PM2.5: More Than Just Dust

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

Most power plant emission control efforts have focused on mercury, NOx and SO2 emissions, but in recent years PM2.5 has risen in importance for Clean Air Act compliance. PM2.5 is a complex and not well understood pollutant, even though it was first regulated via a National Ambient Air Quality Standard (NAAQS) in 1997 (revised in 2006 and 2009). The issue is that for much of that time, PM2.5 was not quantifiable. In fact, EPA only recently repealed its surrogacy policy, which allowed facilities to assume that PM2.5 emissions were equal to their PM10 emissions.

Particulate matter (PM) is broken into three subgroups for NAAQS compliance: PM, PM10, and PM2.5. PM refers to dust of any size (usually less than 30 microns in diameter) that can be caught on a filter. 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. Because it is hot when created, 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.

Utilities with coal-fired assets have been planning for years to comply with a reincarnation of the Clean Air Interstate Rule (CAIR), if and when it survives ongoing court challenges. It’s well understood that these rules require power plants to reduce NOx and SO2 emissions by installing selective catalytic reduction (SCR) systems and scrubbers. But the primary goal of these rules is not so much to reduce NOx and SO2 emissions as to reduce fine particulates (a.k.a PM2.5) and ozone.

Given the legal troubles of CAIR and its potential replacement rules, environmental groups and EPA are finding that it is more expedient to use NAAQS enforcement to attack coal-fired boilers. For example, the annual PM2.5 NAAQS was recently lowered from 15 micrograms per cubic meter (µg/m3) to 12 µg/m3. EPA predicts that most of the country (except for southern California) will be in attainment with this lower standard. However, EPA fails to point out that PM2.5 background values are usually 80% to 95% of the NAAQS, which leaves little room for new sources of PM2.5. Additionally, PM2.5 has a low threshold for triggering what could be a project-delaying Prevention of Significant Deterioration (PSD) permit action: only 10 tpy of direct PM2.5. PSD is also automatically triggered for PM2.5 whenever SO2 or NOx are subject to PSD review, regardless of the amount of PM2.5 emissions.

As with any NAAQS, in the absence of a monitored exceedance a plant’s compliance status is not known until an air dispersion model is run. Modeling is required for PSD permitting, but also can occur for a pollution control project or for construction by a neighboring facility (within 50 km) of your facility). Since Title V operating permits must be renewed every five years, environmental groups are using the required public comment period as an opportunity to press the case that a power plant does not meet the PM2.5 NAAQS and demand that the state agency require dispersion modeling. EPA’s guidance on how to model secondary PM2.5 is in draft form, evolving, and sure to add confusion, cost, and time to any modeling exercise.

PM2.5 is the biggest health benefit/cost ratio of all air, water or waste issues by far, so it isn’t going away; it will likely have increased attention in the coming years. For now, PM2.5 isn’t something to lose sleep over, but it should be on your radar screen as a potential future compliance and enforcement issue.

FIGURE CAPTION: The family of chemicals that make up Particulate Matter are subdivided into the different regulated pollutants.