Using a Radiative Transfer Model in Conjunction with UV-MFRSR

Irradiance Data for Studying Aerosols in El Paso-Juarez Airshed

by Richard Medina Calderón

Radiative Transfer EquationRadiative Transfer Equation

InstrumentationInstrumentation

Tropospheric Ultraviolet ModelTropospheric Ultraviolet Model

ResultsResults

ObjectivesObjectives

ConclusionsConclusions

Outline

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 UV-MFRSR instrument.

ObjectivesObjectives

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.

ObjectivesObjectives

Notation• Aerosol optical depth

• Angstrom exponent

• θ Solar zenith angle

• φ Azimuth angle

• Ω Solid angle

• jν Emission coefficient

• κν Absortion coefficient

• Sν Source term = jν /κν

Radiative Transfer EquationRadiative Transfer Equation

Radiative Transfer EquationRadiative Transfer Equation

Flux density or irradiance

Total energy passing through a plane (integral of radiance over solid angle)

(Units: W m-2)

20

)( sn̂ )s,r( )n̂,r()n̂,r( sdIdFF

0

)s,r( )s,r( dII

Radiance or intensity (Units: W m-2 sr -1)Is the power per unit area, per unit solid angle at a point , in the direction of the unit vector ; in other words it is the integral of over frequency:

r

s

I

bext aerosol extinction coefficient, aerosol extinction coefficient, zzminmin , z , zmaxmax lower and upper bounds of the lower and upper bounds of the heights of heights of the atmospheric the atmospheric layer. layer.

θθ the solar zenith anglethe solar zenith angle

Radiative Transfer EquationRadiative Transfer Equation

Optical Depth:Optical Depth: dz zb zmax

min

z

z ext ),(),(

os c

Radiative Transfer EquationRadiative Transfer Equation

Time Independent form of radiative transfer equation:

)ˆ,( )ˆ,( )ˆ,( )ˆ,(

rIrrj

s

rI

Consider a small cylindrical element of cross section dσ in a medium with anabsorption coefficient κν and an emission coefficient jν

Radiative Transfer EquationRadiative Transfer Equation

sdsssSsIsIs

)],(-exp[)(])0,(exp[- )0( )(0

General Solution

The equation of transfer for plane-parallel atmospheres

),,( ),,( ),,(

SId

dI

Solution for finite plane atmosphere ( )1 and 0

)(1 dt

e S e I I

and

)(1 dt

e S e I I

t(-

(t-(-

0),,(),,0(),,(

0),,(),,(),,(

)

0

))1

1

1

Single Scattering Albedo (SSA). Measure of particle scattering relative to total extinction(bext) by particles (absorption + scattering).

Radiative Transfer EquationRadiative Transfer Equation

absscatt

scatt

b b

b SSA

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

d

d sin cos

2

1 g

sin)(

)(

0

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.

InstrumentationInstrumentation

•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

UV-MFRSR

InstrumentationInstrumentation

Tropospheric Ultraviolet ModelTropospheric 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 ModelTropospheric Ultraviolet Model

Latitude,Longitude, Altittude,Local Time, AOD,

Angstrom Exponent, O3, NO2, etc.

TUV Ranges of SSA, and Asymmetry Parameter

DDR for each SSA and Asymmetry Parameter

Compare each DDRvalue (TUV) to a measuredDDR value (MFRSR)

Best Fit of SSA , Surface Albedo, and Assymeter

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

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

Table 2: Retrieval values of g

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 1: Retrieval values of ωaer

Tables

ResultsResults

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

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].

ResultsResults

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.

ConclusionsConclusions

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.

ConclusionsConclusions

Thank You