now that i know that… what do i do? (analyzing your microtop solar radiometry data)
TRANSCRIPT
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Now That I Know That…
What Do I Do?
(Analyzing your Microtop Solar Radiometry Data)
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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
τ = the “optical thickness” of the medium.
)exp( T
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Review: Optical Thickness
Optical thickness τ is the (dimensionless) radiative unit of length
where:
n = the number of “extincters” (scatterers or absorbers) per unit volume in the medium
σ = the extinction cross-section (effective area per “extincter”)
s = the geometric path length
dsnd
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Linear Problem Additive τ
A medium typically has several kinds of “extincters”, but their effects are additive:
where:
na1 = the number of the 1st absorber per unit volume
σa1 = the absorption cross-section (effective area per absorber) for the 1st absorber
So:
(Etc., etc.)
dsnnnnd ssssaaaa ...)( 22112211
...2121 ssaa ddddd
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Beer’s 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 Beer’s Law as:
where:
F0 = the spectral solar extraterrestrial irradiance
τs = the “optical path length” of the medium along the solar beam.
)exp(0 sFF
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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).
A S
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Flat Atmosphere?
But as long as the sun is not too near the horizon (say, z < 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.
z
AS
cos
A S
AmS
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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
0
221111 )( dznnnm ssssaas
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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…
)exp(0 sFF
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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)
σa1 = ozone absorption cross-section (from lab measurements)
τs1 = molecular scattering optical depth (using laboratory measured cross-sections, and assuming a standard atmosphere, given your location)
0
2111 ssaas mmdznm )exp(0 sFF
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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.
21
0
11 ssaas mmdznm
0
2111 ssaas mmdznm
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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.”
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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.
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Ozone Cross-Section
σa1
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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:
• α is the “Angstrom coefficient” for the aerosol, and is typically ~ 1 or 2.
• Compare to Rayleigh scattering:
2s
41
s
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Final Adjustments
• Based on the manufacturer’s 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
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TEAM 2
Ozone Data from MICROTOPS 7
TEAM 1
Ozone Data from MICROTOPS 5
TEAM 3
AOT Data from MICROTOPS 5
(340 nm, 380 nm)
TEAM 4
AOT Data from MICROTOPS 7
(340 nm, 380 nm)
Inter-comparison
Diurnal Variation
TrendSatellite Data
Inter-comparison
Diurnal Variation
Trend
Satellite Data
OZONE
AEROSOLS
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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?
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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
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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
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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
305 312 320 340 380
Ozone (1/atm-cm)
5.084 1.977 0.858 0 0
Rayleigh (1/atm)
1.126 1.029 0.930 0.718 0.450