Cloud interception

Cloud droplet capture in the form of intercepted fog appears to be a seasonably important sink for pollutant emissions in the LA Basin. At Henninger Flats up to 50% of the total wet deposition of H", NOo", and 04" may be due to cloud interception low intensity springtime drizzle accounted for 20% of the deposition measured in precipitation. The intercepted cloudwater that deposited on pine... 

Cloud interception, the impaction of cloud droplets on the terrain usually at the top of tall mountains... 

Cloud droplets are typically far more acidic than precipitation droplets collected at the ground. In essence cloud drops are small and have not been subjected to the dilution associated with growth to the size of raindrops, snowflakes, and so on, nor the neutralization as sociated with the capture of surface-derived NH3 and alkaline particles held in layers at lower altitudes. Interception of these droplets therefore provides a route by which concentrated solutions of sulfate and nitrate can be transferred to foliage in high-elevation areas that are exposed to clouds. Only limited areas of the eastern part of the United States are frequently exposed to such deposition, but for these sensitive areas cloud interception is an important acid deposition pathway. 

The literature provides little information on the effects of thermal radiation from flash fires, probably because thermal radiation hazards from burning vapor clouds are considered less significant than possible blast effects. Furthermore, flash combustion of a vapor cloud normally lasts no more than a few tens of seconds. Therefore, the total intercepted radiation by an object near a flash fire is substantially lower than in case of a pool fire. 

In order to compute the thermal radiation effects produced by a burning vapor cloud, it is necessary to know the flame s temperature, size, and dynamics during its propagation through the cloud. Thermal radiation intercepted by an object in the vicinity is determined by the emissive power of the flame (determined by the flame temperature), the flame s emissivity, the view factor, and an atmospheric-attenuation factor. The fundamentals of heat-radiation modeling are described in Section 3.5. 

Radiation effects from a flash fire are now fully determined if vapor cloud composition, as well as the geometry of the flame front (dependent on time), is known. Vapor cloud composition is, of course, place- and time-dependent, and the shape of flame front will greatly depend on cloud shape and ignition site within the cloud. The total radiation intercepted by an object equals the surmnation of contributions by all successive flame positions during flame propagation. This is an impossible value to compute with the simplified approach just described. Because there are many uncertainties (e.g., cloud composition, location of ignition site) which greatly influence the final result, a conservative approach is recommended for practical applications ... 

Incoming radiation from the Sun and backradiation emitted by Earth interacts with the atmosphere. Although about half of the Sun s radiation passes directly to Earth s surface, a portion is reflected back directly into space, while another portion is absorbed by atmospheric gases and reradiated. Figure 18.3 shows the fate of radiation intercepting Earth. About half of the incoming solar radiation actually reaches the surface of Earth. The rest is reflected or absorbed by the atmosphere or clouds. Infiared radiation reflected from Earth s surface is partially absorbed and reflected by the atmosphere and clouds. Some of this radiation is reradiated back toward Earth s... 

For coniferous forests, the calculated v+ increases rapidly as droplet diameter increases to 10 /iva. Also, w is typically several times greater over a forest than over grassland, so the disparity is greater in terms of vt. Lovett Reiners (1986) found vt of cloud droplets to a subalpine fir forest to be 300 mm s-1, increasing possibly to 2000 mm s-1 on the lee side of gaps in the canopy. In these conditions, occult precipitation is the equivalent of 0.1 to 0.3 mm h-1 of rainfall (Lovett, 1984). Much of the intercepted water re-evaporates, but ions dissolved in the droplets remain on the leaves and are potentially damaging. 

If the sensory hair is suddenly immersed in a homogeneous air sample that contains the two chemicals, the ratio of the rates at which the hair takes up the two compounds will be directly proportional to the product of their molecular concentrations and the square root of the ratio of the diffusion coefficients (approximating using the first term in equation 21.16). That is, if ethanol and hexadecanol had similar diffusion coefficients, the 3 1 ratio in their molecular concentration would be reflected in an expected 3 1 ratio in interception by the hair. The diffusion coefficients actually differ by a factor of 5.3, and therefore the odorant with the smaller diffusion coefficient (ethanol, in this case), will be taken up at a rate of approximately 2.3 times what would be expected on the basis of their molecular concentrations. Thus, the 3 1 ethanol hexadecanol ratio would be expected to result in an interception ratio of 6.9 1. This boundary condition corresponds approximately to the case of a filiform antenna suddenly immersed in a cloud of odorant in still air. 

For a consideration of wet deposition mechanism it is useful to make a distinction between processes transferring material to cloud droplets before they begin their descent as a raindrop, known collectively as rain-out and processes transferring material to falling raindrops known as washout, There are five mechanisms [8] by which particulate and gaseous compounds may be captured by cloud or rain drop diffusiophoresis, brownian diffusion, impact and interception, solution and oxidation of gaseous species (notably SO2 and NO2) and the cloud condensation nuclei (CCN) pathway. 

Local Exhaust Ventilation. Local exhaust ventilation is designed to intercept the flammable atmosphere at the source of release and directs it into a system where air is safely separated from the fuel. Correctly designed local exhaust ventilation systems could be very effective in limiting the spread of dust cloud atmospheres beyond the source of release. Local exhaust ventilation is generally less expensive to run than dilution ventilation because less air is used. 

The AFM creates atomic level resolution images of the surfaces of noncon-ductive specimens, using the quantum mechanical phenomenon of repulsive atomic force. The AFM records the repulsive forces that occur when electron clouds of two atoms, one in the microscope probe and the other at the sample surface, are in close proximity to each other. In most AFMs, the sample is mounted on the piezoelectric tube, so the sample moves in relation to a stationary tip. As the sample is very close to the probe, the fluctuations in electric dipole moment of the interacting atoms create a repulsive action between atoms of the specimen and atoms of the probe. As the probe is deflected by the atoms on the surface of the specimen, its movement is intercepted by a laser beam, which transmits the information to the computer for image generation. 

Waldman, J. M., and Hoffmann, M. R. (1987) Depositional aspects of pollutant behavior in fog and intercepted clouds, in Sources and Fates of Aquatic Pollutants, Advances in Chemistry Series, no. 216, R. A. Hites and S. J. Eisenreich, eds., Wiley, New York, pp. 79-129. 

---