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Mecklenburg County Particulate Matter Monitoring

Particulate matter, or PM, is the term for particulate found in the air, including dust, dirt, soot, smoke, and liquid droplets.  National Ambient Air Quality Standards (NAAQS) regulate ambient air concentrations of particulate in two different size fractions, PM 2.5 and PM 10. PM 2.5 is the fine fraction of particulate that is 2.5 microns or smaller in aerodynamic diameter. (PM 10 is the coarse fraction and consists of particulate that is 10 microns or smaller. In comparison, a human hair is about 70 microns in diameter.) 

Health and Environmental Impacts Filter samples
Particulate matter that is microscopic in size is important because it may penetrate deep into the lungs causing a variety of significant health problems, including:  aggravated asthma, increases in respiratory symptoms like coughing and difficult or painful breathing, chronic bronchitis, and decreased lung function.

The most noticeable environmental impact from PM is reduced visibility  (haze) in parts of the United States, including many of our national parks

The Network
Mecklenburg County Air Quality (MCAQ) operates three fine particulate matter (PM 2.5) monitoring sites and four coarse particulate matter (PM 10) monitoring sites. Monitoring is conducted year-round.  Data collected represent concentrations over a wide area.

Two monitoring methods are used, a filter based method and a continuous instrumental method. The Federal Reference Method (FRM) for both coarse and fine particulate is filter based and relies on a direct mass measurement. A known volume of air is drawn through a preconditioned pre-weighed filter media for a 24-hour period.  The exposed filter is reconditioned and reweighed at the end of the sample period and a particulate concentration is calculated in terms of mass particulate concentration per volume of air sampled. The concentration is measured in micrograms per cubic meter (ug/m 3).  From the time the filter is collected until a concentration value is determined is usually one to three weeks.  Data from this type of analysis is used to determine compliance with the NAAQS. 

Near real-time PM 2.5 data is collected using a continuous method of measuring particulate at the Garinger and Montclaire monitoring sites.  Particulate is continuously collected on a filter mounted on the tip of a glass element that oscillates in an applied electric field.  Every few seconds the analyzer determines the particulate concentration in the ambient air. The information is recorded by a data logger and compiled into one minute averages. The one-minute averages are used to compute one-hour averages. The data is downloaded to the MCAQ central computer every hour.  The central computer calculates a running 24-hour average.  Data from both types of monitors compares favorably.

Data from continuous monitors are used to report PM 2.5 concentrations on our public information system, the SMOGLINE (704-333-7664); using the air quality index. The AQI reports the most recent twenty-four hour average.  The reported value is a rolling average assigned to the last hour of the twenty-four hour period.  For example:  an average is calculated from 6/21/02-8:00 am through 6/22/02-7:59 am.  The value is assigned to 6/22/02-7:00 am and reported after the 8:00 am data download. (The update is normally available on the SMOGLINE by 8:10). The best time to call for current air quality information is about 10 minutes after the hour.

PM  Patterns
PM 2.5 concentrations are typically somewhat higher during morning and evening hours, possibly as a result of relatively more stable atmospheric conditions during these periods and increased vehicle emissions.   PM 2.5 may be a problem at any time of the year.  During winter, concentrations may be influenced by an increase in wood burning and a higher incidence of atmospheric inversion.  PM 2.5 concentrations may rise during summer as ozone rises due to formation of particulate in the atmosphere.  Unlike ozone, PM 2.5 concentrations may remain elevated after sunset.

Sources of Particulate Matter
Sources of Particulate Matter

Sources of Particulate Matter
Fine particles (PM 2.5 ) are directly emitted from fuel combustion by motor vehicles, power generation, and industrial facilities, as well as from residential fireplaces and wood stoves. Coarse particles (PM 10) are usually caused by dust from unpaved roads, materials handling, and crushing and grinding operations. Fine PM also may be formed in the air when gases from burning fuels react with sunlight and water vapor.

Products of Fuel Combustion

Watson, J. G., J. C. Chow, D. Dubois, M. Green, N. Frank, and M. Pitchford (1997)).  Guidance for Network Design and Optimum Site Exposure for PM 2.5 and PM 10 .  Prepared for the Office of Air Quality Planning and Standards, United State Environmental Protection Agency, Research Triangle Park, NC 27711.  December 15, 1997.
Weinstock, L (2002).  E-Mail to J. Francis:  Additional PM2.5 FCSTG Guidance.    Forsyth County Environmental Affairs Department, 537 N. Spruce Street, Winston-Salem, NC  27101-1362.  March 25, 2002.

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