GOVT BILASA GIRLS P G Session 2019 2020

GOVT. BILASA GIRL’S P. G. Session – 2019 -2020 COLLEGE Subject – Environmental chemistry Topic – Chemical and Photochemical reaction in atmosphere Submitted by Dr. Madhuri Nanda

CHEMICAL AND PHOTOCHEMICAL REACTION IN ATMOSPHERE CONTENT • Introduction • Reaction of air in stratosphere • Reaction of air in mesosphere and ionosphere

INTRODUCTION The various chemical and photochemical reactions taking place in atmosphere, mostly depend upon the temperature , composition, humidity and intensity of sunlight. Photochemical reaction take place in atmosphere by the absorption of solar radiation in the ultraviolet region. Absorption of photons by chemical species gives rise to electronically excited molecules, which can bring about certain reactions. These reactions are not possible thermal condition except at higher temperatures and in presence of catalysts.

�The electronically excited molecules produced by absorption of a photon may undergo following changes to initiate the chemical steps. 1. 2. 3. 4. 5. Reaction with other molecules on collision. Polymerization. Internal rearrangement. Dissociation. De-excitation by fluorescence or de-activation to return to the original state. • The steps involved in photochemical reactions 1. 2. 3. Absorption of radiation. Primary reactions Secondary reactions

REACTIONS OF AIR IN STRATOSPHERE Atmosphere pollutants are present largely in the troposphere and lower stratosphere. Oxygen plays an important role in troposphere whereas O 3 plays a significant role in stratosphere. Stratospheric Chemistry. Oxygen undergoes photochemical reaction in stratospheric forming ozone. O 2 + hυ (λ 240 -320 nm O+O+M O 2 +M , O+O 2+M O 3 + M (M = N 2 or O 2) Ozone so formed undergoes photodissociation as follows : O 3 + hυ (λ 230 -320 nm) O 2* + O* + Heat O 3 +v O O 2 + O 2

Stratospheric Chemistry The species of oxygen in the upper atmosphere include O 2, O, O*, O+, and O 2* alongwith O 3, i. e. , atoms, ions, molecules and excited species. These are formed by photochemical dissociation and ionization etc. For example, O + hυ O+ + e O 2 + hυ O 2+ + e Ozone concentration is around 10 ppm in the stratosphere compared to 0. 05 ppm in the troposphere. Now ozone depletion is taking place at a tremendous rate by reaction with atomic oxygen, reactive hydroxyl radicals, NOx, SO 2, CH 4 and chlorofluoro carbons. 1. O 3+ O O 2+O 2 2. O 3+ HO˙ O 2 + HOO˙ + O ˙OH +O 2 3. Photochemical decomposition of N 2 produces an electronically excited N(2 D) and a ground state N(4 S). The latter reacts with O 2 to form NO. N 2 + hυ (λ< 126 nm) N(2 D) + N(4 S) + O 2 NO+O

Stratospheric Chemistry Nitric oxide is also produced in the stratosphere below 30 km by the reaction of nitrous oxide with excited oxygen atoms and above 30 km by ionizing radiation on nitrogen. N 2 O +O 2 NO NO + O 3 NO 2 + O 2, NO 2 + O NO + O 2

OZONE DISTRIBUTION Ozone is highly unstable relative to oxygen. At equilibrium, O 2/O 3 ratio is 1025 while it is 3 x 105 at an altitude of 30 km where ozone content is maximum. The actual O 3 concentration is determined by the rate of its formation and rate of destruction. The rate of O 3 formation depends on the flux of ultraviolet solar radiation in stratosphere. The solar flux being variable , O 3 concentration is also subject to variation depending on the altitude. Consider ozone formation and destruction, the reaction rates (k) can be given by O 2 + hυ O+O Reaction rate = k 1[O 2] O+O 2 + M O 3 + M Reaction rate = k 2[O][O 2][M] O 3 O+O 2 Reaction Rate = k 3 [O 3] O+O 3 Reaction rate = k 4[O][O 3] At equilibrium. K 2[O][O 2][M] = k 3[O 3]+k 4[O][O 3]

OZONE DISTRIBUTION The steady state [O 3]/[O 2] ratio given by [O 3] = [O 2] 1/2 k 1 k 2[M] k 3 k 4 This ratio depends on the altitude. The density of air and [M] concentration decrease with rise in the temperature as the altitude increases in the stratosphere. It can be calculated that at an altitude of 30 km : [Air ] = 1017. 7 mol/cm K 1 = 10 -11 , k 2 = 10 -32. 7 , k 3 = 10 -3 and k 4 = 10 -4 [ O 3 ] /[O 2 ] = 10 -4 The calculated value of O 3 concentration differs from the observed value by factor of two. However, other mechanisms are also responsible for ozone destruction.

REACTIONS OF AIR IN MESOPHERE AND IONOSPHERE Mesosphere lies above stratopause and extends upto 85 km. The region above stratosphere, in the altitude range of 50 km to 100 km is called ionosphere. N 2 + O+ N 2+ + O NO+ + N, N 2+ + O 2 N 2 + O 2+ NO+ + e N + O Oxygen undergoes photolytic dissociation generating atomic oxygen. O 2 + hυ (λ< 240 nm) O + O

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