Particles light scattering coefficients

The particle light scattering coefficient has been continuously measured at this location since 1976. Measurements of the particle size distribution have been made daily since 1978, providing the data base necessary to assess the variability of the normalized aerosol volume distribution. 

Aerosol sampling instruments were housed in an air conditioned monitoring trailer operated by NWC. The particle light scattering coefficient, bg, was continuously measured with modified MRI Model 1561 integrating nephelometer. The nephelometer automatically zeroed daily and was calibrated quarterly. 

Particle light-scattering coefficient Integrating nephelometer Scattering intensity from particles within a dehned light path as a function of wavelength -h... 

FIGURE 3-18 Ratio of light-scattering coefficient to mass concentration for uniform spherical particles of unit density. Refractive index, l.S diameter, dp. Reprinted with permission from Hidy. ... 

TABLE 9.7 Some Reported Values of the Light Scattering Coefficients (at) for Common Constituents of Atmospheric Particles"... 

Figure 9.26 shows the calculated light scattering coefficient for NFI4FIS04 (uNii4iiso4 a common con-stitutent of atmospheric particles, as a function of relative humidity (Sloane, 1986). The contribution of this one compound to light scattering increases from 3.7 to 15.8 m2 g 1 as the RH increases from 20 to 80%. 

The uptake of water with increasing RH causes an increase in both mass and radius and a decrease in refractive index the net effect of all these factors is an increase in light scattering. This can be seen in Fig. 9.27, where the light scattering coefficient, bxai 6sp, for some ambient aerosol particles measured using an integrating nephelometer was found to increase with the liquid water content of the aerosols. 

For aerosols in the atmosphere, the light-scattering coefficient empirically is found to be proportional to particle mass concentration in the range of diameter below 10 /urn. This relationship is useful for coarse estimates of airborne particle concentrations. 

Particle mass scattering efficiency m g sp = o- p/M Both the mass concentration and light-scattering coefficient are sensitive to RH, necessitating specification of RH for both quantities often reported for dry material, with RH dependence separately specified... 

Radford, H. E. (1980). The fast reaction of CH2OH with 02. Chem. Phys. Lett. 71, 195-197. Radke, L. F., and P. V. Hobbs (1969). Measurement of cloud condensation nuclei, light scattering coefficient, sodium-containing particles and Aitken nuclei in the Olympic Mountains of Washington. J. Atmos. Sci. 26, 281-288. 

Radke, L. F., P. V. Hobbs, and J. E. Pinnons (1976). Observations of cloud condensation nuclei, sodium containing particles, ice nuclei and the light scattering coefficient near Barrow, Alaska. J. Appl. Meteorol. 15, 982-995. 

Other measurements important to visual air quality are pollutant related, i.e., the size distribution, mass concentration, and number concentration of airborne particles and their chemical composition. From the size distribution, the Mie theory of light scattering can be used to calculate the scattering coefficient (20). Table 14-2 summarizes the different types of visual monitoring methods (21). 

Extinction Coefficient a measure of the ability of particles or gases to absorb and scatter photons from a beam of light a number that is proportional to the number of photons removed from the sight path per unit length. See absorption. Extinction Cross Section the amount of light scattered and absorbed by a particle divided by its physical cross section. 

Scattering Coefficient measure of the ability of particles to scatter light measured in number proportional to the "amount" of light scattered per unit distance. 

The extinction coefficient can be expressed as the sum of terms which account for light scattering and absorption by gases and suspended particles ( ) ... 

To test the applicability of statistical techniques for determination of the species contributions to the scattering coefficient, a one-year study was conducted in 1979 at China Lake, California. Filter samples of aerosol particles smaller than 2 ym aerodynamic diameter were analyzed for total fine mass, major chemical species, and the time average particle absorption coefficient, bg. At the same time and location, bgp was measured with a sensitive nephelometer. A total of 61 samples were analyzed. Multiple regression analysis was applied to the average particle scattering coefficient and mass concentrations for each filter sample to estimate aj and each species contribution to light scattering, bgn-j. Supplementary measurements of the chemical-size distribution were used for theoretical estimates of each b pj as a test of the effectiveness of the statistical approach. 

The quality of statistically inferred species extinction balances can be enhanced with proper aerosol sampling. Due to its important role in light scattering, only the fine aerosol should be sampled. A mass balance should account for all major fine particle species. Ideally the particle scattering coefficient should be measured directly at the location where aerosol is sampled by the filters. The importance of soot and other carbonaceous aerosol contributions to light extinction in arid regions should not be overlooked. 

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