Using a Radiative Transfer Model in Conjunction with

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 • Aerosol optical depth • Angstrom exponent • θ • • φ Ω • jν Emission coefficient • κν • Sν Absortion coefficient Source term = jν /κν Solar zenith angle 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 , in the direction of the unit vector ; in other words it is the integral of over frequency: Flux density or irradiance (Units: W m-2) Total energy passing through a plane (integral of radiance over solid angle)

Radiative Transfer Equation Optical Depth: bext aerosol extinction coefficient, zmin , zmax lower and upper bounds of the heights of the atmospheric layer. 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:

Radiative Transfer Equation General Solution The equation of transfer for plane-parallel atmospheres Solution for finite plane atmosphere ( )

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

Figures

Single Scattering Albedo vs Aerosol Optical Depth Figure 2: Sensitivity study: τaer vs ωaer (332 nm)

Aerosol Optical Depth & Asymmetry Parameter Figure 3: Sensitivity study: τaer vs g (332 nm)

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 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 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 SSA 332 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 p 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, 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