The bi-molecular reaction constants (molecule^-1 cm^3 s^-1) of following reactio

Question

The bi-molecular reaction constants (molecule^-1 cm^3 s^-1) of following reactions can be calculated as; CO + OH rightarrow CO_2 + HO_2 k_1 = 5.4 x 10^-14 (T/298 k)^1.5 exp(2079/RT) CH_4 + OH rightarrow H_2O + CH_3O_2 = k2 = 2.46 times 10^-12 exp(-1.48x10^4/RT) where, R = 8.314 J mole^-1 k^-1 Calculate k_1 and k_2 at 27 degree C and -20 degree C Assume [OH] = 1 times 10^6 molecules cm^-3 then calculate the atmospheric lifetime of CO and CH_4. (FYl. The above reactions are the most dominant CO and CH_4 loss pathways in the troposphere) In general, CH_4 is defined as a well mixed gas. Justify this assumption. The minimum observed CO concentration is around 50 ppb (e.g. in the middle of Antarctica in -20 degree C) and the maximum is around 2 ppm (e.g. in the middle of megacities in 27 degree C), discuss relative importance of CO and CH_4 as a proxy radical production source (Assume CH_4 is well mixed as 1.7 ppm).

Explanation / Answer

Q1.

k1 T = 27°C = 300

k1 = (5.4*10^-14)((300/298)^1.5)*exp(2079/(8.314*300)) = 1.25530*10^-13

k1 T = -20°C = 253

k1 = (5.4*10^-14)((253/298)^1.5)*exp(2079/(8.314*253)) = 1.135 *10^-13

for k2:

k2 = (2.46*10^-12)*exp((-1.48*10^4) /(8.314*T))

at T = 27°C = 300

k2 = (2.46*10^-12)*exp((-1.48*10^4) /(8.314*300))

k2 = 6.515*10^-15

at T = -20°C = 253

k2 = (2.46*10^-12)*exp((-1.48*10^4) /(8.314*253))

k2 = 2.163*10^-15

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