In represents the photon counts measured by the Brewer at wavelength A and corresponds to the photon counts that would be measured at the top of the earth's atmosphere. The sum of aerosol and gas extinction is Tx and m is the air mass defined as the cosecant of the solar zenith angle. Once the I0 has been characterized, the total atmospheric optical depth (ra,x), at a specific wavelength, is calculated using the following:

' r


r I \

v m


Ii is the Brewer photon count as measured by the PS routine. The natural log of the instrument's photon count (7-axis) is plotted against the cosecant of the solar zenith angle (X-axis). The slope of the resulting line is the total optical depth.

The total optical depth can be further broken down into its components: (1) the Rayleigh optical depth, (2) the ozone optical depth, and (3) the aerosol optical depth, as shown in Fig. 11.10. One of the objectives for this chapter is to determine the aerosol optical depth (AOD). Anthropogenic aerosols are one of the largest factors affecting surface UV radiation values, but to determine their optical properties is a challenge and one which has many facets. As shown later, the aerosols have a myriad of physical and chemical properties which affect their capabilities to absorb the radiation. Size, chemical composition, and their ability to adsorb or desorb water vapor all contribute to their interaction with the UV.

320 330 340 Wavelength (nm)

Figure 11.10 The optical depths of the atmosphere

(1) Rayleigh optical depth (Rayleigh scattering): Rayleigh scattering by air molecules is a significant non-pollution contributor to the total optical depth of the atmosphere. It is wavelength dependent, with increased scattering at the shorter UV wavelengths. Rayleigh scattering by the N2 and O2 molecules is more significant in the UV portion of the spectra as is shown in the following equation (Dutton et al., 1994).

1.013 x 105

where A is wavelength in micrometers and Ps is the atmospheric pressure at the site expressed in Pascals.

(2) Gases: NO2 and SO2 also have strong absorption bands in the UV portion of the spectra. The gases can play a small part in determining the total optical depth of the atmosphere, but their concentrations must be extremely high or they must have an extremely long path length. These conditions exist infrequently in the real world. A large SO2 plume may exist at a soft coal fired power plant or smelter where the concentrations may be very high (much greater than the normal parts per billion normally seen in ambient air). Nitrogen dioxide is normally a by-product of auto emissions, so large concentrations may exist under the right atmospheric conditions (inversions), but even then the path length would be very small, less than a few kilometers.

Total column O3 is the major absorber of UV radiation due to its relatively large concentration and path length in comparison to other absorbing gases, SO2 and NO2. Therefore, measurements of OD obtained from measurements of direct irradiance are directly affected by the total column of O3 in the atmosphere.

(3) Aerosols: Once the total atmospheric optical depth has been established, the aerosol optical depth (AOD) in the UV is obtained by subtracting the sum of the Rayleigh and O3 optical depths. Nitrogen dioxide and SO2 may contribute to the optical depth in the UV, although their contribution is relatively small (less than 1%). Changes in the total column ozone and the vertical structure of temperature and pressure of the atmosphere have an effect on the retrieved AOD. This effect is small, but non-negligible, in the retrieved AOD for the UV-B (A< 320 nm) due to Rayleigh scattering.

Keep in mind though that the UV wavelengths we are talking about generally range between 300 nm and 400 nm. This corresponds to the Rayleigh and the Mie portion of the scattering range described above. Particulate matter found in the atmosphere is generally not found in great quantities below the 1 (r range, but smaller quantities can still be very efficient scatterers.

The selective scattering of blue light by air molecules and very small particles can make distant mountains appear blue, such as the Blue Ridge Mountains of Virginia, North Carolina, and Tennessee. In some remote places, a blue haze may cover the landscape. Hydrocarbon emissions from trees reacting with ozone to produce very small particles selectively scatter blue light and create this haze. When small particles, such as fine dust and salt, become suspended in the atmosphere, the color of the sky begins to change from blue to milky white. Although these particles are small, they are large enough to scatter all wavelengths of visible light fairly evenly in all directions. When our eyes are bombarded by all wavelengths of visible light, the sky appears milky white and when the visibility lowers, we call the day "hazy". If the humidity is high enough, hydroscopic particles will create this haze. Thus, the color of the sky indicates the quantity of the aerosol material suspended in the air. The sky will appear a very deep blue from on top of a very high mountain peak and above the aerosol pollution; a result of Rayleigh scattering.

In a simplified example, for scattering, the luminous flux = F, and b is a factor of proportionality; therefore, for a short distance, dl through the atmosphere dF = - bFdl. If we integrate both sides we get F = Fo e-bl. Now if we do the same thing for absorption we get F = Fo e-kl where k is another factor of proportionality associated with absorption. If we now combine these two equations, we get F= Foe-(b+k)l or F = Foe-yl. This last equation is known as the Bouguer/Beer's Law.

Example: In a certain experiment, the luminous flux F is reduced to 36.8% of its original value when a beam of light is passed through an aerosol over a path length of 10 meters. Determine the numerical value of 7 in m-1.

From the equation above:

F=Fo e-(b+k)l=F0e"71 = 0.368 F/Fo = e- 71 = 0.368 -71 = ln(0.368) Since l = 10 m - 71 = -1.00 and 7 = 0.100 m-1

These are values that a nephelometer may measure under highly polluted conditions. A nephelometer measures the ambient light scattering of the atmosphere. This value can be related to the particulate (aerosol) loading of the atmosphere.

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