Using a Radiative Transfer Model in Conjunction
with UV-MFRSR Irradiance Data for Studying
Aerosols in El Paso-Juarez Airshed
by
Richard Medina Calderón
Outline
Objectives
Radiative Transfer Equation
Tropospheric Ultraviolet Model
Instrumentation
Results
Conclusions
Objectives
1.
Implementing a light-based scattering technique to study in
situ Aerosols in the El Paso-Juarez Airshed using the
Ultraviolet MFRSR.
2.
Modifying and enhancing the Radiative Transfer Model
selected (TUV) to study pollutants in the El Paso-Juarez
Airshed.
3.
Validate the TUV Model using Irradiance Data from UVMFRSR instrument.
Objectives
4.
Performing sensitivity studies on key optical parameters such
as Single Scattering Albedo and Asymmetry Parameter using
the Direct to Diffuse Ratio of Irradiances (DDR) obtained
using the TUV Model.
5.
Use the Radiative Transfer Model as a diagnostic tool to
interpret MFRSR Irradiance data to be used in future
characterizations of pollutants for this Airshed.
Radiative Transfer Equation
Notation
•


•
θ
•
Aerosol optical depth
Angstrom exponent
Solar zenith angle
•
φ
Ω
•
jν
Emission coefficient
•
κν
•
Sν
Absortion coefficient
Source term = jν /κν
•
Azimuth angle
Solid angle
Radiative Transfer Equation
Radiance or intensity (Units: W m-2 sr -1)
Is the power per unit area, per unit solid
angle at a point r , in the direction of the
unit vector s ; in other words it is the integral
of I over frequency:
 
I (r, s) 


0
 
I ( r , s ) d
Flux density or irradiance (Units: W m-2)
Total energy passing through a plane (integral of radiance over solid angle)



 


ˆ
ˆ
ˆ
F ( r , n)   F ( r , n) d   I ( r , s ) n  s d( s )
0
2
Radiative Transfer Equation
Optical Depth:
zmax
 ( z, )   z
bext ( z, ) dz
min
bext
 aerosol extinction coefficient,
zmin , zmax  lower and upper bounds of the heights of
the atmospheric layer.
  cos  θ the solar zenith angle
Radiative Transfer Equation
Consider a small cylindrical element of cross section dσ in a medium with an
absorption coefficient κν and an emission coefficient jν
Time Independent form of radiative transfer equation:
 ˆ
 ˆ
 ˆ
 ˆ
I (r , )
  j (r , 
)    (r , 
) I (r , 
)
s
Radiative Transfer Equation
General Solution
s
I ( s)  I (0) exp[- ( s,0)]   S ( s) exp[- ( s, s)]  ds
0
The equation of transfer for plane-parallel atmospheres
dI ( ,  ,  )

 I ( ,  ,  )  S ( ,  ,  )
d
Solution for finite plane atmosphere (  0 and   1 )
I ( ,  ,  )  I ( 1 ,  ,  ) e
-(  1  ) 

dt
1

  S ( ,  ,  ) e -(t  ) 
(1    0 )
and
I ( ,  ,  )  I (0,  ,  ) e
 

dt
0

  S ( ,  ,  ) e -(  t ) 
(1    0 )
Radiative Transfer Equation

Single Scattering Albedo (SSA). Measure of particle scattering relative to
total extinction(bext) by particles (absorption + scattering).
SSA 
bscatt
bscatt  babs
Asymmetry parameter (g). Intensity-weighted average of the cosine of the
scattering angle, used to describe the direction in which most of the radiation is

scattered
1 0 cos   () sin d
g 

2
 () sin  d

0
Where Θ is the scattering angle, Ψ(Θ) is intensity.
Values for g range from -1 to +1.
Value of -1 indicate most of the radiation is backscattered.
Value of +1 indicate much of the radiation is forward scattered.
Value of 0 indicate the radiation is scattered isotropically.
Instrumentation
Instrumentation
UV-MFRSR
•Measures solar irradiance at seven narrowband wavelengths
(nominal 300, 305, 311, 317, 325, 332, and 368 nm) in the UV-B
and UV-A regions
•332 nm – 368 nm are sensitive to column aerosols
•317 nm - 325 nm are sensitive to column ozone
Tropospheric Ultraviolet Model
Direct versus Diffuse Radiation
• Shortwave radiation can be either direct (with a specific source in a specific
direction), or diffuse (coming from all directions).
• Direct radiation
• Emanates from the sun, which is typically treated as a point source of
radiation, traveling as a beam.
• Diffuse radiation
• Emanates from the entire hemisphere (above or below), and is scattered
sunlight. e.g., the light coming from a clear blue sky (or a grey cloudy sky).
• Has no specific direction, and is typically treated as uniform.
Tropospheric Ultraviolet Model
Latitude,Longitude,
Altittude,Local T ime, AOD,
Angstrom Exponent, O3,
NO2, etc.
TUV
DDR for each SSA and
Asymmetry Parameter
Compare each DDR
value (T UV) to a measured
DDR value (MFRSR)
Best Fit of SSA , Surface
Albedo, and Assymeter
Parameter
Ranges of SSA, and
Asymmetry Parameter
Figures
Single Scattering Albedo vs Aerosol Optical Depth
Figure 2: Sensitivity study: τaer vs ωaer (332nm)
Aerosol Optical Depth & Asymmetry Parameter
Figure 3: Sensitivity study: τaer vs g (332nm)
Retrieval of Single Scattering Albedo (Clean Day)
Figure 4: Retrieval of ωaer for 332 nm, clean day
Retrieval of Single Scattering Albedo (Dirty Day)
Figure 5: Retrieval of ωaer for 332 nm, dirty day
Retrieval of Asymmetry Parameter (Clean Day)
Figure 6: Comparison of g for 332 nm, clean day
Retrieval of Asymmetry Parameter (Dirty Day)
Figure 7: Comparison of g for 332 nm, dirty day
Tables
Table 1: Retrieval values of ωaer
Date (mmddyy)
λ(nm)
ωaer range
τaer range
τaer average
012809 (Clean Day)
332
0.66 - 0.81
0.072 - 0.308
0.097
020509 (Dirty Day)
332
0.58 - 0.70
0.100 - 0.264
0.150
Table 2: Retrieval values of g
Date (mmddyy)
λ(nm)
g range
τaer range
τaer average
012809 (Clean Day)
332
0.6 -0.8
0.072-0.308
0.097
020509 (Dirty Day)
332
0.6 -0.8
0.100-0.264
0.150
Results
The retrieved SSA332 value for the dirty day is in the range 0.6-0.7,
which justifies the presence of both soot and mineral dust particles
present in the atmosphere [Petters et al., 2003, Aerosol single scattering
albedo retrieved from measurements of surface UV irradiance and a
radiative transfer model].
For soot (absorptive): 0.58-0.48
For mineral dust (reflective): 0.67-0.95
Results

According to table 2, retrieval values of g using the DDR
method for clean and dirty days are in good agreement
with values of g for atmospheric aerosols, which range from
0.6 to 0.8, [Madronich, Environmental UV Photobiology,
Plenum Press, New York, New York, 1993].
Conclusions
1.
Sensitivity studies were performed to determine the
impact of numerous physical parameters on the Model’s
Irradiance results. The studies showed a larger influence in
the aerosol optical depth parameter.
2.
A new methodology was developed to use the Radiative
Transfer Model as a diagnostic tool to interpret MFRSR
data.
Conclusions
4.
Preliminary results show the presence of both small and
large size particles in our Airshed, even under no high
wind conditions, which is syntomatic of an interface region,
between an urban and a desert region, such as the El
Paso-Juarez Airshed.
5.
All the studies performed in this work will have an impact
on improving the air quality and consequently, the quality
of life for the El Paso-Juarez Airshed.
Thank You