lecture 6: spectroscopy and photochemistry ii - welcome |...
TRANSCRIPT
![Page 1: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/1.jpg)
1
Required Reading: FP Chapter 3Suggested Reading: SP Chapter 3
Atmospheric ChemistryCHEM-5151 / ATOC-5151
Spring 2005Prof. Jose-Luis Jimenez
Lecture 6: Spectroscopy and Photochemistry II
Outline of Lecture• The Sun as a radiation source• Attenuation from the atmosphere
– Scattering by gases & aerosols– Absorption by gases
• Beer-Lamber law
• Atmospheric photochemistry– Calculation of photolysis rates– Radiation fluxes– Radiation models
![Page 2: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/2.jpg)
2
Reminder of EM Spectrum
Blackbody RadiationLinear Scale Log Scale
From R.P. Turco, Earth Under Siege: From Air Pollution to Global Change, Oxford UP, 2002.
![Page 3: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/3.jpg)
3
Solar & Earth Radiation Spectra• Sun is a radiation source with
an effective blackbody temperature of about 5800 K
• Earth receives circa 1368 W/m2 of energy from solar radiation
• Question: are relative vertical scales ok in right plot?
From TurcoFrom S. Nidkorodov
Solar Radiation Spectrum II
From Turco
From F-P&P
•Solar spectrum is strongly modulated by atmospheric scattering and absorption
![Page 4: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/4.jpg)
4
Solar Radiation Spectrum III
From Turco
UVC B A Photon Energy ↑
O3
O2
Solar Radiation Spectrum IV• Solar spectrum is strongly
modulated by atmospheric absorptions
• Remember that UV photons have most energy– O2 absorbs extreme UV in
mesosphere; O3 absorbs most UV in stratosphere
– Chemistry of those regions partially driven by those absorptions
– Only light with λ>290 nm penetrates into the lower troposphere
– Biomolecules have same bonds (e.g. C-H), bonds can break with UV absorption => damage to life
• Importance of protection provided by O3 layerFrom F-P&P
![Page 5: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/5.jpg)
5
Solar Radiation Spectrum vs. altitude
• Very high energy photons are depleted high up in the atmosphere• Some photochemistry is possible in stratosphere but not in troposphere
• Only λ > 290 nm in trop.
From F-P&P
Solar Zenith Angle
sechL
pathlength Verticalpathlength Actualm Mass"Air " θ=≈==
S o la r F lu x
W a v ele n g th [n m ]1 0 0 0
Sola
r Fl
ux [P
hoto
ns c
m-2
s-1 n
m-1
]
1 0 1 2
1 0 1 3
1 0 1 4
1 0 1 5
S Z A = 0 ºS Z A = 8 6 º
3 0 0 4 0 0 5 0 0 7 0 0
At large SZA very little UV-B radiation
reaches the troposphere
• Aside form the altitude, the path length through the atmosphere critically depends on the time of day and geographical location.
• Path length can be calculated using the flat atmosphere approximation for zenith angles under 80º. Beyond that, Earth curvature and atmospheric refraction start to matter.
![Page 6: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/6.jpg)
6
Direct Attenuation of Radiation
I ≡ radiation intensity (e.g., F)I0 ≡ radiation intensity above atmospherem ≡ air masst ≡ attenuation coefficient due to
– absorption by gases (ag)– scattering by gases (sg)– scattering by particles (sp)– absorption by particles (ap)
apspagsg
m-t
ttttt eI I
+++=×= ×
0
Rayleigh scatteringtsg ∝ λ-4
Deep UV – O, N2, O2Mid UV & visible – O3
Near IR – H2OInfrared – CO2, H2O, others
tag ∝ σ
tsp ∝ λ-n
much more
complex
From F-P&P & S. Nidkorodov
Scattering by Gases• Purely physical
process, not absorption
• Approximation:
• Strongly increases as λdecreases
• Reason why “sky is blue” during the day
From Turco
420
5 /)1(10044.1 λλ −⋅⋅= ntsg
![Page 7: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/7.jpg)
7
Scattering & Absorption by Particles• Particles can scatter
and absorb radiation• Scattering efficiency is
very strong function of particle size– For a given wavelength
• Visible: λ ~ 0.5 µm – Particles 0.5-2 µm are
most efficient scatterers!
• Will discuss in more detail in aerosol lectures
From Jacob
Gas Absorption: Beer-Lambert Law I
• Allows the calculation of the decay in intensity of a light beam due to absorption by the molecules in a medium
Definitions:• A = ln(I0/I) = Absorbance = σ⋅L⋅N(also “optical depth”)• σ ≡ absorption cross section
[cm2/molec]• L ≡ absorption path length [cm]• n ≡ density of the absorber
[molec/cm3]
)exp(0 NLII ⋅⋅−= σ
Solve in class: Show that in the small absorption limit the relative change in light
intensity is approximately equal to absorbance.
From F-P&P & S. Nidkorodov
![Page 8: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/8.jpg)
8
Beer-Lambert Law IIPitfalls:• Other units are frequently used to express absorbance,
for example:A = ln(I0/I) = ε×L×C A = ln(I0/I) = α×L×Pε ≡ extinction coefficient [L mol-1 cm-1]α ≡ absorption coefficient [atm-1 cm-1]C ≡ density of the absorber [mol L-1]P ≡ partial pressure [atm]
• Base 10 is used in most commercial spectrometers instead of the natural base:Abase 10 = log(I0/I) = Abase e/ln(10)
Physical interpretation of σ• σ , absorption cross section (cm2 / molecule)
– Effective area of the molecule that photon needs to traverse in order to be absorbed.
– The larger the absorption cross section, the easier it is to photoexcite the molecule.
– E.g., pernitric acid HNO4
Collisionsσ ≈ 10-15 cm2/molec
Light absorptionσ ≈ 10-18 cm2/molec From S. Nidkorodov
![Page 9: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/9.jpg)
9
Measurement of absorption cross sections is, in principle, trivial. We need a light source, such as a lamp (UV), a cell to contain the molecule of interest, a spectral filter (such as a monochromator) and a detector that is sensitive and responds linearly to the frequency of radiation of interest:
Measurements are repeated for a number of concentrations at each wavelength of interest.
Although seemingly trivial, in practice such measurements are difficult because of impurities, especially when it comes to very small cross sections (< 10-20 cm2/molec)
Gas cell
n [#/cm3]DetectorFilter
L
I0
L
I
Solve in class: Sample contains 1 Torr of molecules of interest with σ=3x10-21 cm2/molec
and 1 mTorr of impurity with σ=2x10-18 cm2/molec. What is the total absorbance in a 50 cm cell?
Measurement of Absorption Cross Sections
From S. Nidkorodov
dzzmz
eI I
dzzA
A eI I
z
z-
z
mA-
∫
∫
∞
∞
××
××=
≡×=
=
≡×=
)](O[)(),(
depth optical where)()(
:nely writteAlternativ
)](O[
densitycolumn where)()(
3
),(0
3
)(0
λσλτ
τλλ
λλ
λτ
λσ
Attenuation coefficient is dominated by O3absorption in the 200-300 nm window. Therefore, direct attenuation can be easily calculated from known absorption cross sections of O3. Similar formulas apply to attenuation by O2 in 120-180 nm window.
Solve in class: Using barometric law estimate column density of O2 in the atmosphere. By how
much does atmospheric O2 attenuate solar radiation at around 170 nm (σ ≈ 10-17 cm2/molec) at noon (m = 1)? Assume that O2 fraction (21%) is
independent of altitude and T = 270 K.Ans: 4x1024; by exp(-107)
Example: UV Attenuation by O3 and O2
From S. Nidkorodov
![Page 10: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/10.jpg)
10
Solar Radiation IntensityTo calculate solar
spectral distribution in any given volume of air at any given time and location one must know the following:– Solar spectral
distribution outside the atmosphere
– Path length through the atmosphere
– Wavelength dependent attenuation by atmospheric molecules
– Amount of radiation indirectly scattered by the earth surface, clouds, aerosols, and other volumes of air
From F-P&P
Surface Albedo
• Wavelength dependent!
• Question: for the same incident UV solar flux, will you tan faster over snow or over a desert?
)Radiation(Incident )Radiation( Reflected)(
λλλ =Albedo
From F-P&P
![Page 11: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/11.jpg)
11
Total vs. Downwelling Radiation• If atmosphere was
completely transparent and surface completely absorbing (albedo = 0)– FU = 0– FD =FT= 1
• Due to gas + aerosol scattering and surface reflection– FU can be large– FT > solar flux!
From Warneck
Calculation of Photolysis Rates I
• A “first-order process”• What does JA depend on?• JA depends on
– Light intensity from all directions• “Actinic flux”
– Absorption cross section (σ)– Quantum yield for photodissociation (φ)– All are functions of wavelength
][][ AJdtAd
A−=
Generic reaction: A + hν → B + C
![Page 12: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/12.jpg)
12
Calculation of Photolysis Rates II
][)()()(][][ AdFAJdtAd
AAA ×−=−= ∫ λλλφλσ
Generic reaction: A + hν → B + C
JA – first order photolysis rate of A (s-1)
σA(λ) – wavelength dependent cross section of A (cm2/#)
φA(λ) – wavelength dependent quantum yield for photolysis
F(λ) – spectral actinic flux density (#/cm2/s)
So, what are the smallest cross sections that matter?The solar actinic flux (photons cm-2 s-1 nm-1) is of order 1014. In many cases, we need to know whether the photolytic lifetime of a molecule is 10 days (J=10-6 s-1), or 100 days (J=10-7 s-1). This means that cross sections as small as 10-20 cm2 or even smaller are potentially interesting. Such small cross sections are very challenging to measure with sufficient accuracy.
J m-2 s-1Irradiance: same as flux but for a unit area with a fixed orientationE
J m-2 s-1 nm-1Spectral irradiance: same as radiance but per unit wavelength rangeE(λ)
J m-2 s-1 sr-1Radiance: radiant flux density per unit solid angleL(θ, ϕ)
J m-2 s-1 nm-1 sr-1Spectral radiance: same as radiance but per unit wavelength rangeL(θ, ϕ, λ)
J m-2 s-1 nm-1Spectral actinic flux density: same as flux but per unit wavelengthF(λ)
J m-2 s-1Actinic flux density: energy crossing a unit area per unit time without consideration of direction (we do not care where photons come from)F
UnitsDescriptionQuantity
∫ ∫∫
∫ ∫∫
==
==
π π
ω
π π
ω
ϕθθϕθωϕθ
ϕθθθϕθωθϕθ
2
0 0
2
0 0
sin),(),(
sincos),(cos),(
ddLdLF
ddLdLE
Radiance as a function of direction gives a complete descriptionof the radiation field. When L is independent of direction, the field is called isotropic, in which case E = π L and F = 2π L.
Solve in class: There are 109 photons flying into a 0.01 cm diameter opening every second. What is F with respect to this opening in units of #/cm2/s?
Radiation Fluxes Definitions
From F-P&P& S. Nidkorodov
![Page 13: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/13.jpg)
13
Radiation Mesurements
• Radiation does not just come directly from the sun– scattered radiation is just as important– Measure total or spectrally-resolved flux– Use models
Flat Plate → Irradiance 2π → ½ of Actinic Flux
From F-P&P
Radiation Models• Predict radiation intensity
– As f(time, altitude, latitude, λ)– Results of Madronich (1998) described in text
• Will use extensively in homeworks & exams
– Typical model results:From F-P&P
![Page 14: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/14.jpg)
14
Example model results
From F-P&P
Q: summer/winter solstices intensity at 445 nm: @ 8 am? @ noon?
Examples of Photolysis Rates
From F-P&P
![Page 15: Lecture 6: Spectroscopy and Photochemistry II - welcome | …cires1.colorado.edu/jimenez/AtmChem/CHEM-5151_S05… · · 2005-01-24Lecture 6: Spectroscopy and ... • Attenuation](https://reader030.vdocuments.us/reader030/viewer/2022020315/5aa66cdf7f8b9a7c1a8eac36/html5/thumbnails/15.jpg)
15
Example: Photolysis of CH3CHO∑∫
=
∆≈=i
nmFdFJ
λ
λ
λλλφλσλλλφλσ290
)()()()()()(CH3CHO + hν → CH3 + HCO (a)
→ CH4 + CO (b)
From F-P&P