Air pollution can be measured in one of two ways. Source monitoring, which measures the pollutants as they are emitted from a source, seeks to determine the mass of polluting material that is contained in a volume of emission (i.e., mass of pollutant/volume of emission or the amount of polluting material that is emitted by a given mass of fuel or other pollution source, i.e., the mass of the pollutant/mass of fuel consumed). Source monitoring is commonly used to determine the amount of polluting material emitted by by such fixed or mobile sources as factories, volcanoes and motor traffic. Ambient air monitoring, on the other hand, measures the concentration of polluting material in the immediate atmosphere and is commonly used to determine the quality of air in major urban areas, with several monitoring stations measuring the presence and/or levels of specific predetermined pollutants. Obtaining samples of air for processing an be done in a number of different ways.
Passive sampling is a simple and inexpensive way to measure pollution levels in the air. Plastic tubes or discs are used for this process. The tubes/discs are open at one end and contain an absorbent material at the other end which is soaked in a chemical that reacts to different pollutants. The open end is exposed to the atmosphere, and, when the atmosphere has reacted with the chemical in the absorbent material, the samples can be sent to a laboratory for analysis. The relative low cost per tube allows samples to taken from several spots so that actual pollution levels can be determined for specific points, and those areas that require remedial actions can be more easily pinpointed. Very similar to passive sampling is a method known as active sampling, where a known volume of air taken from a specific area is pumped through an air filter which isolates polluting matter or through a chemical that reacts with air pollutants. Samples can then be taken to a laboratory, where analysis can be carried out in order to determine what further action, if any, is required.
A more sophisticated method used in determining the presence of pollutants in the air is known as automatic sampling, which can be employed to determine the presence of multiple pollutants at a single site. The instruments used employ such sophisticated measurement techniques as ultraviolet fluorescence and infrared absorption and the results are transmitted online and in real time to a collation center where rapid computer analysis can be undertaken and determinations as to possible redemptive actions can be made in extremely short time. Many urban areas usually have such monitoring points, which measure different pollutants that may be found in the air in urban areas. This is very important for, although clean air is necessary in all locations, urban areas are at greater risk than less populated areas for the sheer amount of pollutants that are released into the atmosphere on a daily basis.
These sampling methods, which may be described as direct sampling, are frequently used in conjunction with more indirect methods. Such indirect methods make use of computer modeling techniques, which attempt to determine the presence and levels of pollutants in the atmosphere. These computer models, which vary greatly in terms of sophistication, accuracy and precision, use previously collected data to predict future pollutant emissions from varied sources and the eventual impact of such emissions on ambient air quality and many different models exist that are used for different sources and types of pollution.