“Particulate matter” (PM) is the general term used to describe solid particles and liquid droplets found in the air. The composition and size of these airborne particles and droplets vary. Some particles are large enough to be seen as dust or dirt, while others are so small they can only be seen using a powerful microscope. Two size ranges, known as PM10 and PM2.5, are widely monitored, both at major emissions sources and in ambient air. PM10 includes particles that have aerodynamic diameters less than or equal to 10 micrometers (μm), approximately equal to one-seventh the diameter of human hair. PM2.5 is the subset of PM10 particles that have aerodynamic diameters less than or equal to 2.5 μm.
Particles within the two size ranges behave differently in the atmosphere. PM2.5, or fine particles, can remain airborne for long periods and travel hundreds of miles. Coarse particles, or the subset of PM10 that is larger than 2.5 μm, do not remain airborne as long and their spatial impact is typically limited because they tend to deposit on the ground downwind of emissions sources. Larger coarse particles are not readily transported across urban or broader areas because they are generally too large to remain suspended in air and they tend to be removed easily on contact with surfaces. In short, as the particle size increases, the amount of time the particles remain airborne decreases. The PM Concentrations indicator describes the various ways PM can harm human health and the environment.
PM can be emitted directly or formed in the atmosphere. “Primary” particles are those released directly to the atmosphere. These include dust from roads and black and/or elemental carbon from combustion sources. In general, coarse PM is composed largely of primary particles. “Secondary” particles, on the other hand, are formed in the atmosphere from chemical reactions involving primary gaseous emissions. Thus, these particles can form at locations distant from the sources that release the precursor gases. Examples include sulfates formed from sulfur dioxide emissions from power plants and industrial facilities and nitrates formed from nitrogen oxides released from power plants, mobile sources, and other combustion sources. Unlike coarse PM, a much greater portion of fine PM (PM2.5) contains secondary particles.
This indicator presents trends in annual average primary PM emissions data tracked by the National Emissions Inventory (NEI). The NEI tracks emissions data, both measured and estimated, for primary particles only and therefore may only be representative of a small fraction of all emissions that serve to form PM2.5. Because secondary particles are not released directly from stacks, the NEI instead tracks the precursors that contribute to formation of secondary particles. These precursors include nitrogen oxides, sulfur dioxide, ammonia, and other gases (e.g., particle-producing organic gases), some of which are addressed in separate indicators (the Nitrogen Oxides Emissions indicator; the Sulfur Dioxide Emissions indicator). As noted above, particles formed through secondary processes are not included in this indicator.
Primary emissions of PM can exist as solid or liquid matter (the “filterable” portion) or as gases (the “condensable” portion). Data for the condensable portion exist only for the years 1999 to 2014. To allow for a valid comparison of emissions trends from 1990 to 2014, only data for the filterable portion of PM10 and PM2.5 are included in the trend graphs for non-mobile sources. Filterable and condensable PM measurements are designed for smokestacks and other industrial sources, not mobile sources. Condensables are, however, included in the pie charts shown in Exhibits 2 and 5.
All emissions data presented in this indicator are taken from the NEI. Primary particulate emissions data are presented for the traditionally inventoried anthropogenic source categories: (1) “Fuel combustion,” which includes emissions from coal-, gas-, and oil-fired power plants and industrial, commercial, and institutional sources, as well as residential heaters and boilers; (2) “Other industrial processes,” which includes chemical production, petroleum refining, metals production, and processes other than fuel combustion; (3) “On-road vehicles,” which includes cars, trucks, buses, and motorcycles; and (4) “Nonroad vehicles and engines,” such as farm and construction equipment, lawnmowers, chainsaws, boats, ships, snowmobiles, and aircraft. For 2014 only, this indicator includes a comparison of these anthropogenic sources with emissions from forest wildfires, fugitive dust from roads and construction activities, and miscellaneous other sources, such as agriculture, forestry, and managed burning.
The NEI is a composite of data from many different sources, with PM data coming primarily from EPA models as well as from state, tribal, and local air quality management agencies. Different data sources use different data collection methods, and many of the emissions data are based on estimates rather than actual measurements. For mobile sources, the data are based on mobile source models for on-road and nonroad vehicles, often using state-supplied model inputs (U.S. EPA, 2018b). Emissions from forest wildfires in 2014 come primarily from an estimation process that bases fire activity and location on satellite detection (U.S. EPA, 2018b). For most fuel combustion sources and industrial sources, emissions are from the state, local, and tribal air quality management agencies and are estimated using emission factors.
NEI data have been compiled since 1990 and cover all 50 states and their counties, D.C., the U.S. territories of Puerto Rico and Virgin Islands, and some of the territories of federally recognized American Indian nations. Data are presented for 1990, 1996, 1999, 2002, 2005, 2008, 2011, and 2014. With the exception of 1993, the NEI data are published on a triennial cycle, thus an annual trend is not readily available. The NEI data are the basis of the national and regional air pollutant emission trends shown in this indicator (U.S. EPA, 2018c).