environmental chemistry chapter 15: atmospheric chemistry copyright © 2011 by dbs

Environmental Chemistry

Chapter 15:Atmospheric Chemistry

Copyright © 2011 by DBS

Contents

• Chemical and Photochemical Reactions in the Atmosphere• Free Radicals• Acid-Base Reactions in the Atmosphere• Inorganic Species in the Atmosphere• Particles in the Atmosphere• The Composition of Inorganic Particles• Carbon Oxides• Sulfur Dioxide Sources and the Sulfur Cycle• Nitrogen Oxides in the Atmosphere• Fluorine, Chlorine and Other Gaseous Compounds• Hydrogen Sulfide, Carbonyl Sulfide, and Carbon Disulfide• Organics in the Atmosphere• Organic Compounds from Natural Sources• Pollutant Hydrocarbons• Nonhydrocarbon Organic Compounds in the Atmosphere

Chemical and Photochemical Reactions in the Atmosphere


Figure 15.1. Important Aspects of Atmospheric Chemical Processes


Important Atmospheric Chemical Species

• Inorganic oxides: CO, CO2, NO2, SO2

• Oxidants: O3, H2O2, HO• radical, HO2• radical, ROO• radicals, NO3 radical

• Reductants: CO, SO2, H2S

• Hydrocarbons: Natural CH4, pollutant alkanes, alkenes, aromatics

• Oxidized organics: Aldehydes, ketones, acids, organic nitrates

• Photochemically active species: NO2, formaldehyde

• Acids: H2SO4, H2SO3, HNO3

• Bases: NH3

• Salts: NH4HSO4

• Unstable reactive species: Electronically excited nitrogen dioxide (NO2*), HO•

Solid and liquid particles in aerosols and clouds

• Sources and sinks for gas-phase species

• Sites for surface reactions on solids

• Aqueous phase reactions in water droplets


Two Very Important Factors in Atmospheric Chemistry:

(i) Radiant solar energy

– Photons put high energy into individual molecules

(ii) Hydroxyl radical, HO•

– Most important highly reactive intermediate


Photochemical Processes

• From photons of energetic solar electromagnetic radiation, h• Produce electronically excited species designated *

NO2 + h NO2*

• Excited species tend to be highly reactive in the atmosphere

• Two other reactive species

– Free radicals with unpaired electrons: H3C• , HO•

– Ions such as O+ (uncommon in lower atmosphere)


• Electronically Excited Species

– Absorption of a photon, usually of ultraviolet radiation, can energize molecules, atoms, or radicals to electronically excited states

Figure 15.2. Electronically excited states where the arrows represent directions of electron spin


Loss of Excitation Energy from Electronically Excited Species

(i) Emission of a photon (light): NO2* NO2 + h

Called luminescence if instantaneous, phosphorescence if slower and

Chemiluminescence when the excited species that emits a photon is formed as the result of a chemical reaction

O3 + NO O2 + NO2* (luminescent species)

(ii) Direct reaction of an excited species

O2* + O3 2O2 + O

(iii) Dissociation

NO2* NO + O (very important tropospheric reaction)

O2* O + O (important in stratosphere leading to O3)

(iv) Photoionization (formation of ions in the ionsphere)

N2* N2+ + e-

Free Radicals

Free Radicals

• Free radicals are atoms, molecules, or molecular fragments with unpaired electrons designated •

• Free radicals are highly chemically reactive because of the strong pairing tendency of their unpaired electrons

– Undergo series of chain reactions generating more free radicals

– Chain termination such as H3C• + H3C• C2H6

• Although highly chemically reactive, free radicals can be quite stable in the upper atmosphere where there are few other species with which they can react

Free Radicals

Hydroxyl (HO•) and Hydroperoxyl (HOO•) Radicals

• HO• is the single most important intermediate species in atmospheric chemical processes

• HO• formed by photolysis of water at higher altitudes

H2O + h HO• + H•

• As the result of ozone photolysis in lower atmosphere

O3 + h ( < 315 nm) O* + O2

O* + H2O 2HO•

Others:O* + CH4 → HO + CH3OHHNO2 → HO + NOH2O2 + h → 2HO

Free Radicals


• HO• reacts with many electron rich (multiple bonded) species in the atmosphere

– CO SO2 NO

• HO• reacts can also react by abstraction of hydrogen, e.g.

CH4 + •OH → •CH3 + H2O

NH3 + •OH → •NH2 + H2O

H2S + •OH → •SH + H2O

CH3Cl + •OH → •CH2Cl + H2O

• HO• commonly removed by reaction with CO or CH4

HO• + CO CO2 + H•

HO• + CH4 H3C• + H2O

Free Radicals


• Predominant fate of methyl radical is ‘add-on’ reaction with O2,

e.g. •CH3 + O2 → CH3OO•

HOO• / HO2• (hydroperoxy) and CH3OO• are called peroxy radicals- Less reactive than other radicals - Do not readily abstract H- Do not react with O atoms due to low conc.

• Main reactions:

HOO• + NO• → •OH + NO2•

R-OO• + NO• → RO• + NO2• (where R = carbon chain)

Free RadicalsOxidation of CH4

• CH4 produced in inefficient (anaerobic) burning of fuels

• Predominant HC in atmosphere

• No multiple bonds

• Not soluble in water, does not absorb sunlight

• Slow oxidation initiated by hydroxyl radical (hydrogen abstraction reaction)

CH4 + •OH → •CH3 + H2O abstraction

•CH3 + O2 → •CH3OO• O2 adds forming peroxyCH3OO• + NO → CH3O• + NO2 transfer of OCH3O• + O2 → H2CO + HOO• O2 abstracts H

…conversion of methane to formaldehyde

H2CO + UV-A (338 nm) → H• + HCO• unstable

H• + O2 → HOO• O2 abstracts

HCO• + O2 → CO +HOO• O2 abstracts

Note: CO is a stable intermediate and can further undergo transformations

C O + OH• → HO-C=O

H-O-C=O + O2 → O=C=O + HOO•

….. Production of CO2 as the final product

CH4 + 5O2+ NO + 2OH• + UV-A →

CO2 + H2O + NO2 + 4HOO•

Notice the radicals consumed

and produced.

What happens to the HO2 produced?

What happens to the NO2 produced?

(see fate of free radicals)

Acid-Base Reactions in the Atmosphere


• Rainwater is naturally slightly acidic

CO2(g) ⇌ CO2(aq)

CO2(aq) + H2O ⇌ H+ + HCO3-

• Stronger acids from acid gases such as SO2

SO2 + H2O ⇌ H+ + HSO3-

• Strong pollutant acids

H2SO4 HNO3 HCl

• Bases are relatively less important than acids in the atmosphere, usually from ash and ground rock (calcium oxide, hydroxide and carbonate)


• Ammonia, the most important base in the atmosphere

– NH3 from bacterial action

NO3-(aq) + 2{CH2O}(biomass) + H+ → NH3(g) + 2CO2 + H2O

– NH3 from industrial pollutant sources:

Ammonia manufacture, coke production, refrigeration systems

• Ammonia reacts with acidic gases to produce corrosive salts

NH3(aq) + HNO3(aq) NH4NO3(aq)

Inorganic Species in the Atmosphere

Inorganic Species in the Atmosphere

• CO2 is the most abundant inorganic compound other than water in the atmosphere

– Natural constituent

– Pollutant in the sense that excess causes excessive global warming

• Oxides of carbon (CO), sulfur, and nitrogen are important inorganic air pollutants

– Oxides of S and N cause pollutant acid precipitation

• Other inorganic pollutants include NH3, HCl, H2S

Particles in the Atmosphere

Particulate MatterA complex mixture of solid particles and liquid droplets found in the air

Sizes of Common Airborne Particles

Sourcese.g NH4Cl, SO4

2- / NO3- salts

Natural: forest fires, volcanoes etc.

Man-made: fossil-fuel combustion, industry

Mineral dust from weathering of rocks and soils

Chemical composition can be used to ID source and fate


Table 15.1 Terms Used to Describe Atmospheric Particles

Term Meaning Condensation aerosol Formed by condensation of vapors or reactions of gases Aerosol Colloidal-sized atmospheric particles

Dispersion aerosol Formed by grinding of solids, atomization of liquids, or dispersion of dusts

Fog Denotes high level of water droplets

Haze Decreased visibility due to particles

Mists Liquid particles

Smoke Particles from incomplete fuel combustion


Chemical Processes for Inorganic Particle Formation

• Solid oxides from inorganic solids in fuels, e.g. coal

3FeS2 + 8O2 Fe3O4(s) + 6SO2(g)

CaCO3 + heat CaO(s) + CO2(g)

• Gases reacting to produce liquid-forming compounds

2SO2(g) + O2 + 2H2O 2H2SO4(aq) (hydroscopic, forms aerosol droplets)

• Salt formation

H2SO4(droplet) + 2NH3(g) (NH4)2SO4(droplet)


Reactions Involving Particles

• Provide active surfaces upon which heterogeneous reactions can occur

• Nucleation bodies for the condensation of water vapor


Reactions Involving Particles (Figure 15.3)

The Composition of Inorganic Particles


• Example: Formation of particulate bound atmospheric Na2SO4:

2SO2(g) + O2 + 2H2O 2H2SO4(aq)

• Followed by reaction with particulate NaCl from ocean spray containing sea salt

H2SO4 + 2NaCl(particulate) Na2SO4(particulate) + 2HCl

Particulate high in SO4- but low in Cl- would

indicate ocean spray origin, plus man-made S content

Figure 15.4. Inorganic Materials and their Origins in Particles

Chemical composition can be used to ID source and fate


Origins of Some Atmospheric Particle Constituents

• Al, Fe, Ca, Si: Soil erosion, rock dust, coal combustion

• C: Incomplete combustion of carbonaceous fuels

• Na, Cl: Marine aerosols, chloride from combustion of organohalide polymers

• Sb, Se: Volatile elements from combustion of oil, coal, refuse

• V combustion of some kinds residual petroleum

• Zn: Combustion sources

• Pb: Combustion of lead-contaminated materials


• Fly ash – Much of the mineral particulate matter in polluted atmosphere is in the form of oxides (aluminum,

calcium, iron and silicon) and other compounds (soot and carbon black) from combustion of fossil fuels

• Toxic metals– Lead formerly in leaded fuels

– Cadmium and mercury in batteries that are burned

• Radioactive particles– Largely polonium from decay of naturally occurring radon gas

– Man-made from combustion of fossil fuels (fly ash)

– Above ground nuclear testing

Carbon Oxides

Carbon Oxides

Carbon monoxide, CO

• From partial combustion of carbon-containing fuels

• Toxic by reaction with blood hemoglobin

• Localized pollutant, such as in high-traffic areas

• Destroyed by reaction with hydroxyl radical: CO + HO• CO2 + H•

Carbon dioxide, CO2

• Natural atmospheric constituent, from aerobic respiration, volcanoes, rocks

• IR absorption responsible for atmospheric greenhouse effect

• Excessive amounts of fossil fuel burning will probably cause global warming

• Now at about 380 ppm volume

• Increasing more than 1 ppm/year

Global Warming and the Greenhouse Effect

1958: Keeling began measuring CO2 at Mauna Loa, HI

Carbon Oxides

What’s Up With the Weather (2000)

Carbon Oxides

• What is the significance of the Keeling curve?

What could be responsible for this seasonal up-down fluctuation?

Since 1958 atmospheric carbon dioxide has risen by more than 15%

http://www.cmdl.noaa.gov/ccgg/index.html

http://www.cmdl.noaa.gov/ccgg/index.html

Carbon Oxides

Fate of Atmospheric Carbon Dioxide (sinks)

• Incorporated into biomass by photosynthesis

CO2 + H2O + sunlight energy {CH2O} + O2

• Dissolving and reacting in water

CO2 + H2O H+ + HCO3-

• Reaction with carbonate minerals

CO2 + H2O + CaCO3 Ca2+ + 2HCO3-

Sulfur Dioxide Sources and the Sulfur Cycle

Sulfur Dioxide Sources and the Sulfur Cycle

Sulfur Dioxide Reactions in the Atmosphere

• Influenced by many factors

– Temperature, humidity, light intensity, atmospheric transport

• Results in formation particulate matter

• Ultimately oxidized to sulfuric acid and sulfates

• Effects of Atmospheric Sulfur Dioxide

– Affects respiration

– Phytotoxic

– Acidifies precipitation

– Particulate sulfuric acid and sulfate salts

Nitrogen Oxides in the Atmosphere


• Three oxides of N:

– Nitrous oxide N2O

– Nitrogen monoxide NO

– Nitrogen dioxide NO2

• Microbially generated N2O is relatively unreactive in the atmosphere

• Photolyzed in the stratosphere:

N2O + hv → N2 + O

Also:

N2O + O* → N2 + O2

N2O + O* → NO + NO

• NO participates in stratospheric ozone cycles (see later)

NOx Cycle (simplified)

N2O, N2 Agriculture NO3-, NO2

-

RESERVOIR

Removal negligable (few reactions except O)

RAIN OUT

SINK

HNO3 (inert)

i.e. inactive until transported

SOURCE

N2O + O → 2NO

NOx Cycle

Removes O3

Mankind can alter stratospheric O3 without leaving the ground


• NO and NO2 = NOx

• Natural sources – lightning, biological

• Man-made – burning fossil fuels, automobiles

• Most NO2 is derived from NO generated at high temperatures from internal combustion engines

N2 + O2 → 2NO

• Atmospheric conversion from NO to NO2 is rapid


Atmospheric Reactions of NOX (Figure 15.9)

Fluorine, Chlorine, and Other Gaseous Compounds

Fluorine, Chlorine, and Other Gaseous Compounds

• HF and F2 are extremely toxic

• Gaseous fluorides are highly phytotoxic

• Silicon tetrafluoride, SiF4, generated by reactions used in metal smelting operations

• Sulfur hexafluoride, SF6, is a powerful greenhouse gas used as an atmospheric tracer

– Global warming potential per molecule around 24,000 x CO2

– Extremely stable, lifetime around 3000 years

– Probably destroyed in the ionsphere by reaction with free e-

– Now at levels of around 0.3 parts per trillion

• Elemental chlorine, Cl2, is very toxic

• Hydrogen chloride, HCl, contributes to acid rain

• Produced in combustion of organochlorine polymers

Hydrogen Sulfide, Carbonyl Sulfide, and Carbon Disulfide

Hydrogen Sulfide, Carbonyl Sulfide, and Carbon Disulfide

• Hydrogen sulfide, H2S, from microbial decay of organosulfur compounds, microbial reduction of sulfate, natural gas contaminant

• H2S is very toxic

– 22 people killed in Poza Rica, Mexico, 1950

– 242 fatalities in China in 2003

– Very phytotoxic to some plants

• H2S damages some materials such as exposed copper metal

• Eventually oxidized to sulfate in the atmosphere

• Carbonyl sulfide, COS, emitted by marine phytoplankton

• • Greatest source of atmospheric sulfur

• • Reacts only slowly, eventually producing sulfate

• Carbon disulfide, CS2, produced by some marine phytoplankton

• • Does not last long due to reaction with HO•

Organics in the Atmosphere

Organics in the Atmosphere

• Two important aspects

– Reactions resulting from photon absorption, h– Importance of reactions with hydroxyl radical, HO•

Organic Compounds from Natural Sources


• Natural sources contribute 7/8 ths of total organics to atmosphere

• Huge amounts of methane produced by anaerobic bacteria

2{CH2O} (bacterial action) →CO2(g) + CH4(g)

• Also methane from domesticated animals

• Methane from rice fields

• HC’s produced by living sources are called biogenic hydrocarbons

– Vegetation, microorganisms, forest fires, animal wastes, volcanoes

• Vegetation most important source of non-methane hydrocarbons (NMHCs)


• Most HCs emitted by plants are terpenes

Fig. 15.8: Some common terpenes emmitted to the atmosphere by vegetation

Pollutant Hydrocarbons


• Hydrocarbons are abundant air pollutants because of their widespread use

• • Fuels • Polymer manufacture • Industrial chemicals • Solvents

• Alkanes, such as 2,2,3-trimethylbutane

• Alkenes such as ethylene and propylene


• Alkynes such as acetylene

• Aromatics such as napthalene

Hydrocarbons emitted as engine exhaust byproducts tend to be relatively more reactive because of the unsaturated compounds in them


Figure 15.12. Aromatic hydrocarbons found in the atmosphere; the first six compounds are among the top 50 chemicals manufactured and the last two are multicyclic aromatics (PAHs)

Nonhydrocarbon Organic Compounds in the Atmosphere


Aldehydes and Ketones

• Formaldehyde (right) is the simplest aldehyde, a photochemically reactive species that may be emitted directly or formed by secondary atmospheric reactions

• Unlike most air pollutants, some aldehydes may undergo direct photochemical reactions by absorbing h

Formaldehyde


• Aldehydes and Ketones

Figure 15.13. Some Aldehydes and Ketones that May be Found in the Atmosphere


Miscellaneous Oxygen-Containing Compounds

• • Aliphatic alcohols (R-OH) • Phenols (Ar-OH) • Ethers (R-O-R’)

• Carboxylic acids (R-COOH)

• Among alcohols, methanol, ethanol, isopropanol and ethylene glycol rank among top 50 chemicals

– Methanol and ethanol in atmosphere because of volatility, but are removed due to high water solubility

– Increased levels of ethanol because of use in gasoline


Miscellaneous Oxygen-Containing Compounds

• Phenol is among the top 50 chemicals produced

• Two examples of common ethers that may be found in the atmosphere (right); MTBE is being phased out of use in gasoline due to water solubility


• Organo-oxygen Compounds (Cont.)

• Ethylene oxide and propylene oxide are among the top 50 produced chemicals; ethylene oxide is relatively toxic

• Carboxylic acids (R-COOH)

• Final oxidation products of hydrocarbons in the atmosphere

– Relatively non-volatile, so they tend to occur in particles

– Lower acids (formic and acetic acids) are water-soluble and removed with rainfall


• Organohalides (Figure 15.14)

• Dichloromethane is a very volatile solvent

– Vinyl chloride used to make polyvinylchloride plastic, carcinogen

– Trichloroethylene widely used as a dry cleaning solvent

• PCBs are notorious water pollutants

– Once widely used in electrical applications, as hydraulic fluids, other

– Highly persistent and bioaccumulative, manufacture now banned

– Hudson River sediments badly contaminated due to dumping from industrial equipment manufacture


Chlorofluorocarbons (Freons)

• Compounds such as dichlorodifluoromethane in which all Hs have been replaced by Cl and F

– Extreme stability

– Very low toxicity

– Now constituents of the global atmosphere

– Banned because of destruction of stratospheric ozone

– Replaced by compounds with at least one H-C bond

– H-C bond attacked by tropospheric HO•

• Halons, such as Halon-1211, CBrClF2

– Fire extinguishers on aircraft

– Damage stratospheric ozone

– Much lower production levels than past chlorofluorocarbons


Organosulfur Compounds

• Some, such as thiols, notable for bad odors

• Dimethylsulfide released in large quantities by marine phytoplankton

Figure 15.13. Common Organosulfur Compounds Associated with Air Pollution


Organonitrogen Compounds

• • Amines • Amides • Nitriles • Nitro compounds • Heterocyclics

• Some amines, such as the methylamines, are widely used, toxic, noxious substances

– Some aromatic amines are known human carcinogens

– Some oxygenated -NO2 compounds, such as peroxyacetyl nitrate (PAN, Chapter 16), are strong oxidants that are harmful air pollutants

Figure 15.16. Potential Air Pollutant Organonitrogen Compounds


Organonitrogen Compounds

• Some oxygenated -NO2 compounds, such as peroxyacetyl nitrate (PAN, Chapter 16), are strong oxidants that are harmful air pollutants

(peroxyacetylnitrate)

Peroxyacetylnitrate is eye irritant and toxic to plants

(acetyl)

environmental chemistry chapter 15: atmospheric chemistry copyright © 2011 by dbs

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