Meteorological and Pollutant Parameters

Meteorological Measurements

  • Temperature: The ambient air temperature reported in degrees Fahrenheit. To convert to Celsius, subtract 32 from degrees Fahrenheit and divide by 1.8. Temperature sensors are often mounted on a lightweight tower at either 2 meters (2m) or 10 meters (10m) above the ground surface.

  • Relative Humidity: The ratio of actual water vapor in the ambient air to the maximum amount that could occur at the same ambient air temperature. The ratio is reported as a percentage.

  • Precipitation: Amount of rainfall reported in inches per hour.

  • Wind Speed: Wind velocity is reported in miles per hour. To convert to meters per second, divide by 2.24.

  • Wind Direction: Wind direction is the cardinal or intercardinal direction from which the wind is blowing. Wind directions are also reported in degrees azimuth, a compass scale of 0° to 360°. The cardinal directions are North (0° or 360° azimuth), East (90°), South (180°) and West (270°). See diagram below.

          Wind Directions

  • Solar Radiation: The amount of atmospheric solar radiation with a wavelength of 0.4 to 1.1 micrometers in length reported in watts per square meter. These wavelengths are in the infrared and the visible light range of the electromagnetic spectrum.


Pollutant Concentration Measurements

Two categories of air pollutants are monitored and shown here: Suspended Particulates and Gaseous Pollutants. Suspended particulates are generally measured and reported in units of micrograms per cubic meter (µg/m3). Gaseous pollutants are reported in either parts per billion (ppb) or parts per million (ppm), depending on the numerical precision required and the sensitivity of the monitoring equipment. All pollutants are shown as one-hour averages, except as noted below.

  • Suspended Particulates

    • PM-10: Suspended particulate matter with an aerodynamic diameter of 10 micrometers or less. Particulate matter is unhealthy to breathe, particularly for children, the elderly and people with respiratory ailments. It also contributes to haze that can obscure views, and it can reduce photosynthesis in plants. The PM10 value shown is a one-hour average, but there is no hourly ambient air standard. The 24-hour standard is 150 mg/m3.

    • PM-2.5: Suspended particulate matter with an aerodynamic diameter of 2.5 micrometers or less. Fine particulates are an even greater health concern than larger particulate matter, because fine particulates are not easily trapped by the respiratory system and can penetrate more deeply into the lungs. Fine particles also play a large role in reducing visibility. The PM2.5value shown is a one-hour average, but the 24-hour standard is 35 µg/m3. (See Note 1 for detailed information about the relationship between one-hour and 24-hour averages of PM2.5).

  • Gaseous Pollutants

    • Ozone: A reactive chemical compound of 3 oxygen atoms found at all levels of the atmosphere. Ozone is unhealthy to breathe, particularly for children and other sensitive groups, and it can damage trees and crops. Power plants and motor vehicles are the main sources of ozone-forming pollution in North Carolina. Ozone values are shown both as one-hour averages and eight-hour averages. Historically, there was an ambient air standard for one-hour averages, but it was revoked nationally in June, 2005 and phased out for all areas of North Carolina by April, 2009. The 8-hour average ambient air standard is 0.076 ppm (76 ppb).

    • Carbon Monoxide (CO): The most commonly occurring air pollutant. Carbon monoxide (CO) is a colorless and poisonous gas produced by incomplete burning of carbon-containing fuel. Breathing carbon monoxide affects the oxygen-carrying capacity of the blood. Hemoglobin in the blood binds with CO more readily than with oxygen, starving the body of vital oxygen. Individuals with anemia, lung and heart diseases are particularly sensitive to CO effects. Low concentrations affect mental function, vision and alertness. High concentrations can cause fatigue, reduced work capacity and may adversely affect fetal development. Chronic exposure to CO at concentrations of 70 ppm (70,000 ppb) or greater can cause cardiac damage. Other health effects associated with exposure to CO include central nervous system effects and pulmonary function difficulties. The CO value shown is a one-hour average. The 8-hour standard is 9 ppm (9,000 ppb), and the 1-hour standard is 35 ppm (35,000 ppb).

    • Ammonia: A colorless gas with a pungent and suffocating odor. and the most abundant alkaline gaseous component in our atmosphere. Ammonia is incompatible with, and thus reactive with, strong oxidizers, acids, halogens, and salts of silver and zinc. It is corrosive to copper and galvanized surfaces. Health hazards associated with ammonia are chest pain, skin burns and frostbite. The vapors are extremely irritating and corrosive. There is no ambient air standard for ammonia.

    • Sulfur Dioxide: A colorless, corrosive, harmful gas with a pungent odor. Smaller concentrations of sulfur trioxide and other sulfate compounds are also found in sulfur dioxide (SO2) emissions. Sulfur oxides contribute to the formation of acid rain and the formation of particles that reduce visibility. The most obvious health effect of SO2 is irritation and inflammation of body tissues brought in contact with the gas. SO2 can increase the severity of existing respiratory diseases such as asthma, bronchitis, and emphysema. The one-hour health-related standard is 75 ppb. SO2 also has a separate "secondary" standard for protection from adverse effects on vegetation, animals, materials, and other aspects of "well-being" distinct from human health. The secondary standard is a 3-hour average of 500 ppb (0.50 ppm).

    • Nitrogen Oxides: Several gaseous oxides of nitrogen are normally found in the atmosphere, including nitrous oxide (N2O), nitric oxide (NO) and nitrogen dioxide (NO2). The combination of NO and NO2 is sometimes generically referred to as NOx. NO and NO2 are monitored extensively because they are precursors of ozone formation. N2O a stable gas with anesthetic characteristics and typical ambient concentrations well below the threshold concentration for a biological effect. NO2 is reddish-brown but is not usually visible at typical ambient concentrations. NO is a colorless gas with ambient concentrations generally low enough to have no significant biological effect. It is rapidly converted in air to NO2. NO reacts with fluorine, combustible materials, ozone, ammonia, chlorinated hydrocarbons, metals and carbon disulfide. It causes eye, nose and throat irritation. NO2 is the only nitrogen oxide subject to an ambient standard. The one-hour ambient standard is 100 ppb. The annual standard for NO2 is an arithmetic mean one-hour average of 53 ppb.

    • Reactive oxides of nitrogen: Oxidation of NOx produces other compounds in the atmosphere, and in combination these are referred to as "reactive nitrogen oxides" and usually abbreviated to "NOy". value shown is a one-hour average. There is no ambient hourly air standard for NOy. health effects associated with NOy are similar to those of NO. There is no ambient air standard for NOy.

Note 1:
The 24-hour standard for PM2.5 is required to be evaluated using specially designated monitors termed "federal reference method (abbreviated "FRM") or federal equivalent method" ("FEM") monitors, FRM monitors currently are not capable of reporting one-hour averages and are not available for real-time reports. For real-time reports, we acquire PM2.5 one-hour averages using either a "tapered element oscillating microbalance" instrument ("TEOM") or a beta attenuation method instrument ("BAM").

Each TEOM is paired with an FRM at the same monitoring station, and we periodically compare 24-hour averages calculated from the TEOM data to the corresponding 24-hour averages measured directly by its companion FRM. BAM monitors are operable in a configuration that is FEM, but we pair new ones similarly with an FRM for a probationary period (typically, 12 months), and where there is a TEOM collocated with a BAM, we use the TEOM data for real-time reports. For reporting purposes, TEOM averages are adjusted in a manner explained in the next paragraph.

There is a strong, though imperfect, linear relationship between the TEOM and FRM data. Once we have accumulated at least 12 months of paired averages at a site, we can estimate that relationship using inverse regression and use it to compute "FRM-adjusted" TEOM averages representing our best prediction of the one-hour averages that would have been reported by a FRM. The regression equation can be stored in the TEOM's data logger so that FRM-adjusted one-hour averages are computed in real time. We use the FRM-adjusted averages in online real-time data reports and current Air Quality Index reports. In general, the TEOM-FRM relationship varies with each season of the year, so at most sites, we cycle annually through 4 different regression equations.