Properties of Particulate Matter

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Properties of Particulate Matter

• Physical, Chemical and Optical Properties
• Size Range of Particulate Matter
• Mass Distribution of PM vs. Size PM10, PM2.5
• Fine and Coarse Particles
• Fine Particles - PM2.5
• Coarse Particle Fraction - PM10-PM2.5
• Chemical Composition of PM vs. Size
• Optical Properties of PM
• Resource Links

Contact Rudolf Husar, rhusar_at_mecf.wustl.edu
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Physical, Chemical and Optical Properties

• PM is characterized by its physical, chemical and
  optical properties
• The physical properties include particle size and
  particle shape. The particle size refers to
  particle diameter or equivalent diameter for
  odd shaped particles. The particle shape of may
  be liquid droplets, regular or irregular shaped
  crystals or aggregates of odd shape.
• Their chemical composition may also vary from
  dilute water solution of acids or salts, organic
  liquids, to earth's crust materials (dust), soot
  (unburned carbon) and toxic metals.
• The optical properties determine the visual
  appearance of dust, smoke and haze and include
  light extinction, scattering and absorption . The
  optical properties are determined by the physical
  and chemical properties of the ambient PM.
• Each PM source type produces particles with
  specific physical, chemical and optical
  signature. Hence, PM may be viewed as several
  pollutants since each aerosol type has its own
  properties, sources and requires different
  control control

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Size Range of Particulate Matter

• The size of aerosol particles ranges from about
  tens of nanometers (nm) which corresponds to
  molecular aggregates to tens of microns of the
  size of human hair.
• The smallest particles are generally more
  numerous and the number distribution of particles
  generally peaks below 0.1 um. The size range
  below 0.1 um is also referred to as ultrafine
  range.
• The largest particles (0.1-10 um) are small in
  number but contain most of the aerosol volume
  (mass). The volume (mass) distribution can have
  two or tree peaks (modes). The bi-modal mass
  distribution has two peaks.
• The peak of the aerosol surface area distribution
  is always between the number and the volume peaks
  

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Mass Distribution of PM vs. Size PM10, PM2.5

• Usually, the PM mass is plotted vs. the log of
  particle diameter
• The mass distribution tends to be bi-modal with
  the saddle in the 1-3 um size rage
• PM10 refers to the fraction of the PM mass less
  than 10 um in diameter
• PM2.5 or fine mass are less than 2.5 um in size.
• The difference between PM10 and PM2.5 constitutes
  the coarse fraction
• The fine and coarse particles have different
  sources, properties and effects. Many of the
  known environmental impacts (health, visibility,
  acid deposition) are attributed to PM2.5.

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Fine and Coarse Particles

• There is a natural division of atmospheric
  particulates into Fine and Coarse fraction based
  on particle size.
• The fine and coarse particles originate from
  different sources, their formation mechanisms,
  transport distance, their properties and effects
  are also different.
• Many of the known environmental impacts on
  health, acid deposition, visibility, and
  corrosion are associated with the fine particles

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Fine Particles - PM2.5

• The majority (over 90) of the PM2.5 mass over
  the US is of secondary origin, formed within the
  atmosphere through gas-particle conversion of
  precursor gases such as sulfur oxides, nitrogen
  oxides and organics. The resulting secondary
  aerosol products are sulfates, organics and
  nitrates.
• Some PM2.5 is emitted as primary emissions from
  industrial activities and motor vehicles
  including soot (unburned carbon), trace metals
  and oily residues.
• Fine particles are mostly droplets except for
  soot which is in the form of chain aggregates.
• Over the industrialized regions of the US
  anthropogenic emissions from fossil fuel
  combustion contribute most of the PM2.5. In
  remote areas biomass burning, windblown dust, and
  sea salt also contribute.

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Coarse Particles - PM10-PM2.5

• Coarse particles are primary in that they are
  emitted as windblown dust and sea spray in
  coastal areas. Anthropogenic coarse particle
  sources include flyash from coal combustion and
  road dust from automobiles
• The chemical composition of the coarse particle
  fraction is similar to that of the earth's crust
  or the sea but sometimes coarse particles also
  carry trace metals and nitrates.
• Coarse particles are removed from the atmosphere
  by settling, impaction to surfaces and by
  precipitation. Their atmospheric residence time
  is generally less than a day, and their typical
  transport distance is below a few hundred km.
  Some dust storms tend to lift the dust to several
  km altitude, which increase the transport
  distance to many thousand km.

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Chemical Composition of PM vs. Size

• The chemical species that make up the PM occur at
  different sizes.
• For example in Los Angeles, ammonium and sulfate
  occur in the fine mode, lt2.5 um in diameter.
  Carbonaceous soot, organic compounds and trace
  metals tend to be in the fine particle mode
• The sea salt components, sodium and chloride
  occur in the coarse fraction, gt 2.5 um.
  Wind-blown and fugitive dust also mainly in the
  coarse mode.
• Nitrates may occur in fine and coarse modes.

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Optical Properties of PM

• Particles effectively scatter and absorb solar
  radiation.
• The scattering efficiency per aerosol mass is
  highest at about 0.5 um. This is why, say, 10 ug
  of fine particles (0.2ltDlt1 um) scatter over ten
  times more than 10 ug of coarse particles (Dgt2.5
  um)

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Resource Links

• Workbook Table of Contents
• Comment and Feedback Page
• Applications / Reports
• Data sets used in the Applications
• Methods and tools used in the Applications