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Radiation Transfer in the Atmosphere: Why the sky is blue (mostly)and sunsets are red (some are redder).: 

Radiation Transfer in the Atmosphere: Why the sky is blue (mostly) and sunsets are red (some are redder).

Today’s Lecture: 

Today’s Lecture The big picture: A little spectroscopy Aerosols and scattering Global warming Derivation of Radiation Transfer Equation More on aerosols: blue skies and red sunsets Measuring atmospheric opacity and aerosols This will be On Thursday!

A little spectroscopy: 1: 

A little spectroscopy: 1 Why do atoms absorb light? Quantum Mechanics says only discrete (fairly discrete) energy states are permitted for electrons bound in atoms. Only frequencies corresponding to the energy differences between two states can be absorbed (or emitted) DE = hf. Each atom has a unique fingerprint spectrum Electronic absorptions in the Ultraviolet, Visible and Infrared

Slide4: 

A little spectroscopy: 2 Why do molecules absorb light? Quantum Mechanics says only discrete (fairly discrete) energy states are permitted for vibration and rotation of molecules. Only frequencies corresponding to the energy differences between two states can be absorbed (or emitted) DE = hf. Each molecule has a unique fingerprint spectrum Molecular vibration and rotation absorptions are in the infrared and far infrared

Molecular vibration and rotation: 

Molecular vibration and rotation http://science.widener.edu/svb/ftir/ir_co2.html O O O O O O O O C C C C

Aerosols 1: 

Aerosols 1 What is an aerosol? Anything larger than a single molecule that is suspended in the atmosphere for times equal to or longer than a day. Particulate matter in the atmosphere Ultrafine: radius from .001 to .01 mm Fine: radius from .01 to 1 mm (accumulation mode) Coarse: radius > 1 mm A large clump of gas particles growing to an aggregate particle, with or without water, possibly organic or carbon (soot) or sulfate, or mineral dust

Aerosols 2: 

Aerosols 2 Where do aerosols come from? Ultrafine: condensation from gas Fine: coagulation from Ultrafine and condensation from gas Coarse: mineral dust, ice particles, soot Sources: Anthropogenic and Natural Volcanoes, deserts, fires, combustion, industry Where do aerosols go: Ultrafine and Fine: evaporate or scavenged by raindrops (wet deposition) Coarse: Fall out by sedimentation (too heavy) (dry deposition) or rainout (wet deposition)

Aerosols 3: 

Aerosols 3 What do aerosol spectra look like? Aerosols can absorb as their constituent molecules would (black carbon looks black!) Aerosols can scatter light: redirect its path Scattering? Partly geometrical optics: reflection and refraction Partly wave optics: diffraction Wavelength > radius  Rayleigh scattering ~ l-4 Wavelength < radius  Mie scattering Mie (1908) solved Maxwell’s Equations for spherical particles

Aerosols 4: Scattering 1: 

Aerosols 4: Scattering 1 From Daniel Jacob, Introduction to Atmospheric Chemistry, Princeton, 1999.

Aerosols 5: Scattering 2: 

Aerosols 5: Scattering 2 From Daniel Jacob, Introduction to Atmospheric Chemistry, Princeton, 1999.

Aerosols 6: Scattering 3:More or Less Light?: 

Aerosols 6: Scattering 3: More or Less Light?

Aerosols 6: Scattering 3:More or Less Light?: 

Aerosols 6: Scattering 3: More or Less Light?

Aerosols and Global Warming 1: 

Aerosols and Global Warming 1 What if aerosols reflect incoming sunlight back to space? Net cooling below the aerosols Called the Direct effect What if dissolved aerosols in cloud droplets changes their size or optical properties? Could enhance or decrease scattering leading to cooling or warming. Called the Indirect Effect What if aerosols absorb sunlight? Net cooling below aerosols Warming of the air around the aerosols

Aerosols and Global Warming 2: 

Aerosols and Global Warming 2 Recent studies indicate about 50% of the expected Global Warming from greenhouse gases has been masked by aerosol cooling via the Direct Effect Removing aerosol particulate pollution could lead to an increase in global warming Not removing aerosol particulate pollution will lead to more cases of Asthma and other respiratory illnesses Rainfall patterns are disrupted via the Indirect Effect

Slide15: 

Radiation Transfer Equation 1

Slide16: 

Radiation Transfer Equation 2

Slide17: 

Radiation Transfer Equation 3 Absorption (-dI)  mass of absorber (r dz)  Incident Radiation (I) This is known as Beer’s Law (or Lambert’s or Bouguet’s)

Radiation Transfer Equation 4: 

Radiation Transfer Equation 4 Beer’s Law Absorption  (mass of absorber)  (Incident Radiation) -dI = k (r dz) I proportionality constant Absorption coefficient

Slide19: 

Beer’s Law -dI = k (r dz) I dI/I = - k (r dz) With solution: I = Ioe-krdz k = absorption coefficient (cm2/g) Radiation Transfer Equation 5

Slide20: 

Beer’s Law for single scattering -dI = s (r dz) I With solution: I = Ioe- sr dz = scattering coefficient (cm2/g) is a material property of the aerosol! Optical depth = d ≡  sr dz (unitless, from 0 to ) Integrate from observation point toward source Transmittance = T ≡ e-d (from 1 to 0) Radiation Transfer Equation 6

Slide21: 

Plane parallel atmosphere approximation Radiation Transfer Equation 7 Io I dz atmosphere (r) -dI I  dz cos  Optical depth = d ≡  s r dz cos 

Slide22: 

Plane parallel atmosphere assumptions Horizontally uniform atmosphere Vertically uniform layer(s) Definitions Solar Zenith Angle =  AOT: Aerosol Optical Thickness (optical depth) Always with reference to vertical optical depth (dz) Air Mass (or air mass factor) = Radiation Transfer Equation 8

Slide23: 

Rayleigh scattering off air molecules: ~ l-4 Blue scattered more than red Scattered light is polarized! Why is the Sky Blue?

Slide24: 

Rayleigh scattering of all BUT red light Large air mass factor as   90°! Why are sunsets red? I -dI  dz cos 

How do we measure aerosol attenuation?: 

How do we measure aerosol attenuation? If you know Io, then measure I(l,t) Find AOT from I/Io If you don’t know Io? Measure I(l,t,) Plot air mass vs. I(l,t,) (x vs. y) This is called a Langley Plot For air mass = 0, intercept = ? Slope = ? What assumptions do you have to make here?

How do we measure I(l,t,)?: 

How do we measure I(l,t,)? Use a Sun Photometer Measures solar flux at different wavelengths High quality sun photometers from NASA Already calibrated: known Io Pre-programmed for Lat,lon Records data, time, pressure, etc. Homemade sun photometers you will build Simple circuit with LED detector Uncalibrated! You have to write everything down!

Class Lab Project: Measure I(l,t,)!: 

Class Lab Project: Measure I(l,t,)! Three groups of three students each Build homemade sun photometers Go test homemade sun photometers Check out a NASA sun photometer Compare NASA vs. homemade Calibrate homemade Make measurements over several hours Different groups make measurements from different locations Make Langley plots Plot NASA solar flux vs. wavelength Plot AOT vs wavelength, time Experiment!

Using NASA Sun Photometer: 

Using NASA Sun Photometer Put on sunglasses! Turn it on and keep front cover closed Wait for RDY on screen, then open cover Point at sun and line-up white dot in circle Press Scan to make observations Takes 5-8 sec for measurements Press right arrow to see observations Up, down to scroll through observations Press escape to go back to RDY Measures flux at 5 wavelengths (340,380,440,500,870 nm)

Homework Assignment: 

Homework Assignment Problem 8-2, #1 in Jacob’s book Problem 8-2, #2 in Jacob’s book Writing Beer’s Law as: I = Ioe-d/cos where d =  s r dz How can you find d from two measurements of I at different , assuming d is constant between the two measurements? Plot measurements from your homemade sun photometer using the relation from A (perhaps air mass vs. ln V) and compute d from each pair of measurements. You might need to know V for no sunlight (the background term). If d truly were constant, what would the slope give you? Compare your measurements of d with those from the NASA sun photometer at 500 nm (a plot and a table would be good ideas). Can you calibrate your homemade sun photometer?