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Before undertaking air monitoring for Particulate Matter (PM) [identify the end goals of monitoring for your community. Monitoring airborne particles can be prohibitively expensive and actionable data for regulators can take years to collect. The precision of collected data should be appropriate for its end use. A community may want to collect data to:

Airborne particles are clustered into three rough size ranges, or modes, of particles in the air, each with distinct sources. While droplets and ultrafines are largely combustion by-products, dust is broken off of larger materials. No single method of PM monitoring method all categories.

[3 modes chart from /wiki/pm]

Dust is the most established particle mode to monitor. Because of the ubiquity of background dust, however, industrial dust emissions can be difficult to track back to a source.

Droplets are difficult to monitor in real-time, especially because of the effects of humidity on optical PM monitors. Only a fraction are captured in most filter-based PM-monitors. measures of “secondary” droplets are added in during regulatory monitoring of NAAQS.

The study of ultrafines is fairly new. There are no regulatory categories that apply to ultrafines, and no inexpensive means to monitor them. Ultrafine exposure is associated with proximity to combustion, especially of diesel and marine fuels.


NIEHS EHP lesson: Size makes all the Difference, pg 4

Because of the varied challenges of accurate monitoring, determining the precision needed for a specific end goal is important.

proposed EPA precision categories for citizen monitoring

State and federal regulators are empowered make judgements based on [visual assessments of particle pollution] (smoke school), but at present regulators have no statutory guidance or authority to interact with PM data collected with instruments other than their (very expensive) regulatory monitors. This can lead to curt rejections of scientifically sound data. Federal regulators recognize this issue and are working to propose categories of precision for community-collected data:


These categories are prospective (except for regulatory monitoring, Category V) and should only be treated as guidelines for technologies in development. Watch a video presentation by Ron Wilson of the EPA on Citizen Science PM data

Getting judgements about airborne particles

Given that regulators are currently unlikely to make judgements based any data other than visual monitoring and regulatory monitoring, community-based PM monitoring should focus on:

  • highlight a problem for the purposes of [organizing for action](/wiki/frac-sand-action-oriented-
  • resources)
  • identify emissions hotspots for more monitoring
  • identify times for visual monitoring
  • compel industry to pay for community monitors to become certified in visual monitoring (Smoke School).
  • compel regulatory monitoring through suggestive data

regulatory grade PM monitoring

Regulatory monitors cost $20-60,000 to buy, ~$100/day to analyze, and 1-3 years of daily data to get a judgement. Only a few states have pollutant-specific regulations, and a failure to demonstrate an exceedance of PM2.5 or PM10 levels does necessarily indicate safe conditions. Airborne silica, for example, can be dangerous at 5-10% of exceedance levels.

smoke school

Visible emissions and can be measured by their effects on the opacity of the air. Opacity can be monitored through visual assessment with only a stopwatch. Opacity is expressed as the percentage of light that is blocked by emissions. Examples of pollutants that change opacity are smoke stack emissions and dust.

certifying community observers in EPA Method 9 can be written into a facility’s permits, read more in advocacy leverage points.

Types of monitoring equipment

Most monitors give a mass-based particle concentration for all particles in a size category. Only systems that capture and save particulate matter can identify, or ‘speciate’ particles.

Filter-based systems

Used for: regulatory monitoring, supplementary monitoring

Filter-bases systems can collect particles for lab methods of speciation, and are the basis of Federal Reference Methods. Data can only be analyzed after collection. Usually 24-hour averages are produced. Filter-based systems are usually the most precise measurements of PM.

optical systems

Used for: personal exposure monitoring, supplementary monitoring, hotspot identification, hotspot characterization, education

optical-electronic systems offer the possibility of real-time particle counts which are valuable in hotspot identification, pinpointing emissions events, and identifying when air may pose a health threat. Their data is heavily effected by humidity, and more precise monitors usually include a filter-based system to correct data after collection.

passive systems

Used for: personal exposure monitoring, supplementary monitoring, education, hotspot characterization, education

Passive systems have no moving parts and are easy to deploy for long-term monitoring without electricity. They can approach the precision of regulatory monitoring and are within the range necessary for supplementary monitoring.

Because particles are collected, Passive monitors have the potential to do some limited speciation of particles. They generally require longer monitoring periods (3-10 days) and are better used to characterize hotspots than identify them.

Read more on monitoring silica