Springtime Airmass Transport Pathways to the US

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Springtime Airmass Transport Pathways to the US
Prepared by Bret A. Schichteland Rudolf B. Husar
Center for Air Pollution Impact and Trend
Analysis (CAPITA) Washington University Saint
Louis, Missouri Submitted to Joseph Pinto,
USEAPA December 1, 1999
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Introduction

• Anthropogenic and natural pollutants generated in
  one country are regularly transported to other
  countries adding to their air quality burden.
• Generally, the concentration of transboundary
  pollutants impacting the US is a small addition
  to the natural background. However, under the
  conditions of combination of high emissions and
  favorable meteorological conditions transboundary
  pollutants can significantly add to the U.S. air
  quality burden.
• This work is a first step toward assessing the
  transboundary transport of pollutants to the US.
  The global scale transport pathways to the
  borders of the United States are explored during
  the Spring (February, March, April) 1999.
• The transport analysis is conducted using airmass
  histories.

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Airmass History Definition

• An airmass history is an estimate of the 3-D
  transport pathway (trajectory) of an airmass
  prior to arriving at a specific receptor location
  and arrival time.
• Meteorological state variables, e.g. temperature
  and humidity are saved along the airmass
  trajectory.
• Multiple particles are used to simulate each
  airmass. Horizontal and vertical mixing are
  included in the airmass history calculation
  causing particles emitted at the same time to
  follow different trajectories.

The history of an airmass arriving at Acadia NP,
ME on 7/19/95 noon
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US Airmass History Database

• Ten day airmass histories for 9 receptor sites
  were calculated between February and April 1999.
  The airmass histories were calculated using the
  CAPITA Monte Carlo Model driven by the FNL global
  meteorological data.

• Each airmass history is composed of 15
  trajectories which are tracked at two hour time
  increments back in time. Approximately, 12,000
  trajectories per receptor were calculated
• The back-trajectory starting heights are within
  the mixing layer.
• Temperature, Relative Humidity, Cloud coverage,
  Precipitation rate, and Mixing height are also
  saved out along each trajectory.

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Residence Time AnalysisThe probable airmass
pathway to the receptor

• The residence time is the time all airmasses
  reside over a grid cell. This analysis used a
  1.8o X 1.3o grid over the northern hemisphere.
• The grid level residence times are divided by the
  total time the airmasses reside over the entire
  domain. Thus, the residence times become a
  probability of an airmass passing over a given
  area prior to impacting the receptor.
• The residence time probability fields are
  displayed as isopleth plots where each shaded
  area contains 20 of the residence time
  probability, with isopleths bounding the smallest
  areas accounting for 20, 40, 60, 80 and 100
  of the total probability. The most probable
  pathways occur in the inner 20 region (red)
  while the least probable pathways occur in the
  80-100 region (light blue).

Illustration of the Residence Time Analysis
Calculation
Figures adapted from Poirot et al. 1999
Underhill, VT trajectories overlaid a residence
time tracking grid
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Particle Height Distribution vs. Airmass Age

• To examine the vertical distribution of the
  particles making up the airmass prior to
  impacting the receptor, cumulative particle
  height distribution functions were calculated for
  each receptor.
• The particle height is dependent on their age,
  so the distributions were calculated for particle
  age bins in 6 hour increments.
• The 10th, 50th and 90th percentiles of the age
  segregated particle height distributions are
  plotted for each receptor site.

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Airmass Transport to the US West Coast

• At all three west coast sites, Seattle, San
  Francisco, and San Diego, the most probable near
  field transport pathways are along the west
  coast.
• The most probable long range transport occurs
  across the Pacific from the Russian coast
  eastward.
• Both Seattle and San Francisco are also impacted
  from airmass traveling up (north) the west coast.
  Airmass transport up the west coast south of San
  Diego does not occur.
• At all three sites, the airmass height
  distribution continually increases with age with
  the 90th percentile of the airmass height 5 km
  10 days prior to impacting the receptor.

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Airmass History Transport to the West Coast
Probabilistic Transport Pathways to
Particle Height Distribution
Seattle, WA
San Francisco, CA
San Diego, CA
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Airmass Transport to the Midwest

• At Minneapolis, the most probable transport
  occurs from the north over Canada.
• Airmass transport to St. Louis is most likely to
  come from the north. However, transport from the
  south also occurs a significant fraction of the
  time.
• In the south at San Antonia, the most probable
  transport pathways are from the Gulf of Mexico
  and along the coast of Central America and
  Mexico. This receptor is also likely to be
  impacted by airmasses from the north traversing
  the Great Plains and the west from the Pacific
  Ocean
• At all sites, the particle height distributions
  continually increase with age. However, the
  vertical distribution increases to the south with
  a maximum particle 90th percentile height of 4 km
  at Minneapolis and 6 km at San Antonio.

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Airmass History Transport to the Midwest
Probabilistic Transport Pathways to
Particle Height Distribution
Minneapolis, MN
St. Louis, MO
San Antonio, TX
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Airmass History Transport to the East Coast

• At Boston, the most probable transport occurs
  from the north, similar to Minneapolis.
• Airmass transport to Norfolk mostly likely comes
  from the north but transport from the south is
  also likely to occur.
• The most probable transport pathways to Miami
  occurs from the east over the Atlantic Ocean.
  Significant airmass transport also occurs over
  the Eastern US prior to impacting Miami.
• The particle height distributions increase to
  the south with the maximum particle 90th
  percentile height of 4 and 8 km at Boston and
  Miami respectively. The particle height
  distributions at Norfolk and Miami leveled out at
  airmass ages of 5 to 7 days.

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Airmass History Transport to the East Coast
Probabilistic Transport Pathways to
Particle Height Distribution
Boston, MA
Norfolk, VA
Miami, FL
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Conclusions

• The analysis examined the most probable airmass
  transport pathways to the US using 10 day airmass
  histories from receptors along the US borders
  during the spring of 1999.
• The transboundary transport pattern is
  geographically dependent such that
• The west coast is most likely to be impacted by
  airmasses that had traversed Russia and Northern
  China and the west coast of Canada.
• Airmasses impacting the north and central US east
  of the Rocky Mnts likely traversed Canada.
• Airmasses impacting Texas likely traversed
  Central America and Texas.

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Desirable Future Analysis and Refinements

• Some back-trajectories never pass through the
  mixing layer but descend from high altitudes
  without being exposed to surface-based emissions.
  Subsequent analyses will need to examine
  separately the high and low elevation airmass
  transport.
• Quantifying the airmass elevation is critical to
  understanding the long range transport. The
  particle height distributions vs age used in this
  analysis is only a crude method for exploring the
  airmass elevation. For example, this analysis
  provides no indication of the geographic
  variability in elevation. New methods for
  quantifying and displaying the airmass elevation
  need to be devised.