air, water and land pollution chapter 2: the atmosphere copyright © 2009 by dbs

Air, Water and Land Pollution

Chapter 2:The Atmosphere

Copyright © 2009 by DBS

Contents

• The Global Atmosphere• Atmospheric Transport and Dispersion• Emissions to Atmosphere and Air Quality• Gas Phase Reactions and Photochemical Ozone• Particles and Acid Deposition

The AtmosphereThe Global Atmosphere

Atmospheric Structure• Troposphere and stratosphere

– Most of atmosphere < 100 km (homosphere)


Dominant, permanent gases Variable, trace gases


Atmospheric Structure• Trace gases

– Variable in time and space– Due to variations in emission rate, chemistry and removal processes

e.g. water vapor 4 % in tropics, < 0.00001 % at the poles

– Residence time is a measure of the time a gas spends in the atmosphere– Water vapor is around 11 days (see coursework)


Atmospheric Structure• Layers of the atmosphere – divided based on temperature

– Troposphere 0 – 10 km– Stratosphere 10 – 50 km– Mesosphere 50 – 90 km– Thermosphere 90 – 500 km– Exosphere > 500 km

(1 km = 0.62 miles)


Atmospheric Structure• Due to air pressure 99 % of the total mass of the

atmosphere resides in the troposphere and stratosphere


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Troposphere:From 0 to 10 km (6 mi)

[1000 - 200 mb]

1. Temperature decreases

2. Winds increase with height to the jet stream

3. Moisture decreases (VP)

4. Sun’s heat warms the surface and is transported up by convection

5. Weather!

6. Depth depends on latitude(18 km at equator, 8 km at poles)


Stratosphere• Dry and stable

– less turbulence• O3 rich• UV induced

photochemistry dominates• Inversion –

temperature increases with height

• Earth’s ‘sunscreen’

Turco, 2002


Atmospheric Structure• Tropospheric pollutants have limited lifetime before removal

– washout by rain– chemical reaction or – deposition to ground

• Stratospheric pollutants have longer residence times – due to slow downward mixing

e.g. major volcanic eruptions injecting fine dust can reduce solar energy for more than a year after the event


Atmospheric Structure• Atmospheric Circulation

– Energy from the sun and the Earth’s rotation– Meridional circulation, zonal circulation and jet streams

– Affected by Earth’s albedo, evaporation, cloud formation (condensation) and precipitation


• Winds and ocean currents transfer energy around the globe

• Hot air at equator moves north, replaced by cold air from poles

• Motion is broken by Coriolis Force into 3 cells

• Drives wind belts and jet streams


Atmospheric Structure• The Boundary Layer

– Mechanical forces generate turbulence as air flows over uneven ground – rough surfaces reduce wind speed

– Ground also warms and cools the air resulting in convection– Effect of friction with height is to change wind direction - generates wind shear


Atmospheric Structure• The Boundary Layer

– Area affected by surface effects ~ 1 km– Vertical mixing of pollutants determined by stability– Mixing is relatively rapid compared to remainder of troposphere– Mixing depth for modeling purposes (pollutants are retained and transport over

long distances)


Greenhouse Gases and Climate• Global Energy Balance

– Amount of energy that reaches Earth determines climate– Without atmosphere Earth surface temperature would be 255 K (-18 °C)– Incoming solar radiation is absorbed, scattered and reflected by gases

100/340 = albedo

Surface Albedo

Snow 0.8-0.95

Dry sand 0.4

Forests 0.2

Calm sea water

0.05

Asphalt 0.05

Smith, 2001


11

Radiation emitted from the ground lies in infra-red region…


Greenhouse Gases and Climate– IR radiation emitted from ground is absorbed by gases:

CO2, H2O, O3, CH4, N2O, CFC’s

– ‘Atmospheric greenhouse effect’

– Net effect is a warmer planet (global average 288 K, 15 °C)


% = pph = ppm / 10,000ppm / 10,0000.0350 = 350/10,000


Greenhouse Gases and Climate

• The Carbon Dioxide Cycle– Determines atmospheric concentration

– Man-made CO2 input:

Fossil fuels 6.3 x 109 tons yr-1

Deforestation 1.6 x 109 tons yr-1

– May eventually modify climate

– Compare to 750 x 109 tons already in atmosphere~ 360 ppmv

– Compare to 280 ppmv pre-industrial level

– Ocean is main sink

Question

How much more CO2 does the ocean store than the atmosphere?

39000 / 720 = 50


Greenhouse Gases and Climate• Global Warming

– Rate of increase

– GWP (radiative forcing relative to CO2) – ‘heat trapping ability’

– CO2 is most important (largest concentration)

– Other gases contribute ~ half overall radiative forcing


• Forcing: GH gases reduce amount of heat radiated to space

– Climate system adjusts

– Earth’s surface warms to compensate – maintain equilibrium

Houghton, 2004

F = Fin – Fout = 0 at equilibrium

Fin = Fout


Greenhouse Gases and Climate• Climate Change

– Mean surface temperature increasing at a rate above natural variability– Climatological consequences are not well understood– Requires climate models– Feedbacks and ocean-atmosphere coupling– Ocean atmosphere global circulation models (OAGCMs)

predict rise of 1.5 – 4.5 °C if CO2 doubles

– Cloud and aerosol feedbacks tend to cause most uncertainty


Greenhouse Gases and Climate• Climate Change

– Predictions limited by accuracy of assumptions regarding future economic and social change

– Emissions?– IPCC gives several scenarios

– Changes in sea level calculated anywhere between 0.09 – 0.88 m by 2100


Greenhouse Gases and Climate• International Response

– Limit greenhouse emissions– Kyoto Protocol of 1997 – reduce GH emissions by between 0-8 % of 1990 levels

by 2010– US did not agree, Australia finally signed in 2007

– Reduction process – energy efficiency, protection of sinks and reservoirs (forests), sustainable agriculture, increased use of renewable energy, CO2 sequestration, economic measures (phase out of tax exemptions and subsidies)

– Developing countries were exempt, encouraged to participate in ‘clean development mechanisms’ (CDM), to earn emission reduction credits that could then be sold in order to finance their projects


Depletion of Stratospheric Ozone• The Ozone Layer

– Temperature increases with height in stratosphere

– UV induced photochemistry of ozone dominates

– Meteorology is influenced by heat generated

– Earth’s ‘sunscreen’

Turco, 2002



– 90 % total atmospheric O3 in stratosphere

– Filters UV from the sun removing most of the high energy UV below 300 nm



– Depletion of stratospheric O3 leads to larger UV flux at Earth’s surface and increased risk of cancer

– Disruption of biological communities



– Chlorofluorocarbon (CFC) catalytic destruction of stratospheric O3

CFCl3 + hν → CFCl2 • + Cl•

Cl• + O3 → ClO • + O2

ClO• + O• → Cl • + O2

O3 + O• → 2O2

1974



– Chlorofluorocarbons (CFC’s, Freons,…)– Used as aerosol propellants, refrigerants and blown plastics– non-toxic, non-flammable, non-carcinogenic

– Inert in troposphere (no sinks!), not soluble in water– Resistant to attack by molecules, radicals or UV in

the troposphere


1950 – 50,000 tonnes

1976 – 725,000 tonnes

90% of emissions already in the atmosphere, remainder emitted when equipment is discarded


CFC11 – 0 to 268 ppt

Atmospheric concentration is small – 1 ppb

CFC12 – 0 to 533 ppt


Depletion of Stratospheric Ozone• Ozone Depletion

– Ozone is formed from the dissociation of molecular oxygen by short wave length UV radiation in the stratosphere

Chapman TheoryAbove stratosphere oxygen absorbs UV-C and exists as O atoms

O2 + hν → O• + O• ΔH = 495 kJ/mol (<241 nm) (1)

Oxygen atom could react with oxygen molecule to form O3

O• + O2 + M → O3 ΔH = -100 kJ/mol (2)

O3 formed could react with O atoms or absorb solar radiation

O3 + hν → O2 + O• (<320 nm) (3)

O• + O3 → 2O2 ΔH = -390 kJ/mol (4)

- A third molecule ‘M’ (N2 or H2O) facilitates as a heat energy carrier (is not required when there is more than one molecule produced)

- Enthalpies show a great deal of heat is generated

+ O3

- O3


Depletion of Stratospheric Ozone• Ozone Depletion

– For about 40 years, it was generally accepted that this sequence explained the full cycle of stratospheric ozone…

– Measurements of the vertical profile of ozone in the atmosphere showed the Chapman mechanism over estimated the amount

– Must be another sink…


• Seen this before!• X can be either NO•, •OH, Br• or Cl•• X is recycled

• These cycles compete with production by sunlight to produce the O3 distribution

NB: Both NOx and HOx cycles are natural cycles…pollution may add

Interaction with Other Cycles

• Free radicals are short-lived and are readily converted into stable forms – so called reservoir species that are catalytically inactive

•ClO + NO2 ⇌ ClONO2 (chlorine nitrate)

•Cl + CH4 ⇌ HCl + •CH3

• HCl and ClONO2 are inactive since they do not react directly with O3…chlorine reservoirs…transported out of stratosphere?

• When it was realized in 1980s that the chlorine in the atmosphere exists in the inactive form , the predicted loss of ozone in the stratosphere was lowered

sunlight


The Antarctic Ozone “Hole”• Farman et al. dramatic and

unpredicted decline in stratospheric O3 in a surprising location

– Antarctica

– Shocked the world

– Showed dramatic decline in springtime O3 starting in 1970’s

30% by 1985

70% by 2000

Min O3 at Antarctic in Spring (Sep-Nov)


• Occurs at the beginning of Southern Hemisphere spring (August-October)

• The average concentration of O3 in the atmosphere is about 300 Dobson Units

Not a “hole” but a region of depleted O3 over the Antarctic

Ozone is ‘thinning’ out

Any area where O3 < 220 DU is part of the O3 hole


The Antarctic Ozone “Hole”• Strong westerly circulation in Antarctic winter develops into a vortex• Isolates the air over Antarctica• Formation of Polar Stratospheric Clouds (PSCs) – comprised of

nitric acid trihydrate (HNO3.3H2O)

• Heterogeneous reactions on ice crystals alters the chemistry of the stratosphere

• Stratosphere in winter is chemically ‘preconditioned’ so that in the spring rapid depletion occurs

Why are Cl Concentrations So High?

During Polar winterSpecial vortex conditions

+Low temperature

+Denitrification of ClONO2

On PSC

Cl2

•Cl

sunlight

Stratospheric ‘containment vessel’ over S. pole


HCl + ClONO2 → Cl2 + HNO3

Ice gas gas ice

• The crystals persist in the polar season even in springtime due to low temperature in the lower stratosphere (-80 °C)

• Exposure of sunlight in the early spring initiates destruction of O3

Activation of Cl On Ice Particles • Cl resides in stable "reservoir"

species, HCl and ClONO2

• PSC’s ‘denitrify’ (remove NO2 from the atmosphere) as HNO3, which prevents the newly formed ClO from being converted back into ClONO2

Cl2 + hν → 2 Cl•2 Cl• + O3 → ClO• + O2


O3 and •ClO are anticorrelated


Step 1: •Cl + O3 → ClO• + O2

Step 2: 2ClO• → Cl-O-O-Cl

Step 2b: Cl-O-O-Cl → •Cl + ClOO

Step 2c: ClOO → •Cl + O2

Step 2 net: 2ClO• → ClOOCl + hν → 2 •Cl + O2

Step 1 and 2 represent Mech II:

ClO dimer formation

Occurs when [O] (needed for Mech I)is low

controls seasonNet: 2O3 → 3O2

One molecule of chlorine can degrade over 100,000 molecules of ozone before it is removed from the stratosphere or becomes part of an inactive compound

These inactive compounds, for example ClONO2, are collectively called 'reservoirs'. They hold chlorine in an inactive form but can release an active chlorine when struck by sunlight

Nearly 75% of the ozone depletion in the antartica occurs by this mechanism (Cl. As a catalyst)


• NASA FACTS http://ozonewatch.gsfc.nasa.gov/meteorology/index.html

Antarctic Hole Size and Minimum O3

http://ozonewatch.gsfc.nasa.gov/meteorology/index.html

http://ozonewatch.gsfc.nasa.gov/meteorology/index.html

Mid-lattitudes

• Slow, steady decline, of about 3% per decade during the past twenty years

• Enhanced by volcanic eruptions (Mt. Pinatubo)

Kerr, 2002


• A


Depletion of Stratospheric Ozone• Effects of International Control Measures

– 1985 UN Convention on the Protection of the Ozone Layer (Vienna Convention)(Adopted prior to hole being discovered)

– 1987 Montreal Protocol Final objective to eliminate ozone depleting substances– More than 160 countries– CFCs replaced with HCFCs



– HCFCs have shorter lifetimes than CFCs– HCFCs react with •OH– Growth rate of ozone depleting substances slowed



– Expected that total stratospheric chlorine load will peak in the early 21st century



– Global ozone losses and the Antarctic hole are predicted to recover around 2045

Cartoon

‘Air pollution is not stationary. It does not sit where it is formed. Rather, it visits other places, carried on the winds across state lines and national borders. Polluted air produced in Czechoslovakia migrates to Austria. Sulfur dioxide emitted by power plants in Ohio falls as acid rain in New York.’

`Because of this easy mobility, it is essential to understand the relationship between the motions of the atmosphere and the distribution of pollutants. We must not only determine the degree to which air quality is degraded, but also identify the sources and devise measures to control them.’

Turco, 2002

The AtmosphereAtmospheric Transport and Dispersion

Dispersion processes – diffusion, advection and convection

• Turbulent diffusion results in eddies

• Convection is driven by buoyancy

• Advection = wind

Turco, 2002


Wind Speed and Direction• Localized pollution is significantly affected by:

– Low wind speeds result in high pollution– Stability – unstable well mixed atmosphere

• Wind speed in the boundary layer drops overnight, picks up in early morning hours

• Emissions follow the same pattern• Boundary layer is shallower during the night and early morning• Much less volume for mixing pollutants

Results…


Wind Speed and Direction

• Highest pollution levels occur in the morning– Emissions increase– Stable atmosphere– Low wind speeds– Boundary layer is shallow

• Most at risk population are those down-wind of major sources or in path of major air masses


Wind Speed and Direction

• Most at risk population are those down-wind• Prevailing wind direction is important (in short-term), also long-range

transport over continental land masses (long-term, 1-3 d)


Atmospheric Stability - Vertical mixing depends on stability• Lapse rate

– Thermal buoyancy - ascending air expands and cools as pressure decreases

(a) ELR > ALR (b) ELR < ALR

Worksheet


Atmospheric Stability• Temperature Inversions

– Rapid radiative cooling of the ground at night leads to inversions– Heat is transferred from air to colder ground via conduction– “Radiation inversion” forms– Very stable as cooler dense air lies beneath warm air– Ground level emissions become trapped– Reversed by surface warming



– High pollution levels also due to lowered wind speeds– Surface layers become isolated from faster winds aloft– Surface air may become stagnant– Dew, frost or fog formation slows break-up of overnight inversions since solar

radiation is reflected away and does not warm surfacee.g. London Fog (1952), Donora Fog (1948)



– Subsidence inversion– Forms during anticyclonic conditions (High pressure at surface)– Subsiding air is compressed and warms– Develops elevated inversion layer– Air may is well mixed below inversion– SI provide ideal conditions for long-range transport of pollution


Atmospheric Stability• Los Angeles

– Geography and meteorology exaggerates pollution problems– Basin surrounded by San Gabriel Mountains to the east of the city– Leads to high incidence of inversions– Limits mixing of pollutants out of the city



– Sea breezes from cool water to warmer land – recirculation of pollutants– Subsiding air of the subtropical Pacific high pressure system is compressed

creating a warm layer of air aloft – subsidence inversion

Turco, 2002



– Some of the worst polluted cities of the world are situated in the Pacific basin region

e.g. Los Angeles, Sao Paulo, Mexico City, Jakarta (Mage et al, 1996)

The AtmosphereEmissions to Atmosphere and Air Quality

Natural Emissions

• Introduction– N2 (78.1 %), O2 (20.9 %), Ar (0.9 %), CO2 (0.035

%), variable: H2O (0.5 – 3 %)

– + Trace gases (many)– Many pollutants however have natural sources– Pollutant = presence of a contaminant above the

natural background concentration resulting in unacceptable adverse consequences to human health and/or the environment


Natural Emissions

• Introduction– Natural emissions may be comparable to human emissions on a global scale


Natural Emissions

• Introduction


Natural Emissions

• Sulfur Species (SO2, H2S, Dimethyl sulfide (DMS) etc.)

– Largest source of SO2 is volcanoes

– Largest source of H2S decay of organic matter

– All S sources oxidized to SO2 in atmosphere

– Sulfate aerosol component (from sea-salt spray) unknown

1 Tg = 1 x 106 tons


Natural Emissions

• Nitrogen Species (NOx, N2O, NH3, HNO3 etc.)

– Oxides of N produced by microorganisms, lightning and burning– Oxidation of ammonia in the troposphere

– Stratospheric HNO3

1 Tg = 1 x 106 tons

N-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


Natural Emissions

• Hydrocarbons (Methane, isoprene, α and β-pinene and other terpenes)– Anaerobic fermentation of organic material in rice paddies and wetlands,

ruminants– Total Methane (natural+manmade): 300-550 Tg yr-1

– Biogenic VOC’s 1150 Tg C yr-1 mostly from trees and shrubs


Anthropogenic Emissions of Primary Pollutants• CO and HC’s

– Internal Combustion Engines (ICE)– Incomplete combustion leads to high CO and HC emissions– Reduction: Introduction of CC technology and emissions limits on vehicles


Anthropogenic Emissions of Primary Pollutants• VOCs

– Another term for volatile HC’se.g. aldehydes, ketones, etc.

– Definition may exclude CH4 (NMVOC or NMHC)

– Sources: combustion, solvents, paints, evaporation of fuels– undergoes photochemical reactions


Anthropogenic Emissions of Primary Pollutants• NOx

– Main source is combusiton (some from production of nitric acid)– Thermal NOx (air derived) and fuel NOx (fuel derived)– ICE NOx is thermal derived; fossil fuel NOx is both air and thermal derived

– NO produced is oxidized to NO2 in the atmosphere


Anthropogenic Emissions of Primary Pollutants• SOx

– From fossil fuel burning (1-2 % wt in coal, 2-3 % in heavy fuel oils), sulfuric acid production and non-ferrous smelting

– Sulfur content of Diesel fuel higher than gasoline (which produces very little SO2)

– High S fuel oils flue gas emissions ~ 2000 ppm vs 1200 ppm for coal– Reduced using desulfurization technology


Anthropogenic Emissions of Primary Pollutants• Particulate Matter (PM)

– Sources: quarrying, MTR mining, digging, traffic– ‘Fugitive’ emissions – unintended/irregular– Soot - formed from incomplete combustion of volatile matter, measured as

‘smoke’– Particulates more of a problem with Diesel engines



– Smoke – PM assessed in terms of blackness or reflectance (not mass)– TSP – total suspended particulate matter– PM10 – inhalable fraction – measured using size selective inlet (50% efficiency

for 10 μm particles)– PM2.5 – respirable fraction – measured using size selective inlet (50% efficiency

for 2.5 μm particles)



– Total number of particles is dominated by ultrafine particles (0.01-0.05 μm), total mass is dominated by larger particles



– More significant relationship between PM2.5 and health effects than PM10– Increase the risk of cardiovascular diseases and mortality– Particles penetrate the lungs, blocking and irritating air passages– Ultra-fine particles may be potentially more toxic due to trace metals or organics

present in the particles


Anthropogenic Emissions of Primary Pollutants• Emissions Limits

– Industrial emissions controlled and authorizedby EPA


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Anthropogenic Emissions of Primary Pollutants• Emissions Inventories

– National emissions by country


Emissions of Primary Pollutants• AQ Standards

– NAAQS set by EPA

Question

Show that the US NO2 annual standard (0.053 ppm) is approximately twice the UK standard of 40 μg/m3

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

(0.053 ppm x Molar mass) / 24.0 = 0.102 mg m-3

= 102 μg m-3

UK NO2 std. is 40 μg/m3 which is approx. half this amount


Source: http://www.epa.gov/air/criteria.html, http://www.airquality.co.uk/archive/standards.php


Air Quality

• AQ Monitoring– Continuous monitoring for all

atmospheric pollutants– Diurnal patterns


Air Quality

• AQ Monitoring– Global Environment Monitoring System (GEMS)– Global assessment of levels and trends in urban air quality– 47 countries, 80 ‘Megacities’


Air Quality

• AQ Trends– Cities in developing world following same trends as industrialized nations– Pollution increases with population– Industrial development and energy use increase air pollution levels

The AtmosphereEmissions to Atmosphere and

Air Quality

Air Quality

• Vehicular Emissions – CO + HC’s– Largest input– Trend is down due to CC use– ‘Double hump’ – AM/PM travel


Air Quality

• Vehicular Emissions – NOx– Trend is down due to tighter emissions controls– Flattened out 21st century– NO similar pattern to CO (same source)

(NO2 is sec. pollutant)


Air Quality

• Vehicular Emissions – SOx– Small component– Mainly from coal combustion, oil and gas


Air Quality

• Vehicular Emissions – PM– 40 – 50 % from vehicles– Non-attainment of AQ standard in many large cities– Lack of info on health effects– Benefits of PM2.5 regulations


Air Quality

• Vehicular Emissions – Heavy Metals– Lead from tetra-alkyl lead anti-knock additives (octane improvers)– Now banned in developed countries– Significant reduction in airbourne lead– WHO limits exceeded in developing countries


Air Quality

• Vehicular Emissions – Toxic Organics– Present in vapor phase or adsorbed onto PM– Polynuclear aromatics, high mol. wt. HC’s found in soot – carcinogenic– Polychlorinated aromatics (PCB’s, Furans and Dioxins)

Photochemical Smog

Photochemical Smog

NASA October 2000

Lewis Structures of Free Radicals

• Free radicals possess an unpaired e-

• The unpaired e- is not in actual use as a bonding e-

• Carbon centered radical in which the carbon atom has one unpaired e- forms 3 bonds rather than four

• Oxygen forms one rather than 2 bonds:

•O – H

• A halogen forms no bonds:

Cl•

•H―C―H | H

The AtmosphereGas Phase Reactions and Photochemical Ozone

Gas Phase Chemistry in the Troposphere

• Atmospheric Photochemistry and Oxidation– Emission – dispersion – chemical reaction - deposition



• Atmospheric Photochemistry and Oxidation– Homogeneous (gas phase) and heterogeneous (aqueous droplet phase)

chemical reactions– Transformation of primary pollutants to secondary pollutants– Many reactions are photochemical (powered by the sun)



• Atmospheric Photochemistry and Oxidation– Photochemistry– Photons of light initiate chemical reactions that would other-wise not take place– Produce free radicals such as:

hydroxyl radical (•OH), hydroperoxy radical (HO2•) and methyl radical (•CH3)


Gas Phase Chemistry in the Troposphere• Atmospheric Photochemistry and Oxidation

e.g. Hydroxyl Radicals (•OH) produced in the environment serves as an oxidant (Conc. In the atmosphere is small 106-107 radicals per cm3 and it is very short-lived)

O3 + UV-B → O2 + O*

H2O + O* → 2 •OH

•OH radical is referred as Troposphere vacuum cleaner or detergent e.g. oxidation of various species (note O2 is not oxidant!)

Life-times of other species highly dependent on [•OH]

[•OH] drops quickly at night



• Atmospheric Photochemistry and Oxidation– Results of oxidation

CO + •OH → CO2 + H•

NO2 + •OH → HNO3

SO2 + •OH → H2SO3•

HC’s → aldehydes → CO(may be a number of intermediate steps)

When free radical is left over, hydroxyl radical is eventually regenerated (see table 10)



• Atmospheric Photochemistry and Oxidation

Methylperoxy radical

Methoxy radical



• Ozone– Present at 20-40 ppb natural background level– Secondary pollutant– Main source for •OH radicals

O3 + UV-B → O2 + O*

H2O + O* → 2 •OH

– (•OH also produced from photolysis of aldehydes, RCHO to produce H atoms, see Table 10)

H• + O2 + M → HO2• + M

HO2• + NO → •OH + NO2


Gas Phase Chemistry in the Troposphere• Ozone

– Photolysis of NO2 (by UV < 420 nm) produces excited O atoms

– First step is slow oxidation of NO by molecular oxygen:

O2 + 2NO → 2NO2

NO2 + UV → NO + O*

O2 + O* → O3

– Reversed by reaction: O3 + NO → O2 + NO2

– Net result: natural O3 in equilibrium with NOx and dependent on UV intensity

– Higher NO2 and UV leads to higher O3

– Transfer of O-atom from VOC produced radical species catalyzes the NO to NO2 reaction (see table 10)

– So higher VOC’s and higher NO2 leads to more O3 above background



• Ozone– Concentrations of O3 and NO2 vary diurnally and seasonally

AM peak in NO and HC followed by conversion to NO2 and rise of O3



• Ozone– Concentrations of O3 and NO2 vary diurnally and seasonally

– Greater in summer due to higher rate of photolysis

– HC chemistry is complex

– In addition to reactions with •OH and O2 (table 10) HC’s attacked by O* and by O3

– Also produce lachrymatory peroxyacetyl nitrate (PAN) and peroxybenzoyl nitrate (PBzN)

e.g. OH O2 NO2

CH3CHO → CH3CO• → CH3CO-O-O• → CH3CO-O-O-NO2


Summary of Photochemical smog formation steps:

1) Nitrogen oxides generate oxygen atoms

2) Oxygen atoms form ozone and hydroxyl radicals

3) Hydroxyl radicals generate hydrocarbon radicals

4) Hydrocarbon radicals form hydrocarbon peroxides

5) Hydrocarbon peroxides form aldehydes

6) Aldehydes form aldehyde peroxides

7) Aldehyde peroxides form peroxyacylnitrates

Urban atmospheres have been referred to as chemical soups!

The AtmosphereParticles and Acid Deposition

Particle Formation and Properties

• Particle Formation– HNO3 and H2SO4 formed in gas phase reactions absorbed into water droplets

– React with solid particulates to form sulfates and nitrates

e.g. CaCO3 converted to CaSO4

e.g. NaCl (sea-salt) converted to NaSO4 or NaNO3 with evolution of HCl gas

– Most common reactions with NH3:

NH3 + HCl NH⇌ 4Cl

NH3 + HNO3 NH⇌ 4NO3

NH3 + H2SO4 → NH4HSO4

NH3 + NH4HSO4 → (NH4)2SO4 (natural fertilizer!)



• Particle Formation– Initially small (<0.1 μm)– Grow by accumulation and coagulation– 0.1 – 2.0 remain airborne for days– 2-50 μm coarse



• Particle Composition– Urban area source

– Fine – mostly NH4SO4 and NO3- and carbon (elemental and organic material)

– Coarse – dominated by wind-blown dust (clays, silica, limestone) and sea-salt, much less C and SO4

2-

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 – more basic

Fine – more acidic

Fin

eC

oa

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1 nm



• Deliquescent Behavior– Particles comprising water soluble compounds of sulfates, nitrates and chlorides

will exist either as particles or liquid droplets depending on relative humidity– Particles are important starting points for formation of clouds – condensation

nuclei



• Optical Properties– Fine particles 0.1 – 2 μm scatter light, soot will absorb light– Reduce visibility– In clean air visibility can exceed 50 km (30 miles)– Polluted air severely reduces visibility– 200-300 μg m-3 will reduce visibility ro below 5 km (3 miles)


Droplets and Aqueous Phase Chemistry• Water droplets accumulate pollutants

– Adsorption of gases and/or particulates– Chemical reactions within the droplets

e.g. solution of SO2 results in SO32-, HSO3

- and H2SO3 mixture

– typical cloudwater pH HSO42- is dominant species

SO2 + H2O H⇌ + + HSO3-

– Most important oxidants are O3 and H2O2

(formed from two HO2 radicals)

– [H+] concentration controls the overall concentration of HSO3

- - pH dependent

H+ + SO42-


Droplets and Aqueous Phase Chemistry

• Water droplets accumulate pollutants

– O3 + HSO3- → H+ + SO4

2- + O2

– H2O2 + HSO3- → H+ + SO4

2- + H2O

– Acidity of the droplet has effect on the rate of SO2 oxidation

– At pH below 5 H2O2 dominates oxidation and above pH 5 ozone or other catalytic reactions (radicals) dominate the oxidation

– Difficult to distinguish between photochemical formation of H2SO4 followed by adsorption of acid gas into water droplets and this aqueous phase route

H+ + SO42-


Deposition Mechanisms

• Dry Deposition of Gases– Understanding rates and mechanisms of deposition is important for assessing

the environmental impact of pollution– Concentration (μg m-3) and rate of deposition (μg m-3 s-1)

Depositional velocity = deposition rate (μg m-2 s-1) = (m s-1)

concentration in air (μg m-3)

– Higher the ground level conc. The more rapid the deposition– Depositional velocity is a measure of the efficiency of the deposition process

(adsorption to a surface and downwind mixing of gases)

Depositional Fluxes

Deposition Mechanisms• Dry Deposition Rate of Gases

– Combine concentration measurement with meteorological data (depositional velocity)

– Collection of particles settling from air is dependent on surface type

– Depositional velocity is enhanced for moist surfaces

– Values around 2-5 mm s-1 for SO2, 1 mm s-1 for NO2 and 40 mm s-1 for HNO3

http://lepo.it.da.ut.ee/~olli/eutr/html/htmlBook_17.html



• Wet Deposition– Rainfall or snow– ‘rain out’ = in-cloud absorption followed by precipitation– ‘wash-out’ = below cloud absorption (as rain falls)– Inc. with rainfall– Rate of washout is lower for NOx due to reduced solubility in water



• Wet Deposition– Rainfall or snow– ‘rain out’ = in-cloud absorption followed by precipitation– ‘wash-out’ = below cloud absorption (as rain falls)– Inc. with rainfall – measured using scavenging ratio

(fractional loss of pollutant from the gas phase per second)– Rate of washout is lower for NOx due to reduced solubility in water

– For S dry:wet ratio is 40:60– For N dry:wet ratio is 27:73 (depends on sources)



• Deposition of Particles– Large particles with diameter > 10 μm settle out– Particles > 150 μm falling at over 1 m s-1 not considered air pollutants– Particles < 5 μm have low settling velocity, movement determined by turbulence– Particles 1 – 10 μm removed by impaction onto surfaces– Particles 0.1 – 1 μm removed slowly by dry deposition (1 mm s-1) – lower than

SO2

– Most likely removal route is rain-out following water vapor condensation and droplet growth in clouds


Acid Rain

• Rainwater Composition and Effects– Naturally acidic due to dissolved CO2

– Acid rain has pH ~5

Three particular effects:

(i) Acidification of lakes and streams – associated loss of wildlife

(ii) Damage to forests, e.g. Germanys Black Forest

(iii) Attack on stonework and buildings made of limestone


Acidity from the rain deteriorates soil by removing plant nutrients:

K+, Ca2+, Mg2+ attached to –ve sites on clay and organic matter

H+ trades places and is retained

‘Base cations’ K+, Ca2+, Mg2+ leached into subsoil or washed away

CaCO3(s) + H+ → Ca2+ + HCO3-(aq)

HCO3-(aq) + H+(aq) → H2CO3(aq) → CO2(g) + H2O(aq)

The Atmosphere Summary

• Global atmosphere

• Transport and dispersion

• Emissions to atmsophere and air quality

• Gas phase reactions and and ozone

• Particles and acid deposition

References

• Baird, C. (2005) Environmental Chemistry. W.H. Freeman.• Harrison, R.M. (2006) Introduction to Pollution Science. The Royal Society of

Chemistry, London.• Dunnivant, F.M. and Anders, E. (2006) A Basic Introduction to Pollutant Fate

and Transport: An Integrated Approach with Chemistry, Modeling, Risk Assessment, and Environmental Legislation. Wiley-Interscience, New Jersey.

• Turco (2002) Earth Under Siege. Oxford University Press.

air, water and land pollution chapter 2: the atmosphere copyright © 2009 by dbs

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