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					North American Visibility
rdyswth
Seasonal Bext
                              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 (a1*Dust + a2*Smoke + a3*Haze + a4*Salt + a5*Soot)
The weights a1…a5 are the mass extinction efficiencies for each species.
• The chemical species concentrations are obtained from filter samples followed by
   chemical analysis
           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.
WMO Global Surface Meteorological Network
Visibility over North America: A Global Perspective
Surface Visibility
AVHRR
ShipObs
POLDER
              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).
3D SeaWiFS May 14, 1998
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.
Fire Locations
                    Surface Ozone Concentration




Superposition of daily maximum ozone and aerosol extinction maps derived from surface visibility.
                  Visibility Module:
Calculates visibility from aerosol concentration and humidity data
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.
                       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.
                              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.
                     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     NOAA’s Operational Significant
Satellite Program (DMSP) satellite data   Event Imagery (OSEI)
            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.
         Smoke passes over Eastern North America


               GOES 8 Visible Imagery




May 12          May 14         May 15              May 16
                TOMS Aerosol Index
                    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
                  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.
             Light Extinction Trends of the 75th and 90th Percentiles




              Trend Statistics
                                          Eastern US         Northeastern US          Southeastern US
                                  75th %-ile   90th %-ile   75th %-ile   90th %-ile   75th %-ile 90th %-ile
             BEXT (1/km)            0.015        0.042       0.014        0.042        0.024      0.053
               ('80 - '95)
           Stnd Error (1/km)        0.0095        0.018       0.014        0.023        0.011      0.023
                 /Error             1.58          2.29         1          1.83         2.14       2.32
         Trend Confidence            85%          95%         65%          90%          95%        95%
         limit (t distribution)

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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