Removal Process of Air Borne Pollutants, Effects of Meteorological Parameters

Airborne Pollutants Removal Process

Most of the airborne pollutants are eventually remove from the atmosphere by sedimentation, reaction, dry deposition, wet deposition, interaction with earth surface.

1: Sedimentation

This process is also called as the settling of the gravity particles having the size less than 20μm are treated as the dispersing gases and effect due to their fall velocity are generally ignored. Particles greater than the 20μm have the applicable settling velocity. The fall velocity of the smooth sphere and the function of particle size particles in the range of 2200μm are assumed to be dispersed approximately as gases with their centre moving downward in the atmosphere according to the fall velocity. For these particles, the deposition of the ground is

ω=V_g (X,Y,Z)

Were X= air concentration is evaluated for height above the ground Z of about 1m

V_g= fall velocity

Particles larger than 100μm fall through the atmosphere so rapidly that turbulent have less chance to act upon as dispersed them.

2: Wet removal by precipitation or Wet deposition

The scavenging of the particles or gases by the wet deposition has two distinct mechanisms, takes place in clouds i.e rain out and below cloud i.e watch out by precipitation. By the precipitation, the first includes various processes taking place where the contaminants such as condensation supplied on which droplets condense the second mechanism refers to the removal of pollutants below the cloud level by falling rain.

Wet deposition is one of the most scavenging processes for both the particulate matter and the gaseous pollutants in the global sense. The rain out mechanism is particularly effective for those whose size is less than 0.3µm. These particles are being captured by the cloud droplets and the droplets grow in size by the coalescence and then precipitation. Wash out is the most effective in removing particles larger than 2µm each scavenging effectively. However, is influenced by the raindrop size and the intensity of the rainfall. A scavenging ratio or wash out ratio (ω) can be defined as;

ω= K.Ρ/x

where

K= concentration of contaminants in precipitation µg/gm

χ= concentration in the air in µg/m³

Ρ= density of air ( nearly equal to 1200gm/m³⁾

3: Chemical reaction in the atmosphere

Many of the pollutants i.e especially gaseous pollutants undergo chemical reaction within the atmosphere and form the new compound. This process is called the process of great importance for the removal of the sulphur dioxide. A large part of the sulphur dioxide in the atmosphere is oxidised to form sulphur dioxide which quickly combines with the atmospheric moisture to form sulphuric acid. The sulphur acid droplets may, in turn, react with the metal salt such as NaCl or metal oxide like MgO. Fe₂O_3,ZnO etc. to produce the sulphates.

2SO₂ + 2H₂O + O₂→ 2H₂SO₄

NaCl + H₂SO₄→ NaSO₄ + 2HCl

MgO + H₂SO₄→MgSO ₄+ H_2O

Both the acid droplets and the sulphates particles are rapidly removed from the atmosphere by the wet deposition. Carbon monoxide (CO) react with the atmospheric oxygen in the presence of the sunlight is found to be very slow and the accounts for the removal of only 0.1% of available CO for each of the hour of sunlight but CO relatively reacts with the hydroxyl radicals present in the atmosphere.

CO + OH^-→ CO₂ + H

4: Dry deposition

The process of the dry deposition is also called the process of the fall out. This process is applicable to the fine particles than those which take par in the sedimentation. In this case, deposition velocity (Vd) has been used to the account for the removal due to the amplification with the vegetation near the surface or chemical reaction with the surface.

According to the strokes law

V_d = [gdp² (Ρp-Ρa) (1=2c/ (dp.p)]/18μa]

where,

V_d= terminal settling velocity

dp=particle diameter

(Ρp-Ρa)= density or particle and air respectively

Ρ= air pressure

C= constant ( when Ρ is given in MB and up in cm, c= 0.0034)

From the above equation, it is seen that the rate of the sedimentation is strongly influenced by the particle size but the density of the particles is less dependable for the settling velocity of particles is less dependable for the settling velocity. The energy deposition rate (ω) is proportional to the immediate ground level air concentration. i.e

(ω)=V_d χ (x, y, 0) = ground level concentration

We can apply it to the gas vapours as well as to the small particles, SO_2,O_3.

Iodine and CO have deposition velocity

SO₂= 0.5-1.2 m/s

O₃=0.1-2.0 m/s

Idinw=0.7-0.8 m/s

CO= negligible

5: Interaction at the earth surface

Gaseous pollutants can be transported to the earth surface by the atmosphere turbulence where they interact with the ocean surface, vegetation and the upper layers of the soil are removed by the process of absorption or chemical reaction.

At the ocean surface carbon dioxide first diffused through the gas phase and the change into the liquid and finally diffuse into the oceans vegetation and the upper layers of soil also act as sinks for sulphur dioxide. Sulphur dioxide first diffuses to the external surface of the solid and then penetrates into the pores of the solute and subsequently absorbed on the porous side.

For the CO, the biological action in the soil seems to be the most important sinking of the carbon monoxide. The role played by the soil on the removal of the CO has been finally established. Soil contains certain bacteria which can make use of the carbon monoxide in their metabolism producing either carbon dioxide or the methane.

CO+1/2 O₂→ CO₂

CO+3H₂ →CH₄ +H₂O

Effects of meteorological parameter on air pollution

Any addition to the environmental components (i.e. air, water etc.) that threatens the health survival and activities of humans and another living organism is called pollution. Pollutants can enter to the environment from natural as well as anthropogenic activities.

Meteorological parameters that affect the atmospheric pollutions can be grouped into two categories. They are primary parameters and the secondary parameters. The wind speed turbulence is under primary parameters whereas the temperature, humidity, precipitation, solar radiation, pressure, facilities are the secondary parameters. The primary parameters are responsible for the dispersion and dilution of pollutants whereas secondary parameters affect the primary parameters. Even though the discharge of the contaminants into the atmosphere in a given area remains constant the degree of air pollution may vary from day to day because of difference in the meteorological conditions.

References:

Miller, Jr. G.T. Living in the Environment. Wadsworth Publication, 2003.

S.C., Santee. Environmental Science. India, New Center: New Center Book Agency (P) Ltd, 2004.

T., Richard. Environmental Science Towards a Sustainable Future. India: PHI (P) Ltd., 2008.

Lal, D.S. Climatology, Sharda Pustak Bhawan, Allahabad.(2010)