gaseous pollutants
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Gaseous Pollutants
Carbon oxides
Sulfur compounds
Nitrogen compounds Hydrocarbon compounds
Photochemical oxidants
Chap 2.3
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Carbon Oxides
Two major carbon oxides
Carbon dioxide (CO2) Carbon monoxide (CO)
CO2 Natural atmospheric constituent
Sources:
Natural
Aerobic biological processes, combustion and weathering of
carbonates in rock and soil Anthropogenic:
Combustion of fossil fuels
Land use conversion
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CO2
Essential atmospheric gas
Present in variable concentrations Not considered to be toxic
Environmental concerns are relatively new
Changes in atmospheric concentrations
Geological time
The modern period
1.5-1.7 ppmv/yr
Long atmospheric lifetime(~100 years)
Figure 2.2
Whats the impact if there is
no CO2 in the atmosphere?
Is CO2 emission regulated?
Should it be?
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Figure 2.3
CO2
Majorsink processes
Oceans
Forests
More discussion in Atmospheric Effects
Pre-industrial revolution: 98% of exchangeable CO2
were in the oceans and 2% in the atmosphere; for
anthropogenic CO2, only 42% dissolves in oceans
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CO Sink processes
Photochemistry with OH (hydroxyl radical)
Soil uptake
Atmospheric lifetime (1 month in the tropics and 4months in mid-latitudes)
222
2
HOCOOHCO
HCOOHCO
Adverse effects on the consumption of OH?
Formation of O3
MOMOPO
PONOhNO
OHNONOHO
MHOMOH
32
3
3
2
22
22
)(
)(
Overall 322OCOhOCO
Increase CH4 concentration thus
enhancing global warming
M: an energy absorbing molecule, e.g.
N2 or O2
OH: hydroperoxyl radical
O(3P): ground-state atomic oxygen
h: a photon of light energy
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CO
Background level concentration
Vary with latitude, lower in the tropics and higher inthe northern middle latitudes
Average 110 ppbv
Increasing 1%/yr, mostly in the northern middle
latitudes
Urban/suburban levels
Vary from few ppmv to 60 ppmv: mainly associated
with transportation emissions Average highs (10-20 ppmv)
Higher concentrations in higher altitude cities
Why higher in higher latitudes and altitudes?
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Sulfur Compounds
Sulfur Oxides: Sulfur trioxide (SO3), Sulfur dioxide (SO2)
Reduced sulfur compounds (COS, CS2, H2S)
Natural sources
Volcanoes
Oxidation of reduced S
compounds
Sulfur Oxides
Anthropogenic sources
Combustion of S-containing fuels
Smelting of metal ores
SO2 Colorless, sulfurous odor gas
Major sulfur oxide in the atmosphere
Produced on S oxidation
May be converted to SO3
Produced from SO2 oxidation
Rapidly reacts with water
Very short atmospheric lifetime
SO3
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Data from http://www.uea.ac.uk/~e490/su/sulfur.htm
Historical Sulfur Emission
Year
1840 1860 1880 1900 1920 1940 1960 1980 2000
kT
on
0
2000
4000
6000
8000
10000
12000
14000
16000
US
UK
USSRChina
India
Japan
Egypt
Brazil
What is the overall picture?
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sec)infast(very
22
4223
322
22
MSOHMOHSO
SOOSO
SOOS
Sink processes: SO2 oxidized in gas & liquid phase
reactions; can be direct, photochemical or catalytic
Gas phase
Reaction with OH
(major), O3, HO2, RO2, O(3
P)
Liquid phase
It can be further oxidized to H2SO4 by reaction with HNO2,
O3, H2O2, RO2 and catalysis by Fe and Mn
4223
3222
22
SOHOHSO
SOHOOHOSO
HOSOOHSO
3222 SOHOHSO
H2SO4: sulfuric acid
H2SO3: sulfurous acid
HNO2: nitrous acid
H2O2: hydrogen peroxide
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SO2 concentration
Background levels: ~20 pptv over marine surface to 16-
pptv over clean areas of US
Historical urban 1-hour highs: 1-500 ppbv
Highest 1 hr near non-ferrous metal smelters: 1.5-2.3
ppmv
Removal processes
Aerosol formation by nucleation/condensation
Sulfuric acid reacts with ammonia: forms sulfate salts SO2 + aerosols removed by wet & dry deposition
processes
SO2 atmospheric lifetime (1-7 days)
What is the consequence of the deposition?
More discussion in Welfare Effects
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Reduced S compounds
COS (Carbonyl sulfide)
Most abundant S species inatmosphere
Produced biogenically
Background levels (0.5 ppbv)
Limited reactivity Atmospheric lifetime ( 44 years)
CS2 (Carbon disulfide)
Produced biogenically
Photochemically reactive Global concentrations range (15-
190 ppbv)
Atmospheric lifetime (12 days)
(CH3)2S (Dimethyl sulfide)
Released from oceans inlarge quantities
Short atmospheric lifetime
(0.6 days) by rapid
conversion to SO2
Mercaptans
Source of malodors:
Rotting cabbage
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H2S
Major environmental and health concern (toxic):
characteristic malodor (rotten egg odor, threshold of 500pptv)
Sources:
Natural: primarily by biological decomposition
Anthropogenic sources: Oil & gas extraction, Petroleumrefining, Coke ovens, Kraft paper mills
Short atmospheric lifetime (4.4 days): Oxidized to SO2
Background concentrations( 30-100 pptv);
concentrations in industrial and surrounding ambientenvironments can be above the odor threshold
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Nitrogen Compounds
Gas phase Nitrogen (N2)
Nitrous oxide (N2O)
Nitric oxide (NO)
Nitrogen dioxide (NO2)
Nitrate radical (NO3)
Dinitrogen pentoxide(N2O5)
Peroxyacyl nitrate
(CH3COO2NO2; PAN) Ammonia (NH3)
Hydrogen cyanide (HCN)
Gas/Liquid phase
Nitrous acid (HNO2)
Nitric acid (HNO3)
Nitrite (NO2-)
Nitrate (NO3-
) Ammonium (NH4
+)
NOx: NO and NO2
NOy: NOx and their
atmospheric oxidation
products
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Nitrous Oxide (N2O)
Colorless, slightly sweet non-toxic gas
Also called laughing gas because human exposure toelevated concentrations produces a kind of hysteria
Atmospheric concentration increasing: (0.8 ppbv/yr)
Sources:
Natural: by nitrification and denitrification processes biogenically
Anthropogenic sources: Soil disturbance, Agricultural fertilizers
No known sink in the troposphere: atmospheric lifetime of
150 years
Stratosphere is only sink: photolysis and subsequentoxidation by singlet oxygen (O(1D))
So, why do we care about its increase in the atmosphere?
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Nitric oxide (NO)
Colorless, odorless, relatively non-toxic gas
Natural sources: Anaerobic biological processes
Biomass burning processes, lightning
Oxidation of NH3
Photochemical reactions in stratosphere and transportfrom there into the troposphere
Anthropogenic sources
Fuel combustion (transportation, coal-fired power plants,
boilers, incinerators, home space heating) Product of high temperature combustion; concentration
depends on temperature and cooling rate
NOON 222
More details about NO formation in Reaction/Kinetics
So, why do we careabout NO emission?
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Nitrogen Dioxide (NO2)
Brown colored, relatively toxic gas with a pungent and
irritating odor Absorbs light and promotes atmospheric photochemistry
Peak levels occur in mid morning
Production by chemical reactions
Direct oxidation
Photochemical reactions
)combustioninfastambient,in(slow22 22 NOONO
OHNOHONO
RONORONO
ONOONO
22
22
223
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NOx concentrations
Remote locations: 20-80 pptv
Rural locations: 20 pptv -10ppbv Urban/suburban areas: 10 ppbv - 1 ppmv
Diurnal variation
Weekly pattern?
Seasonal pattern?
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NOx Sink Processes
Chemical reactions convert
NO to NO2 to HNO3
Major sink process reaction with OH
Nighttime reactions involving O3
Reactions with organic compounds
Neutralized by ammonia to form salts
HNO3 serves as a reservoir and carrier for NOx
MHNOMOHNO 32
RHNORHNO
CHOHNOHCHONO
33
33
3252
5232
2332
2HNOOHON
ONNONO
ONOONO
(Reverse reaction under sunlight)
3433 NONHHNONH (removed by dry & wet deposition)
OHNOhHNO 23
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Reduced N Compounds
NH3 (Ammonia)
Sources: anaerobic decomposition of organic matter,animals and their wastes, biomass burning, soil humusformation, fertilizer application, coal combustion, industrialemissions
Background levels (0.1-10 ppbv)
Sink processes: reaction with acids, absorption by waterand soil surface
Atmospheric lifetime (10 days)
Very important neutralizer for strong acids Example?
Other N Compounds HCN (Hydrogen cyanide)
Organic nitrate compounds: Peroxyacyl nitrate (PAN),Peroxyproprionyl nitrate (PPN), Peroxybutyl nitrate (PBN)
potent eye irritants
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Hydrocarbons
Comprise a large number of chemical substances
Basic structure includes only carbon & hydrogencovalently bonded
Serves as a base for a number of derivative compounds
May be straight, chained, branched or cyclic
May be Saturated (single bonds, C-C)
Unsaturated (double/triple bonds, C = C)
Unsaturated HCs more reactive
May be gas, liquid or solid phase, depending on thenumber of carbons: gases 1-4 C; volatile liquids 5-12 C;
semivolatile liquids or solids > 12 C
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Hydrocarbons
Types
Aliphatic Paraffins/Alkanes - single bond
Olefins/Alkenes - have 1 double bond
Alkynes have 1 triple bond
Aromatic Have at least one benzene ring
Benzene
Toluene
Xylene
Lifetime Paraffins days
Olefins hours
Alkyenes weeks
Benzene (12 days), toluene (2 days), m-xylene (7 hr)
Example?
http://en.wikipedia.org/wiki/Benzene
http://en.wikipedia.org/wiki/Toluene
http://en.wikipedia.org/wiki/Xylene
http://en.wikipedia.org/wiki/Benzenehttp://en.wikipedia.org/wiki/Benzenehttp://en.wikipedia.org/wiki/Toluenehttp://en.wikipedia.org/wiki/Xylenehttp://en.wikipedia.org/wiki/Xylenehttp://en.wikipedia.org/wiki/Toluenehttp://en.wikipedia.org/wiki/Benzene -
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Hydrocarbons
Polycyclic aromatic HCs (PAHs)
Multiple benzene rings Solids under ambient conditions
Produced in combustion processes
Components of atmospheric aerosol
Potent carcinogens
Classification by volatility
VVOC (Very Volatile Organic Compounds): BP up to 50-100 oC
VOC (Volatile Organic Compounds): BP 50-100 to 240-260 oC
SVOC (Semi-Volatile Organic Compounds): BP 240-260 to 380-
400 oC SOC (Solid Organic Compounds): above 400 oC
NMHCs: Non-Methane HydroCarbons; Methane is
excluded because of its low reactivity in the atmosphere
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Hydrocarbon Derivatives
Formed from reactions with O2, N2, S or halogens
Derivatives of major atmospheric concern include: Oxyhydrocarbons
Halogenated hydrocarbons
Include
Aldehydes (C=O)
Acids (-COOH)
Alcohols (-OH)
Ketones (CO)
Ethers (C-O-C)
Esters (R-CO-OR)
Oxyhydrocarbons Direct emissions from
industrial/commercial use:
adhesives, solvents
By-products of combustion
Produced from photochemical
reactions
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Nonmethane Hydrocarbons
Primary focus of air quality regulation
Biogenic sources Trees (isoterpenes, monoterpenes)
Grasslands (light paraffins; higher HCs)
Soils (ethane)
Ocean water (light paraffins, olefins, C9-C
28paraffins)
Order of magnitude higher than anthropogenic
Question of their significance
Anthropogenic emission estimates
40% transportation
32% solvent use
38% industrial manufacturing/fuel combustion
Identification is challenging; concentration of
individual NMHC is not commonly measured
Why?
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NMHC Sink Processes
Oxidation by OH or O3
Produce alkylperoxyradicals (ROO) ROO is converted to alkoxy radical (RO) by reacting
with NO
RO reacts with O2 to produce aldehyde
Longer chained NMHCs result in ketones Ethane reaction
CHOCHOOHC
NOOHCNOOOHC
OOHCOHC
HCOHOHHC
3252
25252
52252
52262
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Oxidation of HCHO
Acetaldehyde more reactive than ethane
Acetaldehyde oxidized to HCHO through a series ofreactions with OH
HCHO can decompose by ultraviolet (UV) light in the range
of 330-350 nm and produce CO
OHNONOHO
COHOOHCO
MHOMOH
HHCOhHCHO
22
22
22
1st pathway produces OH
for oxidizing other NMHC
COHHCOH 2
2nd pathway
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Photochemical Precursors
CO (above) can be eventually converted to CO2
Aldehydes/ketones removed by wet/dry deposition
Longer chained HCs may produce condensible products
These oxidation products (e.g. ROO, RO, HO2 and
CO) serve as major reactants in forming smog; they also
serve to produce elevated tropospheric O3
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Methane (CH4)
Most abundant HC in atmosphere
Low reactivity with OH
Little significance in urban/suburban photochemistry;
hence, levels subtracted from total HC concentration
Can affect downwind of urbansources
Thermal absorber - global
warming concern Concentrations average ~
1.75 ppmv
Significant increases over timesince industrial revolution
So, why do we care about CH4?
Figure 2.5
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Methane
Natural Sources
Anaerobic decomposition inswamps, lakes and sewage wastes
Rice paddies
Ruminant/termite digestion
Anthropogenic Sources
Coal/lignite mining
Oil/gas extraction Petroleum refining
Transmission line leakage
Automobile exhaust
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Methane
Sink processes
In the troposphere, reaction with OH
Produces HCHO, CO & ultimately CO2
Competes with CO for OH Photodecomposition in stratosphere
Produces H2O
Major source of water in stratosphere
Levels in atmosphere increase with increasing CO Atmospheric lifetime (~10 years)
OHCHOHCH 234
Why?
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Halogenated Hydrocarbons
Contain one or more atoms of halogen (Cl, Br, or F);
include a variety of compounds Chlorinated HCs
Brominated HCs
Chlorofluoro HCs
Remarkable persistence (i.e. low reactivity) Include both natural/anthropogenic sources; both volatile
and semi-volatile compounds
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Volatile Halogenated HCs
Methyl Chloride (CH3Cl)
Methyl Bromide (CH3Br) Methyl Chloroform (CH3CCl3)
Trichloroethylene(CH2CCl3)
Perchloroethylene(C2Cl4)
Carbon tetrachloride (CCl4)
Semi-volatile Halogenated HCs
Chlorinated pesticides (DDT, Dieldrin, Aldrin) Polychlorinated biphenyls (PCBs)
Polybrominated biphenyls (PBBs)
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Chlorofluoro HCs (CFCs)
Trichlorofluoromethane (CFCl3): CFC-11
Dichlorodifluoromethane (CF2Cl2): CFC-12
Trichlorotrifluoroethane (C2Cl3F3): CFC-13
Characterized by Low reactivity
Low mammalian toxicity
Strong thermal absorption properties
Good solvent properties
So, why do we care about them?
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Halogenated HCs
Most halogenated HCs have tropospheric sinks
CFCs have no tropospheric sinks. Atmospheric Lifetimes
CH3Cl, CH3Br ~ 1 year
CH3CCl3 ~ 6.3 years
CCl4 ~ 40 yearsCFCl3 ~ 75 years
CF2Cl2 ~ 111-170 years
Concentrations vary spatially, with highest in source
regions over the northern hemisphere.
Concentrations in both the troposphere and stratosphere
have been increasing until the early 1990s.
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Photochemical Oxidants
Produced in chemical reactions involving:
Sunlight Nitrogen oxides
Oxygen
Hydrocarbons
Include
Ozone
Nitrogen dioxide
Peroxyacyl nitrate Odd hydrogen compounds (OH, HO2, H2O2)
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Photochemical oxidants: O3 Ozone the major photochemical oxidant of concern
Atmospheric O3 formation
Requires source of O(3P): through photolysis of NO2 at
wavelengths of 280-430 nm
Nitric oxide quickly
destroys O3
Steady-state concentration of20 ppb under solar noon
conditions in mid-latitudes
MOMPOO 33
2 )(
)(3
2 PONOhNO
223 ONOONO
Figure 2.6This doesnt explain the high
level O3in smog! Whats wrong?
Is O3 level high or low
at a highway tollbooth?
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Tropospheric O3 Formation
Elevated O3 levels occur as a result of reactions
that convert NO to NO2 without consuming O3! Role of peroxy compounds (ROO) derived from
photochemical oxidation of HCs
32
32
3
3
2
2
:
)(
)(
OROhOROONet
MOMOPO
PONOhNO
RONONOROO
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Tropospheric O3 formation
Rate of O3 formation depends on ROO availability
ROO produced when OH and HOx react with HCs OH is formed by photo-dissociation of O3, aldehydes
and HNO2
NOOHhHNO
OHnmhOH
OHOHDO
ODOhO
2
22
2
12
1
3
2)350(
2)()(
In summary, what are the important parameters in
determining O3 level?
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Tropospheric O3 Concentrations
Remote Locations (20-50 ppbv, summer
months) Photochemical processes
Stratospheric intrusion
Populated locations
Peak concentrations (50 ppbv - 600 ppbv) In urban areas concentrations decline at night
In rural areas peak concentrations occur at night
Elevated rural levels associated with long-rangetransport (Yosemite NP,http://www2.nature.nps.gov/air/webcams/parks/yosecam/yosecam.cfm) Transport of O3 aloft
Transport of low reactivity paraffins
http://www2.nature.nps.gov/air/webcams/parks/yosecam/yosecam.cfmhttp://www2.nature.nps.gov/air/webcams/parks/yosecam/yosecam.cfm -
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Tropospheric O3 levels
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Ozone Sink Mechanisms
Photo-dissociation
Reaction with NO in polluted area
Reaction with NO2 at night time
Surface destruction: reaction with plants, bare
land, ice/snow and man-made structures
OHOHDO
ODOhO
2)()(
2
1
2
1
3
223 ONOONO
3252
5232
2332
2HNOOHON
ONNONO
ONOONO
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