Gaseous Pollutants

7/27/2019 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?


10/438/18/201310

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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