Factors influencing dispersion patterns

Atmospheric factors influencing dispersion

In this chapter, we delve into the influence of atmospheric factors on dispersion patterns. Our aim is to elucidate how various atmospheric factors, such as wind speed, wind direction, surface roughness, and atmospheric stability, shape the spread of hazardous materials in the atmosphere following industrial accidents. By dissecting the role of these atmospheric factors, we provide a structured understanding of their impact on dispersion dynamics

Wind speed and direction

Wind speed and direction are fundamental to understanding dispersion patterns. Wind data for a geographical area are typically presented using a wind rose, a graphical representation of wind directions and velocities. The north is at 0 (or 360) degrees and the west is at 270 degrees. Wind direction indicates where the wind is coming from; for example, a north wind (N) blows from north to south. This information can also be presented in tabular form (Bosch, 2005).

Surface roughness

Wind velocity increases with height and decreases close to the ground surface, creating a velocity gradient. Earth’s surface roughness interacts with this velocity gradient and influences the wind velocity profile.

Surface roughness is expressed as a surface roughness length, categorised as follows:

In dispersion calculations, the wind velocity is typically measured at 10 meters, which is relevant to the height at which toxic or flammable clouds disperse. Since wind speed changes with height, this standard measurement is used to calculate wind speed at specific heights.

Atmospheric stability

Atmospheric stability refers to the atmosphere’s tendency to resist vertical motion and is influenced by wind velocity and solar radiation. Stability depends on the relationship between the temperature of an air parcel and the surrounding environmental temperature at different altitudes.

Day vs Night conditions

• Day: During the day the earth's surface is heated by the sun, causing warmer air patches to rise. This generates more turbulence and causes unstable conditions. This enhances the dispersion of pollutants in the atmosphere.

• Night: During the night the earth’s surface cools down, causing the air at ground level to cool down and stabilise. This stable atmospheric condition leads to the formation of an inversion layer, preventing turbulence and vertical movement.

The Pasquill scheme is commonly used to determine stability in a mixed layer, categorising it into classes ranging from A to F, as illustrated in the table provided below (Bosch, 2005).

The combination of Pasquil stability class, wind speed, and surface roughness length determines the turbulence in the atmosphere.

Conclusion

A comprehensive understanding of atmospheric conditions is crucial for evaluating the dispersion of hazardous materials in industrial settings. Factors such as wind speed and direction, surface roughness, and atmospheric stability play pivotal roles in determining the spread of pollutants following accidental releases. Daytime heating and nighttime cooling further influence atmospheric stability, impacting dispersion behaviours. Comprehensively analysing these atmospheric conditions, helps better assess the potential risks associated with industrial accidents and develop effective mitigation strategies to safeguard both human health and the environment.

References

Bosch, C. v. (2005). Methods for the calculation of physical effects 'Yellow book' CPR 14E. The Hague: Ministerie van Verkeer en Waterstaat.

Donker, J., Van Maarseveen, M. and Ruessink, G. (2018). Spatio-Temporal Variations in Foredune Dynamics Determined with Mobile Laser Scanning, Journal of Marine Science and Engineering, 6(4), 126. https://doi.org/10.3390/jmse6040126