19 Particles span many sizes and shapes and consist of hundreds of different chemicals.

20 Particles are emitted directly from sources and also are formed through atmospheric chemical

21 reactions and often are referred to as primary and secondary particles, respectively. Particle

22 pollution also varies by time of year and location and is affected by several aspects of weather

23 such as temperature, clouds, humidity, and wind. Further complicating particles is the shifting

24 between solid/liquid and gaseous phases influenced by concentration and meteorology,

25 especially temperature.


27 Particles are made up of different chemical components. The major components, or

28 species, are carbon, sulfate and nitrate compounds, and crustal materials such as soil and ash

29 (Figure A-2). The different components that make up particle pollution come from specific

30 sources and are often formed in the atmosphere. Particulate matter includes both “primary” PM,

31 which is directly emitted into the air, and “secondary” PM, which forms indirectly from fuel

32 combustion and other sources. Primary PM consists of carbon (soot)—emitted from cars, trucks,

33 heavy equipment, forest fires, and burning waste—and crustal material from unpaved roads,

34 stone crushing, construction sites, and metallurgical operations. Secondary PM forms in the

35 atmosphere from gases. Some of these reactions require sunlight and/or water vapor. Secondary

36 PM includes:

37 Sulfates formed from sulfur dioxide emissions from power plants and industrial

38 facilities;

39 Nitrates formed from nitrogen oxide emissions from cars, trucks, industrial facilities,

40 and power plants; and

41 Carbon formed from reactive organic gas emissions from cars, trucks, industrial


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