3(g) 9: (NH4)2SO4(aq) The ammonia that results from animal urine originates in the liquid as urea, CO(NH2)2, which subsequently hydrolyzes: CO(NH2)2 ϩ H2O 9: NH3 ϩ CO2 The neutralization of acidity by ammonia gas released into the air from livestock, from the use of fertilizers, and by carbonate ion suspended in air from the dust raised by farming activities are the main reason that precipitation over the central United States is not particularly acidic, and similarly for that over regions of China However, some acidification results from the ionization of the ammonium ion, NH4ϩ, a weak acid that is produced by ammonia neutralization: 0! NH4ϩ !1 NH3 ϩ Hϩ Although the nitrate and sulfate salts initially are formed from acids in aqueous particles, evaporation of the water can result in the production of solid particles The predominant ions in fine particles are the anions sulfate, SO42Ϫ, bisulfate, HSO4Ϫ, and nitrate, NO3Ϫ, and the cations ammonium, NH4ϩ, and hydrogen ion, Hϩ Aerosols dominated by oxidized sulfur compounds are called sulfate aerosols NH3 HNO3 NH4NO3 (NH4)2SO4 NH4ϩ NO3 Ϫ NH4HSO4 SO4 Hϩ HSO4Ϫ NH3 2Ϫ NH3 H2SO4 sulfate aerosol particle baird_ch03.indd 123 2/2/12 11:40 AM 124 Chapter The Chemistry of Ground-Level Air Pollution On the west coast of North America, nitrate rather than sulfate is the predominant anion since more pollution results initially from nitrogen oxides than from sulfur dioxide, since coal mined in the western United States tends be low in sulfur In Great Britain, most of the fine particles in the winter months originate as soot from car exhaust and pollution from industry, whereas in the summer they arise from the oxidation of sulfur and nitrogen oxides If there is substantial ammonia gas in the air, nitric acid will react with it to form ammonium nitrate solid in the particulate phase Recent simulations of smog formation in Southern California indicate that, although reductions in VOC concentrations without any change in NOX would reduce ozone formation, the production of nitrate-based particulates would actually increase because more nitrogen dioxide would then react to produce nitric acid and then nitrate ion The simultaneous control of ozone and particulates presents regulators with a formidable challenge! In summary, coarse particles are usually either soot or inorganic (soillike) in nature, whereas fine ones are mainly either soot, sulfate, or nitrate aerosols Fine particles are usually acidic due to the presence of unneutralized acids and are eventually neutralized by ammonia, whereas coarse ones are usually basic also because of their soil content 3.26 Smoke from Wood Stoves The burning of wood in domestic fireplaces and wood stoves produces large quantities of particulates, which are emitted from the chimneys into outdoor air Indeed, in residential neighborhoods where wood is the predominant fuel used for heating, wood stoves contribute up to 80% of the fine particles in the air during the winter months Some municipalities have controversially limited the number of wood stoves permitted in a given geographical subregion in order to deal with the problem Outdoor wood-fired boilers, used to heat water for saunas and swimming pools, have grown so much in popularity that the particulates they emit have become a significant problem Newer wood stoves and boilers have emission controls incorporated into their design, which greatly reduce the quantity of particulates and gases they release into the air For example, U.S EPA-certified wood stoves reduce the amount of smoke from 15–30 (uncontrolled) down to 2–7 grams per hour of operation The control is accomplished in one of two ways: • A catalytic converter, consisting of a coated ceramic honeycomb structure is located near the top of the firebox Smoke and gases from the wood fire below are passed through it and more fully oxidized before continuing upward to the chimney • More commonly, the firebox is designed so that near its top, secondary combustion of the smoke occurs before it is emitted into the outside air This is accomplished by having some of the air entering the stove diverted through baird_ch03.indd 124 2/2/12 11:40 AM 125 Particulates in Air Pollution tubes under and around the fire, thereby heating it (see Figure 3-20) The hot air is released near the top of the firebox, where the smoke combusts A baffle is used above this area to divert the air and maximize the amount of time it spends in the secondary combustion area before being released up the chimney Chimney Because these new stove designs combust much more of the fuel value of the wood, rather than emitting much of it as smoke, they are about 50% more energyefficient than those of older design 3.27 Smoke over Large Areas of Land Baffle Outside air Secondary combustion zone Preheated air Smoke Outside air FIGURE 3-20 Noncatalytic wood stove (schematic) with preheated Serious episodes of smoky haze pollution air and baffle for more complete combustion over large areas of land have occurred in recent years in Southeast Asia, especially in Malaysia, Singapore, and Indonesia The smoke originates mainly from forest fires—many of them in Indonesia—that are intentionally started in order to clear land that can be subsequently used for agriculture and to grow trees for their rubber, palm oil, or pulp content A secondary source of the smoke is the smoldering underground fires that slowly burn in underground coal and peat deposits Indeed, there are estimated to be a quarter million individual coal fires currently burning in Indonesia, as well as many in China and India, and there are also many peat fires in Malaysia The fires are initiated when an outcropping of coal or a peat deposit that has dried after draining is ignited, typically during one of the fires set to clear the land Fires can also be ignited by lightning strikes in coal exposed to the air, and even by spontaneous combustion when the surface pyrite is oxidized and the heat released by this reaction sets the carbon ablaze These underground fires can continue to burn for decades after the original forest fires have stopped A so-called atmospheric brown cloud of particles and gases from forest fires, vehicle exhausts, and domestic cookers—especially in rural areas—that burn wood, dung, and agricultural waste overhangs most of East and Southeast Asia annually from December to May, the main season for home heating The brown cloud over the Indian Ocean consists mainly of smoke from the burning of dried manure in cooking fires This haze lowers sunlight levels at the surface by up to 15%, with a corresponding decline in the yield of crops such as rice and an alteration to rainfall and monsoon patterns In contrast to the pollution aerosol over North America and Europe, to which it is comparable in magnitude, the “black carbon” content of the brown cloud is significant The baird_ch03.indd 125 2/2/12 11:40 AM 126 Chapter The Chemistry of Ground-Level Air Pollution absorption of sunlight by this elemental carbon alters the local hydrological cycle and hence the weather over the northern Indian Ocean The lack of nitric oxide produced in the low-temperature flames of burning biomass currently limits ozone production over the area, but that will likely be reversed in the future with increased use of fossil fuels for vehicles Large forest fires in northern Canada produce huge quantities of carbon monoxide and VOCs, which have been found to travel as far as the U.S Southeast and which may well increase ozone and particulate concentrations in the air of this region Air Quality Indices and Size Characteristics for Particulate Matter As we shall see in Chapter 4, the effect of particles suspended in air upon human health depends significantly upon the size of the particles involved In the material that follows, we investigate the pollution indexes used by governmental agencies to characterize the level of particulate air pollution present in an air sample as well as the effect of particle size on visibility through air masses 3.28 The PM Indices When air quality is monitored, the most common measure of the concentration of suspended particles is the PM index, which is the amount of particulate matter that is present in a given volume Since the matter involved usually is not homogeneous, no molar mass for it can be quoted and thus concentrations are given in terms of the mass, rather than the number of moles, of particles present The usual units are micrograms of particulate matter per cubic meter of air, i.e., ␮g mϪ3 Because smaller particles have a greater detrimental effect on human health than larger ones, as we shall see later in this chapter, usually only those having a specified diameter or less are collected and reported; this cut-off diameter, in ␮m, is listed as the subscript to PM Government agencies in many countries, including the United States and Canada, monitor PM10, which is the total concentration of all particles having diameters less than 10 ␮m, which corresponds to all of the fineparticle range plus the smallest members of the coarse range These are called inhalable particles since they can be breathed into the lungs A typical value for PM10 in an urban setting is 20–30 ␮g mϪ3 More commonly, regulators in developed countries now use the PM2.5 index, i.e., that which includes all and only fine particles, which are also called respirable particles The respirable range includes only particles that can penetrate deep into the lungs, where there are no natural mechanisms such as the cilia that line the walls of bronchial tubes to catch particles and move them up and out Urban PM2.5 values are usually in the 10–20 ␮g mϪ3 range in North America, although background concentrations are only 1–5 ␮g mϪ3 baird_ch03.indd 126 2/2/12 11:40 AM Air Quality Indices and Size Characteristics for Particulate Matter 127 Some researchers have introduced the term PM10-2.5, which, although termed “coarse” corresponds only to the smallest of the particles covered by the conventional definition of coarse particles The new term ultrafine is applied to particles with very small diameters, usually taken to be less than 0.1 ␮m Nanoparticles are still smaller, less than about 0.02 ␮m (Figure in Box 3-3 shows the distribution of particles in an air mass and visually summarizes the size definitions.) Most ultrafine particles are anthropogenic in origin In the past, the total suspended particulates, abbreviated TSP, which is the concentration of all particulates suspended in air, was often reported instead of a PM index As the importance of suspended particulate matter to human health (discussed in Chapter 4) became more and more apparent in the late twentieth century, some governments set air quality standards for PM10 Now that PM2.5 is recognized to be more relevant than PM10 and the instrumentation for its measurement is widely available, standards for it have been implemented in some countries such as the United States and have been proposed for others, and in some cases have replaced PM10 regulation The status of PM regulations, as of 2010, is summarized in Table 3-2 Governments generally not attempt to regulate overall emissions of particulate matter since so much of it is secondary in origin, not primary An exception is the emission of particulates from vehicles Particulate air pollution globally is a serious problem, especially in cities in developing countries; as indicated in Figure 3-21, few of the world’s megacities currently come close to meeting the World Health Organization annual standard for PM10 (horizontal green line) of 20 ␮g mϪ3 Several other Chinese and Indian cities and Cairo are also among the most particulatepolluted cities in the world Given the continuing increase in vehicles, most lacking emission controls, in developing countries, and the fact that many of TABLE 3-2 Country or Organization Particulate Matter Air Standards, in units of ␮g mϪ3 PM2.5 24-hour PM2.5 Annual PM10 24-hour PM10 Annual WHO 25 10 50 20 USA 35 15 150 — (25) 50 (25) (8) 50 China — — 50–250 40–150 India 60 40 100 60 Canada EU Australia (30) 40 Note: Figures in round brackets are tentative standards baird_ch03.indd 127 2/2/12 11:40 AM 128 Chapter The Chemistry of Ground-Level Air Pollution FIGURE 3-21 Particulate D D Parish and T Zhu, Science 326 (2009): 674–675.] 250 200 PM10 (mg m–3) air quality in major world cities Note: Values are annual PM10 means The green horizontal line is the WHO air quality guideline for this parameter [Source: 150 100 50 K ar ac h D i el Bu Cal hi en cut os ta A ir Be es iji n Ja g ka Sh rta an g M hai M um ex b ic o Ci t La y go Is s ta nb M ul a Sa ni o la Pa ol o Se ou Lo To l s A ky n o N gele ew s Yo rk them are diesel-powered, it seems likely that PM levels will increase over time Most Chinese cities failed to meet the daily and annual WHO guidelines for PM2.5, Beijing and Shanghai by factors of ten and six, respectively, in the early 2000s Ironically, although levels of primary particulate matter and that formed from SO2 decreased over China in the late 2000s due to government controls and an economic downturn, by 2009 particulate levels were again increasing due to their formation as secondary pollutants from reaction of NOX emissions from the increasing number of vehicles on the road Almost all sites in the United States meet its annual PM10 standard, although not the lower limit more generally adopted, and there are still some areas that miss the PM2.5 limit, notwithstanding gradual improvement over the last decade Many large cities in sub-Saharan Africa suffer from high levels of particulate air pollution from vehicle exhaust, although little regular monitoring is yet in place there In Nairobi, Kenya, for example, the air sampling during the mid2000s revealed very high levels of PM2.5, with enhanced black carbon Most of the vehicles in Nairobi at the time had been imported from Asia as used cars and had no pollution control devices PROBLEM 3-17 What would be the correct PM symbol for an index that included only ultrafine particles? What would be the PM symbol for the TSP index? Numerically, would the value for the ultrafine component of a given air mass be ● larger or smaller than its TSP? baird_ch03.indd 128 2/2/12 11:40 AM 129 Air Quality Indices and Size Characteristics for Particulate Matter As discussed in detail in Box 3-3, the distribution of particles suspended in air peaks in the micrometer region because smaller particles coagulate to form ones of this size and further growth is slow, and because much larger ones rapidly settle out The large increase in surface area that occurs when a large particle is split into smaller ones is explored in Problem 3-18 Review Questions 18–21 are based on material in the preceding sections PROBLEM 3-18 Let k be a given measure of length; then suppose a cubic particle of dimension 3k ϫ 3k ϫ 3k is split up into 27 cubes of size k ϫ k ϫ k Calculate the relative increase in surface area when this occurs by comparing the surface area (length times width) of the six faces of the larger cube to the sum of all those of the smaller ones From your answer, deduce whether the total surface area of a given mass of atmospheric particle is larger or smaller when it occurs as a large number of small particles rather than a small number of ● large ones BOX 3-3 B The Distribution of Particle Sizes in an Urban Air Sample ecause particles suspended in the atmosphere are of different origins and compositions, and were often formed and interacted over a period of time in haphazard ways, there is a wide distribution of particle sizes present in any air mass One way of looking at the distribution of sizes is to plot the number of particles having a given diameter against the diameter; this is done in the dark green curve of Figure for a typical urban air sample The peak in the distribution occurs at just under 0.01 ␮m The oddly shaped net distribution is presumably the sum of several symmetrical (“bell shaped”) distributions having peaks at different diameters The particles of the distribution at the smallest diameter are primary in nature, formed by the condensation of vapors of pollutants formed by chemical reactions, such as the sulfuric acid formed by the oxidation of gaseous sulfur dioxide and the soot particles formed by combustion The coagulation of such particles into larger ones (which can occur in minutes) and the deposition of gas molecules onto them result in the broad distribution of secondary particles Particles of this size also are created when water in aqueous droplets containing dissolved solids evaporate Growth beyond this size is slow because the larger the particle, the slower it moves and thus the less likely it is to encounter and coagulate with particles of comparable size Growth by condensation of gases is also slow for larger particles since their surface-to-mass ratio is smaller than for small particles The particles associated with the rightside tail of the distribution are mainly soot or consist of material produced by mechanical disintegration of soil particles, etc Numerically, there are few particles of mass larger than a few microns in diameter because each has such a large mass and because the larger ones quickly settle out of the air (Stokes’ law), although particles that then settle on (continued on p 130) baird_ch03.indd 129 2/2/12 11:40 AM 130 Chapter The Chemistry of Ground-Level Air Pollution roadways often become resuspended by the action of vehicular traffic The plots of particle numbers can be misleading for some purposes because tiny particles of very small mass and surface area dominate the samples and thus the distributions One alternative way to represent the data in a more meaningful way is to plot the total mass of all particles of a given size in an air sample against the diameter to see how mass is distributed among the different sizes This type of plot is shown by the light green curve in Figure The distribution function for mass is displaced to larger diameters compared to that for particle numbers for the following reason: the mass (or volume) of a particle is proportional to the cube of its diameter d (since for a sphere volume is proportional to the cube of the radius), so the height of the curve at any diameter of the distribution in the light green curve in Figure corresponds to the value for the number distribution for this air mass times d3 Very small particles, even though numerous, not collectively have a large mass Consequently, in the mass distributions, the peak heights for larger particles are higher than are those for smaller ones; the whole distribution appears to have shifted to higher diameters Two symmetrical distribution curves, one centered in the fine region at about 0.4 ␮m and the other in the coarse region at about ␮m, appear to be superimposed to produce the final bimodal “double-humped” distribution Notice that the total mass of the coarse-particle range (i.e., the sum of the area under the dark green curve for d Ͼ 2.5 ␮m) in Figure is greater than that for the fine region; this ratio is even larger for clean, rural air masses PROBLEM PROB PR O LE OB LEM M1 The distribution of particle surface area against size is proportional to the square of the diameter times the number distribution, since the surface area of a sphere is proportional to the square of its radius or diameter Given the distribution for the particle volumes and masses (Figure 1), which is d3 times the number distribution rather than d2 times it, qualitatively predict the shape of the distribution curve and approximate location of peak(s) for total particle surface area Number FIGURE The distribution of particle numbers and of particle masses versus particle size for airborne particles Particles in the condensates, soot, and brake-wear categories originate mainly from vehicular traffic The mineral dust comes from road abrasion and from agricultural and natural sources [Source: Modified from Condensates and other primary particles Mass Secondary particles Soot 0.01 0.001 Nanoparticles Mineral dust Brake wear 0.1 10 100 (µm) Ultrafine particles PM2,5 (fine particles) PM10 J Fenger, Atmospheric Environment 43 (2009): 13–22.] baird_ch03.indd 130 2/2/12 11:40 AM Green Chemistry Questions 131 Review Questions In the “micrograms per cubic meter” concentration scale, to what substances micrograms and cubic meters refer? 11 Describe the manner in which emissions from diesel-powered vehicles can be controlled, including the use of catalytic converters What chemical substance initiates the air oxidation of stable molecules? How is it initially formed, and how is it reformed? 12 Describe the reaction used in the selective catalytic reduction of nitrogen oxides What other techniques are used for NOX emission control from power plants? In general terms, what is meant by photochemical smog? What are the initial reactants in the process? Why is sunlight required? 13 What are the main anthropogenic sources of sulfur dioxide? Describe the strategies by which these emissions can be reduced What is the Claus reaction? What is meant by a primary pollutant and by a secondary pollutant? Give examples 14 What species are included in the air pollution index called total reduced sulfur? How does OH react with a stable molecule containing a C RC bond? With an alkane? 15 Describe the three strategies used in clean coal What is meant by the term synergism? Give an example What is the chemical reaction by which thermal NO is produced? From which two sources does most urban NO arise? What is meant by the term NOX? What is meant by fuel NO? Describe the strategies by which reduction of urban ozone levels has been attempted What difficulties have been encountered in these efforts? Is photochemical smog strictly a localized urban problem? What is meant by geographic regions that are VOC-limited? NOX-limited? 10 Describe the operation of the three-way catalyst in transforming emissions released by an automobile engine Does the catalyst operate when the engine is cold? Why is it important for converters that the level of sulfur in gasoline be minimized? 16 What two species, other than O2, are active oxidizing agents of sulfur dioxide in atmospheric water droplets? 17 State Henry’s law 18 Define the term aerosol, and differentiate between coarse and fine particulates What are the usual origins of these two types of atmospheric particles? 19 What are the usual chemical components of a sulfate aerosol? 20 Write a balanced equation illustrating the reactions that occur between one molecule of ammonia and (a) one molecule of nitric acid and (b) one molecule of sulfuric acid 21 What are the usual concentration units for suspended particulates? What would the designation PM40 mean? What the terms respirable and ultrafine mean? Green Chemistry Questions The development of a low-VOC coalescent for paints by ADM won a Presidential Green Chemistry Challenge Award baird_ch03.indd 131 (a) Into which of the three focus areas (see page xxviii) for these awards does this award best fit? 2/2/12 11:40 AM 132 Chapter The Chemistry of Ground-Level Air Pollution (b) List two of the twelve principles of green chemistry that are addressed by this discovery Justify each of your answers What are the environmental advantages of the coalescent developed by ADM? Consider the structure (Figure 3-11b) of the coalescent developed by ADM Using your knowledge of organic chemistry, explain why this molecule undergoes reaction with oxygen so readily Would you expect linoleic acid to react in a similar manner? Stearic acid? PERC replaced gasoline and kerosene in the dry-cleaning process (a) Describe any environmental problems or worker hazards that would be associated with these solvents (b) Would these same environmental problems or workers hazards be eliminated by the use of PERC? (c) By the use of carbon dioxide? The development of surfactants for carbon dioxide by Joseph DeSimone won a Presidential Green Chemistry Challenge Award (a) Which of the three focus areas (see page xxviii) for these awards does this award best fit into? (b) List two of the twelve principles of green chemistry (see pages xxiii–xxiv) that are addressed by the green chemistry developed by DeSimone The ions in ionic liquids (ILs) have weak ionic attractions for one another This weak interaction is due to one or more factors including • the presence of bulky nonpolar groups which prevent the close interaction of the charged regions of the ions, and • delocalized and/or dispersed charges resulting in low charge density Inspect the (ILs) in Figure 3-15 and discuss the structural features of these compounds that result in weak interactions between the oppositely charged ions The discovery of the dissolution of cellulose with ionic liquids and the formation of various cellulose composites by Robin Rogers won a Presidential Green Chemistry Challenge Award (a) Which of the three focus areas (see page xxviii) for these awards does this award best fit into? (b) The use of an abundant, naturally occurring polymer, a microwave heat source, and ionic liquids are three important green chemistry aspects of this study For each of these aspects list at least two of the twelve principles of green chemistry (see pages xxiii–xxiv) that are addressed in this study Additional Problems The rate constant for the oxidation of nitric oxide by ozone is ϫ 10Ϫ14 moleculeϪ1 cm3 secϪ1, whereas that for the competing reaction in which it is oxidized by oxygen, i.e., NO ϩ O2 9: NO2 is ϫ 10Ϫ38 moleculeϪ2 cm6 secϪ1 For typical concentrations encountered in morning smog episodes, namely 40 ppb for ozone and 80 ppb baird_ch03.indd 132 for nitric oxide, deduce the rates of these two reactions and decide which one is the dominant process [Hint: The concentrations of the reactants must be expressed in units appropriate to the rate constant.] In a particular air mass, the concentration of OH was found to be 8.7 ϫ 106 molecules cmϪ3, and that of carbon monoxide was 20 ppm 2/2/12 2:25 PM Additional Problems (a) Calculate the rate of the reaction of OH with atmospheric CO at 30°C, given that the rate constant for the process is ϫ 10Ϫ13 eϪ300/T moleculeϪ1 cm3 secϪ1 (b) Estimate the half-life of an OH molecule in air, assuming that its lifetime is determined by its reaction with CO [Hint: Re-express the rate law as a pseudo-first-order process with the level of CO fixed at 20 ppm Consult your introductory chemistry textbook to find the relationship between the half-life of a substance and the rate constant for its first-order decay.] In the overall reaction that produces nitric oxide from N2 and O2, the slow step in the mechanism is the reaction between atomic oxygen and molecular nitrogen to produce nitric oxide and atomic nitrogen (a) Write out the chemical equation for the slow step and the rate law equation for it (b) Given that its rate constant at 800°C is 9.7 ϫ 1010 L molϪ1 secϪ1, and that its activation energy is 315 kJ molϪ1, calculate the amount by which the rate constant increases if the temperature is raised to 1100°C At combustion temperatures, the equilibrium constant for the reaction of N2 with O2 is about 10Ϫ14 Calculate the concentration of nitric oxide that is in equilibrium with atmospheric levels of nitrogen and oxygen Repeat the calculation for normal atmospheric temperatures, at which the equilibrium constant is about 10Ϫ30 Given that the concentration of NO that exits from the combustion zone in a vehicle is much higher than this latter equilibrium value, what does that imply about equilibrium in the reaction mixture? [Hint: Use the stoichiometry of the reaction to reduce the number of unknowns in the expression for K.] The concentration of ozone in ground-level air can be determined by allowing the gas to react with an aqueous solution of potassium iodide, KI, in a redox reaction that produces molecular iodine, molecular oxygen, and potassium hydroxide baird_ch03.indd 133 133 (a) Deduce the balanced reaction for the overall process (b) Determine the ozone concentration, in ppb, in a 10.0-L sample of outdoor air if it required 17.0 ␮g of KI to react with it The percentage of sulfur in coal can be determined by burning a sample of the solid and passing the resulting sulfur dioxide gas into a solution of hydrogen peroxide, which oxidizes it to sulfuric acid, and then titrating the acid Calculate the mass percent of sulfur in a sample if the gas from an 8.05-g sample required 44.1 mL of 0.114 M NaOH in the titration of the diprotic acid Calculate the volume, at 20°C and 1.00 atm, of SO2 produced by the conventional roasting of 1.00 tonnes (1,000 kg) of nickel sulfide ore, NiS What mass of pure sulfuric acid could be produced from this amount of SO2? Ironically, SO2 could be extracted from gas emissions by passing it through a solution of sulfite ion, SO32Ϫ (a) Assuming sulfite ion acts as a base and the sulfur dioxide is present in water initially as sulfurous acid, write an acid–base reaction between the species (b) Devise a scheme by which dilute sulfur dioxide in an emission gas could be captured by an aqueous solution of sulfite ion, and later released as a concentrated stream of SO2 Assuming its concentration in air is 2.0 ppb, calculate the molar solubility of SO2 in raindrops whose pH is fixed (by the presence of strong acids) to be 4.0, 5.0, and 6.0 The data required for the calculations is present in Section 3.21 of the text 10 The sulfur species that undergoes oxidation in water droplets is the bisulfite ion, HSO3Ϫ, so the rate of oxidation is proportional to its concentration multiplied by that of the oxidizing agent Predict how changes in pH in the droplet will affect the rate of oxidation (a) if O3 reacts with bisulfite ion, and (b) if hydrogen peroxide in 2/2/12 11:40 AM 134 Chapter The Chemistry of Ground-Level Air Pollution the protonated form H3O2ϩ, formed in the equilibrium 0! H2O2 ϩ Hϩ !1 H3O2ϩ is the species that reacts with bisulfite 11 The settling rate of particulates in air is directly proportional to the square of their baird_ch03.indd 134 diameters (Stokes’ law), provided that their densities are equal If emitted particulates with a specific diameter are found to settle out after two days, how long would it take particulates of the same material with half the diameter to settle out if they are emitted from the same tall chimney? 2/2/12 11:40 AM ... Instructor Environmental Chemistry, Fifth Edition, has been revised, updated, and expanded in line with comments and suggestions made by a variety of users and reviewers of the fourth edition Since... the content of this book from instructors and students Please contact Colin Baird at ncolinbaird@gmail.com and Michael Cann at cannm1@scranton.edu Acknowledgments The authors wish to express their... intentionally left blank ENVIRONMENTAL CHEMISTRY baird_fm.indd i 22/02/12 9:45 AM this page left intentionally blank baird_fm.indd ii 22/02/12 9:45 AM ENVIRONMENTAL CHEMISTRY Fifth Edition Colin Baird University