Emission of methane to the atmosphere is largely biogenic and the individual sou

Question

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. Methane is removed from the troposphere by oxidation, and the corresponding lifetime of methane is known to be 9 years (as will be seen in chapter 11). Based on this lifetime, would you expect methane to be well mixed in the troposphere? Briefly explain. The present-day methane concentration in the troposphere is 1700 ppbv and is rising at the rate of 10 ppbv yr-1. Using a mass balance equation for methane in the troposphere, show that the present-day emission of methane is E = 3.0 Times 1013 moles per year. For this calculation, take 150 hPa as the top of the troposphere and neglect transport of methane to the stratosphere. We now refine our estimate by accounting for the chemical loss of methane in the stratosphere. The mixing ratio C of methane above the tropopause (altitude zt) decreases exponentially with altitude, with a scale height h = 60 km. Using a turbulent diffusion formulation for the vertical flux and assuming steady state for methane in the stratosphere, show that: where the turbulent diffusion coefficient Kz, the air density na, and the methane mixing ratio are evaluated just above the tropopause; A is the surface area of the Earth; and Lstrat is the total chemical loss of methane in the stratosphere. Calculate Lstrat assuming a turbulent diffusion coefficient Kz = 7 Times 103 cm^2 s-1 and an air density na = 5 Times 1018 molecules cm-3 just above the tropopause. From this result, derive an improved estimate of the 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.

Related Questions

Navigate

• Browse (All)
• Browse E
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.