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6. Atmospheric Photochemical Reactions

6. Atmospheric Photochemical Reactions. 6.1 Introduction 6.2 Thermodynamics of photochemical reactions 6.3 Monatomic oxygen and ozone formation 6.4 Role of oxides of nitrogen in photo-oxidation 6-5. Hydrocarbons in atmospheric photochemistry 6-6 Hydrocarbon reactivity

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6. Atmospheric Photochemical Reactions

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  1. 6. Atmospheric Photochemical Reactions 6.1 Introduction 6.2 Thermodynamics of photochemical reactions 6.3 Monatomic oxygen and ozone formation 6.4 Role of oxides of nitrogen in photo-oxidation 6-5. Hydrocarbons in atmospheric photochemistry 6-6 Hydrocarbon reactivity 6-7 Development of control strategies 6-8 Daily history of pollutants in photochemical smog

  2. 6.1 Introduction • Smog: arises from the interaction of sunlight with various constituents of the atmosphere • characterized chemically by a relatively high level of oxidants. 자료출처 Reuter videos

  3. 6.2 Thermodynamics of photochemical reactions The energy of photon  : frequency c:the speed of light Photochemical dissociation: Two-step process : excited state

  4. 6.3 Monatomic oxygen and ozone formation In the upper atmosphere :monoatomic oxygen in the ground state M: an energy-accepting third body an excited oxygen atom

  5. Ozone layer : acts as filter to ultraviolet radiation trying to reach the earth’s surface. http://www.cec.org/ods/EN/module01/cec_odspolicy_m01t01p01_e.asp?print=1 Image from NASA

  6. 6.4 Role of oxides of nitrogen in photo-oxidation 1) Ozone formation near the earth’s surface fast reaction At steady state

  7. 2) Nitric acid formation reaction During the daytime During nighttime The rapid photolyzation of NO3 to NO and NO2 results in a reduction of NO3 to a very low concentration at dawn and during the daytime

  8. 6-5. Hydrocarbons in atmospheric photochemistry Radical: 쌍을 이루지 못한 전자를 갖고 있는 원자나 분자로 반응성이 큼 1) Hydroxyl radical (OH) The formation of the Hydroxyl radical Reaction with reactive VOCs The mechanism for increasing the Ozone concentration

  9. 6-6 Hydrocarbon reactivity • Incremental reacitivity: the amount of ozone formed per unit amount of VOC added to a VOC mixture • Incremental reactivity= • : the function of ratio of VOC: NOx

  10. 6-7 Development of control strategies EKMA( empirical kinetic modeling approach) model : peak O3 concentration in ppm resulting from the irradiation of mixtures of VOCs and NOx at the initial concentration 1) NOx limited zone: VOC/NOx >8 the changes in the VOC concentration results in little to no change in ozone. Typical of suburban and rural areas 2)VOC limited zone : VOC/NOX <8 Decreasing the VOC concentration is effective control strategy Typical of polluted air EKMA approach provides a 1-day simulation episode but is severly limited in its ability to predict multi-day episodes. => transport should be considered

  11. 6-8 Daily history of pollutants in photochemical smog Hydrocarbon concentration: a peak 8 a.m. due to increased traffic. CO and NO: a peak at early morning due to increased traffic Ozone is formed as a result of photolysis of NO2 => NO2 decreases and O3 increases O3 concentration has a peak in the afternoon.

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