Low-pressure systems, with their associated wet and windy conditions, generally lead to a higher mixing height

 Low-pressure systems, with their associated wet and windy conditions, generally lead to a higher mixing height and thus better dispersion of air pollutants, while high-pressure systems can lead to stagnant conditions and lower mixing heights, trapping pollutants near the surface. 

Here's a more detailed explanation:

Mixing Height:

The mixing height, or mixing layer height, is the height above the surface to which pollutants can be dispersed. 

Low Pressure Systems:

Low-pressure systems are characterized by rising air, which can lead to cloud formation and precipitation. 

The associated winds can help to disperse pollutants, and rain can wash pollutants out of the atmosphere. 

This results in a higher mixing height, allowing pollutants to be distributed over a larger volume of air. 

High Pressure Systems:

High-pressure systems are characterized by sinking air, which leads to stable atmospheric conditions. 

These systems can lead to stagnant air, which can trap pollutants near the surface. 

This results in a lower mixing height, leading to a buildup of pollutants at ground level. 

Examples:

During a low-pressure system, pollutants might be dispersed and transported away by wind and rain, leading to cleaner air. 

During a high-pressure system, pollutants might accumulate near the surface, leading to smog or haze. 

Mixing height and air pollution:

The atmospheric mixing layer height (MLH) determines the space in which pollutants diffuse and is thus conducive to the estimation of the pollutant concentration near the surface. 

The dispersion of air pollutants is often confined within the mixing layer, the depth of the mixing layer, which is usually referred to as the “mixing height”, is one of the most critical parameters in air quality studies.