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