Ambient Air Quality Monitoring System Principle

 AAQMS (ambient air quality monitoring System) for continuous monitoring of pollutant gases like NO2, SO2, O3, CO and suspended particulate matter like PM10 and PM2.5

Ambient Air Quality Monitoring Parameters As per norms below mentioned parameters are mandatory for air quality.

CO : Carbon Monoxide

SO2: Sulphur Dioxide

NO2 : Nitrogen Dioxide

O3: Ozone

PM10: Particulate Matter

The following provides information about the basic principles used for measurement.

Sulfur dioxide (SO2) :

Sulfur dioxide (SO2) is measured continuously by instruments using the fundamental principle of fluorescence. Fluorescent analyzers irradiate an ambient air sample with ultraviolet light at 213.8 nm. Sulfur dioxide gas molecules absorb a portion of this energy, then re-emit the energy at a characteristic wavelength of light. This light energy emitted by SO2 molecules is sensed by a photomultiplier tube and converted to an electronic signal proportional to the concentration of SO2present.

Nitrogen dioxide (NO2)  :

Continuous monitoring of nitrogen dioxide (NO2) is based on a chemiluminescent reaction between nitric oxide (NO) and ozone (O3). When these two gases react, light energy at a specific wavelength is produced. In the monitor, ambient air is drawn along two paths. In the first path, the air is reacted directly with ozone, and the light energy produced is proportional to the amount of NO in the air. In the second path, the air is reacted with ozone after it passes through a catalytic reduction surface. The reduction surface converts NO2 to NO and the light energy produced is a measure of the total oxides of nitrogen in the air sample. The electronic difference of these two signals yields the concentration of NO2. All concentrations for NO2 are given in parts per million (ppm).

Carbon monoxide (CO) is monitored continuously by analyzers that operate on the infrared absorption principle. Ambient air is drawn into a sample chamber and a beam of infrared light is passed through it. CO absorbs infrared radiation, and any decrease in the intensity of the beam is due to the presence of CO molecules. This decrease is directly related to the concentration of CO in the air. A special detector measures the difference in the radiation between this beam and a duplicate beam passing through a reference chamber with no CO present. This difference in intensity is electronically translated into a reading of the CO present in the ambient air, measured in parts per million (ppm).

Ozone :

Ozone is measured continuously by analyzers that operate using the monochromatic ultraviolet absorption spectrophotometry principle. As ultraviolet light at 253.7 nm is passed through the optic bench, a fixed quantity of “zero air” and ambient air are drawn into the bench. The intensity of the ultraviolet radiation traversing the optics bench is attenuated by the ozone present in the ambient sample. This attenuated signal is detected and compared with the unattenuated signal from the “zero air” cycle. This difference in intensity is electronically translated into a reading of ozone present in the ambient air.

Lead (Pb) :

Lead (Pb) concentrations in ambient air are determined by the reference method promulgated by the U.S. EPA. The Pb sample is collected on a filter using a high-volume air sampler and the TSP method. In this method, two 3/4″ X 8″ portions of the teaspoon filters are washed with hot, diluted nitric acid. The Pb compounds are dissolved into the acid solutions. The solution is then analyzed by the atomic absorption technique to determine the amount of lead.

Particulate Matter :

The shorthand PM10 is defined as particulate matter with an aerodynamic diameter less than or equal to a nominal 10 micrometers. The reference method for the measurement of atmospheric particulate matter as PM10 is based on selection of PM10 particles by internal separation, followed by filtration and gravimetric determination of the PM10 mass on a filter substrate. Selection of PM10 is accomplished by a size selective head that is symmetrical designed and contains nine circular acceleration nozzles. By virtue of their larger momentum, particles greater than 10 micron aerodynamic diameter impact onto a greased impaction shim. The PM10 particles smaller than 10 microns are carried vertically upward by the air flow and down 16 vent tubes to a 8-by-10 inch quartz-fiber filter, where they are collected.

Source : epa.ohio