4 The record articles

PM2.5 Update

Posted: January 11th, 2012

Author: All4 Staff 


Airborne particles that are less than or equal to 2.5 micrometers in diameter are considered to be ‘fine particles’ and are also referred to as PM2.5.  A micrometer is one-millionth of a meter, and 2.5 micrometers is less than one-seventh the average width of a human hair!  Fine particles in the atmosphere are comprised of a complex mixture of components.  Common constituents include: sulfate (SO4); nitrate (NO3); ammonium; elemental carbon; volatile organic compounds (VOC); and other inorganic materials including metals, dust, and trace elements.

“Primary” fine particles (also referred to as filterable particulate matter) are emitted directly into the air as a solid or liquid particle.  For example, primary fine particles could include elemental carbon emitted from diesel engines or  other inorganic materials emitted from material handling operations.

“Secondary” fine particles are collectively referred to as condensable particulate matter because they exist as a gas or vapor “in stack” and form tiny particles upon cooling in the atmosphere or as a result of various chemical reactions.  Sources of secondary fine particles include such operations as fuel-burning sources, metal processing operations, and cooking operations.


Yes.  The United States Environmental Protection Agency (U.S. EPA) established a 24-hour and an annual National Ambient Air Quality Standard (NAAQS) for PM2.5 in 1997 based on evidence from numerous health studies demonstrating that serious health effects are associated with exposures to elevated levels of PM2.5.  In April 2005, 39 areas in the U.S. were designated as not meeting the NAAQS for PM2.5.

Under the 1997 standard, each state having a nonattainment area is required to submit an attainment demonstration and adopted regulations to U.S. EPA ensuring that the area(s) will attain the PM2.5 standards as expeditiously as practicable. The demonstration and regulations must be submitted to U.S. EPA by April 5, 2008.  States must be in attainment with the 1997 PM2.5 ambient standards by no later than 2015.

In October 2006, U.S. EPA tightened the 24-hour PM2.5 ambient air quality standard, thus resulting in a more stringent short-term standard and offering greater health protection benefits.  As a result of this action, the number of areas not in attainment with the ambient air quality standard for PM2.5 has increased.  States with areas that are not currently attaining the 24-hour PM2.5 standard will now have a more difficult time achieving attainment status.  State-proposed area designations under the 2006 standard were due to U.S. EPA late last year.


U.S. EPA has promulgated several rules in recent years to curb emissions of gas-phase PM2.5 precursors such as sulfur dioxide (SO2) and oxides of nitrogen (NOX).  In March 2005, U.S. EPA promulgated the Clean Air Interstate Rule (CAIR) to address the transport of SO2 and NOX emissions from power plants and several new regulations addressing emissions from cars, trucks, buses, and non-road diesel engines have been passed in recent years.

U.S. EPA has not yet developed New Source Review (NSR) applicability regulations for PM2.5.  Such regulations, when proposed, are expected to address PM2.5 precursor pollutants (i.e., SO2, NOX, VOC, and ammonium).  Once the PM2.5  attainment designations became affective on April 5, 2005, states were required to begin including PM2.5 limitations in any NSR permits issued, using PM10 emissions as a surrogate for PM2.5 emissions.  Under the 1997 standard, states have until April 5, 2008 to revise their State Implementation Plans (SIPs) to include PM2.5 in their NSR programs. The final implementing PM2.5 NSR regulations from U.S. EPA will not likely be available by the April 5, 2008 SIP deadline.  States will need to submit supplemental plans to incorporate the implementing PM2.5 NSR regulations when they are finalized.

Given the above guidance, facilities that are located in areas that are designated as inattainment or unclassifiable with the PM2.5 NAAQS must carefully evaluate applicability with the Prevention of Significant Deterioration (PSD) air permitting requirements.  Facilities that are located in areas that are designated as nonattainment with the PM2.5 NAAQS must carefully evaluate applicability with Nonattainment New Source Review (NNSR) air permitting requirements.  Facilities triggering major NNSR applicability may be obligated to obtain PM10 emission offsets as a surrogate for PM2.5 emission offsets. The lack of available PM10 emission offsets could hinder the permitting of projects in PM2.5 nonattainment areas.


Under the 1997 standard, states having areas designated as nonattainment for PM2.5 are required to submit a plan to U.S. EPA by April 5, 2008 that describes their plans for bringing their areas into attainment.  For existing sources, states have the authority to require sources to implement Reasonably Available Control Technology (RACT) for not only filterable and condensable PM2.5 emissions, but also PM2.5 precursors such as SO2, NOX, VOC, and ammonium.  Each state can decide which emissions units must evaluate RACT-level controls, and to determine what level of emission reduction is considered to be RACT.  Similar to a Best Available Control Technology (BACT) evaluation, a RACT evaluation must evaluate the cost-effectiveness of available control technologies or alternative fuels and raw materials, and can consider technical feasibility and cost.  Because each state can determine what represents RACT, an emissions unit in one state may be required to implement a RACT control strategy while a similar emissions unit in another state may not.  In addition, states have the authority to develop more stringent PM2.5, SO2, NOX, VOC, or other pollutant emission standards for new and/or modified sources to ensure that nonattainment areas within the state will meet the 2015 PM2.5 attainment deadline.


Yes.  However, at this time, U.S. EPA is still in the process of evaluating test methods for measuring the filterable fraction of PM2.5, and has issued guidance that instructs sources to use U.S. EPA Test Method M201A, developed for measuring filterable PM10, as a surrogate until a test method of filterable PM2.5 is approved.  Since all condensable particulate matter is assumed to be less than 2.5 micrometers in diameter, the PM2.5 condensable fraction can be measured using U.S. EPA Test Method M202.

Emissions of the “filterable” fraction of PM2.5 can typically be calculated for most operations by using emission factors such as those published in U.S. EPA’s AP-42: Compilation of Air Pollutant Emission Factors.  However, very few emission factors have been developed that quantify the condensable fraction of total PM2.5 emissions.  This is important to note because compliance with any new PM2.5 emission limit established through a new regulation or through air permitting must be demonstrated through testing and must include both the filterable and condensable fractions of PM2.5.  Establishing a new PM2.5 emission limit based on an emission rate estimated only for filterable PM2.5 could result in a future compliance issue following a performance test.

If you have a facility located in a PM2.5 nonattainment area and are planning a major modification in the near future, you should consider testing for fine particulate matter to develop emission factors for affected emissions units.  This information will be invaluable for compiling baseline facility PM2.5 emissions and for ensuring that your construction application for new or modified equipment adequately addresses the control of both filterable and condensable fine particulate matter emissions.


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