Scattering aerosol

Measurements of ozone concentration in the ozone layer in the stratosphere are made in the less intense Huggins band to avoid complete absorption of the laser radiation. Again, the two or three wavelength DIAL method is used to make allowance for background aerosol scattering. A suitable laser for these measurements is the XeCl pulsed excimer laser (see Section 9.2.8) with a wavelength of 308 nm, close to the peak absorption of the Huggins... 

The aerosol scattering coefficient distribution was calculated from the aerosol volume distribution, using the method described in Friedlander (] ] ). The resultant distribution is plotted in Figure 3. The contribution of the fine aerosol to visibility degradation at China Lake is seen in this figure. 

Fine Aerosol Scattering Coefficient Balances Statistical and theoretical Results... 

Howell, S. G., and B. J. Huebert, Determining Marine Aerosol Scattering Characteristics at Ambient Humidity from Size-Resolved Chemical Composition, J. Geophys. Res., 103, 1391-1404 (1998). 

The effect of aerosol on the UV flux reaching the earth s surface is reviewed. Under cloudless conditions UV is scattered or absorbed by aerosol. The net effect depends on the optical properties of the aerosol (scattering or absorbing). In cloudy conditions the UV flux attenuation is more pronounced but depends also on the cloud type. In most instances of clear or cloudy skies an attenuation occurs, with a noteworthy departure the case of partly cloudy sky in a rural area. 

The value of 10 is determined by molecular and particulate (cloud and aerosol) scattering, and surface reflection. A small fraction of the molecular scattering is the non-conservative Rotational Raman scattering (RRS) that partially fills the solar Fraunhofer lines in the scattered radiation, creating what is commonly known as the Ring effect [15] As a result, the ratio Iq/F, where F is the extraterrestrial solar flux, contains structure that is correlated with solar Fraunhofer lines. By separating these effects, one can write... 

In some works involving aerosol scattering the term albedo is used to describe the extinction of light by a particle or a system of particles. Albedo is defined as the fraction of incident light or radiant energy that is reflected or scattered by the particle or system of particles. It is a dimensionless fraction commonly used to describe the light reflected from the earth back into space. 

The two important atmospheric processes are molecular absorption and molecular and aerosol scattering. Absorption by aerosols, both liquid and particulate, is not thought to be important. 

As shown in Table IV, for a given altitude in the troposphere there are significant seasonal and latitudinal fluctuations in the total column of ozone, and these lead to similar fluctuations in the ultraviolet flux. The maximum ozone column and minimum transmitted flux occur in late winter and early spring, while the minimum column and maximum flux occur in the fall. This effect is most pronounced at high latitudes, and much less so near the equator [cf. Junge (128)]. The seasonal and latitudinal variation in Rayleigh and aerosol scattering is much less and is not important to this discussion. 

It is important to understand the sources and loss mechanisms of stratospheric sulfate aerosols. These aerosols are linked to the decrease in ozone at mid-latitudes because they hydrolyse N2O5, reducing the amount of NOx that would otherwise limit the efficiency of chlorine-catalysed ozone depletion. In addition these aerosols scatter light, cooling the planet [127]. Their concentration increases dramatically following major volcanic eruptions however they are always present at background levels. The source of these background aerosols is a matter of debate. In 1976 Paul Crutzen presented the idea that sulfate aerosols result from the photolysis of carbonyl sulphide [128] ... 

The aerosol scattering extinction coefficients given in Elterman s model (15) were, however, three times larger than ours. This suggests that our results are closer to "background level" values, and that Elterman s model represents a period of high aerosol concentration, due to volcanic eruptions. 

FIGURE 15.4 Aerosol scattering phase function for (NH SQ particles at 80% relative humidity at X = 550 nm (Nemesure et al. 1995). Incident beam enters from the left. 

Aerosol scattering, absorption, and extinction coefficients are functions of the particle size, the complex refractive index of the particles m, and the wavelength X of the incident light. The extinction coefficient for a monodisperse ensemble of particles was given in terms of the dimensionless extinction efficiency by (15.27). With a population of differentsized particles of identical refractive index m with a number size distribution function of n(Dp), the extinction coefficient is given by1... 

---