Group 4. Local Scattering Closure (Rood)

Input parameters needed to test for local light scattering closure were initially discussed. Those inputs are: measured aerosol particle (1) light scattering coefficient (2) refractive index, (3) mass concentration, (4) mass density, and (5) wavelength of light scattered. Such information is needed as a function of particle size. Once refractive index and number concentration of the particles are known as a function of actual size, then Mie light scattering theory can be used to calculate the aerosols' light scattering coefficients. Calculated and measured light scattering coefficients can then be compared to evaluate closure.

Measured input parameters available by measurements made at Cape Grim include:

1. light scattering coefficient dependence on two upper particle size cuts, wavelength of scattered light, direction of light scattered, and controlled relative humidity using an integrating nephelometer

2. Dry particle number distribution based on size using a differential mobility analyzer

3. Particle size increase due to increasing relative humidity conditions using a controlled relative humidity tandem differential mobility analyzer

4. Particle mass and composition distribution using impactors, gravimetric analysis and ion chromatography

All of the results listed above were submitted and made available on Codiac by the completion of the workshop.

Future research will occur to analyze the impactor results to determine the composition, hence refractive index, and then density of the particles based on their particle size.

Future work:

Evaluate the differences between the mass and ion chromatographic results of the impactor. Integrate data set to calculate light scattering coefficients. Compare experimental and modeled results.

If the dry light scattering closure is encouraging, then it would be useful evaluate light scattering closure as a function of relative humidity. There were discussions to use measured particle size growth curves, thermodynamic modeling, and metastable state modeling to describe particle growth dependence on relative humidity. These results could then be used with Mie Light scattering theory to predict light scattering coefficient dependence on controlled relative humidity. Closure could then be compared between modeled and measured light scattering coefficient dependence on controlled relative humidity.

Regional Characterization of Scattering Coefficients

Scattering coefficient results were compared between nephelometer measurements at Cape Grim, Discover, and the C-130 aircraft. Scattering coefficients [Mm^-1] at 550 nm and particle diameters < 1 µm were 0.6, 0.7, and 0.8, respectively, when the three platforms were operating as close to each other as possible. The arithmetic mean and standard deviation for the scattering coefficients at 550 nm and particle diameter < 1 µm during all measurements made at Cape Grim and Discoverer were 3.6± 1.9, and 4.1± 2.8, respectively. The arithmetic mean and standard deviation for the scattering coefficients at 550 nm and particle diameter < 10 µm during all measurements made at Cape Grim and Discoverer were 15.4 ± 7.9, and 18 ± 12, respectively.

Future Work:

Data sets describing light scattering coefficients from Cape Grim, Discover, and C-130 need to be carefully evaluated and integrated to see how well the measurements were able to characterize the region.