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.
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. 2) Peroxyacetyl nitrate (PANs) CH3CHO +OH-> CH3CO+H2O CH3CO+O2-> CH3C(O)O2 CH3C(O)O2+NO2+M↔CH3C(O)O2NO2+M In the upper troposphere PANs are quite stabel and can be transported long distances.

10. Photolysis rate 𝑑[ 𝑂 2 ] 𝑑𝑡 = 𝑗 𝐴 [ 𝑂 3 ]
𝑗 𝐴 = 𝜆 1 𝜆 2 𝜎 𝐴 𝜆,𝑇 𝜙 𝐴 𝜆,𝑇 𝐼 𝜆 𝑑𝜆
𝜎 𝐴 : absorption cross section (cm2 molecule-1)
𝜙 𝐴 :quantum yield for photolysis rate
𝐼 𝜆 : spectral actnic flux (molecules cm-2 nm-1)
Actnic flux: the radiant energy incident at a point from all direction
The actnic flux for one hemisphere
𝐼 𝜆 ↓= 0 2𝜋 0 𝜋/2 𝐿 𝜆 𝑑𝜃𝑑𝜙
𝐿 𝜆 : radiance

11. Chemical kinetics
The rate of reaction is expressed in a form that accounts for both the
frequency of collisions and the fraction that exceed the required energy.
𝑟=𝐴 𝑇 exp⁡(− 𝐸 𝑅𝑇 ) 𝑐 𝑖 𝑐 𝑗
𝑘=𝐴 𝑇 exp⁡(− 𝐸 𝑅𝑇 )
If 𝐴(𝑇) is independent of 𝑇, we have the Arrhenius form, 𝑘=𝐴 exp⁡(− 𝐸 𝑅𝑇 )
𝐸: Activation energy
𝑅: Universal gas constant

12. 6-6 Hydrocarbon reactivity
Incremental reactivity: the amount of ozone formed per unit amount of VOC added to a VOC mixture
Incremental reactivity=
: the function of ratio of VOC: NOx

13. 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 severely limited in its ability to predict multi-day episodes. => transport should be considered

15. 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.