In this article we will discuss about the estimation of particulate and gaseous pollutants in air.
Estimating the Particulate Concentration in Air:
The selection of equipment or method for sampling of particulate pollutants is based on the size of particulates to be sampled. If the size is greater than 10 microns, sedimentation based method may be adopted and the size is less than 10 microns, filtration based method etc. may be used.
1. Dust Fall Collector:
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It is used generally for particles greater than 10 microns to estimate particulate pollutant concentration in air. This equipment is inexpensive and simple to operate. It consists of a plastic or metal jar of about 200 to 300 mm height and 100 to 150 mm in diameter at the base with a tapered of wall from top to bottom.
Water is poured up to 100 to 250 mm height, so that insoluble dust reaches and retains at the bottom of the jar. To avoid algal growth in the jar, an algicide (1 mg of copper sulphate per litre of water) is added to water. During winter, isopropyl alcohol may be added as antifreegent to prevent the formation of ice in the jar. This equipment is kept at the required location for a period of 30 days.
Just after that, dust is separated from the jar, evaporated and weighed in milligrams. The dust fall rate is expressed in milligrams per square centimeter per 30 days (mg/cm2/month or tons/km2/month). Even though the equipment is inexpensive and simple to operate, the values are not accurate and not reproducible. Hence they can be used to measure the relative pollution level or trends in an area over a period of time.
2. High Volume Air Sampler:
The high volume air sampler is a popular and frequently used equipment for the determination of suspended particulate matter (SPM) in air. The principle involved in this method is that the particles are filtered from known volume of an air sample by a suction apparatus (a vacuum pump) and the particles are made to deposit on a porous filter paper. The commonly used high volume sample (‘Micrometal’ make, Bombay and other similar makes).
The high volume air sampler consists of the following, as shown in Fig. 4.2:
(a) Face plate and gasket
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(b) Filter paper (preferably Whatman make), and fitting assembly
(c) Vacuum pump (air sucker) with air flow measuring device and
(d) Casing with a roof lpm
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The filter plate provides the base for sitting the filter paper of size 200 mm x 250 mm, through which air sample is collected by creating suction in the filter area. This suction is created by a vacuum pump with a flow rate of 25 lpm. These conditions will permit the sampling of abient air for a period of 8-h. The suspended particles of size less than 10 microns and greater than 3 microns are retained on the filter.
The duration of sampling is measured in an elapsed time meter which is placed in series with the blower. A rotameter or a manometer is provided to measure the volume of air passed through the filter. Generally ‘Whatman make’ filter papers are widely used. The concentration of the pollutants is expressed as micrograms per cubic meter which is calculated from the observed data as given below.
Calculation of the result:
3. Other Methods:
To collect the particulate matter from air, samplers like paper tape air samplers, Durham sampler, Anderson sampler, Greenlarg-impinger and cyclone sampler are used. The principle involved in these methods is based on inertial impingement. In this technique, an obstacle is placed against the polluted air flow to change the direction of flow, but the particles continue to travel in the inertial direction. When the particles hit the obstacle, there they attach to the adhesive surface of the obstacle. The particles are separated by immersing the obstacle in a liquid (scrubber).
Estimating the Gaseous Pollutants in Air:
The sampling methods of gaseous pollutants are classified into three types which are as follows:
1. Absorption
2. Adsorption, and
3. Condensation
1. Absorption:
In this method, a gaseous pollutant in air is removed by providing through contact with a chemical which can absorb the pollutant in a closed container under specific conditions and a new substance is formed. After sampling, the gaseous pollutant is estimated based on any convenient characteristic of newly formed substance (e.g., colour). The results are expressed into micrograms per cubic meter. This method can be used for sampling sulphur dioxide, oxides of nitrogen, hydrogen sulphide, ammonia and aldehydes in air using absorbing reagents for each gas.
2. Adsorption:
In this method, gaseous pollutant are made to attract to the surfaces of a solid media (adsorbent) and retained there. The efficiency of the system mostly depended on the surface area of the adsorbent surface and several other factors. By this method, ammonia, oxides of nitrogen, carbon monoxide, hydrogen sulphide and sulphur dioxide etc. may be sampled.
Following are the advantages of using adsorption instead of absorption in the sampling of gaseous pollutants:
(a) Due to colour formation, pollutant are easily identified.
(b) The sample can be easily transported because pollutant is retained in solid media.
(c) Highly volatile gases with high concentrations are also completed collected.
3. Condensation:
This method is used for sampling of hydrocarbons, radioactive gases etc. The gas is sucked through collection chambers (sampling train) in which temperature is progressively lowered. If the temperature in the collection chamber is equal to the boiling temperature, the gas turns to liquid and condensate. Sampling trains with freezing facility enables to collect several gases at the same period.