A Field Guide to Particle Pollution

Particulate matter (PM) is one of six criteria pollutants regulated by the Clean Air Act.  Last week, we told you about how the Division for Air Quality monitors for PM2.5 .  You may recall that PM2.5 is a mixture of both solid particles and liquid droplets measuring 2.5 microns or smaller in size.  Fine particulates can be emitted directly from a source, or they may form in the atmosphere when pollutants chemically combine.

Naturally, if you want to control air pollution, it helps to know which sources contribute to it.  But particle pollution is tricky.  Often, half or more of the PM2.5 mass is comprised of secondarily formed species – in other words, particles that formed through chemical reactions between other pollutants.  This makes it difficult to figure out where those pollutants came from.

Realizing this, the EPA established the Speciation Trends Network in order to assist in identifying the compounds associated with fine particulates.  PM2.5 is composed of many different components or species.  The Speciation Trends Network is used to provide data on a target group of chemical species known to be significant contributors to PM2.5.

Chemical speciation involves both sampling and analysis of species. If you’re not a chemist, the names of the chemical species target groups may sound a bit strange: cations, anions, carbon species, and trace elements.  Each of these target groups requires its own specific approach for sampling and analysis.

Once the PM2.5 speciation samples are collected on specialized filters, they are sent to an EPA contract laboratory for analysis.  Listed below are the techniques used to analyze samples for each group of species:

  •  Trace Elements: X-ray fluorescence and particle induced X-ray emission
  • Anion and Cations: Ion chromatography
  • Carbon: Controlled-combustion/thermal optical

Once analysis is complete, the species are further separated into the target chemical species listed below:

  • Nitrate (total)
  • Sulfate
  • Ammonium
  • Organic Carbon
  • Elemental Carbon
  • Crustal Component (trace elements, fine soil)
  • Other (PM2.5 mass unaccounted for by analytical methods)

The speciated data in turn can be linked to source categories, which ultimately help us develop control strategies for reducing ambient levels of PM2.5 in the air.