To effectively regulate pollution, planners must understand the physical journey of a pollutant from its source to the surrounding community. This journey is primarily governed by three meteorological processes: the initial rise of the effluent, its subsequent transport by local winds, and its eventual dilution through atmospheric dispersion.

3.1 Effective Emission Height: The Initial Ascent

The initial rise of an effluent plume from its source is determined by a combination of source characteristics and meteorological conditions. Atmospheric stability, particularly the presence of an inversion layer, can limit the vertical ascent of a plume, causing it to spread out horizontally. Conversely, in unstable air, a plume may continue rising until it encounters a stable layer. Wind speed also plays a role, with stronger winds tending to reduce the final height of the smoke rise. The final altitude achieved, known as the “effective emission height,” is a critical input for all subsequent calculations of transport and dispersion. While numerous empirical formulas exist to predict this height, those based on dimensional analysis, such as the work by Briggs, have a more robust physical foundation. For example, in neutrally stable air, the theory predicts that the rise should be proportional to horizontal distance to the 2/3 power, which is in good agreement with observations.

3.2 Pollutant Transport: The Role of Local Wind Patterns

Once a pollutant reaches its effective emission height, it travels with the wind. A significant challenge for air quality management is the general scarcity of wind data, especially for altitudes above 10 meters and in the areas between widely spaced airport weather stations. To address this, the field of mesometeorology studies local wind patterns on the scale of several kilometers to 100 kilometers. These local patterns are strongly influenced by surface features like hills, lakes, and the urban landscape itself, which create complex flows such as sea breezes or mountain-valley winds. Understanding these unique, localized wind patterns is essential for accurately modeling where pollutants will be transported.

3.3 Atmospheric Dispersion: The Dilution of Pollutants

The final concentration of a pollutant at a receptor depends on its dilution, or dispersion, in the atmosphere. This process is governed by two primary mechanisms: wind speed and turbulence.

• Wind Speed: Higher wind speeds lead to greater initial dilution. For example, if a stack emits one puff of smoke per second, a 10 m/sec wind will space the puffs 10 meters apart, while a 5 m/sec wind will space them only 5 meters apart. The greater the wind speed, the lower the initial concentration.
• Turbulence: Atmospheric turbulence consists of eddies that mix polluted air with surrounding clean air, decreasing concentrations within the plume. This turbulence originates from two sources:
  1. Convective Turbulence: Generated by heating from below, which occurs when the air temperature decreases rapidly with height. This is common on clear days.
  2. Mechanical Turbulence: Generated by wind shear (changes in wind speed with height) and the physical friction of wind moving over the ground. Cities, with their buildings and varied structures, have a high “roughness length” and inherently generate more mechanical turbulence than smooth, open terrain.

The relative importance of these turbulence types is characterized by the Richardson number (Ri); specifically, –Ri is a measure of the relative rate of production of convective and mechanical energy. As summarized in Table 1, its value indicates the prevailing dispersion regime.

These physical principles are incorporated into quantitative models designed to predict pollution levels and inform regulatory decisions.