Acid Rain and Photochemical Smog

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Acid Rain and Photochemical Smog Powered By Docstoc
					    Choi Man Ho
Chan Kwun Keung

         Acid Rain and
      Photochemical Smog
         What is Acid Rain?
• Unpolluted rainwater is slightly acidic
  (pH 5.6) because of the carbon dioxide from
  air dissolved in it. CO2 + H2O => H2CO3
• Rainwater with a pH as low as 2.5 has been
  recorded in some parts of the world. They
  are commonly known as acid rain.
         What is Acid Rain?
• "Acid rain" is also a broad term used to
  describe several ways that acids fall out of
  the atmosphere.
• A more precise term is acid deposition,
  which has two parts: wet and dry.
• Wet deposition: acidic rain, fog, and snow.
• Dry deposition: acidic gases and particles
         Causes of Acid Rain
• Sulphur dioxide (SO2) and nitrogen
  oxides (NOx) are the primary causes of acid
• Acid rain occurs when these gases react in
  the atmosphere with water, oxygen, and
  other chemicals to form various acidic
• Sunlight increases the rate of most of
  these reactions. The result is a mild solution
  of sulfuric acid and nitric acid.
Causes of Acid Rain
Causes of Acid Rain
   Formation of Sulphuric Acid
• Sulphur dioxide (from the burning of fossil
  fuels in power plants and industrial plants)
  is the primary cause of acid rain.
• In the atmosphere, sulphur dioxide is slowly
  oxidized to sulphur trioxide which
  dissolves readily in water droplets to form
  sulphuric (vi) acid.
• 2SO2+O2 => 2SO3
• SO3 + H2O => H2SO4
   Formation of Sulphuric Acid
• The actual pathways are more complex.
• The formation of SO3 from SO2 is
  influenced by the prevailing atmospheric
  conditions: sunlight, temperature,
  humidity, and the presence of
  hydrocarbons, nitrogen oxides and
  particulates in the atmosphere.
   Formation of Sulphuric Acid
• Sulphuric (iv) acid is also formed when
  SO2 dissolves in rainwater:
• SO2 + H2O => H2SO3
Formation of Nitric Acid (HNO3)
• Nitrogen oxides (from the burning of fossil
  fuels in automobiles and power plants) also
  cause the formation of acid rain.
• When released to the atmosphere, nitrogen
  monoxide combines with atmospheric
  oxygen to form nitrogen dioxide:
• 2NO + O2 --> 2NO2
Formation of Nitric Acid (HNO3)
• In a series of complex reaction, nitrogen
  dioxide combines with oxygen and water
  vapour to form nitric (v) acid.
• 4NO2 + 2H2O + O2 --> 4HNO3
      Environmental Problems
• (i)In water of pH less than 4.5, calcium
  metabolism in fresh water fish will be
  affected, leading to poor health and
  stunted growth.
• As a result, diversity and population of
  some fresh water species will be reduced.
      Environmental Problems
• (ii)In soil of pH less than 4.5, absorption of
  cations by plants will be affected, resulting
  in death of plants.
• (iii)Inflow of acidic water containing
  poisonous metal ions from soil will kill the
  fish and water plants
• (iv)Acid rain corrodes metals and
  accelerates the deterioration of building,
  rock and statue.
How do we Measure Acid Rain?
• Acid rain's pH (and the chemicals that cause
  acid rain) is monitored by two networks,
  both supported by EPA.
• The National Atmospheric Deposition
  Program measures wet deposition, and its
  Web site features maps of rainfall pH and
  other important precipitation chemistry
How do we Measure Acid Rain?
• The Clean Air Status and Trends
  Network (CASTNET) measures dry
  deposition. Its Web site features
  information about the data it collects, the
  measuring sites, and the kinds of equipment
  it uses.
 How do we Reduce Acid Rain?
• Understand acid deposition's causes and
• Clean up smokestacks and exhaust pipes
• Use alternative energy sources
• Restore a damaged environment
• Be green
 How do we Reduce Acid Rain?
• EPA's Acid Rain Program limits, or
  "caps," sulphur dioxide (SO2) emissions
  from power plants at 8.95 million tons
  annually, allows those plants to trade SO2
  allowances, and reduces nitrogen oxide
  emission rates.
  What is Photochemical Smog?
• Photochemical smog is a mixture of
  pollutants which includes particulates,
  nitrogen oxides, ozones, aldehydes,
  peroxyacetyl nitrate (PAN), unreacted
  hydrocarbons, etc.
• A brownish haze and painful eyes are
  often indicators of photochemical smog.
  Nitrogen dioxide is responsible for the
  brownish colour of the haze.
 Causes of Photochemical Smog
• The reactions that lead to the formation of
  photochemical smog are irritated by
  sunlight and involve hydrocarbons and
  nitrogen oxides emitted from automobiles,
  the combination of sunlight, the catalysis
  by particulates and the abundant
  pollutants present in modern cities provide
  favourable conditions for smog formation.
 Causes of Photochemical Smog
• Nitrogen dioxide from automobile exhaust
  first absorbs sunlight and breaks down
  into nitrogen monoxide and reactive
  oxygen atom:
• NO2 => NO + O
 Causes of Photochemical Smog
• The oxygen atom reacts with other
  components of automobile exhaust (e.g.
  unburnt hydrocarbons) and those of the
  atmosphere (e.g. oxygen) in a series of
  complex reactions to produce a variety of
  lachrymatory and toxic chemicals (e.g.
  peroxyacetyl nitrate).
 Causes of Photochemical Smog
• O + O2 => O3
• O + hydrocarbons => aldehydes
  O3 + hydrocarbons => aldehydes
• NO2 + O2 + hydrocarbons => lachrymatory
  substances, including peroxyacetyl nitrate
  Formation of Nitrogen Oxides
• The most natural way of forming nitrogen
  oxides [NOx, where x may be 1 or 2] is
• Atmospheric nitrogen reacts with nearby
  oxygen to form nitrogen monoxide.
• N2 + O2 => 2NO
  Formation of Nitrogen Oxides
• Nitrogen monoxide further reacts with
  oxygen to form nitrogen dioxide.
• 2NO + O2 => NO2
• But this only generate limited amount of
  nitrogen oxides.
• The pollution caused can be accounted for
  the engines of vehicles and furnaces. Due
  to their high temperature, these gases are
  evolved easily. The great number of cars on
  Hong Kong roads deepens the problem.
      Formation of Ozone (O3)
• Ozone [O3(g)] is a pale blue gas (exists in
  the stratosphere of our atmosphere as the
  ozone layer).
• It is formed from atmospheric oxygen by
  the absorption of ultraviolet radiation of
  the right energy (wavelength 250 nm).
  O2 => 2O
  O + O2 => O3
     Formation of Ozone (O3)
• Ozone itself undergoes photodissociation
  with 215-295 nm ultraviolet radiation.
  O3 => O2 + O
• Thus, ozone is constantly created and
  destroyed. The above reaction is responsible
  for the vital screening effect of ozone.
     Formation of Ozone (O3)
• Ozone can also be formed in the lower part
  of the atmosphere.
• It is formed by reactions between nitrogen
  oxides and hydrocarbons under sunlight,
  or by electric sparks which occur in car
  engines or electrical appliances like
• In nature, the gas can be generated during
   Formation of Hydrocarbons
• Hydrocarbons have both the elements
  carbon and hydrogen. Motors of cars do not
  always burn the fuel completely. There is
  tiny amounts of unburnt hydrocarbons in
  car exhaust.
• Petrols and organic solvents are left
  unattended and exposed to air. Since those
  solvents are volatile in nature, these
  hydrocarbons pose danger to our health.
      Formation of Particulates
• Particulates make up smoke.
• They may be ashes from burning of fuels.
• If fossil fuels (like coal and oil) are burnt,
  the tiny particules formed are mainly soot
  (carbon) from incomplete combustion.
• Incineration plants, factories and diesel
  vehicles are sources of emission.
     Environmental Problems
• (i)It can cause headaches, eye, nose and
  throat irritations, impaired lung function,
  coughing and wheezing.
• (ii)It can cause rubbers and fabrics to
• (iii)It can cause damage plants, leading to
  the loss of crops.
                  Another Smog
Characteristics    Industrial smog     Photochemical
Typical city       London              Los Angeles

Climate            Cool and humid      Warm and dry

Pollutants         SOx, particulates   NOx, O3,
                                       aldehydes, PAN,
                   Another Smog
Characteristics     Industrial smog    Photochemical
Major sources       Industrial and      Motor vehicles
                    burning of coal
Major effects on    Irritation of lungs Eye irritation
humans              and throat
Times when          Winter months      Summer months
worst episodes      (especially in     (maximum effect
occur               early morning)     around noon)
Cases of Smog (photochemical
           smog )
• China's Pearl River Delta region is
  starting to suffer photochemical smog
  due to heavy air pollution in the cities.
• Emissions from factories, power
  plants and the rapid growth of vehicles
  in cities are providing the raw
  materials for photochemical smog.
• The first case of photochemical smog
  happened in Los Angles as early as in
Cases of Smog (photochemical
           smog )

 Pearl River
 Delta region

                Hong Kong
Cases of Smog (Industrial smog)
• On 9 December 1952, foggy conditions
  developed over London.
• Being very cold, most houses kept fires
  burning, with coal as the major fuel.
• The smoke from these fires mixed with the
  fog and was unable to disperse, resulting in
  a smog which persisted for 4 days.
Cases of Smog (Industrial smog)
• The pH of air during the Great London
  Smog was as low as 1.6. During this period
  some 4000 more people died than would
  expected at this time of the year. Most of
  these additional deaths were due to
  respiratory disorders.
Cases of Smog (Industrial smog)

     Before          After smog
           New York City
        How do we Reduce
       Photochemical smog?
• To address the problem of photochemical
  smog, we have to reduce the emissions of
  NOx and VOC.
• Reduce the emissions from motor vehicles.
        Sources of Information
• Chem notes by Ng sir
       The End

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