Photochemical Smog PDF

Chapter 8 PHOTOCHEMICAL SMOG Dr PP Mpungose 1  INTRODUCTION Smog is air pollution that reduces visibility. The term "smog" was first used in the early 1900s to describe a mix of smoke and fog. The smoke usually came from burning coal. Smog was co...

Chapter 8 PHOTOCHEMICAL SMOG Dr PP Mpungose 1  INTRODUCTION Smog is air pollution that reduces visibility. The term "smog" was first used in the early 1900s to describe a mix of smoke and fog. The smoke usually came from burning coal. Smog was common in industrial areas, and remains a familiar sight in cities today. https://www.youtube.com/watch?v=CdbBwIgq4rs 2  INTRODUCTION Today, most of the smog we see is photochemical smog. Photochemical smog is produced when sunlight reacts with nitrogen oxides and at least one volatile organic compound (VOC) in the atmosphere. Nitrogen oxides come from car exhaust, coal power plants, and factory emissions. VOCs are released from gasoline, paints, and many cleaning solvents. When sunlight hits these chemicals, they form airborne particles and ground-level ozone or smog. 3 FORMATION OF PHOTOCHEMACAL SMOG 4 5  INTRODUCTION Ozone can be helpful or harmful. The ozone layer high up in the atmosphere protects us from the sun’s dangerous ultraviolet radiation. But when ozone is close to the ground, it is bad for human health. Ozone can damage lung tissue, and it is especially dangerous to people with respiratory illnesses like asthma. Ozone can also cause itchy, burning eyes. 6  INTRODUCTION Smog is unhealthy to humans and animals, and it can kill plants. Smog is also ugly. It makes the sky brown or gray. Smog is common in big cities with a lot of industry and traffic. Many countries have created laws to reduce smog. Some laws include restrictions on what chemicals a factory can release into the atmosphere, or when the factory can release them. 7  SHANGHAI TOWERS ABOVE THE SMOG The tallest towers of Shanghai, China, rise above the haze. Shanghai's smog is a mixture of pollution from coal, the primary source of energy for most homes and businesses in the region, as well as emissions from vehicles 8  SMOG OVER LOS ANGELES Los Angeles, California, is known for its smog. Like other smog-infested cities, such as Shanghai, China, and Athens, Greece, Los Angeles is surrounded by mountains. This allows the smog to build up instead of spread out. Wildfires make smog (a combination of smoke and fog) worse. 9  SMOG-COVERED CAIRO Old and new Egypt are represented underneath this blanket of smog. The pyramids loom in the background, and the smog of industrialization hangs in the front. Heat, common in Egypt and other smog-infested cities such as Los Angeles, California, and New Delhi, India, contributes to smog. Warm air high in the atmosphere does not allow air below to circulate. The smog stays in the lower atmosphere. 10 PHOTOCHEMICAL SMOG IN SOUTH AFRICA South Africa has not generally seen the development of severe photochemical smog though it does, in certain areas such as Cape Town, experience the effects of smog caused by a combination of inversions and excessive pollution during the winter. 11  INTRODUCTION Smog is still a problem in many places. Everyone can do their part to reduce smog by changing a few behaviors, such as: Drive less. Walk, bike, carpool, and use public transportation whenever possible. Take care of cars. Getting regular tune-ups, changing oil on schedule, and inflating tires to the proper level can improve gas mileage and reduce emissions. Fuel up during the cooler hours of the day—night or early morning. This prevents gas fumes from heating up and producing ozone. Avoid products that release high levels of VOCs. For example, use low-VOC paints. Avoid gas-powered yard equipment, like lawn mowers. Use electric appliances instead. 12  INTRODUCTION This chapter deals with oxidizing photochemical smog characterized by Eye irritation Low visibility at low humidity Presence of oxidants including O3 Photochemical smog forms in the troposphere’s planetary boundary layer Extends up to about 1 km Region of maximum interaction between tropospheric air and Earth’s surface Location of temperature inversions in which photochemical smog forms 13  SMOG-FORMING EMISSIONS Automobile a prime source of smog forming emissions Major sources of smog-forming hydrocarbons from an automobile before emission controls were put into effect Fuel tank Carburetor (15% of hydrocarbons from evaporation) Exhaust (65% of Crankcase (20% of hydrocarbons produced) hydrocarbons produced) Exhaust hydrocarbons, especially unsaturated ones, are especially reactive in smog formation 14 Automobile also source of NO required for smog  CAR CATALYTIC CONVERTER A catalytic converter is an exhaust emission control device that converts toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction. Catalytic converters are usually used with internal combustion engines fueled by gasoline or diesel. 15 Control of operational parameters of the four-cycle automobile engine important in smog control Steps in operation of the four-cycle automobile engine Spark plug Air in Exhaust gases out Intake Compression Ignition/Power Exhaust 16  ENGINE CONTROL TO LIMIT SMOG-FORMING EMISSIONS Stoichiometric ratio Computerized control of timing, Fuel-rich Fuel-lean Relative pollutant emissions air/fuel ratio, (see Figure), other parameters limit emissions of NO, NO hydrocarbons (HC), CO Catalytic converters oxidize HC and CO and reduce NO Hyd roc arbon s Mixture cycles rapidly between slightly rich and slightly lean CO 12 13 14 15 16 17 Air/fuel ratio (mass/mass) Table 12.1 shows trends in allowable automobile emissions (g/mile) Before controls: HC, 10.6 CO, 84.0 NOx, 4.1 1970: HC, 4.1 CO, 34.0 NOx, --- 2008: HC, 0.41 CO, 3.4 NOx, 0.4 After 2008: Continued reductions in emissions, diesel emissions 17 regulated  POLLUTING GREEN PLANTS Plants are high contributors to reactive atmospheric HCs Highly reactive terpenes such as a-pinene Most abundant is isoprene H CH3 H C C C C Isoprene H H H Oxidized to carbonyls and other products H CH3 O CH3 H C C C C C C H H O H H Methacrolein Methylvinyl ketone Isoprene nitrates formed from reactions with HO , NOx, NO3 radical 18  SMOG-FORMING REACTIONS OF ORGANIC COMPOUNDS IN THE ATMOSPHERE Hydrocarbons undergo photochemical oxidation in the atmosphere to produce CO2 Organic solids Water-soluble aldehydes Inorganic byproducts including O3 and HNO3 19  OVERVIEW OF PHOTOCHEMICAL SMOG FORMATION Smog-forming conditions Ø Hydrocarbon pollution Ø NO pollution Ø Intense sunlight Ø Stagnant air Photochemical smog evidenced by 1. Gross photochemical oxidant that oxidizes I- to I3- Ø Main photochemical oxidant is ozone, O3 Ø Other oxidants include Ø H2O2 & Peroxides (ROOR’) 2. Organic hydroperoxides (ROOH) H O 3. Peroxyacyl nitrates H C C OO NO2 20 H Peroxyacetyl nitrate (PAN) GENERALIZED SCHEME FOR SMOG FORMATION Solar energy input h! NO2 Absorption of solar energy by NO2 produces NO O O2 and atomic oxygen, O. NO reacts with NO O3 or RO2. to produce NO2. O Atomic oxygen, HO and O3. O3 react with hydrocarbons O reacts with O3 to produce highly reactive O2 , yielding hydrocarbon free radicals. ozone, O3 Hydrocarbon free radicals Hydrocarbon free radicals NO2 react further with species such as NO to produce n PAN, aldehydes, and other r o c arbo s 2 smog components. Hyd radical free s c t ive rbon a a NO Re droc hy 21 Chapter 8 PHOTOCHEMICAL SMOG Dr PP Mpungose 22 23  GENERALIZED PLOT OF SPECIES IN THE ATMOSPHERE DURING A SMOGGY DAY 0.4 Non-methane hydrocarbons Pollutant level, ppm by volume 0.3 Aldehydes 0.2 NO2 NO 0.1 oxidant 0.0 M 4 A.M 8 A.M. N 4 P.M. 8 P.M. M Time of day Smoggy atmospheres show characteristic variations with time of day in levels of NO, NO2, hydrocarbons, aldehydes, and oxidants. Examination of the figure shows that shortly after sunrise, the level of NO in the atmosphere decreases markedly, a decrease that is accompanied by a peak in the concentration of NO2. During midday (significantly, after the concentration of NO has fallen to a very low level), the levels of aldehydes and oxidants become relatively high. The concentration of total hydrocarbons in the atmosphere peaks sharply in the morning and then decreases 24 during the remaining daylight hours.  REACTIONS OF METHANE TO ILLUSTRATE MAJOR KINDS OF SMOG-FORMING REACTIONS CH4 + O (from NO2 dissociation) → H3C + HO An abstraction reaction involving the removal of an atom, usually H, by a reactive species such as O or HO Rapid reaction of hydroxyl radical v CH4 + HO → H3C + H2O v H3C + O2 + M → H3COO + M Regeneration of NO2, which can undergo further photodissociation v H3COO + NO → H3CO + NO2 Production of hydroperoxyl radical v H3CO + O2 → CH2O + HOO v HO and HOO are odd hydrogen radicals that are ubiquitous intermediates in atmospheric chain reactions 25 CH2O is photochemically active formaldehyde  ADDITION REACTIONS OF UNSATURATED COMPOUNDS Ø Addition of HO across double bond H H H H H HO + H C C C H C C C OH H H H H H Ø Addition reactions with ozone H H H H O O H H C C C + O3 H C C C H O H H H Primary photochemical reactions of organics, especially aldehydes H O H H C C H + hν H C + HCO 26 H H  REACTIONS OF ORGANIC FREE RADICALS Ø Example: Generation of HO from organic peroxyl radicals Ø Chain reactions with many steps Ø Hydroxyl radical key species in sustaining chain reactions Ø Chain branching Ø Chain termination Ø Two radicals react: HO + HO → H2O2 Ø Radical adding to NOx (stable free radical) HO + NO2 + M → HNO3 + M Ø Radical adding to solid surface 27  MAJOR KINDS OF REACTIONS FOR SMOG FORMATION 1. Primary photochemical reaction producing oxygen atoms NO2 + hν → NO + O 2. Reactions involving oxygen species O2 + O + M → O3 + M O3 + NO → O2 + NO2 3. Production of free radicals from hydrocarbons, RH RH + O → R + HO RH + O3 → R + other products 4. Chain propagation, branching, and termination by a variety of reactions such as NO + ROO → NO2 + and/or other products NO2 + R → products (for example, PAN) (common chain-terminating reaction, NO2 is a stable free 28 radical species)  MAJOR KINDS OF REACTIONS FOR SMOG FORMATION Hydroxyl radical, HO , is a very important species in propagating chains and generating products in photochemical smog v HO + NO2 → HNO3 v Oxidation of CO by hydroxyl radical v HO + CO + O2 → CO2 + HOO Responsible for removal of CO and production of HOO Ø HOO important in oxidation of NO to photochemically active NO2 Ø HOO + NO → NO2 + HO 29  ABSTRACTION OF HYDROGEN FROM ALKANES LEADING TO SMOG FORMATION Ø RH + O + O2 → ROO + HO Ø RH + HO + O2 → ROO + H2O Addition reactions of HO across double bonds in alkenes are very rapid R R R R Very rapid C C + HO HO C C Oxidation R R products R R Radical adduct ------------------------------------------------------------------------------------------------------------------------------- R R O R C C + O R C C Oxidation R R products R R Biradical ------------------------------------------------------------------------------------------------------------------------------- O R R O O Oxidation C C + O3 R C C R products R R R R Ø Because of addition reactions, alkenes are very reactive in 30 photochemical smog formation REACTION OF AROMATIC HYDROCARBONS WITH HO H OH + HO H OH OH + O2 + HOO 31  ALDEHYDE REACTIONS Aldehyde reactions: Ø With HO O O R C H + HO + O2 R C OO + H2O H C O + HO + 3O2 CO2 + HOO + H2O 2 H Ø Photochemical O R C H + hν + 2O2 ROO + CO + HOO 32  SEQUENCE OF REACTIONS LEADING TO PHOTOCHEMICALLY ACTIVE NO2 Ø Key to smog-forming process RH + HO → R + H2O R + O2 → ROO ROO + NO → RO + NO2 Ø Chain reactions re-initiated by NO2 + hν → NO + O RH + O → R + HO 33  PEROXYACYL NITRATE FORMATION Peroxyacyl nitrate formation O O R C OO + NO2 R C OO NO2 When R is CH3, peroxyacetyl nitrate is the product Peroxyacyl nitrates are significant air pollutants Ø Characteristic of photochemical smog Ø Eye irritants and mutagens Ø Potent phytotoxins that adversely affect plants Formation of alkyl nitrates and nitrites Ø RO + NO2 → RONO2 34 Ø RO + NO → RONO  NITRATE RADICAL NO3 is an important species in smog formation, especially at night. Rapid photodissociation in daylight Ø NO3 + hν (λ < 700 nm) → NO + O2 Ø NO3 + hν (λ < 580 nm) → NO2 + O NO3 rapidly adds across double bonds in alkenes 35  PHOTOLYZABLE COMPOUNDS IN THE ATMOSPHERE Most important is NO2 Ø NO2 + hν (λ < 394 nm) → NO + O Photodissociation of carbonyls, especially formaldehyde Ø CH2O + hν (λ < 335 nm) → H + HCO Photodissociation of hydrogen peroxide Ø HOOH + hν (λ < 350 nm) → HO + HO Photodissociation of organic peroxides Ø H3COOH + hν (λ < 350 nm) → H3CO + HO 36  REACTIVITY OF HYDROCARBONS Reactivity based on speed of reaction with hydroxyl radical Ø CH4 least reactive, but still important in smog formation because of abundance Ø Benzene, ethene, and n-hexane examples of intermediate reactivity Ø β-pinene from conifer trees about 9000 times as reactive as methane Ø d-limonene from orange rind about 19,000 times as reactive as methane 37  IMPORTANCE OF HOX/VOC RATIOS Whether ground level ozone in smog is decreased by decreasing VOC or reducing NOx depends upon their relative abundances A single HO radical can produce numerous ozone molecules, a process of propagation Radical propagation is stopped by termination processes These processes and the production of ground-level ozone depend upon HOx/VOC ratios 38  INORGANIC PRODUCTS FROM SMOG Two major classes are sulfates and nitrates These inorganics contribute to: Acidic precipitation Corrosion Reduced visibility Adverse health effects Atmospheric sulfur from SO2 emissions Ø SO2 rapidly oxidized in photochemical smog Ø By oxidant compounds 1) O3 2) NO3 3) N2O5 Ø By radicals (especially hydroxyl) 1) HO 2) HOO 3) RO 4) ROO Formation of inorganic nitrates and nitric acid Ø HO + NO2 → HNO3 Ø H2O + N2O5 → 2HNO3 Nitrates and HNO3 are very damaging smog products 39 Corrosive Toxic to plants  EFFECTS OF SMOG 1. Human health and comfort Ø Especially respiratory effects of ozone 2. Damage to materials (such as ozone attack on rubber) H CH3 H H O O CH3 C C C C + O3 R C C C R' O H H H n O O Rubber polymer R C OH + H3C C R' 3. Effects on the atmosphere Ø Especially reduction of visibility 4. Toxicity to plants Ø From ozone Ø From organic oxidants such as peroxyacetyl nitrate 40  REPRESENTATION OF CHLOROTIC STIPPLING OF A LEMON LEAF DUE TO EXPOSURE TO PHOTOCHEMICAL SMOG Damage largely from ozone and organic oxidants San Francisco Geographic distribution of smog damage to plants in California Los Angeles 41