Atmospheric sampling deals with the pollutants within the total air mass surrounding the earth.
The purpose of atmospheric sampling is to develop air quality criteria, which are the basis for setting air quality standards. Atmospheric sampling measures pollution, provides control data, and renders background information for use in trend evolution and source detection.
Sampling Procedures:
The complete sampling procedure is based on the pollutant sampled, the techniques used in collecting the pollutant, the device chosen, and the method of analysis. Atmospheric sampling is related to analysis in that a volume of air, for example, is collected. This volume of air is analyzed to determine quantity of air pollutant collected, which is measured in microgram (Hg). The pollution concentration in the air sample is, then, expressed in micrograms per cubic meter
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Collection Techniques:
Collection techniques are determined by particulate involved indicates these particulate sampling devices and a summary of the samples of particulates is listed in
Gravity Technique:
The gravity technique is used to collect settle able particulates that settle out of the atmosphere as a result of the force of gravitational pull. Devices that use this technique are referred to as dust fall sampling instruments.
Dust fall sampling may be done with a dust fall brush or cellophane tape coated with a material that captures dust or its sticky surface, but is most often done with a dust fall bucket. Dust is rinsed from the bucket, evaporated to dryness, and, then, weighed in milligrams.
The amount of air sampled is based on the measurement of the area of the bucket. The standard collection period is 30 days. The results are reported in milligrams per square cm per 30 days (mg. /cm2/ month).
Filtration Technique:
This technique is used to collect suspended particulates that do not settle out of the air. Particulates are removed from air sample by a suction apparatus, e.g., vacuum pump and are deposited on a porous filter. The high-volume sampler is most frequently used when sampling for suspended particulates.
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The paper tape sample is another filtration instrument used for suspended particulates. It is most suitably adopted to collect fine or soiling materials.
In the inertial technique, total particulates are collected. In this technique, a polluted air stream containing particulates is drawn into a sampler where obstacles are placed across the path of the airstream. The obstacle causes the airstream to change direction but particles continue to travel in the initial direction and collide with the obstacle.
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If the obstacle has an adhesive surface, the particulates are impacted on it. If the obstacle is immersed in a fluid, the particulates are collected by impingement in the liquid.
Precipitation Techniques:
These techniques are divided into thermal and electrostatic precipitation. Thermal precipitation uses a heated wire to drive radioactive particulates by thermal convection and molecular bombardment out of gaseous stream into a cold collecting surface. Particles 0.01m to 10m adhere best to a collecting surface.