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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( l ), vs. wavelength, altitude, etc. Photochemistry as fate of a molecule Photolysis as radical source

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## Spectrometry and Photochemistry

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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(l), 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 I Io A = slc s= absorption cross-section (per molecule) cm2/molecule (s= e 3.8 10-21) c = concentration (molecules cm-3) l in cm**A =3.3**Example Between 40 and 50 km, [O3] ~ 3 x 1011 molecules cm-3 s254 nm = 1.1 x 10-17 cm2 molecule-1 Calculate Absorbance over the 10 km (106 cm)**Light Intensity**Solar Zenith Angle – angle from perpendicular (season, time of day, latitude: see Spreadsheet) Other Factors Clouds Albedo (reflectivity) Eccentricity**Why SZA Matters- Pathlength**Absorbance & scatter lo l = lo /cos(SZA) SZA=0 SZA=40**Ozone UV Spectrum**Wavelength in nm**Photolysis Rate Constant, J**Solar flux: F(l) Absorption cross section:s(l) Quantum yield for photolysis: f(l) (fraction of photons absorbed that cause decomposition) Ozone Photolysis Rate = JO3[O3]**Numerical Integration**Spreadsheet: www.esf.edu/chemistry/dibble/fch511/calculateJ.xls Exercise: Calculate J for O3 or HOOH at ground level Use absorption cross-sections from JPL Data Evaluation #14 Assume f =1**Photolytic Production of Radicals**O3 + hn→ O2 (3S)+ O(3P) ground state products O3 + hn → O2 (1D)+ 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) =**Quantum Yield for O (1D) from O3**Explain the altitude dependence of J(O(1D)) vs. J(O(3P))**Key Points**• Ozone UV absorption dominatesF(l) • F(l) depends on SZA • Photolysis rate constants readily calculable

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