Sources, Types & Distribution of Air Pollution

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Chapter 2

• Sources, Types Distribution of Air Pollution

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Major Sources of Air Pollution

• The number of different types of pollution
  sources in modern society is almost endless.
• We look at only the most significant sources of
  air pollutants.
• Mobile (50 - 70), and stationary sources.
• 15-25 from heavy industrial stationary sources
  and as much as 25 from other stationary sources.

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Major Sources of Air Pollution
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Major Sources of Air Pollution

• The table is from USA. The total amount emitted
  in Australia will be far less
• A significant reduction in the amounts of CO and
  H/Cs when compared to the previous decade
• levels of other pollutants has been steady or
  shown only a slight increase

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Transportation Combustion Sources

• The most important transportation sources at
  present are major polluters
• Motor vehicles CO, CO2 H/Cs, NOx and small
  amounts of SOx
• Motor vehicle exhaust accounts for 40 of all H/C
  air pollutants and 90 of all NO2

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Transportation Combustion Sources

• Motor vehicles pollutants react to form more
  reactive (and dangerous) pollutants such as
  photochemical smog.
• diesel fuel a source of very dangerous H/Cs
  (PAHs).
• Pb has decreased in significance, and according
  to the latest national SoE report, is no longer
  considered a problem
• vehicles running on unleaded fuels emit lower
  levels of NOx and SOx

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Transportation Combustion Sources

• Aircraft and trains are less significant sources
  of pollution compared with road transport
  vehicles.
• Aircraft run on kerosene, burnt efficiently, but
  they fly very high in the atmosphere the
  pollutants most of which are H/Cs are spread
  and diluted in the upper atmosphere. DISCUSS NEW
  PROBLEM
• Trains mostly run on electricity contribute
  very little to air pollution (except CO2 some
  ozone)

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Stationary Combustion Sources

• Some of the more important sources include
• furnaces - and their combustion of carbonaceous
  fuels
• boilers
• ovens and dryers
• process systems which produce volatile chemicals,
  gases, etc.

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Stationary Combustion Sources

• Solvent evaporation (fugitive) from
• solvent-based materials
• leaking pipe joints
• maintenance work
• spills, unloading /loading procedures
• an important part of photochemical pollution

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Stack Emissions

• Emission of waste gases, fumes, vapours and
  smokes to the atmosphere are usually by the use
  of a smoke stack or chimney.
• stack emission becomes a plume in the atmosphere.
  
• The plume is an area of concentrated waste
  emissions that slowly become diluted with the
  other atmospheric gases.

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Stack Emissions

• How dilution happens depends on many factors
• Nature of the waste emission
• Toxic emissions need to be very dilute
• Volume of the waste
• Is emission constant or only at certain times in
  the process.
• Local topography
• Many cities located in areas surrounded by hills
  or mountains.
• low wind and cooler temperatures photochemical
  smog.

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Stack Emissions

• Prevailing climate
• direction of prevailing winds
• e.g. Queenstown, Tasmania
• The Existing Atmosphere
• In very polluted cities, more stack emissions not
  desirable.
• e.g. build power stations in the country away
  from NOx from cars

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Plume Behaviour

• Effects of plumes are considered local within 500
  metres of the stack, and regional beyond this.
• Mixing or dispersion of the waste gases and
  products into the atmosphere plume behaviour.

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Types of Plumes

• Fanning plumes
• Looping plumes
• Coning plumes
• Fumigating
• Lofting

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The Fanning Plume

• Fanning Plumes
• Require stable air and slow vertical movement of
  the emission
• common after calm clear nights
• temperature inversion limits the rise of the
  plume into the upper atmosphere

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The Fanning Plume

• creates a higher conc. of polluted air at lower
  levels
• exists for several hours
• Commonly seen from Eraring Power station

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Looping plumes

• Looping plumes
• Require windy conditions which cause the plume
  can swirl up and down
• common in the afternoon.
• Moderate and strong winds are formed on sunny
  days creating unstable conditions
• Exists for several hours.

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Coning plumes

• Coning plumes
• Require moderate winds and overcast days
• wider than it is deep, and is elliptical in shape
  
• exists for several hours.

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Fumigating plume

• Fumigating plume
• Is short-lived (fraction of an hour), but reaches
  the earth's surface.
• occur when the conditions move from stable to
  unstable
• A fanning plume develops overnight under stable
  conditions but as the day heats up, unstable air
  is produced

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Fumigating Plume

• Fumigating plume (cont)
• unstable air causes the plume to move up and down
  - can cause localised pollution.
• become looping or coning plumes as the air
  conditions stabilise.

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Lofting plume

• Lofting plume
• When plume is above the inversion layer (or there
  is no inversion), it becomes a lofting plume.
• Normal wind direction and speed will disperse the
  plume into the atmosphere without effect from
  ground warming or cooling.

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Stack emissions

• factors used to establish the amount of stack
  emission allowed, and its conc. to the atmosphere
  include
• smoke stack (chimney) height,
• local topography,
• temperature,
• emission rates,
• chemical reactivity, and
• existing air pollution problems
• wind allow rapid dispersal of pollutants.

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Fugitive Emissions

• Fugitive emissions are emissions which escape
  from a process rather than being discharged
• They often have serious consequences because
  their levels are not monitored and they are
  untreated when entering the atmosphere

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Fugitive Emissions

• There are many sources of fugitive emissions
  including
• industrial sources (particulate fluorides from
  aluminium smelters)
• small business (e.g. dry cleaning solvents)
• agriculture (e.g. dust from ploughing)
• natural sources (e.g. volcanoes, forest fires)

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Fugitive emissions

• Often the result of poor maintenance of plant and
  equipment
• Can be eliminated by SOPs that involve timed
  maintenance and quality control checks
• Some are almost impossible to control (e.g.
  natural sources)

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Types of air pollutants

• There are four types of air pollutants
• particulate pollutants and
• gaseous pollutants,
• odour and
• noise.

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Primary vs Secondary pollutants

• Not all of the pollutants found in the atmosphere
  are the direct result of emissions.
• Many pollutants arise from chemical reactions in
  the atmosphere with other substances or light
  (photochemical reactions).

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1 vs 2 pollutants

• Pollutant substances that are directly emitted
  into the atmosphere primary pollutants.
• Substances not directly emitted into the
  atmosphere, formed by chemical reactions in the
  atmosphere secondary pollutants.

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Particulate Pollutants

• Very small solid or liquid particles
• Individual particles may vary in size, geometry,
  chemical composition and physical properties
• May be of natural origin (pollen or sea spray) or
  man made (dust, fume and soot)

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Particulate Pollutants

• Provide a reactive surface for gases and vapours
  in the formation of secondary pollutants
• Particles also diffuse light reducing visibility
• Come from stack emissions, dusty processes,
  unsealed roads, construction work and many other
  sources

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Particulate Pollutants

• Dusts
• large solid particles
• Fume
• solid particles (metallic oxides) formed by
  condensation of vapours from a chemical reaction
  process or physical separation process

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Particulate Pollutants

• Mist
• liquid particles formed by condensation of
  vapours or chemical reaction.
• SO3 H2O H2SO4
• Smoke
• solid particles formed as a result of incomplete
  combustion of carbonaceous materials.
• Spray
• a liquid particle formed by the atomisation of a
  parent liquid.

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Particle Size

• Particles range in size from 0.005 - 500?m.
• Smallest of these are clusters of molecules
  whilst the largest are easily visible with the
  naked eye.
• Sizes given are not the physical size, but rather
  the aerodynamic equivalent diameter which relates
  the particle to the behaviour of an equivalent
  spherical particle.

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Particle Size

• Particles less than 1?m in diameter behave like
  gases (remain suspended, may coalesce, move in
  fluid streams),
• Larger particles act like solids (affected by
  gravity, dont stay suspended long, dont
  coalesce).
• Smaller particles generally derive from chemical
  reactions, whereas the larger particles (10?m or
  greater) are usually generated mechanically and
  tend to be basic.

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Particle Size

• Smaller particles most dangerous to health,
• In urban areas there is an approx. even
  distribution between fine and coarse particles,
  this is weather dependent.
• Calm conditions more fine particles than coarse,
• Fine particulate matter spread over much greater
  distances

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Particle behaviour in the atmosphere

• Particles can undergo many physical and chemical
  changes
• grow in size,
• absorb or desorb gases from their surfaces,
• change electrical charge,

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Particle behaviour in the atmosphere

• Particles can undergo many physical and chemical
  changes
• collide or adhere with other particles,
• absorb water.
• changes the particle size and affect its
  atmospheric lifetime.

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Total Suspended Particles (TSP)

• Most particles concentrated into three main size
  groups
• Larger particles around 10?m in size
• Smaller particles in size groups centred around
  0.2 and 0.02?m.

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TSP

• Only particles of lt10?m penetrate into the human
  lung
• Analyse air for only this fraction to estimate
  its potential danger to human health PM10
  sampling.
• Particles lt2.5?m in size can penetrate deep into
  the lung tissue and are especially dangerous
  PM2.5 sampling

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Organic Particulates

• PAH most significant
• Found on soot and dust particles, and are formed
  from smaller H/Cs at high temperatures (coal
  furnace effluent may contain 1mg/m3 of PAH
  cigarette smoke 0.1mg/m3)
• Urban atmospheres PAH levels 20 ug/m3 but is
  highly variable

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Lead Particulates

• Was the most serious atmospheric heavy-metal
  pollutant, but is no longer
• primary source was exhaust from vehicles

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Gaseous Pollutants

• CO, H/Cs, H2S, NOx, O3 and other oxidants, and
  SOx
• Measured in micrograms per cubic meter (ug/m3) or
  parts per million (ppm).
• 1 ppm 1 volume of gaseous pollutant
• 106 volumes of (pollutant air)

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Gaseous Particulates

• At 25C and 101.3 kPa the relationship between
  ppm and ug/m3 is
• ug/m3 ppm x molecular weight x 103
• 24.5

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Carbon Monoxide

• a colourless, odourless and tasteless gas.
• atmosphere has an avg. burden of around 530
  million tonnes (about 0.00001),
• avg. residence time of 36 to 100 days.
• Much of the CO in the atmosphere occurs naturally
  from volcanic eruptions, photolysis of methane
  and terpenes, decomposition of chlorophyll,
  forest fires and microbial action in oceans.

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Carbon Monoxide

• Anthropogenic sources transportation, solid
  waste disposal, agricultural burning, steel
  production, etc.
• emitted directly into the atmosphere through the
  inefficient combustion of fossil fuels.
• removed by reactions in the atmosphere which
  change it to CO2 and by absorption by plants and
  soil micro-organisms.

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Carbon Monoxide

• It is removed by reactions in the atmosphere
  which change it to CO2 and by absorption by
  plants and soil micro-organisms.
• In combustion, carbon is oxidised to CO2 in a two
  step process.
• 2C O2 2CO
• 2CO O2 2CO2

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Carbon Monoxide

• Typical conc's
• Background levels of CO tend to vary greatly
  depending on location.
• avg. global levels 0.2ppm.
• Peak conc's during autumn months when large
  volumes are generated by the decomposition of
  chlorophyll in leaves.
• In urban areas diurnal conc. pattern

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Carbon Monoxide

• The internal combustion engine contributes much
  of the anthropogenic CO (up to 90 in the Sydney
  region)
• Maximum levels of this gas tend to occur in
  congested urban areas at times when the maximum
  number of people are exposed, such as during rush
  hours.
• At such times, CO levels in the atmosphere may
  become as high as 50-100ppm.

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Carbon Dioxide

• Since the Clean Air Act in NSW in 1972 (and
  subsequent acts), the levels of CO in Sydney have
  dropped from an avg. of 25ppm to around 10ppm
• The accepted standard is 9ppm over an eight-hour
  period
• http//www.environment.nsw.gov.au/air/24hr.htm

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Carbon Monoxide

• Sinks
• CO is removed from the air mostly by conversion
  to CO2
• This may occur through aerial oxidation or
  through the action of soil micro-organisms
• The reason for very high conc's occurring in
  urban areas is that high emission rates are
  combined with a lack of soil

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Carbon Dioxide

• Carbon dioxide is produced when organic matter
  is
• combusted
• weathered
• biologically decomposed
• It is removed from the atmosphere by plants in
  photosynthesis and released by biological
  reactions

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Carbon Dioxide

• Over hundreds of millions of years CO2 has been
  withdrawn from the atmosphere and stored in coal,
  oil and natural gas.
• The intensive use of these fuels has resulted in
  significant CO2 emissions and an increase of
  atmospheric conc's
• Since 1958, CO2 values measured at Mauna Loa
  Observatory in Hawaii have increased from 310 to
  more than 350ppm.

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Carbon Dioxide

• Significant seasonal variations are also observed
  to occur in CO2 levels
• This seasonal variability appears to be
  associated with growing season photosynthetic
  needs and metabolic releases of CO2 in excess of
  plant uptake at the end of the growing season.

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Carbon Dioxide

• Not all CO2 emitted to the atmosphere from
  anthropogenic sources contributes to increased
  atmospheric levels.
• Because of its solubility in water, the oceans
  are a major sink for CO2, absorbing 50 of all
  man made emissions.
• The world's forests, particularly tropical
  forests, also serve as a sink.

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Carbon Dioxide

• As a thermal absorber (read greenhouse gas), CO2
  prevents some IR emissions from the Earth being
  radiated back to space
• Greenhouse Effect.

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Sulfur Compounds

• A variety of sulfur compounds are released to the
  atmosphere from both natural and anthropogenic
  sources
• The most important of these are the sulfur oxides
  (SOx) and hydrogen sulfide (H2S)
• Significant SOx emissions may occur from volcanic
  eruptions and other natural sources
• Man made emissions are responsible for much of
  the atmospheric emissions

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Sulfur Oxides

• These are produced by roasting metal sulfide ores
  and by combustion of fossil fuels containing
  appreciable inorganic sulfides and organic sulfur
  
• Of the four known sulfur oxides, only SO2 is
  found at appreciable levels in the atmosphere.

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Sulfur Oxides

• Sulfur trioxide (SO3) is emitted directly into
  the atmosphere in ore smelting and fossil fuel
  combustion and is produced by the oxidation of
  SO2.
• Because it has a high affinity for water, it is
  rapidly converted to sulfuric acid.

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Sulfur Oxides

• The formation of SO2, SO3, and sulfuric acid in
  the atmosphere is summarised in the following
  equations.
• S O2 SO2
• 2 SO2 O2 2SO3
• SO3 H2O H2SO4

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Sulfur Dioxide

• Sulfur dioxide may be directly absorbed by water
  bodies such as the oceans to form sulfurous acid.
  
• This is one of the sources of acid rain, which
  has dramatically affected the environment in
  Europe and North America.

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Sulfur Dioxide

• SO2 is an acidic colourless gas which may remain
  in the atmosphere for periods up to several weeks
• It can be detected by taste and odour in the
  conc. range of 0.38 - 1.15ppm
• Above 3 ppm, it has a pungent, irritating odour

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Sulfur Dioxide

• It is estimated that 65 million tonnes of SO2 per
  year enter the atmosphere as a result of man's
  activities, primarily from the combustion of
  fossil fuels.
• Of these, coal (and oil) is by far the greatest
  contributor, even in Australia

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Sulfur Dioxide

• Background levels of SO2 are very low, about 1ppb
  
• In urban areas maximum hourly conc's vary from
  less than 0.1 to more than 0.5ppm.

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Sulfur Dioxide

• Sinks
• SO2 is removed from the atmosphere by both dry
  and wet deposition processes.
• It is believed that plants are responsible for
  most SO2 removal that occurs by dry deposition.
• SO2 can also dissolve in water to form a dilute
  solution of sulfurous acid (H2SO3). This water
  can be in clouds, in rain droplets, or at the
  surface.

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Sulfur Dioxide

• A major sink process for SO2 is its gas-phase
  oxidation to H2SO4 and subsequent aerosol
  formation by nucleation or condensation
• Sulfuric acid will react with ammonia (NH3) to
  form a variety of salts

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Sulfur Dioxide

• About 30 of atmospheric SO2 is converted to
  sulfate aerosol
• Sulfate aerosols are removed from the atmosphere
  by dry and wet deposition processes.
• In dry deposition, aerosol particles settle out
  or impact on surfaces.
• In wet deposition, sulfate aerosol is removed
  from the atmosphere by forming rain droplets (in
  cloud) or being captured by falling rain droplets
  (below cloud).
• These removal processes are called rainout and
  washout.

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Hydrogen Sulfide

• H2S is a very toxic gas with a characteristic
  rotten egg odour.
• The principal concerns associated with H2S are
  its smell (foul) toxicity (same as HCN)

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Hydrogen Sulfide

• Background levels of H2S are approx. 0.05ppb
• Natural sources, which include anaerobic
  decomposition of organic matter, natural hot
  springs and volcanoes
• Anthropogenic sources include oil and gas
  extraction, petroleum refining, paper mills,
  rayon manufacture, and coke ovens

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Hydrogen Sulfide

• The major sink process for H2S is its atmospheric
  conversion to SO2.
• This SO2 is then removed from the atmosphere in
  the gas phase or as an aerosol by wet or dry
  deposition processes.

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Nitrogen Compounds

• There are five major gaseous forms of nitrogen in
  the atmosphere.
• These include
• molecular nitrogen (N2),
• ammonia (NH3),
• nitrous oxide (N2O),
• nitric oxide (NO), and
• nitrogen dioxide (NO2).

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Nitrogen Compounds

• N2 the major gas in the atmosphere.
• N2O present in unpolluted air due to microbial
  action
• NO and NO2 significant air pollutants
• NH3 not considered a significant man made
  pollutant, but enormous quantities generated
  through natural emissions.

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Elemental Nitrogen (N2)

• 78 of the air we breathe
• Relatively inert (unlike O2)
• Significant biological use by microbes

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Nitrous Oxide (N2O)

• colourless, slightly sweet, non-toxic gas.
• natural part of the atmosphere avg. conc.
  0.30ppm.
• used as anaesthetic in medicine and dentistry
  (laughing gas)
• product of natural soil processes, produced by
  anaerobic bacteria.
• photolytically dissociates in stratosphere to NO.

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Nitric Oxide (NO)

• colourless, odourless, tasteless, relatively
  non-toxic gas.
• produced naturally by
• anaerobic biological processes in soil and water,
  
• combustion processes and by photochemical
  destruction of N compounds in stratosphere.

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Nitric Oxide (NO)

• Major anthropogenic sources include
• automobile exhaust
• fossil fuel-fired electric generating stations
• industrial boilers
• incinerators
• home space heaters

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Nitric Oxide

• Nitric oxide is a product of high-temperature
  combustion.
• N2 O2 2NO

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Nitrogen Dioxide (NO2)

• light yellow to orange colour at low concs and
  brown at high concs.
• pungent, irritating odour , and extremely
  corrosive especially in wet environments
• toxic - can cause anoxia

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Nitrogen Dioxide (NO2)

• Some of the NO2 in air produced by direct
  oxidation of NO
• 2NO O2 2NO2

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Nitrogen Dioxide (NO2)

• At low atmospheric NO levels, oxidation is slow,
  accounts for lt25 of NO conversion
• Photochemical reactions involving O3, peroxy
  radical (RO2) and reactive hydrogen species such
  as OH?, HO2, H2O2, are primary means by which NO
  is converted to NO2 in the atmosphere.

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Nitrogen Dioxide (NO2)

• Other NO2 formation mechanisms
• NO O3 NO2 O2
• RO2 NO NO2 RO
• HO2 NO NO2 OH

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Nitrogen Dioxide

• Background concs of NO and NO2 are approx. 0.5
  and 1ppb respectively
• In urban areas, 1 hour avg. concs of NO may
  reach 1-2ppm, with max NO2 levels of approx.
  0.5ppm.
• decay of NO rapid as polluted air moves from
  urban to rural areas, with concs dropping to
  near background levels.

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Nitrogen Dioxide

• Atmospheric NO level related to transport/work
  cycle.
• Peak conc's observed in early morning hours, with
  a second smaller peak late in the day (See Figure
  2.8).
• Peak morning NO conc's followed several hours
  later by peak levels of NO2 produced by the
  chemical and photochemical oxidation of NO.

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Nitrogen Dioxide

• Atmospheric levels of NO and NO2 also show
  seasonal trends
• Emissions of NO greater during winter when there
  is increased use of heating fuels
• Since the conversion of NO to NO2 is related to
  solar intensity, higher NO2 levels occur on warm
  sunny days.

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Nitrogen Dioxide

• NOx in vehicle exhausts controlled by legislation
  as with CO
• catalytic converter in the exhaust system
  increases reduction of NOx to N2.
• Australian Design Rules limit emission of NOx
  from exhausts to 1.9g/km
• to maintain the levels in Sydney below the
  recommended standard of 0.16ppm (1 hour avg.).

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Figure 2.8 Levels of NO, NO2, and ozone on a
smoggy day in Los Angeles
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Nitrogen oxides (NOx)

• Sinks
• most significant sink for NO is conversion by
  both direct oxidation and photochemical processes
  to NO2
• A major sink process for NO2 is its conversion to
  nitric acid

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Nitrogen Oxides (NOx)

• OH? NO2 M HNO3 M
• M is an energy-absorbing species (generally O2 or
  N2). NO2 is also converted to nitric acid by
  night-time chemical reactions involving O3.
• NO2 O3 NO3? O2
• NO2 NO3? N2O5
• N2O5 H2O 2HNO3

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Nitrogen Oxides (NOx)

• NO3? is nitrate free radical
• key factor in night-time chemistry
• reaction product of NO2 and NO3 is dinitrogen
  pentoxide (N2O5) - reacts with water rapidly to
  produce HNO3

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Nitrogen Oxides (NOx)

• Some of the HNO3 in the atmosphere reacts with
  ammonia (NH3) or other alkaline species to form
  salts such as NH4NO3
• Nitrate aerosol is generally removed by the dry
  and wet deposition processes in much the same way
  as sulfate aerosol

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Ammonia (NH3)

• relatively unimportant man made pollutant
• Most comes from biological decomposition
• Background conc's vary from 1 to 20ppb
• The avg. atmospheric residence time is approx. 7
  days

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Organic Nitrates

• produced in the atmosphere by reaction of NOx and
  hydrocarbons
• Examples are peroxyacyl nitrates (PANs) and
  peroxybutylnitrates (PBNs).
• discussed in detail in photochemical smog section

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Hydrocarbons

• organic materials in the atmosphere.
• In the atmosphere simple hydrocarbons react with
  substances containing
• oxygen,
• nitrogen,
• sulfur,
• chlorine
• bromine
• even some metals (Pb)

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Hydrocarbons

• Atmospheric hydrocarbons exist in gas, liquid and
  solid phases
• gases and volatile liquids the most significant
  pollutants
• Solid hydrocarbons generally of higher MW and
  exist as condensed particles in atmospheric
  aerosols

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Hydrocarbons

• Methane (CH4) most common hydrocarbon in the
  atmosphere - formed from many natural sources
• termites,
• cows
• decomposition of organic matter
• It and the other alkanes found in the atmosphere
  are fairly un-reactive

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Hydrocarbons

• atmospheric hydrocarbons of most significance in
  terms of chemical reactivity are the alkenes
• highly reactive alkene hydrocarbons are formed
  naturally by plants (e.g. terpenes from citrus
  plants and eucalyptus haze)

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Hydrocarbons

• greatest source of non-methane hydrocarbons are
  motor vehicles and petroleum processing
• Alkenes are the major air pollutant responsible
  for photochemical smog and other gross oxidants
  in the atmosphere

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Hydrocarbons

• Once in the atmosphere non-methane H/Cs combine
  with O2 to form many different oxygenated H/Cs
  including
• alkanones
• alkanals
• alkanoic acids
• alkanols
• ethers

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Hydrocarbons

• Aromatic H/Cs not very reactive, but can react
  with other very reactive chemical oxidants to
  form toxic substances, such as
• benzo?pyrene
• poly-aromatic hydrocarbons (PAHs)

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Benzo?pyrene
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Hydrocarbons

• H/Cs emitted from a variety of natural and man
  made sources
• important pollutants because of their role in
  atmospheric photochemistry
• biological and geological processes release
  hydrocarbon compounds naturally

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Hydrocarbons

• Sources include
• plant and animal metabolism
• vaporisation of volatile oils from plant surfaces
• biological decomposition
• emission of volatiles from fossil fuel deposits

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Hydrocarbons

• Sinks
• most important sink processes are
• photochemical conversion of hydrocarbons to CO2
  and H2O or
• to soluble or condensable products such as
  dicarboxylic acids - a major component of
  photochemical aerosol.
• aerosols are removed from the atmosphere by both
  dry and wet deposition processes.

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Methane

• was initially considered an unimportant H/C
• Measurements of total H/C subtracted the conc. of
  CH4
• Hence ambient air quality standard for H/Cs is a
  non-methane hydrocarbons standard

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Methane

• recognised as one of the trace gases that may
  have significant greenhouse effect on global
  climate

108
Methane

• by far the most abundant H/C in the atmosphere,
  with a 1980 conc. of 1.65ppm.
• It has been increasing at a rate of 1.2-1.9 per
  year. The rate itself is also increasing.

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Methane
110
Ozone Photochemical Smog

• O3 a normal component of the atmosphere
• mostly in the middle stratosphere where it
  controls UV light reaching the planets surface
• here depletion of the substance results in air
  pollution loss of ozone is causing
  deterioration in quality of life

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Ozone

• not listed as a major primary air pollutant in
  the lower atmosphere
• high toxicity and involvement in production of
  other pollutants - very important atmospheric
  pollutant
• Over 90 of photochemical smog is ozone

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Ozone

• Sources
• Electrical discharges, e.g. lightning and
  electrical devices
• Light driven upper atmospheric chemical reactions
  e.g. reaction of molecular oxygen with oxygen
  atoms

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Ozone

• O2 O M O3 M
• In this reaction M is any third substance
  (usually O2 or N2) that removes the energy of the
  reaction and stabilises O. In the lower
  atmosphere (troposphere) the only significant
  source of atomic oxygen is the photolysis of NO2.
  
• NO2 h? NO O
• The reaction of O with O2 produces O3, which
  reacts immediately with NO to regenerate NO2.
• NO O3 NO2 O2

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Ozone

• All reactions proceed rapidly with approx. conc.
  of 20ppb
• atmospheric NO2/NO conc. ratios can be equal to 1
• Hence conc's of ozone remain low unless
  imbalances in the levels of NO2 or other
  alternate chemical reactants are available

115
Oh dear! The chemistry!

• We need to look closely at the chemistry we have
  seen thus far.

116
Photochemical Smog

• refers to an atmosphere laden with secondary
  pollutants that form in the presence of sunlight
  as a result of chemical reactions in the
  atmosphere
• arises in urban areas, where there is a heavy
  build-up of vehicle exhausts
• greatly exacerbated by weather conditions

117
Photochemical Smog

• normally primary air pollutants are dispersed
  over a large region or to the upper atmosphere
• A good prevailing wind is important for cities
  and large urban areas to reduce smog
• At certain times of the year, when wind is very
  still, primary pollutants build up over cities.
• Autumn worst for photochemical smog

118
Photochemical Smog
119
Photochemical Smog

• In autumn, days are sunny and warm, with cool
  nights
• Under still conditions, a warm inversion layer
  forms under a layer of higher cooler ai
• Large urban areas store heat, which provides the
  warmth for the inversion layer
• The inversion layer limits air mixing and
  dispersal trapping primary pollutants at lower
  altitudes over urban areas

120
Photochemical Smog
121
Photochemical Smog

• primary pollutants (NOx), and H/Cs trapped in
  the lower atmosphere are subjected to UV
  radiation from the sun photochemical smog
  forms.

122
Photochemical Smog

• products called gross photochemical oxidants,
  defined by their ability to oxidise I- to I2.
• They include
• ozone (O3)
• hydrogen peroxide (H2O2)
• organic peroxides (ROOR')
• organic hydroperoxides (ROOH) and
• by far the most serious to health, peroxyacyl
  nitrates (RCO3NO2), known as PAN's.

123
Photochemical Smog

• The key chemical reactants in the formation of
  photochemical smog are NOx and hydrocarbons.
• The reactions undergone by these substances in
  the atmosphere are many and varied.
• Many of the reaction mechanisms are not well
  understood.

124
Photochemical Smog

• In the lower atmosphere O3 conc's are often much
  higher than those that occur from NO2 photolysis
  alone.
• This is because there are chemical reactions that
  convert NO to NO2 without consuming O3.
• In polluted atmospheres, these changes in O3
  chemistry can be attributed to peroxy radicals
  (RO2) and other species produced by the oxidation
  of hydrocarbons as shown in the reactions below.

125
Photochemical Smog

• RO2 NO NO2 RO
• NO2 h? NO O
• O O2 M O3 M
• Net RO2 O2 h?
  RO O3

126
Photochemical Smog
127
Photochemical Smog

• The rate of O3 formation is closely related to
  the conc. of RO2.
• Peroxy radicals are produced when hydroxy
  radicals OH? and HOx react with hydrocarbons.
• Hydroxy radicals are produced by reactions
  involving the photolysis of O3, carbonyl
  compounds (mostly alkanals), and nitrous acid.

128
Photochemical Smog

• In polluted atmospheres, O3 conc's are directly
  related to
• the intensity of sunlight,
• NO2/NO ratios,
• the hydrocarbon type and conc's,
• and other pollutants, such as alkanals and CO,
  which react photochemically to produce RO2.
• The increase in NO2/NO ratios caused by
  atmospheric reactions involving RO2 results in
  significant increases in lower atmosphere O3
  levels.

129
Photochemical Smog

• summary of reactions in smog formation can be
  compressed into 4 stages.
• explains time variations in levels of H/Cs,
  ozone, NO2 and NO (see Figure 2.13).

130
Photochemical Smog

• 1. Primary photochemical reaction producing
  oxygen atoms
• NO2 h? NO O
• 2. Reactions involving oxygen species (M is an
  energy-absorbing third body)
• O O2 M O3 M
• NO O3 NO2 O2

131
Photochemical Smog

• Because last reaction is rapid, the conc. of O3
  remains low until that of NO falls to a low
  value.
• Automotive emissions of NO tend to keep O3 conc's
  low along freeways.

132
Photochemical Smog

• 3.Production of organic free radicals from
  hydrocarbons, RH
• O RH R? other products
• O3 RH R? and/or other products
• (R? is a free radical that may or may not contain
  oxygen.)

133
Photochemical Smog

• 4. Chain propagation, branching, and termination
  by a variety of reactions such as the following
• NO ROO? NO2 and/or other products
• NO2 R? products (e.g. PAN)

134
Photochemical Smog

• Some of the many other reactions which are known
  to occur in photochemical smog formation are
  listed below.
• O hydrocarbons HO?
• HO? O2 HO3?
• HO3? H alkanals, alkanones
• HO3? NO HO2? NO2
• HO3? O2 O3 HO2?
• HOx? NO2 PAN's

135
Photochemical Smog

• all H/Cs may form smog, but there are
  considerable differences in their reactivities
• methane, very slow to react, having an approx.
  atmospheric lifetime of more than 10 days
• branched alkenes and aromatic compounds the most
  reactive
• naturally-occurring alkenes (d-limonene) the most
  reactive compounds

136
Photochemical Smog

• With complex reactions and changing vehicle
  emissions during a day, conc's of the major
  components vary considerably over a 24-hour
  period.
• typical pattern of variations shown in fig2.13.

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138
Photochemical Smog

• morning rush hour begins, NO rises rapidly,
  followed by NO2.
• NO2 reacts with sunlight giving ozone and other
  oxidants
• H/C level increases in the morning, then
  decreases as compounds are oxidised to form PAN's
  and other species.

139
Photochemical Smog

• air mass moves toward an urban center, picks up
  NO, and H/Cs.
• OH? begins to degrade H/Cs, producing RO2 while
  O3 precursors peak and then decline with
  increasing downwind distance.
• Ozone conc's increase and are sustained over a
  period of 1-5 hours as more reactive alkene and
  aromatic H/Cs are depleted by photochemical
  reactions.

140
Photochemical Smog

• After 5-10 hours, moderately reactive H/Cs play
  a more important role in O3 production
• O3 levels decrease due to dilution, conversion of
  NO2 to HNO3, and surface adsorption
• At night no O3 produced

141
Photochemical Smog

• Under inversion layer, O3 may persist for 80 hrs.
  
• allows O3 to be transported over long distances
• At sunrise, inversion breaks up, bringing O3 and
  other products to the ground, where they mix with
  the pollutants held in by the inversion layer,
  and begin cycle all over again

142
Photochemical Smog

• In unpolluted atmospheres O3 conc's near ground
  are 10-20ppb (0.01-0.02ppm) during the warm
  months
• O3 conc's over landmasses with large motor
  vehicle numbers often well above this even at
  remote sites
• Los Angeles basin 1 hour conc's are 0.20-0.40ppm

143
Photochemical Smog

• warm, sunny NSW central coast means Sydney Basin
  has high photochemical smog production
• (NHMRC) ozone standard of 0.12ppm (1hr avg.)
  should not be exceeded on more than one day per
  year.

144
Photochemical Smog

• Ozone removed from the atmosphere by reactions
  with plants, soil, and man made materials
  (rubber)
• O3 produced in the atmosphere removed by chemical
  processes involving NOx
• principal scavenger of O3 is NO Night reactions
  with NO2 destroy O3

145
Chlorofluorocarbons (CFCs)

• What are they?
• halogenated H/C compounds used as refrigerant
  gases and propellants in aerosol cans

146
Chlorofluorocarbons (CFCs)

• unique because of their environmental persistence
• examples
• DDT, Chlordane, Dieldrin, and Aldrin (pesticides)
  
• polyhalogenated biphenyls (PCBs, PBBs) solvents
  and fire retardants
• dichloromethane, trichloroethene,
  perchloroethene, tetrachloroethene, and
  tetrachloromethane (solvents)
• CFCs - refrigerants, degreasing agents, foaming
  agents, aerosol propellants

147
Chlorofluorocarbons (CFCs)

• serious atmospheric threat because of their great
  stability - leads to damage the O3 layer
• Also absorb IR energy and are greenhouse gases

148
Chlorofluorocarbons (CFCs)

• most commonly used (most common atmospheric
  contaminants) are
• Trichlorofluoromethane (CFC13)
• Dichlorodifluoromethane (CF2C12),
• Trichlorotrifluoroethane (C2C13F3).

149
Chlorofluorocarbons (CFCs)

• no sink in the lower atmosphere - CFC conc's
  increase with time
• For CFC-11 and CFC-12, atmospheric lifetimes are
  75 and 111 years, respectively

150
Chlorofluorocarbons (CFCs)

• Naming CFCs
• The decoding system for CFC-01234a is
• 0 Number of double bonds (omitted if zero)
• 1 Carbon atoms -1 (omitted if zero)
• 2 Hydrogen atoms 1
• 3 Fluorine atoms
• 4 Replaced by Bromine ("B" prefix added)

151
Fluoride

• Aluminium smelters major source of both gaseous
  and particulate fluorides, as are
• brick and glass works
• some smelters
• steel plants and
• coal fired power stations
• Fluoride is a localised problem

152
Minor Gaseous Pollutants

• Hydrogen sulfide
• odour
• noise

153
Odour as air pollution

• odour pollution increasing importance
• from a regulatory point of view, seen as a
  welfare not a health issue this is changing

154
Odour as air pollution

• odour is response to the inhalation of a chemical
  substance - cannot yet be reliably measured by
  chemically
• sensory attributes of odours measured by exposing
  individuals under controlled conditions

155
Odour as air pollution

• Elements of odour subject to measurement are
• detectability
• intensity
• character (quality)
• hedonic tone (pleasantness, unpleasantness)

156
Odour as air pollution

• limit of detection odour threshold
  characterized in 2 ways
• detectable difference from the background
• first conc. at which an observer can positively
  identify quality of odour

157
Odour as air pollution

• characters of a variety of selected chemicals
  summarised in Table 2.2
• For example, dimethylamine is described as fishy,
  phenol as medicinal, 1,4-dihydroxybenzene
  (paracresol) as tar-like.

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159
Odour as air pollution

• olfactory response to an odourant decreases as
  the odourant conc. decreases (nonlinear)
• responses to malodours include
• Nausea and vomiting
• Headaches and other sensory disturbance
• Coughing respiratory ailments
• Depression

160
Odour as air pollution

• Odour Problems
• Bad odours generate complaints to regulatory
  agencies more than any other form of air
  pollution
• A new area of management that deals with odour
  and noise is called modelling

161
Odour as air pollution

• Odour Problems
• Likely sources of bad odours include
• soap-making facilities
• petrochemical plants and refineries
• pulp and paper mills
• food-processing plants
• sewage treatment plants
• abattoirs

162
Odour as air pollution

• Bad odours associated with
• amines
• sulfur gases (e.g. H2S)
• phenol, ammonia etc
• Hydrocarbons
• And many more

163
Odour and the Law

• Legal/Regulated aspects of odour
• Local Council