STN The Speciation Trends Network (STN) began operation in 1999 to provide nationally

consistent speciated PM2.5 data for the assessment of trends at representative sites in urban areas

in the U.S. The STN was established by regulation and is a companion network to the

mass-based Federal Reference Method (FRM) network implemented in support of the PM2.5

NAAQS. As part of a routine monitoring program, the STN quantifies mass concentrations and

PM2.5 constituents, including numerous trace elements, ions (sulfate, nitrate, sodium, potassium,

ammonium), elemental carbon, and organic carbon. There are currently 54 Trends sites

nationally in operation. Trends sites are largely static urban monitoring stations with protocols

for sampling methods; dedicated to characterizing aerosol mass components in urban areas of the

U.S. to discern long-term trends and provide an accountability mechanism to access effectiveness

of control programs. In addition, there are approximately 181 supplemental speciation sites

which are part of the STN network and are SLAMS sites. The STN data at trends sites are

collected 1 in every 3 days, whereas supplemental sites collect data either 1 in every 3 days or 1

in every 6 days. Comprehensive information on the STN and related speciation monitoring can

be found at and

NADP Initiated in the late 1970s, the National Acid Deposition Program (NADP) monitoring

network began as a cooperative program between federal and state agencies, universities, electric

utilities, and other industries to determine geographical patterns and trends in precipitation


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