ATMOSPHERIC CHEMISTRY PROBLEM 4. 8 Global source of methane Emission of methane

Question

ATMOSPHERIC CHEMISTRY PROBLEM

4. 8 Global source of methane Emission of methane to the atmosphere is largely biogenic and the individual sources are difficult to quantify However, one can use a simple mass balance approach to calculate the global source. 1. Methane is removed from the troposphere by oxidation, and the corresponding lifetime ofmethane is lknown to be 9 years. Based on this lifetime, would you expect methane to be well mixed in the troposphere? Briefly explain. 2. The present-day methane concentration in the troposphere is 1700 ppbx and is rising at the rate of 10 ppbs yr-l. Using a mass balance equation for methane in the troposphere, show that the present-day emission of methane is E 3.0x1013 moles per year. For this calculation, take 150 hPa as the top of the troposphere and neglect transport ofmethane to the stratosphere. 3. We now refine our estimate by accounting for the chemical loss of methane in the stratosphere. C2)- C(zt)exp[-(z-z Ce) 1700 ppbv In C 3.1. The mining ratio C of methane above the tropopause (altitude st) decreases exponentially with altitude, with a scale height h- 60 kam. Using a turbulent diffusion formulation for the vertical flux and assuming steady state for methane in the stratosphere, show that dC -Al K znaTZ) tropopause strat where the turbulent diffusion coefficient density n, and the methane mixing ratio are evaluated just above the tropopause: .d is the surface area of the Earth: and 44 is the total chemical loss of methane in the 3.2. CalculateLassuming a turbulent diffusion coefficientK-x103 cm2 s-1 and an air density 5x1018 molecules cm-3 just above the tropopause. From this result, derive an improved estimate ofthe present-day emission of methane.

Explanation / Answer

1)

The first 15 km of the atmosphere is called troposphere . As colder air lies on top of warmer air in this section of the atmosphere, the troposphere is well mixed vertically by convection currents.

So the methane is present at a constant concentration of approximately 1.7 ppm throughout the troposphere.

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