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Greenhouse Gas Basics
A gas molecule of any kind will only absorb radiation at a given wavelength if it can use the energy to increase its internal energy level, or to transition to a higher state. There are three main types of transitions: electronic, rotational, and vibrational. Each type of transition requires different wavelengths of radiation.
The table below shows the wavelength required for each transition type.
Transition type Wavelength required (μm)
Electronic < 0.4
Rotational > 20
Vibrational 0.7 – 20
Ghg undergo vibrational and vibration-rotational transitions.
Be gone, You Colorless, Odorless Gas
The reactions below show the series of reactions that remove methane from the troposphere:
Reaction 5.5: CH4 + OH → CH3 + H2O
Reaction 5.6 : CH3 + O2 → CH3O2
Reaction 5.7: CH3O2 + HO2 → CH3O2H + O2
Reaction 5.8: CH3O2H → CH3O + OH
Reaction 5.9: CH3O + O2 → CH2O + HO2
This reaction results in the production of formaldehyde (CH2O), which is then oxidized to produce carbon monoxide (CO) as shown in reaction 6.0.
Reaction 6.0: CH2O + OH + O2 → CO + H2O + HO2
The carbon monoxide is then oxidized, forming carbon dioxide, as shown in the net reaction below:
Reaction 6.1: CO + O3 → CO2 + O2
Carbon Dioxide: It's Not Just for Photosynthesis
Despite its hellion reputation, CO2 acts like any other greenhouse gas; it absorbs infrared radiation, thus adding to the greenhouse effect. It absorbs radiation in the 14-19 μm and 4.0 to 4.3 μm ranges. It completely blocks radiation between 15 and 16 μm. Warming due to CO2 is estimated to be 50 W/m2.
As we saw earlier in this chapter the loss of methane from the atmosphere is largely due to reactions with OH in the troposphere. The more methane there is, then, the less OH there will be available. An increase of 1% for CH4 means a decrease of 0.32% for OH. The mean global loss rate of methane is estimated to be 507 Tg (CH4) / yr.