Air Pollution Control Equipment Services, Policy & Regulations

Why Particulate Matters

Issue 5 and Volume 118.

By Robynn Andracsek, P.E., Burns & McDonnell

Greenhouse gases (GHGs) may be the newest pollutant regulated by EPA, but since there are no National Ambient Air Quality Standards (NAAQS) for GHGs, a facility is much more likely to be affected by particulate matter restrictions. Dispersion modeling of the various subsets of particulate matter, each with their own distinctly different makeups, often leads to changes in the design of a new facility or retrofits of existing equipment. This is especially true for the smallest size of particulate matter, Pm2.5, since the background air quality for Pm2.5 is often over 80 percent of the NAAQS. This leaves very little room for emissions from your facility. Therefore, it is important to understand the different types of particulate.

fig 1
fig 2
fig 3

Particulate matter (PM) is broken into three subgroups for NAAQS compliance: PM, PM10, and Pm2.5. See Figures 1-3.

  • PM refers to dust of any size (usually less than 30 microns in diameter) that can be caught on a filter. Particles larger than 10 microns (sand and large dust) are not regulated by EPA but may still be included in state regulations.
  • EPA defines PM10 as both filterable (less than 10 microns) and condensable particulate matter. “Condensable particulate matter” refers to gaseous emissions that condense to form particulate matter at ambient temperatures. As such it passes through the filter in a stack test.
  • Finally, Pm2.5 is filterable matter less than 2.5 microns in size plus condensable particulate matter and secondary Pm2.5. It’s this secondary Pm2.5 that is so little understood. Secondary Pm2.5 is formed in the atmosphere after pollutants from fuel combustion leave the stack. In the presence of sunlight and water vapor, some of the SO2, NOx, volatile organic compounds (VOC) and ammonia (NH3) chemically react to form Pm2.5. Therefore, secondary Pm2.5 cannot be controlled directly; it can only be controlled by reducing emissions of its precursors, SO2, NOx, VOC and NH3.

EPA is currently developing guidance for calculating secondary Pm2.5. It is already true that triggering major source (Prevention of Significant Deterioration – PSD) review for NOx or SO2 automatically triggers review for Pm2.5.

Additionally, the logic behind regulating NO2 and SO2 through the Clean Air Interstate Rule (CAIR), the Cross-State Air Pollution Rule (CSAPR), or whatever final iteration of this regulation survives the court challenges, is to control secondary Pm2.5 emissions. These new Pm2.5 PSD requirements, which are separate from the grandson of CAIR rules, were expected to be published during the spring of 2014.

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