Inversions

Three types of inversions develop in the atmosphere: ra-diational (surface), subsidence (aloft), and frontal (aloft).

Radiational Inversions

A radiational inversion occurs at low levels, seldom above a few hundred feet, and dissipates quickly. This type of inversion occurs during periods of clear weather and light to calm winds and is caused by rapid cooling of the ground by radiation. The inversion develops at dusk and continues until the surface warms again the following day. Initially, only the air close to the surface cools, but after several hours, the top of the inversion can extend to 500 ft (see Figure 5.6.9). Pollution emitted during the night is caught under this "inversion lid."

High-Pressure Cell

Temperature

FIG. 5.6.9 Formation of subsidence inversion.

Temperature

Subsiding Air Heated Adiabatically

High-Pressure Cell

Light and Variable Surface Winds

Subsiding Air Heated Adiabatically

Light and Variable Surface Winds

Temperature °C

FIG. 5.6.7 Relationship of the ambient lapse rates to the dry adiabatic rate.

FIG. 5.6.9 Formation of subsidence inversion.

Subsidence Inversions

A subsidence inversion is important in pollution control because it can affect large areas for several days. A subsidence inversion is associated with either a stagnant high-pressure cell or a flow aloft of cold air from an ocean over land surrounded by mountains (Cooper and Alley 1986). Figure 5.6.9 shows the inversion mechanism.

A significant condition is the subsidence inversion that develops with a stagnating high-pressure system (generally associated with fair weather). Under these conditions, the pressure gradient becomes progressively weaker so that winds become light. These light winds greatly reduce the horizontal transport and dispersion of pollutants. At the same time, the subsidence inversion aloft continuously descends, acting as a barrier to the vertical dispersion of the pollutants. These conditions can persist for several days, and the resulting accumulation of pollutants can cause serious health hazards.

Fog almost always accompanies serious air pollution episodes. These tiny droplets of water are detrimental in two ways: (1) fogs makes the conversion of SO3 to H2SO4 possible, and (2) fogs sits in valleys and prevents the sun from warming the valley floor to break inversions, thus prolonging air pollution episodes.

Figure 5.6.10 shows the frequency of stagnation periods of high-pressure cells over the eastern United States.

Frontal Inversions

A frontal inversion usually occurs at high altitudes and results when a warm air mass overruns a cold air mass below. This type of inversion is not important from a pollution control standpoint.

0 0

Post a comment