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环境化学英文课件4

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Air Pollution-Tropospheric Ozone

Good Ozone and Bad Ozone
? Stratospheric ozone protect lives on Earth from harmful effects of UV radiation. ? Tropospheric ozone:
– Causing respiratory distress and eye irritation – Destroying plants – Producing cracks in rubber
Ozone is a strong oxidant, reacts with molecules containing C=C double bonds, forming epoxides.

Two types of air pollutants: primary vs. secondary
? Primary pollutants: released directly from sources
– Examples: CO, SO2, NOx

? Secondary pollutants: formed through chemical reactions of the primary pollutants and the constituents of the unpolluted atmosphere in the air.
– Example: O3

Formation of ozone
NO2 + hv ? NO + O

NO2 is capable of absorbing visible light (<400 nm) to produce O.

(1)

O + O2 + M ? O 3 + M
NO + O3 ? NO2 + O2 NO2 + hv ? NO + O O + O2 + M ? O3 + M HO2. + NO? NO2 + OH RO2. + NO ? NO2 + RO.

(2)
(3) (1) (2) (4) (5)

No net O3 formation

O3 is formed

Net of (1)+(2)+(4): RO2. + O2 ? O3 + RO.

Sources of RO2.: Oxidation of hydrocarbons
RH + OH ? R. + H2O R. + O2 ? RO2.
A single organic radical can produce many peroxy radicals by successive rounds of O2 combination and fragmentation.

Example: Oxidation of carbon monoxide
CO + .OH + O2 ? CO2 + HO2. HO2. + NO ? NO2 + .OH NO2 + hv ? NO + O O + O2 + M ? O3 + M Net: CO + 2 O2 + hv ? CO2 + O3
The net reaction can be viewed as a catalytic oxidation of CO to CO2. Net formation of O3 occurs.

Example: Oxidation of methane
CH4 + .OH + O2 ? CH3OO. + H2O CH3OO. + NO ? CH3O. + NO2 CH3O. + O2 ? HCHO + HO2. HO2. + NO ? .OH + NO2 NO2 + hn ? NO + O (2x) O + O2 + M ? O3 + M (2x) Net: CH4 + 4 O2 ? HCHO + H2O + 2 O3
The net reaction is that for each mole of methane oxidized, 2 moles of O3 is produced.

Necessary ingredients for ozone formation ? Sunlight Production of O atom ? NOx (NO, NO2) ? Hydrocarbons (VOCs: volatile organic carbon) Production of RO2, which reacts
with NO so that O3 could accumulate.

VOCs + NOx + hn ? O3 + other pollutants

Necessary ingredients for ozone formation
CH4 + .OH + O2 ? CH3OO. + H2O CH3OO. + NO ? CH3O. + NO2 CH3O. + O2 ? HCHO + HO2. HO2. + NO ? .OH + NO2 NO2 + hn ? NO + O (2x) O + O2 + M ? O3 + M (2x) Net: CH4 + 4 O2 ? HCHO + H2O + 2 O3
Sunlight

VOC

Formation of oxidants other than O3
? Formation of aldehydes (e.g. formaldehyde) ? Formation of PAN (peroxyacetyl nitrate) and its analogs O O
C H3C H

+ hv
H3C

C

+H

O C H3C

O

+

O2
H3C

C OO

O

O

PAN
OONO2

+ NO2
C H3C OO H3C

C

ROO. + NO2 ? ROONO2 (peroxyalkyl nitrate)

Photochemical smog ? Smog derives from a combination of the words smoke and fog.

London smog and Los Angeles smog
? London smog is characterized by high SO2 and particle concentration in the presence of fog.
– Also referred as sulfurous smog

? Los Angeles smog is characterized by high oxidants (mainly O3). It was first recognized in the Los Angeles area.
– The term smog is misleading in this case, as smoke and fog are not key components. – The appropriate term is photochemical air pollution.

Photochemical air pollution

HO2. Radical: Interconversion of .OH and HO2. OH and HO2 are interconverted through a series of reactions involving hydrocarbons and oxides of nitrogen.
HO2. + NO ? .OH + NO2
.OH + RCH3 ? H2O + RCH2. RCH2. + O2 ? RCH2OO.

RCH2OO. + NO ? NO2 + RCH2O. RCH2O. + O2 ? RCHO + HO2. Sources of OH are in effect sources of HO2. under most tropospheric conditions.

Sources for .OH radicals: Photolysis of O3
Photolysis of O3 forms O1D, followed by its reaction with water.
O3 + hn ? O1D + O2 l < 320 nm O1D + H2O ? 2 .OH

Sources for .OH radicals: Photolysis of HONO
HONO + hn ? .OH + NO l < 400 nm
Possible sources for HONO include

?NO2 +H2O
?OH + NO ?NO + NO2 + H2O ?HO2 + NO2 reaction (possibly a contribution from a minor channel of this reaction) ? direct emissions, for example, from automobiles.

Sources for .OH radicals: Photolysis of H2O2
H2O2 + hn ? 2 .OH l ? 360 nm
H2O2 is formed from the reaction: HO2. + HO2. ? H2O2 + O2

Sources for HO2. Radicals: formaldehyde
Formaldehyde photolysis is a major source of HO2. during the daylight hours.
HCHO + hn ? H. + HCO. l < 370 nm
H. + O2 + M ? HO2. + M HCO. + O2 ? HO2 . + CO Note: Any process that produces HCO. or H. is a source of HO2. in the troposphere.

Nighttime sources for

.OH/HO2.

? Ozone oxidation of alkene species
– Ethene + O3 ? 0.12 OH – Isoprene + O3 ? 0.27 OH

? Thermal decomposition of Peroxyactyl nitrate (PAN) and its analogs of higher carbon.
CH3C(O)OONO2 ? CH3C(O)OO. + NO2 CH3C(O)OO. + NO ? CH3C(O)O. +NO2 CH3C(O)O. ? CH3. + CO2 CH3. + O2 ? CH3OO. CH3OO. + NO ? CH3O. + NO2 CH3O. + O2 ? HCHO + HO2.

Nighttime sources for .OH/HO2. (Continued)
NO3 reaction with hydrocarbons NO3 + RH ? HNO3 + R.

R. + O2 ? ROO.
ROO. + NO ? RO. + NO2

RO. + O2 ? HO2. + R’CHO

Various sources of .OH/HO2. as a function of the time of day

Control strategies for ozone
? O3 is a secondary pollutant? control of O3 requires control of its precursors. ? Control of VOCs
– General too abundant to be brought low enough to be the limiting factor. – In certain areas, VOCs from biological sources could be significant.

? Control of NOx
– Difficult to control as efficient energy conversion requires high combustion temperature.




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