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 (1)
O + O2 + M ? O3 + M (2)
NO + O3 ? NO2 + O2 (3)
NO2 + hv ? NO + O (1)
O + O2 + M ? O3 + M (2)
HO2,+ NO? NO2 + OH (4)
RO2,+ NO ? NO2 + RO,(5)
No net O3
formation
O3 is
formed
Net of (1)+(2)+(4),RO2,+ O2 ? O3 + RO.
NO2 is capable of absorbing visible
light (<400 nm) to produce O.
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
? NOx (NO,NO2)
? Hydrocarbons (VOCs,volatile organic
carbon)
VOCs + NOx + hn?O3 + other pollutants
Production of O atom
Production of RO2,which reacts
with NO so that O3 could
accumulate,
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
VOC
Sunlight
Formation of oxidants other than O3
? Formation of aldehydes (e.g,formaldehyde)
? Formation of PAN (peroxyacetyl nitrate) and its
analogs
ROO,+ NO2 ? ROONO2 (peroxyalkyl nitrate)
O
C
H
3
C H
+ h v
O
C
H
3
C
+ H
O
C
H
3
C
+ O
2
O
C
H
3
C O O
O
C
H
3
C O O
+ N O
2
O
C
H
3
C O O N O
2
PAN
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.
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 (1)
O + O2 + M ? O3 + M (2)
NO + O3 ? NO2 + O2 (3)
NO2 + hv ? NO + O (1)
O + O2 + M ? O3 + M (2)
HO2,+ NO? NO2 + OH (4)
RO2,+ NO ? NO2 + RO,(5)
No net O3
formation
O3 is
formed
Net of (1)+(2)+(4),RO2,+ O2 ? O3 + RO.
NO2 is capable of absorbing visible
light (<400 nm) to produce O.
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
? NOx (NO,NO2)
? Hydrocarbons (VOCs,volatile organic
carbon)
VOCs + NOx + hn?O3 + other pollutants
Production of O atom
Production of RO2,which reacts
with NO so that O3 could
accumulate,
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
VOC
Sunlight
Formation of oxidants other than O3
? Formation of aldehydes (e.g,formaldehyde)
? Formation of PAN (peroxyacetyl nitrate) and its
analogs
ROO,+ NO2 ? ROONO2 (peroxyalkyl nitrate)
O
C
H
3
C H
+ h v
O
C
H
3
C
+ H
O
C
H
3
C
+ O
2
O
C
H
3
C O O
O
C
H
3
C O O
+ N O
2
O
C
H
3
C O O N O
2
PAN
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.
