Lapse rates, inversion, types of inversion, atmospheric stability and dispersion of air pollutants

LAPSE RATE

In well mixed dry air, temperature falls by 3.3F for every increase in 100 ft altitude. This vertical temperature gradient is called 'lapse rate'.

The value mentioned above is called NORMAL LAPSE RATE or ENVIRONMENTAL LAPSE RATE.

INVERSION
If the cold layer of air at ground level is covered by warmer air at a higher level, the phenomenon is called inversion. During inversion, the vertical air movement is stopped and pollutants are concentrated in the inversion layer below. In this state, the atmosphere is stable and very less turbulence or mixing takes place. As a result, the pollutants in the air do not disperse. Inversion occurs typically in the months of October to February. The accumulation of smoke and other pollutants aggravates the problem of pollution by preventing the sun's rays from heating the ground and adjacent air. Fog is generally associated with inversions. Narrow valleys are favorable to inversions as horizontal air movement is restricted. During inversions visibility is greatly reduced and contaminant concentration is maximum.

TYPES OF INVERSION

1. Radiation inversion
• This type of inversion occurs at night
• It occurs when the vertical movement of air is stopped
• Fog forms in this type of inversion if air is moist and temperature is below the dew point
• It is common in winter due to longer nights
• It frequently occurs in valley areas
2. Subsidence inversion
• This type of inversion occurs at modest altitudes and remains for several days
• It is caused due to sinking of air in high pressure areas surrounded by low pressure areas
• As air sinks, it is compressed and gets heated to form a warm dense layer that prevents upward movement of contaminants
• Inversion height varies from ground layer to a height of 1600 m.
• At inversion height of 200 m, extreme pollution occurs.

If radiation and subsidence inversion occur simultaneously, the phenomenon is called "double inversion".

ATMOSPHERIC STABILITY
Stability is an important characteristic of the atmosphere. It is simply the ability to resist vertical motion. Stability affects the ability to disperse pollutants.
Lapse rate is the rate at which atmospheric temperature decreases with an increase in altitude.
Lapse rate is the negative of the rate of temperature change with altitude change.
Three different lapse rates are:

1. Dry adiabatic lapse rate
2. Moist adiabatic lapse rate and
3. Environmental lapse rate
   

The actual temperature profile of ambient air is the environmental lapse rate or atmospheric lapse rate or prevailing lapse rate. The environmental lapse rate varies from time to time and from place to place. A radiosonde is an instrument to measure environmental lapse rate.
Negative lapse rate is an inversion.
Static stability of the atmosphere can be determined by comparing the dry, moist and environmental lapse rates.
IF:

• ELR < MALR then the atmosphere is absolutely stable
• ELR > DALR then the atmosphere is absolutely unstable
• MALR < ELR < DALR then the atmosphere is conditionally unstable

ELR     - Environmental Lapse Rate
MALR   - Moist Adiabatic Lapse Rate
DALR   - Dry Adiabatic Lapse Rate

During the day, the surface of the Earth get heated more due to insolation while at night, a terrestrial radiation loss causes a temperature inversion.
The range of stability indices are:

1. Very stable
2. Stable
3. Marginally unstable
4. Moderately unstable
5. Very unstable and
6. Extremely unstable

DISPERSION OF AIR POLLUTANTS

The degree to which air pollutants are discharged from various sources and their subsequent concentration in a particular area depends on meteorological conditions. Hence, the application of dispersal theory and knowledge of local weather conditions are essential for:

1. determination of required stack height and
2. evaluation of intensity of air pollution

Degree of air pollution varies spatially and temporally due to difference of meteorological conditions.

Adverse weather can trigger an air pollution episode like the "london killer smog". The important meteorological parameters that influence air pollution are:

1. Primary parameters
1. Wind speed and direction
2. Temperature
3. Atmospheric stability and
4. Mixing height
2. Secondary parameters
1. Precipitation
2. Humidity
3. Solar radiation and
4. Visibility

The above parameters vary as a function of

• Latitude
• Season and
• Topography

The following are the effects of air pollution on weather:

1. Reduced visibility
2. Frequent episodes of fog
3. Reduction in incoming solar radiation

Wind direction and speed influence the movement and diffusion of air pollutants discharged at ground level. high wind speeds diffuse pollutants from the source. Dispersed pollutants get rapidly diluted with increasing volume of air.

Gustiness is an important characteristic of surface winds is proportional to speed and determines the extent to which pollutants are mixed and diluted with surrounding air.

Concentration of a pollutant downwind is inversely proportional to wind speed.