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 (PM₁₀) 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/m³).  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.