What is Particulate Matter and How does it Vary?

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What is Particulate Matter and How does it Vary?

• What is Particulate Matter?
• How Does PM Vary?
• The Influence of Emissions, Dilution and
  Transformations
• Resource Links

Contact Rudolf Husar, rhusar_at_mecf.wustl.edu
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Figure 1. Aerosol Size Distribution and Morphology
Click figure to enlarge

• Purposes of the illustration
• Size spectra over 4 decades, modes
• Particle shapes, electron micrographs
• Chemical composition by size

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What is Particulate Matter?

• The term Particulate Matter or aerosol, refers to
  liquid or solid particles suspended in the air.
  Depending on their origin and visual appearance,
  aerosols have acquired different names in the
  everyday language.
• Dust refers to solid airborne material, dispersed
  into aerosol from grainy powders such as soil.
• Combustion processes produce smoke particles, but
  the incombustible residues of coal are called
  flyash.
• In the early days, air pollution had the
  appearance of both smoke and fog, so the term
  smog was created.
• In the open atmosphere, the visibility may often
  be reduced by regional haze, originating from
  various natural or anthropogenic sources.
• Neither water droplets of fog and clouds, snow,
  rain, sleet (hydrometors) nor dust particles
  larger than 100 um (blowing sand) are considered
  to be particulate matter

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On the Origin PM2.5 in the Atmosphere - Fragment

• At this time, the most reliable means of
  identifying the origin of PM2.5 is the chemical
  analysis of the the PM samples which reveals that
  most of PM2.5 is composed of secondary sulfates,
  organics and nitrates.
• Identifying the PM2.5 precursor sources is
  elusive since they occur 100 or 1000 miles from
  the receptor. Also, the chemical transformations
  involve many factors including photochemical
  oxidants and cloud interactions.
• Primary PM2.5 such as soot and fine dust can be
  traced based on chemical signatures since each PM
  source type produces particles with specific
  physical, chemical and optical signature.

Contact Rudolf Husar, rhusar_at_mecf.wustl.edu
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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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How Does PM Vary?Spatially, temporally, with
particle size and by chemical composition

• As all pollutants, the ambient aerosol
  concentration patterns contain endless
  variability in space and time. However, unlike
  gaseous pollutants, particulate matter also
  depends on particle size, shape and chemical
  composition.
• The chemically rich aerosol mix arises from the
  multiplicity of PM sources, each having a unique
  chemical signature at the source.
• The primary aerosol chemical composition is
  further enriched by the addition of secondary
  species during atmospheric transport.
• The effective mixing in the lower atmosphere
  stirs these primary and secondary particles into
  an externally mixed batch with various degrees of
  homogeneity, depending on location and time.
• Lastly, repeated cloud scavenging and evaporation
  tends to mix the particles from different sources
  internally into particles with mixed composition.
• The result is a heterogeneous PM pattern that is
  probably unparalleled in the domain of
  atmospheric sciences. For instance it is common
  to find soot particles within sulfate droplets,
  or nitrate deposited on sea salt particles.

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The Influence of Emissions, Dilution and
Transformations

• The PM concentration, C, at any given location
  and time is determined by the combined
  interaction of emissions, E, atmospheric
  dilution, D, and chemical transformation and
  removal, T, processes
• C f (E, D, T)
• Each of the three processes has its own pattern
  at secular, yearly, weekly, synoptic, diurnal and
  micro time scales.
• The yearly, weekly and the diurnal scales are
  periodic

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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