DHEC has developed an extensive ambient (outdoor) air quality monitoring network for PM2.5, PM10, and other pollutants to determine:
The monitors' locations were selected by DHEC in accordance with EPA monitor-siting requirements found in the Code of Federal Regulations Title 40, Part 58, Appendices D and E.
In 2011, there were 14 PM2.5 monitors strategically located in 10 counties throughout the state.
In 2011, there were 9 PM10 monitors strategically located throughout the state.
The main purpose of PM monitoring is to provide air quality data to local, state and national regulatory programs for determining whether an area has attained the National Ambient Air Quality Standards (standards). To ensure national consistency in air pollution monitoring, a Federal Reference Method (FRM) sampler is used for making comparisons to the standards. The current FRM for measuring PM2.5 in the atmosphere is the gravimetric method. In FRM-based monitoring, ambient air is drawn through special filters at a specified flow rate. The filters are weighed before and after sampling at an appropriate location, and the net mass of particles accumulated on the filter is divided by the total volume of air sampled to compute the time-averaged concentration for the sampling period.
The PM10 standards are expressed as a weight of PM10 particles per volume of air (micrograms per cubic meter). PM10 mass is collected using a high volume sampler. A high volume PM10 sampler draws a known volume of ambient air at a constant flow rate through a size selective inlet and a filter. Particles in the PM10size range are then collected on the filter during the specified 24-hour sampling period.
EPA determines the minimum number of PM2.5 monitors that can adequately represent an area based on theMetropolitan Statistical Area (MSA), most recently measured PM2.5 concentrations, and its population. The MSA may include heavily populated counties and adjacent counties that are closely related economically. The map below shows the mimimum monitoring requirements for the MSAs in our state as of January 1, 2012.
Depending on historical PM2.5 trends in the area, the larger MSAs with populations greater than 1 million are required to have 2 or 3 monitors. Mid-size MSAs with populations of 500,000 to 1 million are required to have 1 or 2 monitors. And smaller MSAs with populations of 50,000 to 500,000 are required to have 0 or 1 monitor.
EPA determines the minimum number of PM10 monitors that are required for an area is based on the MSA, most recently measured PM10 concentrations, and the area's population. The MSA may include heavily populated counties and adjacent counties that are closely related economically.
As the table below indicates, an MSA with a population of at least 250,000 and High PM10 concentrations is required to have 3 or 4 monitors. An MSA with a population less than 250,000 and High concentrations would have 1 or 2 monitors.
|Level of PM10 Concentration|
|High 2||Medium 3||Low 4,5|
|> 1,000,000||6 - 10||4 - 8||2 - 4|
|500,000 - 1,000,000||4 - 8||2 - 4||1 - 2|
|250,000 - 500,000||3 - 4||1 - 2||0 - 1|
|100,000 - 250,000||1 - 2||0 - 1||0|
1 Selection of urban areas and actual numbers of stations per area will be jointly
determined by EPA and the
2 High concentration areas are those for which ambient PM10 data show ambient concentrations exceeding the PM10 standard by 20 percent or more.
3 Medium concentration areas are those for which ambient PM10 data show ambient concentrations exceeding 80 percent of the PM10 standard.
4 Low concentration areas are those for which ambient PM10 data show ambient concentrations less than 80 percent of the PM10 standard.
5These minimum monitoring requirements apply in the absence of a design value.