Chuong hoa hoc khi quyen (5) pptx

1 1 Chapter 13 PHOTOCHEMICAL SMOG Environmental Chemistry, 9th Edition Stanley E. Manahan Taylor and Francis/CRC Press 2010 13.1 Introduction Originally, “smog” referred to reducing, sulfurous smog • Generated from coal smoke and fog in London This chapter deals with oxidizing smog characterized by • Eye irritation • Low visibility at low humidity • Presence of oxidants including O3 2 3 13.2 Smog-Forming Emissions Automobile a prime source of smog forming emissions Figure 13.1 Major sources of smog-forming hydrocarbons from an automobile before emission controls were put into effect Exhaust hydrocarbons, especially unsaturated ones, are especially reactive in smog formation Automobile also source of NO required for smog 4 Control of operational parameters of the four-cycle automobile engine important in smog control Figure 13.2 Steps in operation of the four-cycle automobile engine 5 Engine control to limit smog-forming emissions Table 13.1 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 Computerized control of timing, air/fuel ratio, (Figure 13.3), other parameters limit emissions of NO, hydrocarbons (HC), CO Catalytic converters oxidize HC and CO and reduce NO • Mixture cycles rapidly between slightly rich and slightly lean Polluting Green Plants Plants are high contributors to reactive atmospheric HCs • Highly reactive terpenes such as α-pinene (Figure 12.1) • Most abundant is isoprene 6 • Isoprene nitrates from reactions with HO•, NOx, NO3 radical • Oxidized to carbonyls and other products 7 13.3 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 8 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 CH4 + HO•→ H3C• + H2O • H3C• + O2 + M → H3COO• + M • Regeneration of NO2, which can undergo further photodissociation H3COO• + NO → H3CO• + NO2 • Production of hydroperoxyl radical H3CO• + O2→ CH2O + HOO• • HO• and HOO• are odd hydrogen radicals that are ubiquitous intermediates in atmospheric chain reactions • CH2O is photochemically active formaldehyde 9 Addition Reactions of Unsaturated Compounds • Addition of HO• across double bond Primary photochemical reactions of organics, especially aldehydes • Addition reactions with ozone Reactions of Organic Free Radicals • Example: Generation of HO• from organic peroxyl radicals 10 • Chain reactions with many steps • Hydroxyl radical key species in sustaining chain reactions • Chain branching • Chain termination • Two radicals react: HO• + HO•→ H2O2 • Radical adding to NOx (stable free radical) HO• + NO2 + M → HNO3 + M • Radical adding to solid surface