6.4 Flashcards
what types of molecules trap heat?
2+ atoms (polyatomic)
99.9% of molecules in atmosphere don’t absorb IR radiation including:
- N2 = 78%
- O2= 21%
- Ar = 0.9%
2 molecular requirements for IR absorption:
- energy of radiation matches energy of molecular transition (nuclei/electron vibrations)
- molecular transition (atmospheric molecules) must change electric field (alter molecule’s dipole moment)
why does Ar NOT absorb IR?
no nuclei vibrations
dipole moment =
(vector sum of atomic charges) * (distance from molecule’s center of mass)
why do N2 and O2 NOT absorb IR?
- they have nuclei vibrations BUT
- NO change in dipole moment (bc symmetrical dipole always = 0)
all homonuclear diatomic molecules =
infrared inactive
homonuclear diatomic molecules
molecule w/ 2 of the same atoms (H2, O2, N2, etc.)
all heteronuclear diatomic molecules =
have altered dipole moment = absorb IR
heteronuclear diatomic molecules
molecules w/ 2 diff atoms (CO, NO, HCI)
polyatomic molecules w/ vibrations that change dipole =
IR active
all gasses that contribute to GHG effect =
polyatomic
2 most important GHG molecules =
H2O and CO2
H2O:
3 vibrations = 3 Δ-dipole
CO2:
symmetric = no Δ-dipole
- BUT net Δ-dipole = altered by stretch/bending of vibrations
molecular vibrational transitions allow for absorption of …
specific energies
rates of rotation
for each vibration, IR photons change rotational levels
rotational absorptions = ? and allow for absorption of…
gain/loss of energy during collision
- broad absorption band
CO2: bending vibration
14,922 nm
CO2: main absorption band
15 µ band
1 µ =
1 µm = 1000 nm = 1 x 10^6 m
H2O: bending vibration
6269 nm (major absorption band)
H2O rotational vibrations =
wavelengths > 20,000 nm
why do CO2 and H2O absorb no significant radiation
wavelengths = short
atmospheric window
region 8000-12,000 nm where radiation can escape
chlorofluorocarbons
polyatomic molecules; major destroyers of stratospheric ozone
- CH4, N2O, O3
- major GHGs
CO2 absorptions =
nearly saturated
“saturated” absorption
most radiation emitted w/in absorption bands = already absorbed
CO2 molecules = contribute __ amnt to total absorption
small
CFC molecules = contribute __ amnt to total absorption bc…
large
- they absorb small amnts from window regions
1 CFC molecule =___ effect of 1 CO2 molecule
times thousands
Beer-Lambert Law:
evaluating UV screening effect of ozone layer
any form of radiation passing through absorbing material (beer-lambert equation=…)
T = I/I0 = e-El
Beer-Lambert: T =
% transmitted light thru global layer of absorbing gas
Beer-Lambert: l =
equivalent thickness
Beer-Lambert: E =
absorptivity
ozone layer = Δ-transmitted fraction produced by small change in absorbing gas (beer-lambert equation=…)
dT = -e-El dl
- dT = Δ-light
Δ-% outgoing IR = __ as ↑ GHG
↓
if [initial concentration] = small, [dependence] = __ bc..
linear
- e^-El approaches e^0 =1
effect of outgoing radiation/molecule = greatest @ __ concentrations or __ absorptivity
LOW
main effects of CO2/H2O added =
↓ transmitted light at edges of absorption bands (E ↓ rapidly)
adding molecules found in low concentration =
absorb light in proportion to absorptivity in atmospheric window
2 vibrational modes involved in CO2 absorption
asymmetrical stretching + bending = net Δ-dipole = absorbs IR
