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					             Major Greenhouse Gases and its Characteristics


                                            Relative      Current
              Atm.            Annual
                                          greenhouse    greenhouse       Principal
 Gas      Concentration   Concentration
                                           efficiency   contribution   sources of gas
             (ppm)         Increase (%)
                                           (CO2 = 1)        (%)


                                                                        Fossil fuels,
 CO2          355              0.4            1             57
                                                                        deforestation
                                                                           Foams
                                                                           Foams,
CFCs         0.00225           5              15            25            Aerosols,
                                                                         refrigerants
                                                                        Wetland, rice,
 CH4          1.675            1              25            12         livestock, fossil
                                                                             fuel
Nitrous                                                                Fertilizer, fossil
              0.31             0.2           230             6
Oxide                                                                         fuel
Controversy is what this means for future
Double CO2             ∆T = 6oC
                         Efforts f Global W  i
                         Eff t of Gl b l Warming



                                            sheets,
Melting of the Greenland and Antarctica ice sheets leading to substantial
increase in sea levels.

Intergovernmental panel on climate change (IPCC) reports sea-level rise
up to 7 meters by 2040 if ice sheets continue to melt at this rate.

Siberian permafrost has frozen carbon (1600 billion tonnes) in the soil
 twice               atmosphere.
-twice the amount in atmosphere

Sea absorbs CO2 and increase in CO2 in atmosphere leads to increase in
 ocean acidity.

With rising temperature, air circulation, ocean currents and rain patterns
would change causing violent weather.
                                  4




                  Summary

The Earth’s Atmosphere


Chemistry of Stratosphere


Global W   i     d Cli t Change
Gl b l Warming and Climate Ch
                                                                          5



                     Photochemical Process

              Study of chemical reactions which are caused by
                the absorption of light radiation (or photons).


In thermal chemical reactions, the activation energy,   Ea results
                intermolec lar collisions
from the random intermolecular collisions.


In photochemical reactions, Ea is acquired by the absorption of photons
  f li ht     i t d ith     ti l        l   th
of light associated with particular wavelength.


         Examples:

                - Formation of O3 from O2 in stratosphere

                - Synthesis of Vitamin D
                                            6


            Jablonski Diagram
depicting various photochemical processes
                                                                                        7



          p
On absorption of p         , j p
                   photon, e- jumps from S0 electronic GS to
S1, S2, S3 etc.., excited singlet states.


                              A + hv         A*


For every singlet state, Sn, there exists a corresponding triplet state, Tn except T0


Corresponding Tn state is lower in energy than Sn.


The activated molecule, A*, returns to the GS by dissipating its energy
through various processes.

                           Non-radiative t
                       (1) N                iti
                                 di ti transitions

                       (2) Radiative transitions and

                       (3) Chemical reactions
                                                        8


                     Non-Radiative Transitions

• Internal Conversion (IC)
      - Transitions between states of same spin
                S2 to S1      T2 to T1
                S3 to S1      T3 to T1
                S4 to S1      etc..
      - Radiation less transitions
      - E of excited molecule is dissipated in the
        form of heat thro’ intermolecular collisions.
      - life-time less than 10-11 s
        life time                 s.



• Inter System Crossing (ISC)
      - Transitions between states of different spins
                S1 to T1
                S2 to T2
                S3 to T3 and so on.
                                                                       9
                       Radiative Transitions

• Fluorescence (F)
      - Transitions involving return of singlet ES S1 state to GS S0

                         S1 to S0

      - Accompanied by emission of radiation
                  is ll     d in
      - S1 to S0 i allowed, i spectroscopy
      - life time is less than 10-8 s.


• Phosphorescence (P)
      - Transitions from triplet ES T1 to GS S0

                         T1 to S0

      - Accompanied by emission of radiation.
                                                    spectroscopy.
      - Triplet T1 to S0 is forbidden transition in spectroscopy
      - life time is 10-3 s due to spin inversion.
                                                                        10
• Chemical Reaction
     - Excited state lose energy by undergoing chemical reaction

             S1 to S0 is very fast
             T1 to S0 is slow, hence favorable for chemical reaction.


               Primary Photochemical Processes
                                                           11
• Photodissociation

  - Molecular fragmentation upon absorption of radiation

  Optical Dissociation

       - Direct excitation to a dissociative state




  Predissociation

       - Excitation to a stable state
       - Non-radiative transition
                                                                           12

Laws of Photochemical Processes:

 (1) Grotthus-Draper Law (principle of photochemical activation)

        Only light which is absorbed by a system can bring about
        a photochemical change.

 (2) Start-Einstein Law (principle of quantum activation)

         Molecule is activated by absorption of one quantum of radiation
         in the primary step of the photochemical process.


   Suppose ν is the frequency of the radiation absorbed, then the
   corresponding quantum of energy absorbed per molecule will be,


                              E = hv

                                  mole,
   Quantum of energy absorbed per mole

                              E = N A hv = N A hc λ
                                                                                            13



         ,      gy         p                      g
        E, Energy absorbed per mole of the reacting substance is
        called Einstein.




Quantum Yield:
                                                          as,
        Quantum yield of photochemical process is defined as

         No. of molecules that react                    No. of moles that react
φ=                                         =
            q
     No. of quanta of radiation absorbed       No. of Einsteins of radiation absorbed




Quantum yield for the product formation is,

     No. of molecules of product formed               No. of moles of product formed
φ=                                             =
     No. of quanta of radiation absorbed           No. of Einsteins of radiation absorbed
                                                                      14




The energy of monochromatic radiation (in terms of no. of Einsteins
absorbed) can be measured by Actinometer.


           Quantum yields of photochemical reactions

    Photochemical
                               λ (nm)                 φ
      Reaction
2NH3         N2 + 3H2            210                  0.2
2NO2         2NO + O2            405                  07
                                                      0.7
H2 + Cl2     2HCl                400               104 – 106
CO + Cl2     COCl2             400-436               103
3O2          2O3               170 190
                               170-190                 3
                                                                              15



Chemiluminescence:

      Emission of light as a result of chemical reaction and
      is         f h t h i l
      i reverse of photochemical reactionti




                    Fe



     Luminol
                                              3-Amino phthalate


 - Used by forensic scientists by collecting blood samples.

 - Haemoglobin is Fe containing protein which acts as catalyst for the reaction.

				
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