environmental chemistry chapter 3: the chemistry of ground-level air pollution copyright © 2012 by...

Environmental Chemistry

Chapter 3:The Chemistry of Ground-Level Air Pollution

Copyright © 2012 by DBS

• Most obvious environmental difference amongst world cities is air quality

Source: UNEP/WHO, 1992

http://www.who.int/peh/air/gemair.htm

3.1 Concentration Units for Atmospheric Pollutants

• Most obvious environmental difference amongst world cities is air quality

• Air contains suspended particles and gases

• Particles are usually heterogeneous, no molar mass

• Reported as a mass per unit volume (m/v) μg m-3


• Concentration units for gases (m/v)– Molecules per cubic centimeter (molec. cm-3)– Micrograms per cubic centimeter (µg cm-3)– Moles per liter of air (moles L-1)

• Mole fraction / mixing ratios (v/v)

volume analyte/total volume of sample

Molecule fraction per million or billion

e.g. 100 ppmv CO2 refers to 100 molec. of CO2 per 106 molec. of air

• Partial pressure of gas expressed in units of atmospheres (atm) kilopascal (kPa) or bars (mb),

Ideal gas law relates pressure and temperature to no. moleculesPV = nRT


• Conversion (at 25 ºC and 1 atm) from m/v to v/v:

concentration (ppm) = concentration (mg m-3) x 24.0Molar mass

• Mixing ratio (v/v) is conserved if temperature pressure changes

Question

Express [SO2] = 5.0 x 1012 molecules cm-3 as a volume mixing ratio (ppbv) at 25 ºC, 1 atm.

[SO2] = 5 x 1012 molecules cm-3

= 8.3 x 10-12 mols cm-3

= 8.3 x 10-12 mols cm-3 x 64.1 g/mol = 5.3 x 10-10 g cm-3

= 5.3 x 10-7 mg cm-3 x (1 x 106 cm3 / m3)= 0.53 mg m-3 = 0.53 mg m-3 x 24.0 / 64.1 g mol-1

= 0.199 ppmv = 0.199 x 1000 ppmv/ppbv = 199 ppbv

[Convert to mg m-3 then use m/v to v/v conversion]

3.2 The Chemical fate of Trace Gases in Air

• Sources: fires, lightening, anaerobic biological decay, volcanoes

• Examples: carbon monoxide (CO), nitric oxide (NO), sulfur dioxide (SO2), Ammonia, (NH3), Hydrogen Sulfide (H2S), Methane (CH4)

• Do not build up in clean air due to SINKS

• Sinks are oxidation reactions with hydroxyl radical (•OH) (Ea for reaction with O2 is too high)

(unpaired e- makes free radicals highly reactive)


• Production of •OH (one mech)

O3 → O2 + O* (UV-B)

O* + H2O → 2 OH

• Lifetime of •OH is 1 s, conc. drops quickly at nightfall

• Seeks out molecules containing multiple bonds, reacts to form larger molecules:

•OH + CO → HOCO•

O2 + HOCO• → HOO• + CO2

• Hydroperoxy radical (HOO•) reacts with NO to reform •OH and NO2

HOO• + NO → •OH + NO2

3.2 The Chemical Fate of Trace Gases in Air

Stable gases, O2 (light)

•OH HOO•

NO

• Role and importance of OH only discovered recently

• ‘Tropohpheric detergent’


• Most NO2 produced in OH/HOO cycle absorbs UV-A and decomposes to NO and O

NO2 + UV-A →NO + O

• O atoms produced react with O2 to form ozone

HOO

NO NO2

O sunlight UV-A

Summary: Oxidation by OH

• The naturally released gases are eventually oxidized to oxides

-but not by direct reaction with O2

• Hydroxyl Radicals (•OH) produced in the environment serves as an oxidant (Conc. In the atmosphere is small and it is very shortlived)

O3 + UV-B → O2 + O*

O* + H2O → 2 •OH

• OH radical is referred as Troposphere vacuum cleaner or detergent

e.g.,

CH4 + 4 •OH → CO2 + 2H2O

• Because the lifetime of hydroxyl radical is short (~1s) constant regeneration is essential

- OH radical concentration drops quickly at night

Formation via O* occurs far quicker than with O

3.3 The Origin and Occurrence of Smog

• Nitric Oxide (NO) and unburnt hydrocarbons (VOCs) are the primary reactants of photochemical smog formation

VOCs + NO + O2 + sunlight → mixture of O3, HNO3, organics, secondary pollutants

Photochemical Smog

Photochemical Smog

NASA October 2000

3.3 The Original and Occurrence of Smog

• Primary pollutant – emitted directly into the air, e.g. NO and VOCs

• Secondary pollutant – produced from chemical reaction of primary, e.g. O3, HNO3, partially oxidized (nitrated) organics

• Atmospheric molecules react with hydroxyl radical (OH)

• Most reactive VOCs are alkenes (C=C) and aldehydes (C=O)

• With non-multi bonded species OH abstracts H leaving a radical fragment

e.g. CH4 + •OH → •CH3 + H2O

• Radical fragments react with O2 and oxidize to CO and CO2


• Primary pollutant – emitted directly into the air, e.g. NO and VOCs

• Secondary pollutant – produced from chemical reaction of primary, e.g. O3, HNO3, partially oxidized (nitrated) organics

• Atmospheric molecules react with hydroxyl radical (OH)

• Most reactive VOCs are alkenes (C=C) and aldehydes (C=O)

• With non-multi bonded species OH abstracts H leaving a radical fragment

e.g. CH4 + •OH → •CH3 + H2O

• Radical fragments react with O2 and oxidize to CO and CO2

• Hydroxyl radical is regenerated as shown before (slide 8)


• Over time the total [OH] and [HOO] free radicals builds up during a smog episode and catalytically accelerates it

• NO and VOCs are said to act in synergism to produce smog

• Combining the reaction sequence of CH4 to CO and CO to CO2 the overall reaction is as follows:

CH4 + 2O2 → CO2 + 2H2O

Oxidation of 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•

3.4 Ground-Level Ozone in Smog

• Conditions for smog:– Substantial vehicular traffic (source of NO, HCs and VOCs)– Warmth and ample sunlight– Little movement of the air

• Geography and dense population:– Los Angeles– Denver– Mexico City– Tokyo– Athens– Sao Paulo– Rome

3.5 An Episode of Photochemical Smog

• NO released from vehicles and power plants• NO slowly oxidizes to NO2

• O3 produced by PC decomposition of NO2 (NO2 → NO + O)

• O3 is low due to destruction via O3 + NO →O2 + NO2

• HCs form intermediate oxidation products - aldehydes

3.5 An Episode of Photochemical Smog

• NO2 peaks in AM but remains due to continuous emissions• NO2 sink via NO2 + OH → HNO3

• NO2 also reacts with HCs to nitrate them

3.6 Nitrogen Oxide Production During Fuel Combustion

• NO/NO2 concentrations in smog are many times natural background

• Important to understand sources (both natural and man-made)


Combustion or Thermal NO: N2 + O2 ⇌ 2 NO H = +180 kJ mol-1

(Occurs only at high temp. (lightning/engines) because of high activation energy)

Fuel NO: NX + O2 → yNOx

(Occurs during combustion of nitrogen compounds)

• NO slowly oxidizes (+O2) to NO2 – takes other routes

• Collectively NO and NO2 are referred as NOxNO2 absorbs in the UV-visible(absorption in the 400 nm region isvisualized by yellow color)

NO2 + UV→ 2NO + OO + O2 + → O3

Thus ozone becomes the main product of smog


• NO/NO2 are collectively referred to as NOx

• Low levels of NOx in clean air result from lightening, biological releases

• Emissions from man-made sources have fallen over the last decade


• Smog story is much more complicated!

• Above analysis indicates O3 as main product of smog, many more products that are far greater health hazards

3.7 Governmental Goals for Reducing Ozone Concentrations

• O3 concentrations in the troposphere vary widely over the Earth’s surface. The more direct the angle of sunlight, the greater its intensity. Where O3 precursors exist, more O3 tends to occur in regions closer to the Equator (lower latitudes) than in regions at the poles (higher latitudes)

• O3 concentrations also vary through time, throughout the day and through the year. The highest O3 concentrations of the year generally occur during summer, when sunlight is most intense

Ozone tolerance levels:In clean air 30 ppbWHO limit 75-100 ppb (50/60 for 8hr)US 75 ppb/8hr period (2008 level)

The ozone levels in West coast and east coast cites, midwest and texas reach as high as 300 ppb at peak

3.8 Photochemical Smog Around the World

• Cities in N. America, Europe, and Japan exceed ozone levels of 120 ppb for 5-10 days each summer

• LA ozone reaches 680 ppb• Pollution control measures have been effective for LA:

3.8 Photochemical Smog Around the World

• In 1990 Mexico city exceeded WHO guidelines on 310 days

• In Mexico city residents can purchase pure oxygen

• Unlike LA Mexico city suffers worst pollution in the winter months

3.7 Governmental Goals for Reducing Ozone Concentrations

• Ozone control is a regional rather than a local air-quality problem (US std. is 75 ppb)

3.9 Limiting VOC and NO Emissions to Reduce Ground Level Ozone

• Control of NOx, double bond containing HCs and other reactive VOCs

• For economic and technical reasons most common strategy is to reduce HCs

• However NOx control is more effective in reducing ozone due to overabundance of HCs

• The same concentration of ozone results from many different ratios of VOCs and NOx

Limiting VOC and NOx Emissions to Reduce Ground Level O3

• If NOx is 0.20 ppm reduction in VOC of 0.5 to 0.4 ppm will result in decrease of O3 from 160 to 80 ppb

Lowering NOx produces more O3

due to OH inc.

• Decreasing VOCs from 1.6 to 1.0 has no effect on O3 concentration – NOx limited

3.10 Catalytic Converters for Gasoline Engines

• Early NOx control for cars was lower burn temperature via reciculation of combustion gases

• Complete control of Nox emissions by use of catalytic converters

• 3 way catalyst (Pt, Rh, Pd):

2NO → N2 + O2 [Reduction]

2CO + O2 → 2CO2 [Oxidation]

CnHm + (n + m/4) O2 → nCO2 + m/2 H2O

CH2O + O2 → CO2 + H2O

• Fuel/air ratio is computer controlled to ensure conversion efficiency


Efficiency of CCs


• Rhodium (reduction) catalyst also reduces SO2 to H2S

• Not efficient at engine start-up temperature

• US EPA regulates car emissions

3.11 Air Quality Standards

• Different jurisdictions differ slightly in the maximum permitted concentrations

• Area is in compliance if these maxima are only rarely exceeded

3.11 Air Quality Standards

• US EPA Air trends:

1980 – 1990

– CO exceeded 9 ppm, national average now 2 ppm (cf. 9 ppm 8 hr std.)

– NO2 / SO2 concentrations have decreased but not as dramatically as CO

– Ozone not improved, many sites exceed quality standard of 75 ppb (8 hr std.)

• India/China standards more strict, air quality has declined

3.12 Catalytic Converters for Diesel Engines

• Catalytic converters used on trucks and buses much less efficient than gasoline vehicles

• Remove only ~ 50 % HCs, mainly due to high sulfur content, and problem with SO2/sulfate production

• Use of low sulfur fuels and engine design have improved this

• No NOx control on diesel engines required since diesel engines operate ‘fuel lean’ with excess O2

• Substantial emissions from train and ship diesel engines

• Significant emissions of particles

3.13 Control of Nitric Oxide Emissions from Power Plants

• Power plants are a large NOx source

• Emission controls:– Special burners to lower flame temperature– Combustion stages– Adding ammonia to gas stream

4NH3 + 4NO + O2 → 4N2 + 6H2O

– Wet scrubbing with NaOH

NO + NO2 + 2NaOH → 2NaNO2 + H2O

3.14 Future Reductions in Smog-Producing Emissions

• Emissions of CO, VOCs, SO2, PM, Pb fell between 1970 – 2000

• Emissions of NOx grew by 20 %

• Energy consumption grew 45 % and vehicle miles grew 143 %

• Emission control strategies have failed to prevent smog episodes

http://www.epa.gov

Introduction

3.14 Future Reductions in Smog-Producing Emissions

• Blackout of 2003 provided interesting data:– 24-hr measurements over PA found SO2 levels down 90 % and

ozone down by 50 %, visibility inc. by 40 km due to less PM

3.18 Sulfur Dioxide and Hydrogen Sulfide Sources and Abatement

• Man-made sources: coal combustion (electricity generation), smelting of ores, petroleum refining, natural gas industry, automobiles

• All sulfur from coal oxidized to sulfur dioxide


• Sulfur in coal may be removed from incombustible mineral content (50%) but not from the organic fraction (1 %)

• Metals refining:– e.g. copper and nickel from sulfide ores

– ‘Roasting’ in air, e.g. 2NiS + 3O2 → 2NiO + 2SO2

– SO2 may be converted to sulfuric acid and sold as a by-product

– May be released directly into air


• Gasoline and Diesel:– Increase in use of gasoline and diesel fuels has not produced large

increase in SO2, due to removal of sulfur by refineries

• Natural gas:– Considerable source of H2S (poisonous)– Converted to elemental S– 2H2S + SO2 → 3S + 2H2O (Claus reaction)

• Petroleum:– Refining of crude oil is a significant source of ‘reduced sulfur’ – H2S,

CH3SH, (CH3)S, and CH3SSCH3

3.19 Clean Coal: Reducing Sulfur Dioxide Emissions

• Power plant emissions of SO2 are strictly controlled

• SO2 is removed by reacting with limestone or lime:

• Flue-gas desulfurization, wet or dry scrubbing:

CaCO3 + SO2 → CaSO3 + CO2

2CaSO3 + O2 → 2CaSO4

3.19 Clean Coal: Reducing Sulfur Dioxide Emissions

• Clean Coal technologies:

(i) Precombustion – mineral S removed by grinding and density separation

(ii) Combustion – modification of combustion conditions, SO2 absorbing substances injected into the fuel, e.g. fluidized-bed combustion (injection of limestone)

(iii) Postcombustion – SNOX process, cooled flue gases mixed with NH3 to remove NO and convert to N2, SO2 oxidized to H2SO4

3.20 Governmental Goals for Reducing SO2 Emissions

• History, by region, of man-made SO2 emissions

3.20 Governmental Goals for Reducing SO2 Emissions

• Global total (sum of curves) reached a maximum in 1970s and was declining, has begun to increase again

• Main sources are 1) Coal, 2) Oil, 3) metal smelting

• p114

3.21 The Oxidation of SO2 in Suspended Water Droplets

• A

3.23 Sources and Composition of Coarse Particles

• A

3.24 Sources and Composition of Fine Particles

• A

3.25 The Neutralization of Acids in Air

• A

3.26 Smoke from Wood Stoves

• A

3.27 Smoke Over Large Areas of Land

• A

Part 3 – Particulate Matter

Glossary

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

"Where Does Tread Rubber Go?"

“It is estimated that more than 600,000 metric tons of tire tread are worn off

American vehicles every year. Instead of leaving black smudges on the

highways, tiny particles of tread are worn off tires and are released into the air.”

by Peggy J. Fisher President of Fleet Tire Consulting, Rochester Hills, MI

Over 80% of respirable particulate matter (PM10) in cities comes from road transport and that tire and brake wear are responsible for the 3-7% emission of it

Tire debris eluates contain zinc, and we have demonstrated that this metal can accumulate in cells, and affect X. laevis embryos [frog]

Gualtieri et al., 2005

http://www.particleandfibretoxicology.com/content/2/1/1

10 μm

1 μm

Sizes of Common Airborne Particles

e.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

Course - basic

Fine - acidic

Fin

eC

oa

rse

1 nm

Sizes of Common Airborne Particles

Settling Velocity

Stoke’s Law

Equating this frictional force with the gravitational force:

vt = 2 r2 g (ρs - ρf )

9 μ

where: Vt = settling velocity, r = radius, g = gravity, ρs = density of the particles, and ρf = density of the medium, = viscosity

Settling velocity increases with square of radius – smaller particles are suspended

Sources of Coarse Particles

• Most coarse particles are primary pollutants

• Natural sources: Soil dusts, (similar composition to soil or rocks – Al, Ca, Si, O), sand, sea salt spray, forest fire debris, leaf litter, pollen, volcanic eruptions

• Man-made sources: vehicle exhaust , stone crushing, land cultivation

• Basic due to high soil content

May begin existence as coarser matter

Sources of Fine Particles

• Most fine particles are secondary pollutants – form via chemical reactions and coagulation / nucleation of smaller species

• Natural sources: similar to course particles + aerosols

• Man-made sources: vehicle exhausts, tires, brakes, metal smelting– Diesel engines produce majority carbon particles (soot), gasoline engines

produce majority VOC’s

• The organic content of fine particles is greater than the coarse ones, e.g. urban smog, they are also more acidic

Because of their small size their settling velocity is very small. Most of them remain suspended in air

Most ultrafine particles in urban air are anthropogenic

Aerosol

• Sulfate aerosol from volcano eruption, fuel combustion and microbial activities

• Ammonium salts from reaction with biologically derived ammonia

• Soot (C) from fuel combustion

• Secondary organic C from volatile organic compounds

Typical composition

Aerosol: A dispersion of microscopic solid or liquid particles dispersed in air

Mt. Pinatubo Eruption

http://www.ngdc.noaa.gov/seg/hazard/stratoguide/pinfact.html

http://vulcan.wr.usgs.gov/Volcanoes/Philippines/Pinatubo/description_pinatubo.html

Largest eruption since 1912

1992 one of the Coolest Years

PM Index• Total suspended particulates – TSP (no longer used)

• Inhalable – PM10,

– diameter < 10 μm (coarse/fine)– Typical urban concentration 10-30 μg m-3

– AQS: 24 hr 150 μg m-3, Annual ave. 50 μg m-3

• Respirable – PM2.5,

– Deeply penetrating (fine and ultrafine)– Typical urban concentration 10-20 μg m-3

– AQS: 24 hr 65 μg m-3 d-1, annual average 15 μg m-3

• Since most of the fine particles in urban air are secondary, their number can only be controlled by reducing primary pollutants (NO, VOCs and SO2)

Distribution of Particle Size

Nuclei Mode: Small particles (0.01 µm) are formed by the condensation of vapors of pollutants (Condensation of H2SO4 and soot particles)

Accumulation Mode: These small particles serve as nuclei and undergo coagulation followed by deposition of gas moecules(Distribution peak 0.1 µm)

Coarse Particle Mode: Particles with distribution peak around 1µm are mainly soot or materials produced from mechanical grinding. Larger particles quickly settle

Distribution of numbers of aerosol particles in a typical urban area

Peak in the m Region

Why no particles down here?

Small particles coagulate

cf. Fig. 2-17

Average Residence Time

Absorbed: dissolvesAdsorbed: stick to the surface

Distribution of Particle Mass

• Number of particles does not represent the actual mass distribution

• Mass (volume) ~ (radius)3

• Larger particles contribute more mass

• Because of their small mass, peak corresponding to nuclei mode is not seen

CoarseUltrafine

cf. Fig. 2-19

3.28 The PM Indices

• A

Chp 3 Homework

• P3-3

• P3-4

• P3-8

• P3-11

• P3-18

environmental chemistry chapter 3: the chemistry of ground-level air pollution copyright © 2012 by...

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