spectrometry and photochemistry theodore s. dibble chemistry department suny-environmental science...
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Spectrometry and Photochemistry
Theodore S. Dibble
Chemistry Department SUNY-Environmental Science and Forestry Syracuse, NY
Role of Spectrometry and Photochemistry
• Light flux, F(), vs. wavelength, altitude, etc.
• Photochemistry as fate of a molecule
• Photolysis as radical source
• Greenhouse gas absorbances
• Concentration Measurement
Beer-Lambert Law
Absorbance A= ln (Io/I)base e not base 10
A = lc= absorption cross-section (per molecule)
cm2/molecule (= 3.8 10-21)c = concentration (molecules cm-3)l in cm
IoI
Example
Between 40 and 50 km, [O3] ~ 3 x 1011 molecules cm-3
254 nm = 1.1 x 10-17 cm2 molecule-1
Calculate Absorbance over the 10 km (106 cm)
2
116
217 10310
101.1
cm
moleculescm
molecule
cmA
A = 3.3
Light Intensity
Solar Zenith Angle – angle from perpendicular
(season, time of day, latitude: see Spreadsheet)
Other FactorsClouds
Albedo (reflectivity)Eccentricity
Why SZA Matters- Pathlength
lo
SZA=0 SZA=40
l = lo /cos(SZA)
Absorbance & scatter
Ozone UV Spectrum
Wavelength in nm
Ground Level Solar Flux, F()
1E+11
1E+12
1E+13
1E+14
1E+15
280 300 320 340 360Wavelength (nm)
Photons / (cm^2 sec^1)
SZA=60
SZA=0
Photolysis Rate Constant, J
Solar flux: F()Absorption cross section: ()
Quantum yield for photolysis: () (fraction of photons absorbed that cause decomposition)
wavelengthissociatedmoleculesd
wavelengthtimearea
photons
time
dFJ
photonmolecule
sectioncross1
)()()(
Ozone Photolysis Rate = JO3[O3]
][1
ionconcentrattimetime
ionconcentrat
Numerical Integration
)()()(FJ
)()()( FJ
Spreadsheet: www.esf.edu/chemistry/dibble/fch511/calculateJ.xls
Exercise: Calculate J for O3 or HOOH at ground levelUse absorption cross-sections from JPL Data Evaluation #14
Assume =1
Photolytic Production of Radicals
O3 + h → O2 (3)+ O(3P) ground state products
O3 + h → O2 (1)+ O(1D) excited state products
O(1D) much more reactive than O(3P)
Rate of production of O(1D) =
Rate of production of O(3P) =
dFODO
)()()(][)(3 1
dFOPO
)()()(][)(3 3
Quantum Yield for O (1D) from O3
Explain the altitude dependence of J(O(1D)) vs. J(O(3P))
1)()( 31
PODO
Key Points
• Ozone UV absorption dominates F()
• F() depends on SZA
• Photolysis rate constants readily calculable