Air pollutant sampling is primarily of two types: 1) Air sampling based on particulate pollutants and 2) Air sampling based on gaseous and vapor pollutants.
Air sampling techniques for particulate pollutants include sedimentation, filtration, impingement, and precipitation.
Sedimentation is used for particulates that settle out of the atmosphere as a result of the force of gravitation. A sampler consists of a simple dust jar with a funnel. A liquid is added to the collector to prevent the solids from blowing out from the jar. The collected dust is evaporated to dryness and then weighed. The sample period is usually 30 days.
The filtration technique is used in the collection of air pollutants of size smaller than 10 micrometers in diameter. The particulates are removed from the air sample by suction apparatus through a porous filter where particulates are deposited. The glass fiber filter of porous size less than 0.1 micrometers is used in a high-volume sampler. The sampling period is typically 24 hours.
Impingement is an inertial technique where particles are collected from a high-velocity air stream directed towards obstacles placed in its path. Particles collide with the obstacles, causing the air to change direction. If the obstacle has an adhesive surface, the particles stick to it, a process known as dry impingement. If the obstacle is wet, the process is referred to as wet impingement.
Precipitation techniques are of two types: a) Thermal precipitation, which is based on the principle that particles move towards the lower temperature region when subjected to a strong temperature gradient. The temperature gradients are typically of the order of 3000 degrees Celsius per centimeter. This technique is used to collect particles of size 0.001 micrometers with high efficiency. b) In electrostatic precipitation, the precipitators are electrically charged to force particulates to migrate out of the air stream onto a collection surface. The precipitators have positive and negative electrodes, and when the air stream with particulates passes through the electrodes, the particles pick up a negative charge and migrate towards the inner post of the precipitators where they get discharged and get deposited on the positive electrodes.
Air sampling for gaseous pollutants includes methods such as 1) Absorption sampling 2) Adsorption sampling and 3) Condensation sampling
In absorption sampling, gaseous pollutants are absorbed in a solvent when both the pollutant and absorbent are in close contact. The liquid reacts with the gaseous pollutant to form a non-gaseous pollutant.
In adsorption sampling, gaseous pollutants are absorbed on a solid surface of activated carbon, silica gel, activated alumina, and molecular sieve. As the surface area increases, adsorption increases.
Condensation sampling is employed to collect radioactive gases, hydrocarbons, and non-reactive vapors. As the gaseous pollutants pass through condensers set at various temperatures, the gaseous pollutant gets trapped in the liquid where the temperature is below the boiling point.
Some new methods of air sampling include bubble sampling and sorbent sampling.
In bubble sampling, air is drawn into an impinger, and forced through a nozzle covered with a liquid such as high-purity water. The pollutant dissolves in the liquid and can be analyzed subsequently.
In sorbent sampling, sorbents are typically contained in a small glass tube with sealed ends. Air is drawn through the sorbent, which captures the molecules orf gas or vapor to be sampled. The trapped contaminants are released using solvent washing. Charcoal is one of the best-known sorbents.