Atmospheric Lifetime and the Range of PM2'5 Transport

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Atmospheric Lifetime and the Range of PM2.5
Transport

• Background and Rationale
• Atmospheric Residence Time and Spatial Scales
• Residence Time Dependence on Height
• Range of Transport
• Resource Links

Contact Rudolf Husar, rhusar_at_mecf.wustl.edu
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Background and Rationale

• Residence time refers to the time span between
  the PM emission (or the emission of their
  precursor gases) and its removal from the
  atmosphere.
• Residence time determines the range of impact of
  a specific sources.

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Atmospheric Residence Time and Spatial Scales

• Atmospheric residence time and transport distance
  are related by the average wind speed, say 5 m/s.
  
• Residence time of several days yields long range
  transport and more uniform spatial pattern.
• On the average, PM2.5 particles are transported
  1000 or more km from the source of their
  precursor gases.

• PM2.5 sulfates reside 3-5 days in the atmosphere
• Ultrafine 0.1 m coagulate while coarse particles
  above 10 m settle out more rapidly.
• PM in the 0.1-1.0 m size range has the longest
  residence time because they neither settle, nor
  coagulate.

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Residence Time Dependence on Height.

• The PM2.5 residence time increased with height.
• Within the atmospheric boundary layer (the lowest
  1-2 km), the residence time is 3-5 days.
• If aerosols are lifted to 1-10 km in the
  troposphere, they are transported for weeks and
  many thousand miles before removal.
• The lifting of boundary layer air into the free
  troposphere occurs by deep convective clouds and
  by converging airmasses near weather fronts.

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Range of Transport

• The residence time determines the range of
  transport. For example, given a residence time of
  4 days (100 hrs) and a mean transport speed of
  10 mph, the transport distance is about 1000
  miles.
• The range of transport determines the region of
  influence of specific sources.

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Transport Mechanisms

• Pollutants are transported by the atmospheric
  flow field which consists of the mean flow and
  the fluctuating turbulent flow

The three major airmass source regions that
influence North America are the northern Pacific,
Arctic, and the tropical Atlantic. During the
summer, the eastern US is influenced by the
tropical airmass, from Gulf of Mexico.
The three transport processes that shape regional
dispersion are wind shear, veer, and eddy motion.
Homogeneous hazy airmasses are created through
shear and veer at night followed by vigorous
vertical mixing during the day.
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Influence of Transport on Source Regions
Horizontal Dilution
Vertical Dilution
Low wind speeds over a source region allows for
pollutants to accumulate. High wind speeds
ventilate a source region preventing local
emissions from accumulating.
In urban areas, during the night and early
morning, the emissions are trapped by poor
ventilation. In the afternoon, vertical mixing
and horizontal transport tend to dilute the
concentrations.
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Plume Transport

• Much of the man-made PM2.5 in the East is from
  SO2 emitted by power plants.

Plume transport varies diurnally from a
ribbon-like layer near the surface at night to
well mixed plume during the daytime. Even during
the daytime mixing, individual power plant plumes
remain coherent and have been tracked for 300 km
from the source. Most of the plume mixing is due
to nighttime lateral dispersion at night followed
by daytime vertical mixing.
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Long Range Transport

• In many remote areas of the US, high
  concentration of PM2.5 have been observed. Such
  events are have been attribute Long range
• Long range transport events occur when there is
  an airmass stagnation over a source region, such
  as the Ohio River Valley and the PM2.5
  accumulates. Following the accumulation, the hazy
  airmass is transported to the receptor areas.
• Satellite and surface observations of fine
  particles in hazy airmasses has provides a clear
  manifestation of long range pollutant transport
  over Eastern N. America.