Air Sampling and Measurements 

Air quality measures or monitoring refers to the sampling and measuring contaminants in the atmosphere. It is an essential component of any campaign to reduce air pollution.

There are two scenarios in which air quality is measured:

1. Measurement of ambient air quality 
2. Stack monitoring

Measurement of ambient air quality – Pollutant levels in the ambient atmosphere are monitored in ambient air quality measurements.

Stack monitoring – Stack sampling is a method of determining the pollutants produced from a source such as a smokestack.

Basic air sampling considerations: 

• In terms of time and geography, the sample taken must be representative.
• The sample volume must be sufficient to allow for proper analysis.
• The sample rate must be such that collecting efficiency is maximized.
• The duration and frequency of sampling should correctly represent the occurrence of pollution level changes.
• During the collecting procedure, the pollutants must not be transformed or altered.

Two types of sampling methods are used:

Short period or Spot sampling – For specific proposals, samples are gathered throughout intervals ranging from less than 30 minutes to many hours.

Continuous sampling – If the data is to be helpful for epidemiological surveys, evaluating the possible dangers to man, animal, or vegetation, and for management programs. These techniques are valuable in systematic investigations of the type and extent of air pollution. It can be done by analyzing the contaminant using proper analytical techniques thereafter.

Collection of Gaseous & Particulate Air Pollutants

Particulate matter refers to all particles floating in the air that pollute and destroy our biosphere. These particles come from various places and come in a variety of sizes and compositions. The size and form of particulate matter significantly impact their pollution-causing potential.

Collection of Gaseous Air Pollutants:

There are four different methods for the collection of gaseous air pollutants.

1. Grab sampling – Grab sampling involves filling an evacuated flask or an inflated bag with a sample. 

2. Absorption in liquid – Absorption removes the desired contaminant from the air by either direct solubility in the absorbing media or chemical reaction. 

3. Adsorption on solids – The tendency of gases to be adsorbed on the surface of solid objects is the basis for this technology. 

4. Freeze-out Sampling – In freeze-out sampling, the air sample is drawn through a succession of cold traps that are kept at increasingly lower temperatures, allowing the contaminants to condense.

Collection of Particulate Pollutants:

• Particle size greater than 10µm – Sedimentation (Dustfall Jar) technique is used. Dust particles settle due to the force of gravity.
• Smaller size – Filtration, Impingement, Electrostatic, and Thermal Precipitation techniques are used. These techniques are used for particles that remain suspended as aerosols.

Collection Of Gaseous Air Pollutants

1. Grab sampling  
2. Absorption in liquid 
3. Adsorption on solids  
4. Freeze-out Sampling

Grab Sampling 

Grab sampling involves filling an evacuated flask or an inflated bag with a sample. Bags made of plastic are commonly used. 

Sampling in a bag:  Disadvantages: Moisture condensation or diffusion through the bag’s walls causes losses. The losses can be reduced by analyzing the data as soon as it is collected.

Grab sampling can be done with glass or stainless-steel solid wall containers.

Absorption in liquid

Absorption removes the desired contaminant from the air by direct solubility in the absorbing media or chemical reaction.

1. Fitted glass absorber – The gas stream is split up into incredibly small bubbles, allowing the gas and liquid to come into close contact. Coarse frits (pore size 50 microns) are utilized for air sampling. After use, the glass frit might become clogged and is difficult to clean. So, before sampling, pre-filter the air.
2. Impingers – The gas stream impinges at high velocity into a flat surface in the Impingers, resulting in good gas-liquid contact. The flat surface might be the collector’s bottom or a custom-made pate. Impingers come in two varieties: wet and dry. 

Adsorption on solids

The sample air is passed through a packed column containing a finely split solid adsorbent, the contaminants of which are retained and concentrated on its surface. The sample gases are desorbed for examination after adsorption. This can be done by either heating the adsorbent to release trapped materials or washing it with a liquid solution. Most organic vapors are analyzed using gas chromatographic methods that directly utilize gas adsorption.

Freezeout sampling

This involves drawing an air sample through a succession of cold traps that are kept at increasingly lower temperatures, allowing contaminants to condense. The traps are taken to the lab, the samples are extracted, and the samples are analyzed using gas chromatography, infrared or ultraviolet spectrophotometers, mass spectrometry, or wet chemical methods.

Collection Of Particulate Air Pollutants

For large particles (≥10µm)

Sedimentation (Dust Fall Jar): Suitable for particles greater than 10 microns. The sedimentation process is used in the jar method for dust fall. The sedimented particles can also be trapped using greasy slides.

For small particles (<10µm)

Filtration: Air particulate matter can be collected by passing it through a filter with a small enough pore size to retain the particles. The filter to use is determined by (1) the sampling objectives and (2) the particle size. Membrane filter morphology and size analysis Particles should be collected for weight measurement using a glass fiber filter (nonhygroscopic).

Impingement in liquid: When the air is deflected after striking the liquid surface, the particles are separated from the air by the force of inertia. Particle collection is done using the same bubblers or impingers used to collect gaseous contaminants.

Devices: (1) Smith standard in green burg (2) Midget Impingers (Midget Impingers)

Limitations: It is not frequently employed for particles due to low sample rates.

Electrostatic Precipitation: A negative charge is transferred to a wire inserted axially inside a positively charged cylinder during operation. When the particle-laden air stream passes through the cylinder, a corona discharge on the center wire gives the particles a negative charge. 

Thermal precipitation: Thermal precipitators work on the idea that tiny particles tend to travel towards the lower temperature and deposit on the cooler of two surfaces when there is a strong temperature difference between them. Temperature differences of this magnitude are usually measured in degrees Celsius per centimeter. 

Stack Sampling

To assess the overall quantity of pollutants discharged to the atmosphere, stack sampling (or source sampling) collects and validates sample sizes of emissions from industrial sources, including pollutant-laden air/gases, at the point of origin of pollutants.

To evaluate the emission rates/or characteristics of contaminants, source sampling is done in a process ventilation stack.

Location Selection for Sampling: The sample point should be as far from any potentially distracting influences as possible, such as elbows. Obstructions such as bends, transition pieces, baffles, and other obstacles. Wherever feasible, the sample size should be 5-10 diameters downstream of any barriers and 3-5 diameters upstream of comparable disturbances.

In every form of sampling technique, deciding on the appropriate sample point is always a personal decision. Stack monitoring is a type of environmental monitoring that includes air quality measures and monitoring.

Size of the sample point: The majority of stacks do not have an opening for sampling. As a result, sample points must be included in the stack. The diameter of the sample point should be between 7 and 10 cm. During non-sampling periods, that extension can also be closed.