Chuong hoa hoc khi quyen (5) pptx

13.1 IntroductionOriginally, “smog” referred to reducing, sulfurous smog • Generated from coal smoke and fog in London This chapter deals with oxidizing smog characterized by • Eye irrit

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

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

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

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

• Isoprene nitrates from reactions with HO • , NOx, NO3 radical

• Oxidized to carbonyls and other products

13.3 Smog-Forming Reactions of Organic

Compounds in the Atmosphere

Hydrocarbons undergo photochemical oxidation in the atmosphere to produce

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

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

• 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

12.4 Overview of Photochemical Smog Formation

Photochemical smog evidenced by

• Gross photochemical oxidant that oxidizes I- to I3

-• Main photochemical oxidant is ozone, O3

• Other oxidants include

• H2O2 • Peroxides (ROOR’)

• Organic hydroperoxides (ROOH)

• Peroxyacyl nitrates

Figure 13.4 Generalized plot of species in the

atmosphere during a smoggy day

Major kinds of reactions for smog formation (Figure 13.5)

1 Primary photochemical reaction producing oxygen atoms

NO + ROO •→ NO2 + and/or other products

NO2+R •→products (for example, PAN)

(common chain-terminating reaction, NO2 is a stable free radical species)

propagating chains and generating products in

photochemical smog

• HO • + NO2→ HNO3

• Oxidation of CO by hydroxyl radical

HO • + CO + O2→ CO2 + HOO •

Responsible for removal of CO and production of HOO •

• HOO • important in oxidation of NO to photochemically active NO2

HOO • + NO → NO2+ HO •

Abstraction of H from alkanes leading to smog

• Because of addition reactions, alkenes are very reactive

in photochemical smog formation

Reaction of Aromatic Hydrocarbons with HO•

Aldehyde reactions

• With HO •

• Photochemical

Sequence of reactions leading to photochemically active NO2

• Key to smog-forming process

Peroxyacyl nitrate formation

• When R is CH3, peroxyacetyl nitrate is the product

Peroxyacyl nitrates are significant air pollutants

• Characteristic of photochemical smog

• Eye irritants and mutagens

• Potent phytotoxins that adversely affect plants

Formation of alkyl nitrates and nitrites

• RO • + NO2→ RONO2

• RO • + NO →RONO

NO3 is an important species in smog formation, especially

Photolyzable Compounds in the Atmosphere

Most important is NO2

13.6 Reactivity of Hydrocarbons

Reactivity based on speed of reaction with hydroxyl radical (see Table 13.2)

• CH4 least reactive, but still important in smog

formation because of abundance

• Benzene, ethene, and n-hexane examples of

13.7 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

• SO2 rapidly oxidized in photochemical smog

• H2O + N2O5→ 2HNO3

Nitrates and HNO3 are very damaging smog products

• Corrosive • Toxic to plants

1 Human health and comfort

• Especially respiratory effects of ozone

2 Damage to materials (such as ozone attack on rubber)

3 Effects on the atmosphere

• Especially reduction of visibility

4 Toxicity to plants

• From ozone

• From organic oxidants such as peroxyacetyl nitrate