Rising smoke plumes

Rising smoke plume phenomena

A specific case of dispersion is the rising phenomenon of hot smoke plumes. Upon a combustion process, a series of combustion products are generated with very high temperatures (convection heat), causing them to rise (Bosch, 2005).

The plume rises if the material is less dense than the surrounding air (e.g., hot smoke from fires) or if the material has upward momentum (e.g., chimney emissions) until it reaches a height where the smoke plume is in equilibrium with the density of the air, leading to passive dispersion.

Warehouse or pool fires can produce smoke plumes that transport toxic combustion products along the wind, posing a risk to the environment and people exposed to these harmful chemicals. This occurs due to the absence of vertical turbulence at the boundary of the mixing layer. Only emissions with sufficient buoyancy, such as those from chimneys, can penetrate this layer and ascend upward.

Influence of the inversion layer

Inversion is a significant phenomenon that drastically influences the dispersion of gases in the atmosphere. It refers to a situation when the normal temperature gradient with altitude is reversed. This inversion creates a layer of warm air above cooler air closer to the ground, which prevents pollutants from dispersing upwards, keeping them trapped in the lower layers of the atmosphere. The formation of the inversion layer is especially dangerous if the released gas has toxic or flammable properties.

Usually, the air near the surface of the Earth is warmer than the air above it, largely because the atmosphere is heated from below as solar radiation warms the Earth's surface, which in turn then warms the layer of the atmosphere directly above it (through convective heat transfer). Air temperature also decreases with an increase in altitude because higher air is at lower pressure, and lower pressure results in a lower temperature.

Temperature inversions, on the other hand, occur in the atmosphere due to a variety of processes that lead to the reversal of the normal temperature gradient, where the temperature increases with altitude rather than decreases. Some of the causes of temperature inversions are as follows:

• Radiation inversion: It happens when the ground loses heat rapidly through radiation after sunset, cooling the air close to the surface. The cooler, denser air stays near the ground, while the warmer air, which is less dense, forms a layer above it. This type of inversion can occur on clear, calm nights when the absence of clouds allows the ground to radiate heat more effectively.

• Advection inversion: It occurs when warm air moves horizontally over a cooler surface, such as cold ocean currents or snow-covered ground. As the warm air passes over the cooler surface, it cools from below, creating an inversion layer. This type of inversion is common along coastal areas where warm air from the ocean moves over the cooler land.

• Topographic inversion: It happens when cold air drains into valleys and low-lying areas from surrounding higher terrain, especially during the night. The cold air accumulates in the valleys, while warmer air remains above. This is often seen in mountainous regions where the terrain traps the cold air.

Temperature inversions can have effects on the air quality (as they trap pollutants close to the ground, leading to poor air quality and health problems), and weather (they can suppress convection, leading to stable and calm weather conditions, but can also lead to fog and low clouds).

Penetration and reflection

The presence of an inversion layer is why some cities have air quality alarms; pollutants get trapped in a thin layer above the city. If there is an inversion layer, a smoke plume could potentially get trapped underneath it, posing an even more dangerous threat to the individuals at ground level. The importance of the plume penetration is that all mass that has risen above the mixing layer will not disperse back below the mixing layer, but will remain trapped above the mixing layer height and will never create chemical exposure at ground level.

Apart from penetration of the mixing layer height, plume reflection should also be considered, especially for plumes remaining below the mixing layer height. Reflection is a phenomenon where concentrations get “bounced back” against a non-permeable boundary.

Importance of rising smoke plume modelling

Imagine a case where the smoke plume has enough convective heat to penetrate the mixing layer height. Because there is no vertical mixing due to the presence of this mixing layer, the plume will never disperse below it, posing no threat to the individuals at ground level. ​

However, if a fire brigade extinguishes the fire, the heat produced is reduced, and the smoke plume no longer has enough momentum to penetrate the mixing layer. This plume now poses a threat to individuals at ground level, creating a hazardous situation for the population.

This is why monitoring the concentrations of different combustion products (such as NO_x, SO₂, HCl, etc.) and simulating the behaviour of the rising plume becomes crucial.

Modelling plume rise phenomena

To simulate the plume rise phenomenon, it is important to identify:

• The tendency of the atmosphere to resist or enhance vertical motion by the corresponding meteorological stability class.​

• The presence of a temperature inversion layer and how it affects the dispersion behaviour of the chemical substance.​

Conclusion

In conclusion, rising smoke plumes from combustion processes, such as warehouse fires or pool fires, ascend due to their high temperature. The atmospheric inversion layer traps pollutants near the ground, exacerbating environmental and health risks. Effective modelling of smoke plume behaviour, including trajectory and interactions with inversion layers, is crucial for predicting if the plume penetrates the inversion layer, hazard distances and mitigating exposure to toxic substances.

References

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

Ynetnews. (2016). Giant fire continues to rage at a Haifa gas refinery. Available at: https://www.ynetnews.com/articles/0,7340,L-4898102,00.html