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IV. Kinetics Introduction (Pseudo) First Order Approx. Steady State Approximation

IV. Kinetics Introduction (Pseudo) First Order Approx. Steady State Approximation. Role of Kinetics. Atmosphere is an open system- equilibrium does not apply Determine chemical fate of a species Determine concentration of a species Compare time scales for processes Chemical Physical

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IV. Kinetics Introduction (Pseudo) First Order Approx. Steady State Approximation

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  1. IV. Kinetics Introduction(Pseudo) First Order Approx.Steady State Approximation

  2. Role of Kinetics • Atmosphere is an open system- equilibrium does not apply • Determine chemical fate of a species • Determine concentration of a species • Compare time scales for processes • Chemical • Physical • Meteorological

  3. Simple Kinetics vs. Complex Models Simple Kinetics (here) • Approximations to give insight Complex Models • Quantitative but • Numerical solutions • Transport • Lack of insight

  4. First Order Kinetics • Photochemistry (J is 1st order rate constant) O3 + hn→ O2 + O Lifetime, tt = 1/J What is the photolysis lifetime of O3 at 10 km?

  5. Elementary versus Complex Reactions • Elementary – balanced reaction reflects molecular-level events (mechanism): CH4 + OH → CH3 + HOH So the Rate Law reflects the mechanism • Complex – multiple elementary reactions CH4 + 2 O2→ CO2 + 2 H2O

  6. Pseudo-First Order Approximation Reduces 2nd order to 1st order [O3] >> [Cl] Major trace species vs. highly reactive radical [O3] effectively constant k’ = pseudo-1st order rate constant

  7. Lifetimes and Halflife photochemistry t = 1/J Pseudo-1st order rxn. t = 1/k’ Halflife (t1/2) = 0.693 t kCl+O3 = 8.8 x 10-12 cm3 molecule-1 s-1 (218 K) [O3] = 5 x 1013 molecule cm-3 (25 km) What is the lifetime of Cl with respect to reaction with O3 ?

  8. Rate Constants Sources – JPL Data Evaluation, IUPAC, NIST Arrhenius (empirical) k(T) = Ae-Ea/RT A = Arrhenius pre-exponential factor Ea = activation energy For Cl + O3: k(T) = 2.9 x 10-11 e-2.2 kJ/mole/RT cm3 molecule-1 s-1 e-Ea/RT = e-(Ea/R)/T k(T) = 2.9 x 10-11 e-260/T cm3 molecule-1 s-1 What is k at 218 Kelvin ?

  9. Steady State Approximation Applied to ClO, means [ClO] changing very slowly:

  10. Steady State Approximation, con’t Local noon, [radicals] at daily maximum, changing slowly  Steady State Approximation okay

  11. Key Points • Pseudo-first order approximation • Steady state approximation • Data sources abound

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