Methane and Ozone

Some time ago, Eli discussed the atmospheric oxidation of methane a topic to which he now returns in the context of ozone pollution and the effects on air quality and human health. Tropospheric ozone is a result of NOx chemistry, where NOx, either referred to as NO ex, or nocks, is the sum of NO (nitric oxide) and NO2 (nitrogen dioxide) in the atmosphere.

Tropospheric ozone is formed by the photodissociation of NO2 between 400 and 300 nm, with the long wavelength limit set by the bond strength of O - NO, and the lower one by the absorption cut off of stratospheric ozone which absorbs all of the solar light below 300 nm (to be fussy 306 nm or so and as long as the bunnies are being fussy somewhere like 420 nm on the high end because of thermal population of excited vibrational levels)

The red and blue lines are the NO2 absorption spectrum at 294 and 220 K, the black line represents the ozone cut off for UV light at the surface.  Essentially no UV light below 300 nm reaches the surface because of strat ozone absorption

NO2 + hν --> NO + O(³P)  [1]

hν stands for a photon in the active wavelength region and O(³P) is the ground state of the oxygen atom, which then reacts with an oxygen molecule in the presence of another molecule in the air, M, (nitrogen or oxygen)

O(³P) + O2 + M --> O3 + M [2]

The reconversion of NO to NO2 by reaction with ozone is fairly slow

NO + O3  --> NO2 + O [3]

So the more NO2 there is, the more ozone will be produced during the day.  The natural source of NO2 is lightening which splits N2 molecules, the anthropic source is high temperature combustion such as in autos. Also the more sunlight, the more photodissociation, so one expects more ozone during the summer.

If Reaction 2 is assumed to be fast (it is because there is so much oxygen and nitrogen around) the rate at which ozone is produced is j [NO2] where j is the flux of UV light and the rate at which it is consumed is k3[NO][O3] where k3 is the rate constant for reaction 3[X] means the concentration of X.  Under steady state conditions when production via Reaction 1 equals destruction via Reaction 3

k3[NO][O3] = j [NO2] 

which can be solved for an estimate of the ozone concentration

[O3] = j [NO2]/k3[NO] 

in terms of the steady state concentrations of NO and NO2

While reaction [3] appears to limit the amount of ozone that can form, other reactions which cycle NO back to NO2 without consuming ozone would lead to much higher ozone concentrations. Volatile organic compounds (VOCs) provide such a path, and methane is the VOC with the highest atmospheric concentration.  Moreover, as a non-condensible gas not very susceptible to rain out,  it spreads worldwide in ways that heavier VOCs do not.  Looking at the reaction scheme for methane oxidation

we see (light blue boxes) two additional paths.  The lower one is shown for methane, although an equivalent exists for other VOCs.  The methane is oxidized by reaction with hydroxy radicals

CH4 + HO --> CH3 + CH4+ H2O [4]

The methyl radical then quickly combines with an oxygen molecule

CH3 + O2  --> CH3O2  [5]

and the methylperoxy radical,  CH3O2 then can react with NO to cycle back to NO2 leaving methoxy

CH3O2 + NO --> CH3O + NO2

The more VOCs, the more ozone. . .