Air Pollution Meteorology and Plume Types

Air Pollution meteorology deals with the meteorological processes near to the earth’s surface which consist of the impacts of meteorology on air pollutants and the effects of pollutants on meteorology. In this blog, I will show you some important terms related to air pollution meteorology, environmental stability and types of plumes. 

If the air is still and pollutants are unable to disperse, the local concentration of pollutants will rise. Strong, turbulent winds, on the other hand, remove pollutants fast, resulting in reduced pollutant concentrations.

Thus, the destiny of air pollutants is influenced by air movements. As a result, any study of air pollution should also include a look at the weather patterns in the area that is meteorology.

The following are some of the benefits of analysing meteorological data:

  1. Identify the source of pollution.
  2. Predict the occurrence of inversions and days with high pollutant concentrations.
  3. Simulate and predict air quality with the help of computer models.

Also read Air Pollution Causes – A Comprehensive Guide

Meteorological Factors affecting Air Pollution

The following factors should be measured while examining air quality because they can help us better understand the chemical reactions that take place in the atmosphere.

Air Pollution Meteorology
Air Pollution Meteorology

Wind speed and direction

Wind data records can be used to estimate the general direction and range of emissions when high pollutant concentrations occur at a monitoring station. Identifying the sources allows for the creation of a plan to decrease the negative effects on air quality.

Temperature

Temperature measurement aids in the assessment, modelling, and forecasting of air quality. The chemical processes that occur in the atmosphere to generate photochemical smog from other pollutants are influenced by temperature and sunlight (solar radiation). 

Humidity

Water vapour plays a key role in a variety of thermal and photochemical reactions in the atmosphere. They can dramatically raise the amount of light scattered by particles suspended in the air if they are bound to particles . If corrosive gases, such as sulphur dioxide, bond to water molecules, the gas will dissolve in the water and generate an acid solution that can harm people and property.

Rainfall

When the rain washes particulate matter out of the atmosphere and dissolves gaseous contaminants, it has a scavenging effect. Visibility is improved by removing particulates. Also, when there is a lot of rain, the air quality is usually improved. Acid rain can arise when rain dissolves gaseous pollutants like sulphur dioxide, causing possible damage to objects and plant.

Solar Radiation

Solar radiation must be monitored for use in modelling photochemical smog occurrences because the intensity of sunlight has a significant impact on the rate of chemical reactions that form smog. The intensity of sunlight is affected by cloudiness in the sky, time of day, and geographic location.

Lapse Rate in Air Pollution Meteorology

  • The rate of change in the measured temperature as we move up through the Earth’s atmosphere is known as the lapse rate.
  • When the temperature drops with elevation, the lapse rate is positive.
  • It is zero when the temperature remains constant with elevation.
  • When the temperature rises with elevation the adiabatic lapse rate is negative. This is called temperature inversion.

Environmental Lapse Rate

The rate of decrease of temperature with altitude in the stationary atmosphere at a given time and location is known as the environmental lapse rate (ELR). The International Civil Aviation Organization (ICAO) has defined an international standard atmosphere (ISA) with a temperature lapse rate of 6.49 K/km (1.98 °C/1,000 ft) from sea level to 11 km.

                                   (dT/dz)env = -6.5 K / 1000 metres

Adiabatic Lapse Rate

The adiabatic lapse rate (ALR) refers to the rate with which the temperature of an air parcel changes in response to compression or expansion associated with elevation change. This process is assumed to be adiabatic, that is, no heat exchange happens in between given air parcel and its surrounding.

                                       (dT/dz)adia  = -g/Cp = -9.86 0C / 1000 metres

Where,

                g = acceleration due to gravity

  Cp = Specific heat at constant pressure

Atmospheric Stability

The degree of atmospheric stability plays a key role in the ability of atmosphere to disperse the pollutants emitted to it. It is determined by comparing ELR and ALR.

When ELR = ALR, the atmosphere is neutrally stable.

If ELR > ALR, the atmosphere is superadiabatic and unstable.

When ELR < ALR, the atmosphere is subadiabatic and stable.

Under an unstable atmospheric condition, the lapse rate is super adiabatic. The actual temperature gradient is more negative than the dry adiabatic temperature gradient. A rising parcel of air gets warmer and tends to travel upwards due to increasing buoyancy. Air from different altitudes mixes thoroughly. There is rapid dispersion of pollutants throughout the entire atmosphere. As a result, this is highly desirable in pollution prevention.

Under a stable atmospheric condition, the lapse rate is subadiabatic. In this condition, a rising parcel of air gets denser, cooler and tends to fall back. The vertical mixing is very less and the dispersion of pollutants is very slow.

Types of Plume

A plume is a column of liquid, gas, or dust that moves through another fluid, gas, or dust. The term plume is commonly used to describe things like smoke rising from a chimney. Depending on the degree of atmospherical instability, exit velocity from a stack and the prevailing wind turbulence, the plume emitted from a stack behave in different ways. Let’s have a look at each one of them.

Plume behaviour in Air Pollution Meteorology
Plume behaviour

1. Coning Plume

  • Formed when horizontal wind velocity exceeds 32 km/h and cloud blocks solar radiation during the day and terrestrial radiation during the night.
  • There is little vertical mixing.
  • The environment is slightly stable under sub-adiabatic conditions (ELR<ALR).
  • The plume shape is vertically symmetrical about the plume line.

2. Fanning Plume

  • Formed at extreme inversion conditions owing to a negative lapse rate.
  • When the environment is under conditions of inversion, a stable environment occurs just above the stack, and the plume moves horizontally rather than upwards. 
  • Occurs more frequently when there is less turbulence.
  • For high stack, fanning is considered a favourable meteorological condition as it doesn’t cause ground pollution.

3. Looping Plume

  • The wavy looping plume arises in a super adiabatic environment (ELR>ALR), resulting in a very unstable atmosphere due to rapid mixing.
  • In an unstable atmosphere, rapid vertical air motions occur both upward and downward, resulting in a looping plume.
  • As a result, large pollution concentrations may arise near the ground.
  • It is preferable to create high stacks where the environment is normally hyper adiabatic to scatter these contaminants.

4. Neutral Plume

  • In neutral atmospheric circumstances (ELR=ALR), a neutral plume forms. 
  • A neutral plume rises vertically in an upward direction.
  • The plume will continue to rise until it reaches a height where the density and temperature of the surrounding air are equal.

5. Lofting Plume

  • Lofting plume is produced by a strong super adiabatic lapse rate immediately above the stack and a negative lapse rate (inversion) immediately below the stack opening.
  • The downward movement is stopped by inversion.
  • This results in a very rapid and turbulent upward mixing of the plume. But the downward mixing is less. 
  • As a result, the dispersion of pollutants becomes quick, and pollutants cannot come down to the ground.
  • Such a plume is good for dispersing air contaminants and providing significant protection to living beings.

6. Fumigating Plume

  • The fumigant plume is the exact opposite of the lofting plume.
  • Formed when there is a negative lapse rate (inversion) just above the stack and a strong super adiabatic lapse rate below the stack.
  • Pollutants cannot escape above the stack under these conditions, thus they settle towards the ground due to turbulence and mixing.
  • As a result, the dispersion of contaminants in a fumigant plume is exceedingly poor.

7. Trapping Plume

  • When an inversion layer exists above and below the stack, the plume does not rise or fall.
  • Rather, it is constrained or trapped between the two inversion levels, resulting in a trapping plume.
  • This plume isn’t optimal for pollution dispersion since it can’t go past a particular height.

That’s it about air pollution meteorology. Hope you found it insightful.

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