Gaseous Pollutant Monitoring

Criteria Pollutants

  Instrument Repair 
A few common air pollutants are found all over the United States (sulfur dioxide, ozone, carbon monoxide, nitrogen dioxide, and fine particulate matter or PM). These pollutants can injure health, harm the environment and cause property damage. EPA calls these pollutants criteria air pollutants because the agency has regulated them by first developing health-based criteria (science-based guidelines) as the basis for setting permissible levels. One set of limits (primary standard) protects health; another set of limits (secondary standard) is intended to prevent environmental and property damage.

This excerpt is from The Plain English Guide to the Clean Air Act. This publication and others can be found at the EPA Office of Air & Radiation publications web site:

In the Office of Air Quality Monitoring, the employees in the Criteria Pollutant Instrumentation Group are responsible for the installation, calibration, troubleshooting and repairing of the continuous instrumentation for the monitoring of gaseous criteria pollutants.

Gaseous Criteria Pollutants

Sulfur dioxide (SO2)

Sulfur Dioxide is formed from combustion processes (i.e., burning of fossil fuels containing sulfur compounds), refining of petroleum, manufacture of sulfuric acid, and smelting of ore containing sulfur. Once in the atmosphere, sulfur dioxide can be oxidized to SO3 (sulfur trioxide). With water vapor, SO3 is converted to sulfuric acid mist. Other basic oxides combine with SO3 to form sulfate aerosols. These compounds can be transported long distances and fall back to earth as acid rain. SO2 damages plant leaves and in moist air forms acids that harm structural materials. The respiratory tract can be also be irritated.

Sulfur dioxide is measured continuously with a fluorescence analyzer. Air is drawn through a sample cell where it is excited by UV light. The SO2 will fluoresce and be detected with a photomultiplier tube. This is converted to an electrical signal related to the SO2 concentration.

Ozone (O3)

Ozone is formed by complex reactions among nitrogen oxides and certain organic compounds under the influence of solar ultraviolet radiation (sunlight). Ozone shows a very strong diurnal (daily) and seasonal (April to October) cyclical character. It injures vegetation, has adverse effects on materials (rubber and fabrics) and is a pulmonary irritant that affects the respiratory mucous membranes, lung tissues and respiratory functions.

Ozone is measured by ultraviolet absorption photometry. Air is drawn continuously through a sample cell where narrow band ultraviolet light (254nm wavelength) passes through it. The proportion of light absorbed by ozone molecules in the air is converted into an electrical signal and recorded.

Carbon monoxide (CO)

Carbon Monoxide is produced by incomplete combustion of carbon compounds, principally in internal combustion engines. Concentrations in the vicinity of heavily traveled highways are higher than ambient concentrations more than 100 meters from any highway. Carbon monoxide is not an irritant, and has little or no effect on plants or materials; however, it reacts in the bloodstream to deprive the heart and brain of oxygen. Moderate concentrations significantly reduce brain function and high concentrations can be lethal.

Carbon monoxide is measured continuously by infrared absorption photometry. Air is drawn through a sample cell where infrared light passes through it. CO molecules in the air absorb part of the infrared light, reducing the intensity of the light reaching a light sensor. This portion of the infrared light absorbed by CO is converted into an electrical signal reflecting the CO concentration, and recorded.

Nitrogen dioxide (NO2)

Formed in the burning of coal, oil, gas, and gasoline, when combustion temperatures are extremely high, NO2 has been established as having a detrimental effect on human health. In high concentration it can cause impairment of dark adaptation, increase airway resistance and respiratory rate, and enhance susceptibility to respiratory infections.

NO2 is measured using chemiluminescense. NO in the air reacts with the ozone to produce NO2, which releases light. The light is detected by a photomultiplier tube. An electrical signal reflects the NO concentration. NOx is measured by passing the air through a converter where any NO2 is converted to NO. By alternating the air directly to the reaction chamber, and through the converter before the reaction chamber, the analyzer reads NO and NOx. The NO2 concentration is the difference between NO and NOx.

Total reactive nitrogen (NOy)

Although not a criteria pollutant, the VA DEQ does sample for ambient concentrations of NOy during the summer months. The following abbreviated explanation is taken from Thermo Environmental Instruments Standard Operating Procedures:

Reactive Nitrogen compounds (NOy) have been identified as precursors for both ozone and fine particulate (PM2.5). NOy consists of all oxides of nitrogen in which the oxidation state of the N atom is +2 or greater, i.e., the sum of all reactive nitrogen oxides including NOx (NO + NO2) and other nitrogen oxides referred to as NOz.

To measure NOy, sample air is passed through a probe-mounted chemical reduction converter and the nitroxyl compounds present are reduced to NO. The sample is then blended with O3 in a reaction chamber causing a chemiluminescent response (emission of light) to occur. The reaction is monitored by an optically filtered high-sensitivity photomultiplier giving an electronic signal output. This output is proportional to the concentration of NOy entering the converter.

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Virginia Department of
Environmental Quality
P.O. Box 1105
Richmond, VA 23218
(804) 698-4000

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