April 14, 2006

The Environmental Protection Agency's Proposed Rule For National Ambient Air Quality Standards For Particulate Matter

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The Regulation

  • The Environmental Protection Agency regulates concentrations of particulate matter (PM) as one of its National Ambient Air Quality Standards (NAAQSs). The smallest form of particulate matter is PM2.5, so called because it refers to particles 2.5 micrometers (mm) across or less, which is about 1/30th the width of a human hair and so small that many thousands of PM2.5 particles could easily fit in the period at the end of this sentence. In 1997, the EPA issued the first standards for PM2.5 setting attainment levels of 15 micrograms per square meter (mg/m³) for annual average concentrations and 65mg/m³ for 24-hour concentrations. The EPA has proposed keeping the annual average standard the same, but lowering allowable 24-hour concentrations to 35mg/m³. The EPA seeks comment on this proposed change, and additional comment on whether the average annual standard should be lower to 14, 13, or 12mg/m³ and the possibility of lowering the 24-hour standard to any alternative point between 65mg/m³ and 25mg/m³.

Our Findings

  • Most PM2.5 owes its existence to chemical reactions in the atmosphere involving three types of molecules - carbon, sulfates, and nitrates, which are refereed to as "PM species." Although these molecules do occur naturally, most are produced by manmade point sources (power plants, factories, oil refineries) and mobile sources (cars, trucks, tractors, etc.). Across the country, carbon is the most dominant PM2.5 species. Nitrates rank second in the West and third in the East, but the role is reversed for sulfates, which place third in the West and second in the East.
  • Although concentrations of PM2.5 have only been measured since 1999, they had fallen 15 percent by 2004 and more reductions appear on the way. These declines are largely due to reductions in emissions from power plants and the modernization of the nation's vehicle fleet.
  • If the EPA finalizes the new daily standard, it will be in September of 2006. By April of 2010, the EPA will have made its final designations of which areas of the country are in attainment, which are not, and which are unlikely to be compliant in 2015. States would then begin the formulation of their state implementation plans (SIPs), which would have to be acceptable to EPA by April of 2013. The States would have until April of 2015 to bring their PM2.5 concentrations into compliance with the new standard.
  • In order to ascertain what pollution reductions are needed to meet future standards, the EPA relies on a model that tries to predict future PM2.5 concentrations and the effects current pollution control measures will have on these future levels. Unfortunately, there are four good reasons why the EPA's model for making these predictions is not only inaccurate but also likely to underestimate the coming reductions in PM2.5 pollution.
  1. While the model is good at predicting future sulfate concentrations, it does not fair as well with nitrates and carbon. As carbon and nitrates define a majority of PM2.5, the EPA's model can only hope to accurately predict part of the problem.
  2. The proposed rule involves reductions in 24-hour PM2.5 levels, but the model is more successful at predicting annual average concentrations and is less suited to make accurate predictions of 24-hour levels.
  3. The effectiveness of existing controls remains uncertain. The EPA estimated that local controls in three urban areas could contribute between 5 to 50 percent of the concentration reductions required to meet the new standard. The breath of this range is so wide that the forecast is almost rendered completely meaningless.
  4. While mobile sources are recognized as major contributors to PM2.5, they receive scant consideration in the model. Emissions from mobile sources are likely to decline sharply, but the complexities and lack of data needed to accurately estimate their declines in emissions have caused them to largely escape consideration.
  • If these problems lead to an inaccurate forecast that underestimates future pollution reductions, then either the EPA or the States will order additional reductions in emissions that are not necessary for attainment.
  • Although the model does little to compensate for reductions in emissions from mobile sources, a dramatic drop in pollution from these sources should materialize in the next few years. EPA's Tier 2 vehicle standards, which came in to effect in 2003, mandated that cars built after 2003 be 90 percent cleaner than those then on the road. Similar standards have also been set for trucks and non-road diesel engines. Furthermore, it has been estimated that 90 percent of all pollution from mobile sources is attributable to the very oldest 10 percent of the vehicle fleet. Not only will most of these vehicles be off the road by 2015, but many younger models will have similarly been replaced by 90 percent cleaner vehicles.


  • The uncertainties in the EPA's model create the real possibility that the agency will underestimate coming reductions in PM2.5 concentrations. As this is the likely outcome, the EPA should consider maintaining the current 24-hour standard of 65mg/m³. The American people are already paying higher prices for cars and trucks, as a result of the EPA's Tier 2 emissions standards. If additional reductions are ordered to meet a lower future standard based on projections provided by a faulty model, then Americans will endure factory closures, higher electricity prices, and billions of dollars in lost economic opportunities and essentially pay twice for compliance with the same air quality standard. In order to avoid this, the EPA should take a wait and see approach and allow time for accurate measurement of current pollution control programs and time to gauge the impact of vehicle fleet modernization.