PM Formation in the Atmosphere

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PM Formation in the Atmosphere

• Primary and Secondary PM
• Sulfate Formation in the Atmosphere
• SO4 Formation in Clouds
• Season SO2-SO4 Transformation rate
• Residence Time of Sulfur and Organics
• Internal and External Mixtures of Particles
• Resource Links

Contact Rudolf Husar, rhusar_at_mecf.wustl.edu
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Primary and Secondary PM

• Primary PM is released into the atmosphere
  directly from the source ( e.g. flyash from coal
  or soot from diesel exhaust).
• Secondary PM is formed within the atmosphere from
  precursor gases, such as SO2, NOx and organics
  through gas-phase photochemical reactions or
  through liquid phase reactions in clouds and fog
  droplets.
• Most of the PM2.5 in the rural atmosphere is
  secondary. In urban areas under poorly ventilated
  winter conditions, primary emissions are also
  important.

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Sulfate Formation in the Atmosphere

• Sulfates constitute about half of the PM2.5 in
  the Eastern US. Virtually all the ambient sulfate
  (99) is secondary, formed within the atmosphere
  from SO2.
• About half of the SO2 oxidation to sulfate occurs
  in the gas phase through photochemical oxidation
  in the daytime. NOx and hydrocarbon emissions
  tend to enhance the photochemical oxidation rate.

• The condensation of H2SO4 molecules results in
  the accumulation and growth of particles in the
  0.1-1.0 size range - hence the name
  accumulation-mode particles.

4
SO4 Formation in Clouds

• At least half of the SO2 oxidation is taking
  place in cloud droplets as air molecules pass
  through convective clouds at least once every
  summer day.
• Within clouds, the soluble pollutant gases such
  as SO2, get scavenged by the water droplets and
  rapidly oxidize to sulfate.

• Only a small fraction of the cloud droplets rain
  out, most droplets evaporate at night and leave a
  sulfate residue or convective debris. Most
  elevated layers above the mixing layer are
  pancake-like cloud residues.
• Such cloud processing is responsible for
  internally mixing aerosol particles from many
  different sources. It is also believed that such
  wet processes are significant in the formation
  of the organic fraction of PM2.5.

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Season SO2-SO4 Transformation rate
Transformation rates derived from the CAPITA
Monte Carlo Model, Schichtel and Husar, 1997
http//capita.wustl.edu/capita/capitareports/mcarl
okinetics/mcrateco4_AWMAPres.html

• The SO2 to SO4 transformation rates are summer
  peaked due to enhanced summer time photochemical
  oxidation and SO2 oxidation in clouds

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Residence Time of Sulfur and Organics.

• SO2 is depleted mostly by dry deposition
  (2-3/hr), and also by conversion to SO4 (1/hr).
  This gives SO2 an atmospheric residence time of
  only 1-1.5 days.
• It takes about a day to form the sulfate aerosol.
  Once formed, SO4 is removed mostly by wet
  deposition at a rate of 1-2 /hr yielding a
  residence time of 3-5 days.
• Overall, sulfur as SO2 and SO4 is removed at a
  rate of 2-3/hr, which corresponds to a residence
  time of 2-4 days.
• These processes have at least a factor of two
  seasonal and geographic variation.
• It is believed that the organics in PM2.5 have a
  similar conversion rate, removal rate and
  atmospheric residence time.

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Internal and External Mixtures of Particles

• During their atmospheric residence of 3-5 days,
  the atmospheric processes trend to mix PM2.5
  particles into external and internal mixtures.
• In an external mixture, particles from different
  sources remain separate i.e not attached or each
  other.
• In an internal mixture, individual particles are
  mixed (aggregated), from particles of different
  types (e.g. a soot particle inside a sulfate
  droplet) as illustrated by the electron
  micrograph below.
• The main cause of internal mixing is cloud
  scavenging and subsequent evaporation.

Electron micrograph of a PM2.5 droplet residue.
Evidently, the droplet contained a solid
particle, possibly soot.