The dispersion of pollutants in the atmosphere is maximum when the environmental lapse rate is greater than the adiabatic lapse rate.

1. Environmental Lapse Rate:
The environmental lapse rate refers to the rate at which the temperature of the atmosphere changes with altitude under normal atmospheric conditions. It is determined by various factors such as solar radiation, air movement, and the presence of pollutants.

2. Adiabatic Lapse Rate:
The adiabatic lapse rate, on the other hand, refers to the rate at which the temperature of a parcel of air changes as it rises or descends in the atmosphere without exchanging heat with its surroundings. There are two types of adiabatic lapse rates - the dry adiabatic lapse rate and the saturated adiabatic lapse rate.

3. Maximum Dispersion of Pollutants:
When the environmental lapse rate is greater than the adiabatic lapse rate, it creates a condition known as an unstable atmosphere. In an unstable atmosphere, the air tends to rise rapidly, carrying pollutants with it and promoting vertical mixing. This vertical mixing helps disperse the pollutants over a larger volume of air, reducing their concentration at any given level.

4. Explanation:
When the environmental lapse rate is greater than the adiabatic lapse rate, the air near the surface gets heated more quickly than the surrounding air as it receives solar radiation. This creates a temperature gradient, with warmer air at the surface and cooler air above it. The warm air near the surface becomes less dense and starts to rise.

As the warm air rises, it expands and cools at the adiabatic lapse rate. If the environmental lapse rate is greater than the adiabatic lapse rate, the rising air remains warmer than the surrounding air, and its buoyancy continues to drive it upwards.

This vertical movement of air promotes the dispersion of pollutants because the rising air carries the pollutants with it. Additionally, as the warm air rises, it mixes with the cooler air above, further spreading and diluting the pollutants.

On the other hand, when the environmental lapse rate is less than the adiabatic lapse rate, or when the environmental lapse rate is equal to the adiabatic lapse rate, the atmosphere is stable. In a stable atmosphere, vertical mixing is limited, and pollutants tend to accumulate near the surface, resulting in poor dispersion and potentially higher pollution levels.

Therefore, the dispersion of pollutants in the atmosphere is maximum when the environmental lapse rate is greater than the adiabatic lapse rate.

Unstable environment means ELR > ALR.