CHEM-5151_S05_L6

CHEM-5151_S05_L6 - Lecture 6: Spectroscopy and...

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1 Required Reading: FP Chapter 3 Suggested Reading: SP Chapter 3 Atmospheric Chemistry CHEM-5151 / ATOC-5151 Spring 2005 Prof. 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
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2 Reminder of EM Spectrum Blackbody Radiation Linear Scale Log Scale From R.P. Turco, Earth Under Siege: From Air Pollution to Global Change, Oxford UP, 2002.
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3 Solar & Earth Radiation Spectra • Sun is a radiation source with an effective blackbody temperature of about 5800 K • Earth receives circa 1368 W/m 2 of energy from solar radiation • Question: are relative vertical scales ok in right plot? From Turco From S. Nidkorodov Solar Radiation Spectrum II From Turco From F-P&P •Solar spectrum is strongly modulated by atmospheric scattering and absorption
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4 Solar Radiation Spectrum III From Turco UV C B A Photon Energy O 3 O 2 Solar Radiation Spectrum IV • Solar spectrum is strongly modulated by atmospheric absorptions • Remember that UV photons have most energy –O 2 absorbs extreme UV in mesosphere; O 3 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 O 3 layer From F-P&P
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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 sec h L pathlength Vertical pathlength Actual m Mass" Air " θ = = = Solar Flux W avelength [nm] 1000 Solar Flux [Photons cm -2 s -1 nm ] 10 12 13 14 15 SZA = 0º SZA = 86º 300 400 500 700 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.
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6 Direct Attenuation of Radiation I radiation intensity (e.g., F ) I 0 radiation intensity above atmosphere m air mass t attenuation coefficient due to – absorption by gases (ag)
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This note was uploaded on 03/03/2012 for the course CHEM 5151 taught by Professor Staff during the Spring '08 term at Colorado.

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CHEM-5151_S05_L6 - Lecture 6: Spectroscopy and...

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