The one main absorbing parameter that must be kept constant if the effects of air pollution are being determined is the known interaction of the UV with the stratospheric ozone. The UV absorbed in this layer is the predominant factor affecting the transmission to the earth's surface. Statistical correlations of surface measured UV with variations in this layer of ozone correlate well above the r = 0.90 level. The importance of ozone as a regulator of UV stems from strong absorption characteristics over the entire ultraviolet region. Ozone exists in two places in the atmosphere; one is where it is supposed to be to protect life on earth: the stratosphere, and the other, the troposphere, where it is a pollutant that can cause severe health problems. Both ozone areas require high energy UV radiation for creation, but in entirely different ways. In one case it is the primary absorber of UV radiation. In the other, UV creates the tropospheric ozone. Stratospheric ozone is produced by a series of reactions with molecular oxygen and singlet oxygen in the presence of high energy UV radiation, UV-C (200 nm - 280 nm). It is primarily produced over the equatorial regions and is slowly transported poleward. The total column ozone (stratospheric and tropospheric) can comprise a major UV absorber under highly polluted conditions. In the troposphere the UV can create the very gas that helps in its own attenuation. Although in general, as will be shown later, the tropospheric gases play a minor UV absorption role in comparison to other aerosol pollutants.
Stratospheric ozone absorbs all of the UV-C radiation. Tropospheric ozone makes up only a small portion of the total column ozone, usually less than 10%. The path length of the stratosphere is much longer than that of the troposphere, but the higher density of the troposphere and its increasing scattering capabilities are believed to enhance the absorptive powers of this region (Bruhl and Crutzen, 1989). Tropospheric ozone in urban/industrialized regions may reduce the surface UV by 3% to 15% as estimated by Frederick et al. (1993) and Ma and Guicherit (1997). Under clear sky conditions total ozone column variations cause the largest variations in UV transmission and has been the focus of numerous studies (e.g., McKenzie et al., 1991; Kerr and McElroy, 1993; Bojkov et al., 1995; Mims et al., 1995;
Varotsos and Kondratyev, 1995; Fioletov and Evans, 1997). This relationship is demonstrated in Fig. 11.2, which depicts the calculated clear-sky spectral UV irradiance for two total column amounts, 200 Dobson Units (DU) and 400 DU, for Raleigh, NC, during the summer at high solar elevation.
It is apparent that a decrease in the total ozone column substantially increases the surface UV-B irradiance over the erythemally important wavelengths. The human skin has a response curve to solar radiation. The particular portion of the sun's spectral region, which is in the UV, causes redness of the skin and is the erythemal region.
I.0E+00 I.0E-01 E" I.0E-02 1.0E-03 5 1.01-01 g" I.0E-05 ■I I.0E-06 | I.0E-07 I.0E-08
280 290 300 310 320 330 340 350
Figure 11.2 UV spectral changes for 200 DU change in stratospheric ozone 22.214.171.124 Cloud Cover
Cloud cover can wreak havoc for making UV measurements. They generally reduce the UV radiation, but it has been shown, Estupinan et al. (1996), and Schafer et al. (1996), that clouds can actually enhance the surface levels by as much as 10% -15%. The physical and chemical makeup of clouds (e.g., thickness, total percent coverage, droplet size distribution, chemical composition, and presence of interstitial absorbing aerosols) causes the cloud's effect on the UV to be highly variable; not only with time, but also with areas of differing pollutant characteristics. Frequently the clouds will act to purge the atmosphere of certain hygroscopic pollutants and in doing so, will alter the atmospheric albedo to reflect UV back to space.
The cloud/physics/chemistry/thermodynamics are not completely understood. Cloud systems vary widely across the world and are dependant upon the cloud type and whether or not they are formed over land or ocean. In one case, they are frequently formed over land where anthropogenic sources have generated thousands of CCN (cloud condensation nuclei) per cubic centimeter, but yet once they are formed (the CCN are hygroscopic, so they collect water vapor as they grow), they become great scavengers of pollution, absorbing the gaseous versions of sulfur
dioxide (SO2), nitrogen dioxide (NO2), and ozone (O3). They may also collect many hydrocarbons, not particularly in solution, but on the droplet surface.
Cloud droplets are formed by condensing water vapor on cloud condensation nuclei. Sizes of these nuclei range from the Aitken nuclei at 0.001 |im up through the large and giant nuclei at 2.0 |im. The smaller nuclei have a much higher concentration in the atmosphere and give rise to clouds with much higher droplet concentrations. These clouds have a higher albedo and provide a cooling effect for the earth's surface by reflecting much of the sunlight back into space. The cloud droplet spectrum broadens as the cloud gets older. For cumulus clouds, this can be caused by collisions and coalescing, condensation and evaporation, turbulence effects, and the mixing of cloud parcels with different histories.
Continental aerosols contain higher concentrations of CCN and thus, cause a different cloud structure over land than over the ocean. The ocean clouds have a broader size distribution of droplet sizes ranging from approximately 6 ^m to 45 ^m. Droplet counts are in the 35 droplets per cubic centimeter range. However, continental clouds have a narrow size range, 5 ^m - 20 ^m, but a much higher concentration of approximately 210 droplets/cm .
Stratus clouds seem to have the most effect on the radiative transfer and the radiation balance, but don't seem to be studied to the extent that cumulus clouds are. Wind velocities in the two types differ considerably. Cumulus clouds contain updrafts that measure in the meters/second range, while the stratus cloud's wind velocities are in tens of centimeters per second. This is also correlated to the extensive coverage of either type of cloud. Stratus clouds extend coverage over tens of km, while cumulus clouds are generally localized events covering just a few kilometers.
Droplet sizes and growth are different in the two types. Stratus cloud droplets are primarily formed through condensation and their growth processes are largely caused by condensation. The droplets enlarge monotonically with height. The droplet size spectra are narrow as compared to the cumulus. This is caused, in part, by these clouds being contained or height inhibited by an inversion layer just above them. This inhibits further vertical growth of both the cloud and the droplets within.
In the stratus case, there is mixing and partial evaporation at the cloud top causing cooling. Radiative cooling also occurs at the cloud top, as discussed earlier, which causes pockets of colder air to fall through the cloud. These clouds can also contain areas of super cooled droplets. Precipitation is more likely to occur if the cloud is thick and contains ice. The probability of precipitation increases with a combination of cloud age, temperature, and vertical extent. In a cumulus cloud, all three of these exist and are related to cloud thickness. Continental clouds must be thicker than maritime clouds for the same probability of precipitation. The fewer droplets in the maritime clouds are larger and can collide and coalesce with each other more readily than the smaller drops in the continental clouds to form precipitation. Less is known about the stratus clouds. Age, temperature, and thickness are still basic requirements for precipitation.
Empirical relations between cloud coverage and surface UV have been developed by Ilyas (1987), Bais et al. (1993), and Frederick and Steele (1995). Differences in the derived empirical relationships demonstrate a variation in regional cloud characteristics. All of these factors combine to create strong UV variabilities at the surface, thus complicating detection of long-term trends (Madronich et al., 1998). Therefore, clear cloudless days are best when attempting to correlate the effects of UV with air pollution.
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