Lecture 6: Spectroscopy and Photochemistry II

Lecture 6: Spectroscopy and Photochemistry II

Required Reading: FP Chapter 3 Suggested Reading: SP Chapter 3

Atmospheric Chemistry CHEM-5151 / ATOC-5151

Spring 2005 Prof. Jose-Luis Jimenez

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

1

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.

2

Solar & Earth Radiation Spectra

From S. Nidkorodov

? 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

From Turco

? Question: are relative vertical scales ok in right plot?

Solar Radiation Spectrum II

From Turco

?Solar spectrum is strongly modulated by atmospheric scattering and absorption

From F-P&P

3

Solar Radiation Spectrum III

UV

C B A

Photon Energy

From Turco

Solar Radiation Spectrum IV

O3 O2

From F-P&P

? 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 >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 layer

4

Solar Radiation Spectrum vs. altitude

From F-P&P

? Very high energy photons are depleted high up in the atmosphere ? Some photochemistry is possible in stratosphere but not in troposphere

? Only > 290 nm in trop.

Solar Zenith Angle

? 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 80?. Beyond that, Earth curvature and atmospheric refraction start to matter.

Solar Flux

1015

"Air Mass" = m = Actual pathlength L = sec Vertical pathlength h

1014 SZA = 0? SZA = 86?

Solar Flux [Photons cm-2 s-1 nm-1]

1013

At large SZA very

little UV-B radiation

reaches the

troposphere

1012

300 400 500 700 W avelength [nm ]

1000

5

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