Air Pollution Dispersion

  Temperature distribution in atmosphere


 

The diagram given above explains the changes in temperature if you vertically go up in the atmosphere. Assume that you are going up in the air (vertically not diagonally) as if you are in an elevator. Initially you will feel a decrease in temperature till 11 km i.e. in troposhere. In stratosphere upto 51 km their will be increase in temperature followed by decrease in temperature in mesosphere. And ultimately temperature will increase in thermosphere. The reading and range of temperature are given in above table. The NLR and PLR represent negative and positive lapse rate.

 

Lapse Rate (LR) is decrease in temperature with increase in altitude. It will be Negative LR is temperature increase with altitude and positive if it decreases with altitude. Thus in all zones its NLR and PLR.

There are two types of lapse rate:

  • Environmental lapse rate – which refers to the actual change of temperature with altitude for the stationary atmosphere.
  • The adiabatic lapse rates – which refer to the change in temperature of a mass of air as it moves upwards. There are two adiabatic rates
    • Dry adiabatic lapse rate
    • Moist adiabatic lapse rate

  Super-adiabatic Lapse Rate


Super-adiabatic Lapse Rate

The condition is said to be super adiabatic when actual decrease in temperature is more than dry adiabatic lapse rate. (ELR>DALR). The environment becomes strongly unstable and favours dispersion of pollutants.

For example in graph given below; if at height 1 km ELR and DALR = 200C; at 2km ELR=70C whereas DALR = 100C. The decrease in ELR temperature is 13 whereas decrease in DALR temperature in 10; it means ELR>DALR. It does nor deal with temperature but the rate of decrease in temperature.

 

Let us assume a parcel of pollutant; plume moving in Environment (on ELR). At 2km altitude the parcel is at 7 degree Celcius temperature however the surrounding air is at 10 degree. Therefore the hot air will take the parcel up in the air and will disperse properly.

Thus, the condition is good for dispersion as the plume will go up in the air.

 

  Neutral Lapse Rate


Neutral Lapse Rate

The equal value of actual environmental lapse rate and dry adiabatic lapse rate makes the environment neutral for dispersion of pollutants (ELR=DALR).

 

Let us assume a parcel of pollutant; plume moving in Environment (on ELR). At 2km altitude the parcel and surrounding air both are at same temperature, therefore the plume will neither move up nor down.

Thus, the condition is not very suitable for dispersion.

 

  Sub-adiabatic Lapse Rate


Sub-adiabatic Lapse Rate

The actual decrease in temperature in environment is slightly less than dry adiabatic lapse rate. The condition favours less vertical mixing of pollutants (ELR<DALR).

 

In the diagram you can see; DALR as always is fixed. At 2km altitude ELR = 13 whereas DALR=10. This means that decrease in temperature at DALR is more that ELR.

Let us assume a parcel of pollutant; plume moving in Environment (on ELR). At 2km altitude the parcel is at 13 degree Celcius temperature however the surrounding air is at 10 degree. Therefore the cold air will sink the parcel down. 

Thus, the condition is not good for dispersion as the plume will come down.

  Inversion


 Inversion

When reverse condition prevails that is temperature increase with increase in altitude. The condition restricts the movement of air parcel and thus dispersion.

 

In the diagram you can see; DALR as always is fixed. At 2km altitude ELR = 25 whereas DALR=10. The condition is opposite to lapse rate.

Let us assume a parcel of pollutant; plume moving in Environment (on ELR). At 2km altitude the parcel is at 25 degree Celcius temperature however the surrounding air is at 10 degree. Therefore, there is lot of difference in temperature and cold air will not let the parcel disperse and will sink the parcel down. 

Thus, the condition is not good for dispersion as the plume will come down. The condition is very prevalent in winters which also leads to smog.

 

  Reasons for Inversion Conditions


Reasons for Inversion Conditions

  • Radiation Inversion
  • Subsidence Inversion
  • Frontal/advective Inversion

Radiation Inversion: Inversion due to difference in solar radiation. Earth cools during night by radiating thermal energy into space. In morning, air near surface becomes cooler than the air above, creating thermal inversion. The condition is very common and frequent, but less problematic and persistent. The inversion layer breaks with the intensity of sunlight and heating of earth surface.

Subsidence Inversion: this type of inversion occurs due to pressure difference in the atmosphere. High pressure mass of air moves towards earth. The air in center gets compressed and heated. The condition is more common in summers and prevails for long time thus restricts pollution dispersion.

Frontal/advective Inversion:

This type of inversion occures due to movement of warm and cold air.

Frontal Inversion: occurs when warm air overrides cooler air.

Advective Inversion: occurs when warm air flows over a cold surface or cold air.

 

 

 

 

  Stability Conditions


Stability Conditions

The atmosphere is dynamic in nature; the actual environmental lapse rate keeps on changing with respect to dry adiabatic lapse rate and thus creates stable and unstable condition for dispersion of pollutants. Pasquill in 1961 divided stability into six main classes named A, B, C, D, E and F with class A being the most unstable or most turbulent class, and class F the most stable or least turbulent class.

 

The conditions vary with various meteorological parameters mainly with the actual value of decrease or increase of temperature with altitude.

  Role of Meteorology in determining Pollution dispersion


The dispersion of pollutants in air significantly depends on the stability condition prevalent at the time of release of pollutants from the stack. Meteorology plays a very imperative role in determining the dispersion of pollutants as it determines the stability condition. Various methods are given by various scientists to determine the stability condition by knowing the meteorological data of that particular locality. The main three methods are:

  • Pasquill-Gifford method

  • Temperature Gradient method

  • Wind direction fluctuation method

 

  Pasquill-Gifford method


Pasquill-Gifford method

Pasquill suggested the absolute combination of various meteorological parameters to determine the stability classification.

The meteorological parameters that he took were:

  • Wind Speed at 10 m height (R= Langley/hr .,1 langley =1 Cal/cm2)
  • Solar Radiation (langley)
  • Cloud cover

 

For example: Wind speed less than 2m/s on a very high solar radiation day (R>50) will lead to A stability condition, however at the same wind speed if sunlight is less intence then the condition can become B and can also become F stability class if there is cloud cover more than 50%. The instrument used to measure cloud cover is called OKTAS.

The table and his study suggest slow to moderate wind speed with high raditions/sunlight and less cloud cover at night are favorable conditions for pollution dispersion.

 

WindSpeed m/s at 10m height

Day time incoming solar radiation

Night time condition

 

Strong

Moderate

Weak

Cloud Cover

 

R>50

R=50-25

R=25-12.5

>50%

<50%

<2

A

A – B

B

E

F

2-3

A – B

B

C

E

F

3-5

B

B – C

C

D

E

5-6

C

C – D

D

D

D

>6

C

D

D

D

D

  Temperature Gradient method


Temperature Gradient method

this method gives the classification of atmospheric stability on the basis of temperature which is the major meteorological parameter. The temperature determines the radiation as well as control wind movement thus this method was a easy method to determine stability based on temperature fluctuations.

the point here to remember is its temperature gradient; the rate of change in temperature that determines ELR. The change can be realted with DALR to further understand dispersion. The negative sign indicate decrease in temperature.

Pasquill Stability Class

Temperature Gradient dt/dz  oC/100m

A

<-1.9

B

-1.9 to -1.7

C

-1.7 to -1.5

D

-1.5 to -0.5

E

-0.5 to 1.5

F

>1.5

  Wind direction fluctuation method


Wind direction fluctuation method

the wind speed and wind direction is also one of the most important meteorological parameter that determines extent of pollution in the atmosphere.

The fluctuation is determined by Sigma theta method = wd/6. The wd is the degree measurement by direction as given in figure.

Stability Class

Average wind direction fluctuation

A

>22.5

B

22.4 to 175

C

17.4 to 12.5

D

12.4 to 7.5

E

7.4 to 3.5

F

<3.5

  Plume Dispersion


Plume Dispersion

A plume is a distribution of pollutant from a continuous source. e.g. smoke from a chimney stack. The dispersion of a plume is influenced by various plume properties including:

  • rate of release
  • temperature of release (buoyancy)
  • height of release

and also by environmental properties including

  • windspeed
  • turbulence
  • atmospheric stability

Based on the above mentioned plume and environmental properties, the plumes are broadly divided into six main categories:

  Looping Plume


Looping Plume

In a well-mixed turbulent boundary layer on a hot day (forced by buoyancy), the turbulent eddies may be large and intense enough effectively disperse the plume in the air.

Conditions:

  • Super-adiabatic conditions
  • Strong solar heating                                    
  • Light wind speed
  • Unstable atmosphere
  • Clear day

 

 

 

  Coning Plume


Coning Plume

This is the kind of form assumed for a Gaussian plume, when the boundary layer is well-mixed and turbulent eddies are smaller than the plume scale. The plume forms a cone downstream.

Conditions:

  • Weakly stable condition
  • Moderate to strong winds
  • Cloudy day
  • Wet condition

 

 

  Lofting Plume


Lofting Plume

At early evening, if a surface inversion is developing, vertical motion may be inhibited below the plume while remaining active above: the plume is diluted but does not reach the ground. This is a favourable situation.

Conditions:

  • Inversion Condition below
  • Early morning
  • Light winds & light turbulence
  • Temperature gradient is positive

 

 

  Fumigation Plume


Fumigation Plume

There is a strong inversion restricting mixing above, and the plume is mixed throughout the boundary layer. This can occur quite rapidly. For example, after sunrise when the nocturnal inversion is being eroded from below by buoyant eddies, plume-level air of high concentration may be brought down to the surface over a wide area.

Conditions:

  • Inversion Condition above
  • Early morning after a stable inversion night
  • Increase Ground Level Concentration (GLC)

 

 

  Trapping Plume


Trapping Plume

 There is a relatively well-mixed layer with inversions above and below it which trap the plume at a particular height. This might occur at night when there is a low-level inversion above the NBL and a higher level inversion left over from the previous days CBL. In between is the relatively well mixed residual layer.

Conditions:

  • Inversion layer exist both above and below the stack
  • Diffuses within limited vertical height

 

  Meteorology & Air Pollution Disasters


Meteorology governs the dispersion of pollutants and thus decides the fate of pollution on environment and people living in the environment. The extent of spread of a plume in its surroundings is the function of meteorology and is decides the total number of people likely to be affected by spread of any time of pollutants in the atmosphere. The strong stability conditions prevent the vertical and horizontal mixing of pollutant in the air and thus trap the pollutants in the air near the surface which affects the well being of the individuals. The accident becomes disasters when people get affected. The trapping of the pollutants in the surrounding air they breathe calls the disaster to happen.

The similar outcome was seen in the major Air Pollution Disasters that occurred in various part of the World. The winters have always increased the intensity of air pollution disasters due to lack of dispersion.

 

Location

Date

Symptoms & effects

London Smog

Dec 5-9, 1952

4000 deaths occurred, irritation in eyes ,throat along with vomiting and headache.

Los Angeles Smog

Feb 4, 1953

Suffered from nausea and breathlessness.

Bhopal Gas Tragedy

3 Dec 1984

2000 deaths and 3000 suffered eye troubles, 50,000 were ill.

The disastrous end of the above mentioned incidence was due to very stable condition that prevails in winters which is governed by various meteorological factors.  Winters are generally dominant in bearing stable atmospheric condition which restrict the dispersion of pollutants in air and thus increase the chances of exposure of people to it.

  Control of Particulate Emission


Control of Particulate Emission

Airborne particles can be removed from a polluted airstream by a variety of physical processes. Once collected, particulates adhere to each other, forming agglomerates that can readily be removed from the equipment and disposed of, usually in a landfill.

Every pollution control device is different with specific pollutant control criteria. It is usually not possible to decide in advance what the best type of particle collection device (or combination of devices) will be; control systems must be designed on a case-by-case basis.

Important particulate characteristics that influence the selection of collection devices:

  • corrosivity
  • reactivity
  • shape
  • density
  • size and size distribution (the range of different particle sizes in the airstream).

Other design factors include airstream characteristics (e.g., pressure, temperature, and viscosity), flow rate, removal efficiency requirements, and allowable resistance to airflow

In general, cyclone collectors are often used to control industrial dust emissions and as pre-cleaners for other kinds of collection devices. Wet scrubbers are usually applied in the control of flammable or explosive dusts or mists from such sources as industrial and chemical processing facilities and hazardous-waste incinerators; they can handle hot airstreams and sticky particles.

Some common air pollution control devices are:

  Model : Air Pollution


Physical models representation of the reality in a reduced scale, simulating processes

Visual models – elaboration of images that represent the environment before and after the development of a project and its alternatives. It can also address the timing dimension (e.g., seasonal changes, vegetation growth).

Mathematic models- maths or statistic simulations applied to the deterministic or probabilistic calculation, based on quantitative values

Cartographic models- representation of reality that will be affected by the project through maps or charts. Cartographic overlaps enable impact predictions

 

  Air Quality Modeling


  • Predict pollutant concentrations at various locations around the source
  • Identify source contribution to air quality problems
  • Assess source impacts and design control strategies
  • Predict future pollutant concentrations from sources after implementation of new regulatory programs

Mathematical and numerical techniques are used in AQM to simulate the dispersion of air pollutant. Modeling is done for:

  • Toxic and odorous substances
  • Single or multiple points
  • Point, Area, or Volume sources

Input data required for Air Quality Modeling

Source characteristics

Meteorological conditions

Site and surrounding condition

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