# Computer Modeling Laboratory 3

Written report due: 25 September

Modeling of optical characteristics with Mie theory.
Analysis of aerosol optical depth retrieved from ground-based sunphotometers.

RELATED MATERIALS:
Lecture 6, S 6.1

TASK 1

To calculate the optical characteristics of a single spherical particle run MIE_sphere

1) Compute and plot (as a function of size parameter in a range from 0.2 to 20) the extinction Qe, scattering Qs, and absorption Qa efficiencies for non-absorbing (m = 1.5), absorbing (m =1.5-i0.01), and strong-absorbing (m=1.5-i0.5) aerosols. Identify and briefly discuss the main differences between Qe, Qs and Qa of these aerosol types.

2) Calculate the total aerosol optical depth and the effective single scattering albedo at the 500 nm wavelength of a 1 km atmospheric layer containing two aerosol types: absorbing and non-absorbing aerosols. Use the refractive indices given above. Assume that all particles have a radius of 0.2 μm, both aerosol types have a particle number concentration of N= 100 #/cm3.

TASK 2

Aerosol Robotic Network (AERONET) is an automatic sun and sky scanning measurement program established by NASA to perform systematic ground-based observations of aerosols. The remote sensing of aerosols from a ground-based sunphotometer utilizes the direct beam of solar radiation transmitted through the atmosphere. The sunphotometer measurements at several wavelength are used to retrieve the spectra aerosol optical depth (AOD)and several other aerosol properties. Read two brief documents describing AOD and the retrieval process:
Measurements of AOD  Inversion products

1) Go to Aerosol Optical Depth => Climatology maps.
By clicking on 500 nm Aerosol Optical Thickness (Level 2.0), you will see animation of AOD climatology.
Identify locations and corresponding months for the largest values of AOD. What type of aerosol is likely to be responsible for these large AODs?
Would you except to find larger or smaller values of the Angstrom exponent for stations located at the West coast of Northern Africa compared to Southern America? Check your guess by examining animation of the Angstrom exponent.

2) Go to Aerosol Optical Depth => Display data. Find station Kuwait_University (see the list of stations below the map). Examine the behavior of AOD during March of 2008 and find the largest observed AOD at the 500 nm (you can estimate max AOD from the plot (click on the plot to zoom in) or download data by clicking on Downloadable products). For this AOD value, calculate the fraction of direct solar radiation that reaches the surface. Hint: don't forget about Rayleigh scattering. Ignore absorption by gases.

Now go to AEROSOL INVERSION and find a value of the single scattering albedo (SSA) when max AOD was observed. Then select another day with smaller AOD and find what SSA was retrieved on that day. Would you expect to see any correlation between AOD and SSA (e.g., smaller AOD corresponds to smaller SSA)? Why or why not?