air, water and land pollution chapter 2: the atmosphere copyright © 2009 by dbs
TRANSCRIPT
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)
The AtmosphereThe Global Atmosphere
Dominant, permanent gases Variable, trace gases
The AtmosphereThe Global Atmosphere
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)
The AtmosphereThe Global Atmosphere
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)
The AtmosphereThe Global Atmosphere
Atmospheric Structure• Due to air pressure 99 % of the total mass of the
atmosphere resides in the troposphere and stratosphere
The AtmosphereThe Global Atmosphere
Atm
osp
he
ric
Str
uc
ture
The AtmosphereThe Global Atmosphere
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)
The AtmosphereThe Global Atmosphere
Stratosphere• Dry and stable
– less turbulence• O3 rich• UV induced
photochemistry dominates• Inversion –
temperature increases with height
• Earth’s ‘sunscreen’
Turco, 2002
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
• 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
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
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)
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
11
Radiation emitted from the ground lies in infra-red region…
The AtmosphereThe Global Atmosphere
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)
The AtmosphereThe Global Atmosphere
% = pph = ppm / 10,000ppm / 10,0000.0350 = 350/10,000
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
• 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
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
Depletion of Stratospheric Ozone• The Ozone Layer
– 90 % total atmospheric O3 in stratosphere
– Filters UV from the sun removing most of the high energy UV below 300 nm
The AtmosphereThe Global Atmosphere
Depletion of Stratospheric Ozone• The Ozone Layer
– Depletion of stratospheric O3 leads to larger UV flux at Earth’s surface and increased risk of cancer
– Disruption of biological communities
The AtmosphereThe Global Atmosphere
Depletion of Stratospheric Ozone• The Ozone Layer
– Chlorofluorocarbon (CFC) catalytic destruction of stratospheric O3
CFCl3 + hν → CFCl2 • + Cl•
Cl• + O3 → ClO • + O2
ClO• + O• → Cl • + O2
O3 + O• → 2O2
1974
The AtmosphereThe Global Atmosphere
Depletion of Stratospheric Ozone• The Ozone Layer
– 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
The AtmosphereThe Global Atmosphere
1950 – 50,000 tonnes
1976 – 725,000 tonnes
90% of emissions already in the atmosphere, remainder emitted when equipment is discarded
The AtmosphereThe Global Atmosphere
CFC11 – 0 to 268 ppt
Atmospheric concentration is small – 1 ppb
CFC12 – 0 to 533 ppt
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
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…
The AtmosphereThe Global Atmosphere
• 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 AtmosphereThe Global Atmosphere
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)
The AtmosphereThe Global Atmosphere
• 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 AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
O3 and •ClO are anticorrelated
The AtmosphereThe Global Atmosphere
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)
The AtmosphereThe Global Atmosphere
• NASA FACTS http://ozonewatch.gsfc.nasa.gov/meteorology/index.html
Antarctic Hole Size and Minimum O3
Mid-lattitudes
• Slow, steady decline, of about 3% per decade during the past twenty years
• Enhanced by volcanic eruptions (Mt. Pinatubo)
Kerr, 2002
The AtmosphereThe Global Atmosphere
• A
The AtmosphereThe Global Atmosphere
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
The AtmosphereThe Global Atmosphere
The AtmosphereThe Global Atmosphere
Depletion of Stratospheric Ozone• Effects of International Control Measures
– HCFCs have shorter lifetimes than CFCs– HCFCs react with •OH– Growth rate of ozone depleting substances slowed
The AtmosphereThe Global Atmosphere
Depletion of Stratospheric Ozone• Effects of International Control Measures
– Expected that total stratospheric chlorine load will peak in the early 21st century
The AtmosphereThe Global Atmosphere
Depletion of Stratospheric Ozone• Effects of International Control Measures
– 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
The AtmosphereAtmospheric Transport and Dispersion
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…
The AtmosphereAtmospheric Transport and Dispersion
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
The AtmosphereAtmospheric Transport and Dispersion
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)
The AtmosphereAtmospheric Transport and Dispersion
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
The AtmosphereAtmospheric Transport and Dispersion
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
The AtmosphereAtmospheric Transport and Dispersion
Atmospheric Stability• Temperature Inversions
– 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)
The AtmosphereAtmospheric Transport and Dispersion
Atmospheric Stability• Temperature Inversions
– 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
The AtmosphereAtmospheric Transport and Dispersion
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
The AtmosphereAtmospheric Transport and Dispersion
Atmospheric Stability• Los Angeles
– 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
The AtmosphereAtmospheric Transport and Dispersion
Atmospheric Stability• Los Angeles
– 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
The AtmosphereEmissions to Atmosphere and Air Quality
Natural Emissions
• Introduction– Natural emissions may be comparable to human emissions on a global scale
The AtmosphereEmissions to Atmosphere and Air Quality
Natural Emissions
• Introduction
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereEmissions to Atmosphere and Air Quality
Anthropogenic Emissions of Primary Pollutants• Particulate Matter (PM)
– 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)
The AtmosphereEmissions to Atmosphere and Air Quality
Anthropogenic Emissions of Primary Pollutants• Particulate Matter (PM)
– Total number of particles is dominated by ultrafine particles (0.01-0.05 μm), total mass is dominated by larger particles
The AtmosphereEmissions to Atmosphere and Air Quality
Anthropogenic Emissions of Primary Pollutants• Particulate Matter (PM)
– 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
The AtmosphereEmissions to Atmosphere and Air Quality
Anthropogenic Emissions of Primary Pollutants• Emissions Limits
– Industrial emissions controlled and authorizedby EPA
The AtmosphereEmissions to Atmosphere and Air Quality
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The AtmosphereEmissions to Atmosphere and Air Quality
Anthropogenic Emissions of Primary Pollutants• Emissions Inventories
– National emissions by country
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereEmissions to Atmosphere and Air Quality
Source: http://www.epa.gov/air/criteria.html, http://www.airquality.co.uk/archive/standards.php
The AtmosphereEmissions to Atmosphere and Air Quality
Air Quality
• AQ Monitoring– Continuous monitoring for all
atmospheric pollutants– Diurnal patterns
The AtmosphereEmissions to Atmosphere and Air Quality
Air Quality
• AQ Monitoring– Global Environment Monitoring System (GEMS)– Global assessment of levels and trends in urban air quality– 47 countries, 80 ‘Megacities’
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereEmissions to Atmosphere and Air Quality
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)
The AtmosphereEmissions to Atmosphere and Air Quality
Air Quality
• Vehicular Emissions – SOx– Small component– Mainly from coal combustion, oil and gas
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereEmissions to Atmosphere and Air Quality
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
The AtmosphereGas Phase Reactions and Photochemical Ozone
Gas Phase Chemistry in the Troposphere
• 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)
The AtmosphereGas Phase Reactions and Photochemical Ozone
Gas Phase Chemistry in the Troposphere
• 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)
The AtmosphereGas Phase Reactions and Photochemical Ozone
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
The AtmosphereGas Phase Reactions and Photochemical Ozone
Gas Phase Chemistry in the Troposphere
• 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)
The AtmosphereGas Phase Reactions and Photochemical Ozone
Gas Phase Chemistry in the Troposphere
• Atmospheric Photochemistry and Oxidation
Methylperoxy radical
Methoxy radical
The AtmosphereGas Phase Reactions and Photochemical Ozone
Gas Phase Chemistry in the Troposphere
• 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
The AtmosphereGas Phase Reactions and Photochemical Ozone
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
The AtmosphereGas Phase Reactions and Photochemical Ozone
Gas Phase Chemistry in the Troposphere
• 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
The AtmosphereGas Phase Reactions and Photochemical Ozone
Gas Phase Chemistry in the Troposphere
• 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
The AtmosphereGas Phase Reactions and Photochemical Ozone
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!)
The AtmosphereParticles and Acid Deposition
Particle Formation and Properties
• Particle Formation– Initially small (<0.1 μm)– Grow by accumulation and coagulation– 0.1 – 2.0 remain airborne for days– 2-50 μm coarse
The AtmosphereParticles and Acid Deposition
Particle Formation and Properties
• 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
rse
1 nm
The AtmosphereParticles and Acid Deposition
Particle Formation and Properties
• 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
The AtmosphereParticles and Acid Deposition
Particle Formation and Properties
• 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)
The AtmosphereParticles and Acid Deposition
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-
The AtmosphereParticles and Acid Deposition
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-
The AtmosphereParticles and Acid Deposition
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
The AtmosphereParticles and Acid Deposition
Deposition Mechanisms
• 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
The AtmosphereParticles and Acid Deposition
Deposition Mechanisms
• 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)
The AtmosphereParticles and Acid Deposition
Deposition Mechanisms
• 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
The AtmosphereParticles and Acid Deposition
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
The AtmosphereParticles and Acid Deposition
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.