North American Visibility

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North American Visibility
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rdyswth
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Seasonal Bext
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Horizontal Visibility Determined by the
Extinction Coefficient, Bext

• Bext can be estimated from surface visual range
  data and WebCams
• The dry BEXT (say RH 50) can also be estimated
  from the weighed sum of the mass concentrations
  of aerosol types
• Bext S (a1Dust a2Smoke a3Haze
  a4Salt a5Soot)
• The weights a1a5 are the mass extinction
  efficiencies for each species.
• The chemical species concentrations are obtained
  from filter samples followed by chemical analysis

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Aerosol Types and Vertical Layering

• At any given geographic location, the aerosol is
  composed of multiple types, e.g. dust, smoke and
  haze
• The aerosol types most frequently reside in
  different layers
• As a consequence, horizontal visibility is
  influenced by the aerosol in the surface layer
  while the vertical and slant visibility is
  determined by the layers in the aerosol column
  containing multiple aerosol types and layers.

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WMO Global Surface Meteorological Network
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Visibility over North America A Global
Perspective
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Surface Visibility
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AVHRR
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ShipObs
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POLDER
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SeaWiFS, TOMS and Surface Extinction
Surface reflectance derived from the SeaWiFS
satellite data for May 14-17 1998. The spectral
reflectance data were rendered as a "true color"
digital image by combining the blue (0.412 ?m),
green (0.550 ?m), and red (0.670 ?m) channels.
The TOMS absorbing aerosol index (green, levels
12 and 30) and the visibility-derived extinction
coefficients are superimposed as green contours
(red, levels 0.2. and 0.4 km-1).
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3D SeaWiFS May 14, 1998
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Average Excess TOMS Index for Mar., Apr., May 1998
Excess TOMS absorbing aerosol index averaged for
March, April, May 1998 compared to 1999. The
insert depicts the 1998 smoke impact from a
global perspective.
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Fire Locations
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Surface Ozone Concentration
Superposition of daily maximum ozone and aerosol
extinction maps derived from surface visibility.
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Visibility ModuleCalculates visibility from
aerosol concentration and humidity data
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SeaWiFS Surface Reflectance on Clear and Smoky
Days
Spectral reflectance data derived from the
SeaWiFS sensor on May 15, 1998 b) Excess aerosol
backscattering over water.
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Synopsis

• During a ten-day period, May 7-17, 1998, smoke
  from fires in Central America drifted northward
  into the USA and Canada.
• The smoke caused exceedances of the PM standard,
  health alerts, and impairment of air traffic, as
  well as major reductions of visual range, and red
  sunsets.
• It was a major air pollution event covered by the
  research community as well as by the national
  media.

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Background

• Throughout the spring of 1998, thousands of fires
  in Central America have been burning as it
  happens every spring but the 1998 fires are said
  to be about twice as intense as the normal year.
• Unlike earlier years, the research community has
  followed with keen interest the 1998 Central
  American fires by a variety of UV, visible and
  infrared remote sensors from satellites.
• This is summary of the Web-based data as
  augmented by surface-based PM10 monitoring data
  by state agencies
• This preliminary and incomplete but timely
  summary is intended for air quality managers and
  researchers interested in pursuing further
  detailed analysis of this unusual event.

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Forest Fires over Central America
Throughout the spring of 1998, thousands of fires
in Central America have been burning with twice
the intensity of normal springtime fires.
Location of fires (red dots) on May 15, 1998,
based on Defense Meteorological Satellite Program
(DMSP) satellite data
NOAAs Operational Significant Event Imagery
(OSEI)
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Smoke from the Central American Fires
Based on SeaWiFS and other satellite imagery,
thick smoke has been lingering over southern
Mexico, Guatemala and Honduras and adjacent
oceans throughout the spring season.
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Smoke passes over Eastern North America
GOES 8 Visible Imagery
May 12
May 14
May 15
May 16
TOMS Aerosol Index
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Preliminary Surface Haze-Ozone Map Comparison

• Surface haze maps show the north and eastward
  transport of smoke aerosol
• Regionally, the smoke does not appear to add
  ozone to the existing values
• Rather, ozone in the smoky airmass tends to be
  lower than the surrounding areas

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US Visibility Trend Maps, 1980 - 1995 Click on
the images to view larger versions
In the Eastern US, throughout the 1980-95 period,
the 75th percentile BEXT exceeded 0.15 or had an
average visibility of less than 10 miles. Most
notable are the hazy regions on both sides of the
Appalachian Mountains where the BEXT exceeds 0.2
1/km. Since the early 1980s the BEXT decreased
10-15 with the largest decreases in the Southern
and Central regions.
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Light Extinction Trends of the 75th and 90th
Percentiles
Over the Eastern US, the 75th percentile BEXT
decreased about 8 percent over the 15 years.
The largest decreases occurred in the Southeast
where the BEXT decreased 12 compared to 8 in
the Northeast.
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