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Instructions
Above is a link to a model
of the Sun Photometer to simulate the function of a
real instrument. Taking
measurements with the Sun Photometer simply requires
aligning the
sunlight
on the LED, using the external guides, and measuring
the
corresponding voltage with a voltmeter. Adjust
the slider controls for the desired conditions.
Once
parameters are set, the CALIPSO sun photometer must
be aimed at the sun. This can be done manually by
clicking
on the sun photometer and dragging to rotate
it. The sun photometer should be positioned such that
the yellow beam
of light passing through the aperture (at the
top left of the photometer) strikes the blue dot on the
target at the
bottom left of the photometer. In addition, the
Sunlight Voltage for both the green channel and red channel
should be at their maximum when the photometer
is
positioned properly.
The sun photometer can also
be positioned automatically by clicking on the
button labeled Move Photometer.
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Slider Controls Explained:
- Atmospheric
Conditions - Level of haze due to aerosols.
- Sun Angle (Degrees) - (90 degrees is straight overhead)
Decreased sun angle early and late in the day increases
the amount of atmosphere that the light
from
the sun passes through before reaching the CALIPSO
sun photometer.
- Air pressure - Changes in
air pressure (due to altitude or weather systems)
change the density
of the air. A decrease
in air pressure (decrease in air
density) causes less light to be scattered by air molecules
and
increases the amount
of light reaching the CALIPSO sun photometer.
- Particle size - The relative
scattering of red vs. green light by aerosol
particles is dependent on the
particle
size.
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How
does Sun Photometer work?
Sun Photometers absorb sunlight
energy with the LED and convert the intensity into a
quantified voltage to measure aerosol
in the atmosphere. The intensity of sunlight at the top
of the earth's atmosphere is constant. While the sunlight
travels
through the atmosphere, though, aerosols can dissipate
the energy by scattering and absorbing the light. More
aerosols in the atmosphere cause more scattering and
less energy
transmitted
to the surface. Therefore, knowing the sunlight's energy
at the top of the atmosphere, the thickness of the
atmosphere, and the amount of sunlight transmitted to
the earth's surface
and can allows us to determine the amount of scattering,
and
thus, the amount of aerosols.
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Additional technical information:
Calculation of AOT:
The simulator can be used
to practice calculating the Aerosol Optical Thickness.
The calibration constant (VO) for the virtual photometer
is
2.117 V for both channels. This is also called the extraterrestrial
constant because it is the voltage that would be produced
by the sun photometer if it were outside of Earth’s
atmosphere.
The decrease in the voltage
measured by the sun photometer is the result of light scattered
by both
the aerosol particles
and the air molecules that make up the atmosphere. Mathematically,
this “total optical thickness” is the sum of
the aerosol optical thickness (AOT) and the non-aerosol
optical thickness. The total optical thickness can be
calculated using the following equation:
Total optical thickness =
sin(q)[lnVO– ln(V – VD)]
Where VO is the
calibration constant described above, V is the sunlight
voltage,
VD is the dark voltage, and
q is the sun angle. Once the total optical thickness
is calculated,
the AOT can be calculated by subtracting the non-aerosol
optical thickness from the total optical thickness:
AOT
= Total optical thickness – Non-aerosol optical
thickness
The non-aerosol thickness
can be read directly from the simulator, or calculated
using the following equation:
Non-aerosol optical thickness
= aR(p/pO)
Where aR is the
Rayleigh scattering (non-aerosol optical thickness)
at standard sea-level atmospheric pressure
(0.138 for the green channel and 0.058 for the
red channel),
p is the actual atmospheric pressure, and pO is
standard sea-level atmospheric pressure (1013 millibars).
Atmospheric Conditions
Due
to the fact that aerosol optical thickness is dependent
on wavelength, this control
setting represents the average
of the AOT at 525 nm (green) and 625
nm (red). The
range is from clear (AOT = 0) to Hazy
(AOT = 2). The actual red
and green AOTs are calculated based on
the average using the Angstrom wavelength coefficient
from the Particle Size
control and the Angstrom turbidity coefficient
expression.
Particle Size
The size of the
aerosol particles affect their ability to scatter
light at
different wavelengths. In
particular,
larger particles scatter red
and green light with about the same efficiency while
smaller particles scatter
more
green light than red light. This
slider
control varies the Angstrom wavelength
exponent from 0 (large particles)
to
2 (small particles)
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Contacts:
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Responsible NASA official:
Melinda Cagle, Science Manager, CALIPSO
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