Now That I Know That

1
Now That I Know That
What Do I Do?
(Analyzing your Microtop Solar Radiometry Data)
2
Review Transmissivity

• The probability that a photon will pass through a
  medium without interacting with it (absorption or
  scattering) is
• where
• T the transmissivity of the medium
• t the optical thickness of the medium.

3
Review Optical Thickness

• Optical thickness t is the (dimensionless)
  radiative unit of length
• where
• n the number of extincters (scatterers or
  absorbers) per unit volume in the medium
• s the extinction cross-section (effective area
  per extincter)
• s the geometric path length

4
Linear Problem ? Additive t

• A medium typically has several kinds of
  extincters, but their effects are additive
• where
• na1 the number of the 1st absorber per unit
  volume
• sa1 the absorption cross-section (effective
  area per absorber) for the 1st absorber
• So
• (Etc., etc.)

5
Beers Law

• Assume that your measurement consists only of
  solar radiation that is transmitted through the
  atmosphere without interacting with it.
• Then the measured spectral irradiance F can be
  described by Beers Law as
• where
• F0 the spectral solar extraterrestrial
  irradiance
• ts the optical path length of the medium
  along the solar beam.

6
Geometry

• Since the sun is not directly overhead, the
  geometric path length along the solar beam (S) is
  longer than a line along the zenith to the same
  altitude (A).

7
Flat Atmosphere?

• But as long as the sun is not too near the
  horizon (say, z lt 80º), the atmosphere can be
  treated as flat, and S is related to A by a
  simple cosine law, with 1/cos z called the air
  mass factor m.

8
Putting it all together

• If we assume that the atmosphere is horizontally
  homogenous, then m is the only difference between
  a zenith line of sight and our slant line of
  sight, and so
• where a1 ozone absorption,
• s1 Rayleigh (molecular) scattering, and
• s2 Mie (aerosol) scattering

9
So what did I measure?

• The only thing the instrument really measures is
  F at 5 wavelengths
• 305, 312, 320, 340 and 380 nm
• (ozone-sensitive) (aerosol-sensitive)
• The instrument did some internal calculations to
  give you more information, however

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And the other bits come from?

• F0 extraterrestrial solar spectral irradiance
  (from independent measurements)
• m air mass factor (from geometry, given your
  location and the local time)
• sa1 ozone absorption cross-section (from lab
  measurements)
• ts1 molecular scattering optical depth (using
  laboratory measured cross-sections, and assuming
  a standard atmosphere, given your location)

11
So what else did I get?

• The instrument therefore also can tell you about
• Ozone column amount (in Dobson
  units)
• and Aerosol Optical Depth (no units)
  
• at each wavelength.

12
And a Dobson Unit is?

• Take all of the ozone in a column above a given
  point at the surface, and compress it to p 1
  atm, T 0ºC.
• The resulting layer of ozone is typically 0.3
  cm thick, which corresponds to 0.3 atm-cm of
  ozone, or 300 Dobson units.

13
Why do I get several ozone estimates?

• The ozone estimate is made by comparing the
  differential absorption between 2 adjacent
  wavelengths whose sensitivity to ozone differs
  significantly.
• Each different estimate uses a different pair of
  wavelengths.
• If ozone is abundant, the weakly absorbed
  wavelengths will give a better ozone estimate if
  ozone is scarce, the strongly absorbed
  wavelengths will give a better estimate.

14
Ozone Cross-Section
sa1
15
Aerosol Cross-Section

• Depends on the nature of the aerosol (size
  distribution, optical properties, etc.).
• For typical tropospheric aerosol, the following
  rule is often useful to estimate the variation
  over small wavelength intervals
• a is the Angstrom coefficient for the aerosol,
  and is typically 1 or 2.
• Compare to Rayleigh scattering

16
Final Adjustments

• Based on the manufacturers calibration, you
  should make the following adjustments prior to
  using your data
• Instrument 5 Instrument 7
• Ozone is 1.2 high Ozone is 1.0 high
• 340nm aer is 0.007 high 340nm aer is 0.009 high
• 380nm aer is 0.048 high 380nm aer is 0.061 high

17
OZONE
Inter-comparison Diurnal Variation Trend Satell
ite Data
TEAM 2 Ozone Data from MICROTOPS 7
TEAM 1 Ozone Data from MICROTOPS 5
AEROSOLS
Inter-comparison Diurnal Variation Trend Sate
llite Data
TEAM 4 AOT Data from MICROTOPS 7 (340 nm, 380
nm)
TEAM 3 AOT Data from MICROTOPS 5 (340 nm, 380
nm)
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THE END
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Team 1

• You have 2 month-long datasets. Make ozone and
  aerosol plots that characterize
• The consistency of the two datasets
• The existence (or not) of diurnal trends in the
  retrieved quantities
• The existence (or not) of longer-term trends
  (weekly? Seasonal?)
• The relative skill of the various students?

20
Team 2

• Make ozone and aerosol plots that characterize
• The accuracy of the two instruments, as compared
  to satellite data (from the OMI instrument)
• Possible sources of disagreement between
  ground-based and satellite-based estimates of
  these quantities

21
Team 3

• Use the ozone and aerosol information to
  calculate the diffuse radiation (as well as the
  direct radiation).
• Comment on the relative contributions of diffuse
  vs direct radiation to the downward irradiance at
  the various wavelengths

22
Team 4

• Use the data to estimate the extraterrestrial
  solar spectral irradiance at each measured
  wavelength
• Compare your results to independent measurements
• This cross-section data might be useful