Vertical motion atmosphere

Wind speed has velocity components in all directions so that there are vertical motions as well as horizontal ones. These random motions of widely different scales and periods are essentially responsible for the movement and diffusion of pollutants about the mean downwind path. These motions can be considered atmospheric turbulence. If the scale of a turbulent motion (i.e., the size of an eddy) is larger than the size of the pollutant plume in its vicinity, the eddy will move that portion of the plume. If an eddy is smaller than the plume, its effect will be to difhise or spread out the plume. This diffusion caused by eddy motion is widely variable in the atmosphere, blit even when the effect of this diffusion is least, it is in the vicinity of three orders of magnitude greater than diffusion by molecular action alone. 

Vertical air motions affect both weather and the mixing processes of importance to air pollution. Upward vertical motions can be caused by lifting over terrain, lifting over weather fronts, and convergence toward low-pressure centers. Downward vertical motions can be caused by sinking to make up for divergence near high-pressure centers. One must know whether the atmosphere enhances or suppresses these vertical motions to... 

Inversions are of considerable interest in relation to air pollution because of their stabilizing influence on the atmosphere, which suppresses the vertical motion that causes the vertical spreading of pollutants. 

The atmosphere is nearly always in motion. The scales and magnitude of these motions extend over a wide range. Although vertical motions certainly occur in the atmosphere and are important to both weather processes and the movement of pollutants, it is convenient to consider wind as only the horizontal component of velocity. 

So far in discussing motion in the atmosphere, we have been emphasizing only horizontal motions. Although of much smaller magnitude than horizontal motions, vertical motions are important both to daily weather formation and to the transport and dispersion of pollutants. 

The vertical motion of the plume to the height where it becomes horizontal is known as the plume rise, (refer back to Figure 1). The plume rise is assumed to be a function primarily of the emission conditions of release, (i.e. velocity and temperature characteristics). A velocity in the vertical plane gives the gases an upward momentum causing the plume to rise until atmospheric turbulence disrupts the integrity of the plume. At this point the plume ceases to rise. This... 

Vertical motions in the atmosphere are caused by a variety of factors ... 

Since the production rates of the cosmic ray radionuclides increase rapidly with increasing altitude in the lower atmosphere, the atmospheric concentrations and ratios of short lived cosmic ray radionuclides can be used to study rapid vertical air motions if the equilibrium concentrations of the radionuclides are known. For example, the concentrations of the short lived cosmic ray radionuclides in air which has moved upward recently from a lower altitude will be less than the equilibrium concentrations. The concentrations of the radionuclides will therefore increase with time until equilibrium is reached. However, the concentration of the shorter lived of two short lived radionuclides will increase more rapidly initially, causing the ratio of the two radionuclides of different half-lives to change with time until equilibrium is reached. Therefore, the time since the air moved from a lower altitude, the speed of the upward motion, and the altitude from which the air originated could be calculated from the concentrations and concentration ratios of cosmic ray radionuclides of different half-lives. Vertical motions of different speeds could be studied since several cosmic ray radionuclides of different half-lives are present in the atmosphere (Table I). Many other radionuclides are produced by cosmic rays in the atmosphere, but they have not yet been detected. Some of these with half-lives of a few minutes could serve as tracers of very short term processes such as post-nucleation scavenging. 

In the real atmosphere horizontal motions along latitude and longitude must also be taken into consideration. Thus, the ozone concentration profile should show a significant derivation near the tropopause due to the downward transport of 03 from the expected profile without vertical eddy diffusion. 

Depending on its explosive yield, a nuclear test may introduce radioactive materials to various heights in the atmosphere. The lowest level of the atmosphere is the troposphere, in which turbulent air movements occur. In addition to prevailing horizontal winds, there is also considerable vertical motion as evidenced by clouds, rain and... 

Temperature inversion In a temperature inversion, the potential temperature of the atmosphere increases with increasing height, creating a stable atmosphere in which vertical motion is suppressed. 

Figure 3 shows the typical shape of a smoke-plume in a stable atmosphere. In this case, there is a vertical temperature gradient such that the air aloft is hotter and less dense then the air near the ground - a so-called temperature inversion - and the resulting absence of ambient vertical motion causes the plume to fan out under the influence of the prevailing horizontal airflow. In comparison with its profile for neutral stability, the flattened cone of the stable-atmosphere plume may be supposed to retain a... 

The distribution of most chemical species in the middle atmosphere results from the influences of both dynamical and chemical processes. In particular, when the rates of formation and destruction of a chemical compound are comparable to the rate at which it is affected by dynamical processes, then transport plays a major role in determining the constituent distribution. In an environment like the Earth s atmosphere, air motions, and hence transport of chemical species, are strongly constrained by density stratification (gravitational force) which resists vertical fluid displacements, and the Earth s rotation (Coriolis force) which is a barrier against meridional displacements. Geophysical fluid dynamics describes how atmospheric motions are produced within these constraints. 

As indicated, the flux may be expressed either in units of molecules/m2 s or in units of kg/m2 s. Here, p and n are the density and number density of air, respectively, and K is called the eddy diffusion coefficient. This quantity must be treated as a tensor because atmospheric diffusion is highly anisotropic due to gravitational constraints on the vertical motion and large-scale variations in the turbulence field. Eddy diffusivity is a property of the flowing medium and not specific to the tracer. Contrary to molecular diffusion, the gradient is applied to the mixing ratio and not to number density, and the eddy diffusion coefficient is independent of the type of trace substance considered. In fact, aerosol particles and trace gases are expected to disperse with similar velocities. 

Table 1-8. Time Constants for Horizontal and Vertical Transport by Atmospheric Mean Motions and Eddy Diffusion11... 

The physical conditions of the atmosphere in the vicinity of the Earth surface are determined by friction and by heat transfer. Friction reduces wind velocity and turbulence as one approaches the surface, so that the eddy transport rate declines. Heating of the surface by solar radiation imparts energy to the overlying air, causing an enhancement of vertical motions and eddy transport owing to convection. An increase of temperature with height in the atmosphere for a certain distance instead of the normal, adiabatic decrease is called a temperature inversion layer. Temperature... 

Vertical motions in the atmosphere result from (1) convection from solar heating of the Earth s surface, (2) convergence or divergence of horizontal flows, (3) horizontal flow over topographic features at the Earth s surface, and (4) buoyancy caused by the release of... 

If A > T, that is if the atmospheric temperature decreases faster than that of the adiabatic rising parcel (Figure 16.2a), then Ta(z) < To and (r/A) — 1 < 0. Therefore the term in brackets in (16.23) is positive, and a > 0. As the parcel rises it will accelerate, rise more rapidly, accelerate more, and so on. Its vertical motion is enhanced by the atmosphere surrounding it and the resulting buoyancy force. 

The lapse rate in the lower portion of the atmosphere has a great influence on the vertical motion of air. Buoyancy can resist or enhance vertical air motion of airmasses, thus affecting the mixing of pollutants. Comparing the environmental lapse rate A with the adiabatic lapse rate T, we can define three regimes of atmospheric stability (Figure 16.2) ... 

Let us compute the temperature change with z of an isolated parcel of air (or possibly other gas) as it rises or falls adiabatically through an atmosphere that is not adiabatic. We assume that conduction or convection of heat across the boundary of the parcel will be slow compared with the rate of vertical motion. Thus an individual parcel is assumed to rise or fall adiabatically, even when the surrounding air is nonadiabatic. 

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