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 Atmosphere
The Global Atmosphere
Atmospheric Structure
• Troposphere and stratosphere
– Most of atmosphere < 100 km (homosphere)
The Atmosphere
The Global Atmosphere
Dominant, permanent gases
Variable, trace gases
The Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The Global Atmosphere
Atmospheric Structure
•
Due to air pressure 99 % of the total mass of the
atmosphere resides in the troposphere and stratosphere
The Atmosphere
Atmospheric Structure
The Global Atmosphere
The Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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
Snow
Albedo
0.8-0.95
Dry sand
0.4
Forests
0.2
Calm sea
water
0.05
Asphalt
0.05
Smith, 2001
The Atmosphere
The Global Atmosphere
Radiation emitted from the ground lies in infra-red region…
11
The Atmosphere
The 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 Atmosphere
% = pph = ppm / 10,000
ppm / 10,000
0.0350 = 350/10,000
The Global Atmosphere
The Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The Global Atmosphere
•
F = Fin – Fout = 0
at equilibrium
Fin = Fout
Forcing: GH gases
reduce amount of heat
radiated to space
– Climate system
adjusts
– Earth’s surface
warms to
compensate –
maintain
equilibrium
Houghton, 2004
The Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The Global Atmosphere
1976 – 725,000 tonnes
1950 – 50,000 tonnes
90% of
emissions
already in the
atmosphere,
remainder
emitted when
equipment is
discarded
The Atmosphere
The Global Atmosphere
Atmospheric
concentration is small
– 1 ppb
CFC12 – 0 to 533 ppt
CFC11 – 0 to 268 ppt
The Atmosphere
The 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 Theory
Above stratosphere oxygen absorbs UV-C and exists as O atoms
O2 + hν → O• + O•
ΔH = 495 kJ/mol (<241 nm)
(1)
+ O3
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•
O• + O3 → 2O2
-
(<320 nm)
ΔH = -390 kJ/mol
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
(3)
- O3
(4)
The Atmosphere
The 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 Atmosphere
The 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
sunlight
•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
The Atmosphere
The 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 Atmosphere
The 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
Any area where
O3 < 220 DU is part of
the O3 hole
Ozone is ‘thinning’ out
Not a “hole” but a region of
depleted O3 over the Antarctic
The Atmosphere
The 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 winter
Special vortex conditions
+
Low temperature
+
Denitrification of ClONO2
On PSC
Cl2
sunlight
Stratospheric ‘containment vessel’ over S. pole
•Cl
The Atmosphere
The Global Atmosphere
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
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
Cl2 + hν → 2 Cl•
2 Cl• + O3 → ClO• + O2
The Atmosphere
The Global Atmosphere
O3 and •ClO are anticorrelated
The Atmosphere
The Global Atmosphere
Step 1
and 2
represent
Mech II:
ClO
dimer
formation
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
Occurs when [O]
(needed for Mech I)
is low
Step 2 net: 2ClO• → ClOOCl + hν → 2 •Cl + O2
Net: 2O3 → 3O2
controls season
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 Atmosphere
The 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 Atmosphere
The Global Atmosphere
• A
The Atmosphere
The 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 Atmosphere
The Global Atmosphere
The Atmosphere
The 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 Atmosphere
The 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 Atmosphere
The 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 Atmosphere
Atmospheric Transport and Dispersion
Dispersion processes – diffusion, advection and convection
•
•
•
Turbulent diffusion results in eddies
Convection is driven by buoyancy
Advection = wind
Turco, 2002
The Atmosphere
Atmospheric 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 Atmosphere
Atmospheric 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 Atmosphere
Atmospheric 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 Atmosphere
Atmospheric 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 Atmosphere
Atmospheric 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 Atmosphere
Atmospheric 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 surface
e.g. London Fog (1952), Donora Fog (1948)
The Atmosphere
Atmospheric 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 Atmosphere
Atmospheric 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 Atmosphere
Atmospheric 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 Atmosphere
Atmospheric 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 Atmosphere
Emissions 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 Atmosphere
Emissions to Atmosphere and Air Quality
Natural Emissions
• Introduction
– Natural emissions may be comparable to human emissions on a global scale
The Atmosphere
Emissions to Atmosphere and Air Quality
Natural Emissions
• Introduction
The Atmosphere
Emissions 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 Atmosphere
Emissions 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)
NOx Cycle
SINK
SOURCE
HNO3 (inert)
N2O + O → 2NO
i.e. inactive until
transported
RAIN OUT
RESERVOIR
Removal negligable (few
reactions except O)
N2O, N2
Agriculture NO3-, NO2-
The Atmosphere
Emissions 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 Atmosphere
Emissions 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 Atmosphere
Emissions to Atmosphere and Air Quality
Anthropogenic Emissions of Primary Pollutants
•
VOCs
– Another term for volatile HC’s
e.g. aldehydes, ketones, etc.
– Definition may exclude CH4 (NMVOC or NMHC)
– Sources: combustion, solvents, paints, evaporation of fuels
– undergoes photochemical reactions
The Atmosphere
Emissions 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 Atmosphere
Emissions 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 Atmosphere
Emissions 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 Atmosphere
Emissions 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 Atmosphere
Emissions 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 Atmosphere
Emissions 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 Atmosphere
Emissions to Atmosphere and Air Quality
Anthropogenic Emissions of Primary Pollutants
•
Emissions Limits
– Industrial emissions controlled and authorizedby EPA
•
Emissions Inventories
– UK breakdown by sector
Anthropogenic Emissions of Primary Pollutants
The Atmosphere
Emissions to Atmosphere and Air Quality
The Atmosphere
Emissions to Atmosphere and Air Quality
Anthropogenic Emissions of Primary Pollutants
•
Emissions Inventories
– National emissions by country
The Atmosphere
Emissions 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 24.0
Molar 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 Atmosphere
Emissions to Atmosphere and Air Quality
Source: http://www.epa.gov/air/criteria.html,
http://www.airquality.co.uk/archive/standards.php
The Atmosphere
Emissions to Atmosphere and Air Quality
Air Quality
• AQ Monitoring
– Continuous monitoring for all
atmospheric pollutants
– Diurnal patterns
The Atmosphere
Emissions 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 Atmosphere
Emissions 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 Atmosphere
Emissions 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 Atmosphere
Emissions 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 Atmosphere
Emissions to Atmosphere and Air Quality
Air Quality
• Vehicular Emissions – SOx
– Small component
– Mainly from coal combustion, oil and gas
The Atmosphere
Emissions 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 Atmosphere
Emissions 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 Atmosphere
Emissions 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
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Free radicals possess an unpaired eThe unpaired e- is not in actual use as a bonding eCarbon centered radical in which the carbon atom has one unpaired e- forms 3
bonds rather than four
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H―C―H
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H
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Oxygen forms one rather than 2 bonds:
•O – H
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A halogen forms no bonds:
Cl•
The Atmosphere
Gas Phase Reactions and Photochemical Ozone
Gas Phase Chemistry in the Troposphere
• Atmospheric Photochemistry and Oxidation
– Emission – dispersion – chemical reaction - deposition
The Atmosphere
Gas 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 Atmosphere
Gas 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 Atmosphere
Gas 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 shortlived)
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 Atmosphere
Gas 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 Atmosphere
Gas Phase Reactions and Photochemical Ozone
Gas Phase Chemistry in the Troposphere
• Atmospheric Photochemistry and Oxidation
Methylperoxy radical
Methoxy radical
The Atmosphere
Gas 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 Atmosphere
Gas 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
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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 Atmosphere
Gas 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 Atmosphere
Gas 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 Atmosphere
Gas 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 Atmosphere
Particles 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 ⇌ NH4Cl
NH3 + HNO3 ⇌ NH4NO3
NH3 + H2SO4 → NH4HSO4
NH3 + NH4HSO4 → (NH4)2SO4 (natural fertilizer!)
The Atmosphere
Particles and Acid Deposition
Particle Formation and Properties
• Particle Formation
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Initially small (<0.1 μm)
Grow by accumulation and coagulation
0.1 – 2.0 remain airborne for days
2-50 μm coarse
The Atmosphere
Particles 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 SO42-
Sizes of Common Airborne Particles
e.g NH4Cl,
SO42- / NO3- salts
Mineral dust from
weathering of rocks
and soils
Fine
Coarse
1 nm
Natural: forest fires,
volcanoes etc.
Man-made: fossil-fuel
combustion, industry
Chemical composition
can be used to ID source
Course – more basic
Fine – more acidic
The Atmosphere
Particles 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 Atmosphere
Particles and Acid Deposition
Particle Formation and Properties
• Optical Properties
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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 Atmosphere
Particles 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 Atmosphere
Particles and Acid Deposition
H+ + SO42-
Droplets and Aqueous Phase Chemistry
• Water droplets accumulate pollutants
– O3 + HSO3- → H+ + SO42- + O2
– H2O2 + HSO3- → H+ + SO42- + 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
The Atmosphere
Particles 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
concentration in air
(μg m-2 s-1) = (m s-1)
(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 Atmosphere
Particles and Acid Deposition
Deposition Mechanisms
• Wet Deposition
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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 Atmosphere
Particles and Acid Deposition
Deposition Mechanisms
• Wet Deposition
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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 Atmosphere
Particles and Acid Deposition
Deposition Mechanisms
• Deposition of Particles
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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 Atmosphere
Particles 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 Atmosphere
Particles and Acid Deposition
CaCO3(s) + H+ → Ca2+ + HCO3-(aq)
HCO3-(aq) + H+(aq) → H2CO3(aq) → CO2(g) + H2O(aq)
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
The Atmosphere
Summary
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Global atmosphere
Transport and dispersion
Emissions to atmsophere and air quality
Gas phase reactions and and ozone
Particles and acid deposition
References
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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.