52
October 2016
4.2 Particle characterization
It is possible to determine the distribution in number and size of the particles
constituting the aerosol. These particles whose diameter is between 10-3 and 100
microns are particularly concentrated over the industrialized regions of the
Northern Hemisphere.
The Ångström coefficient
is a sensitive index to the size distribution of aerosols.
It is inversely related to the average particle size of the aerosol particles are the
more smaller, the exponent is high.
This coefficient is also a good indicator of the proportion of atmospheric
precipitable water, where the aerosol concentration plays now recognized as a
very important role. It allows to anticipate the volume expected in a season
precipitation. Depending on the concentration of water present in the atmosphere,
a higher coefficient of favoring the concentration of the clouds and rain.
Calculation :
The Angström coefficient
α
is calculated with Optical thickness data (
τ
a
n
) taken at
two different wavelength
λ
1
and λ
2
:
τ
a
1
=
β
.
λ
1
-α
τ
a
2
=
β
.
λ
2
-α
<=>
τ
a
1
/
τ
a
2
= λ
1
-α
/ λ
2
-α
<=>
log(
τ
a
1
/
τ
a
2
) = -α . log ( λ
1
/ λ
2
)
<=>
log(
τ
a
1
/
τ
a
2
) = α . log ( λ
2
/ λ
1
)
<=>
α = log(
τ
a
1
/
τ
a
2
) / log ( λ
2
/ λ
1
)
The typical value range
α
is from 0.5 to 2.5 with an average value of 1.3 for natural
atmosphere.
Example :
Seysses, september 1
st
2010 at 12h11:19 UT.
λ
1
= 0.675 µm
τ
a
1
= 0.10
λ
2
= 0.532 µm
τ
a
2
= 0.13
Calculation of
α
:
α = log(0,100 / 0,135) / log ( 0,532 / 0,675 ) =
1,126
Note: Both neperian logarithms and decimal logarithms can be used. For
consistency with the programs of Calitoo and the PC software which have their
own computational constraints, the "Ln" have been replaced here by "Log" from the
2016 version of this document.
Aerosol global map :
Aerosol Size GlobalMaps
Summary of Contents for CALITOO
Page 49: ...October 2016 49 4 Appendix...
