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@mecf.wustl.edu
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.
Atmospheric Residence Time and Spatial Scales
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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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Atmospheric residence time and transport
distance are related by the average wind speed,
say 5 m/s.
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Residence time of several days yields ‘long
range transport’ and more uniform spatial
pattern.
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On the average, PM2.5 particles are transported
1000 or more km from the source of their
precursor gases.
Residence Time Dependence on Height.
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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.
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.
Transport Mechanisms
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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.
Influence of Transport on Source Regions
Horizontal 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.
Vertical Dilution
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.
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.
Long Range Transport
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In many remote areas of the US, high concentration of PM2.5 have been observed. Such
events are have been attributhe 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.
Resource Links
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Workbook Table of Contents
Comment and Feedback Page
Applications / Reports
Data sets used in the Applications
Methods and tools used in the Applications