Lecture 8 - Green House Gases
1. Why is methane H bonded more in the N. pole rather than in the southpole?
2. The reaction with the Hydroxyl radical. Hydroxyl radical is the single most important radical to
keep the atmosphere cleansed.
3. Generally, in biological entities like methane, N20, gasoline from engines, hydrocarbons. So we
get chemically reduced compounds in the atmosphere.
4. In the atmosphere 20% is Oxygen. There is a huge huge driving force to get oxidized the C to co2
5. This is what happens. The carbon in methyl oxidizes into CO2. Atmopshere does oxidize the
6. The mechanism which this happens is through radicals, and not through O.
7. Hydroxyl is there about 0.1ppt. There is a little concentration of it, but it is very active, it drives
the oxidation reaction.
8. The source of hydroxyl atoms:
a. We have ozone in the atmosphere. And it gets photolized. We need wavelengths lesser
than 310nm through this. Just a little silvery UV light is enough. Then we form O2 molecules + an electron excited molecule. O*. This O atom is a high energy O atom,
which is much more reactive.
b. This molecule reacts with water and creates Hydroxyl radicals. Oxygen molecule reacts
with water and generates H20.
c. The rate of loss = k. [OH] [CH4]
d. The rate constant for [OH] with [CH4] is
e. [OH] is really small = 10^6.
f. When you plug all these together you can get the life time = about roughly 10 years.
9. The image with the mixing ratios of methane : Slide
a. The reason for not having a lot of variation around the whole hemisphere is because
methane’s life time is long. If we had a molecule which has a life time of an hour then,
you would see more localized concentrations. There wouldn’t be anything near the
north poles, but only around their sources.
b. Because if the life time is short, you don’t have time to move around. If the life time is
10 years, you have plenty of time to move.
c. Why do you see even higher concentrations in the North Pole rather than in the
source is because there isn’t that much hydroxyl in the NP. So the sink is low.
d. Why don’t you have much hydroxyls in the NP because there aren’t that much
sunlight to photolyze ozon.
Ntirous Oxide (N2O) – Laughing Gas
1. Like CO2 and CH4 it is in pre-emit concentrations
2. Tendency is that molecules which have a tendency to be naturally to distribute in the
atmosphere, naturally increase their abundance in the atmosphere
3. Started increasing their abundance 2-300 years ago
4. Coded science with large scale agricultural industry
5. Largely release biological molecule. Mostly from microbial reaction from the soil. Small amounts
from the oceans. These reactions are
6. Most of N20 is coming from soils. Used as fertilizers to the ground.
7. Sources of n2o
a. De-nitrification processes – reductive
b. Nitrification process – oxidizing
c. Ammonium/ammonia – where N is chemically reduced
d. Nitrate – N is oxidized
8. When you have a reduction process occurring you are generally having it occur in an anaerobic
9. When you have oxidation processes you have aerobic processes
10. When you have a microbial process going on and you have little bit of oxygen creeping in, then