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					Aerosol Pattern over Southern North America

           Janja D.Husar and Rudolf B. Husar
         Center for Air Pollution Impact and Trend Analysis (CAPITA)
                     Washington University, St. Louis, MO

                      Tropospheric Aerosols:
        Science and Decisions in an International Community
           A NARSTO Technical Symposium on Aerosol Science
                             Queretero, Mexico
                            October 23-26, 2000
• Background:
   – North America is part of the global aerosol system.
   – The aerosol pattern and trends over the US and southern Canada are
     reasonably well documented .
   – The aerosol levels and characteristics over the southern part of North
     America are not well documented.
• Goal: Explore the Regional Aerosol Pattern Over North America, esp.
  Central America
• Approach:
   – Use surface visibility
   – Multiple satellite data
   – Combine visibility and satellite data to obtain aerosol pattern
• Results:
   – Seasonal aerosol maps and charts
   – Long-term trends
   – Smoke event
   Regional Aerosol Detection by Surface Visibility and Satellites
• At any given geographic
location, the aerosol is composed
of multiple types, e.g. dust, smoke
and haze.
• Horizontal visibility is influenced
by the aerosol in the surface layer
while the vertical optical depth is
determined by the aerosol column
• The data from multiple sensors
yield complementary information
on aerosols:

     • Light extinction (visibility),
     • Scattering (AVHRR, SeaWiFS)
     • aerosol absorption (TOMS)
• The data from multiple sensors
yield only semi-quantitative
aerosol pattern
             Oceanic Aerosol – AVHRR satellite sensor

•   During the winter and fall season the optical depth (AOD) is low surrounding N. America
•   In the spring, elevated AOD is observed adjacent to Mexico
•   In the summer an aerosol plume is evident near the Eastern US
                 POLDER Aerosol Polarization Index
                        (fine particles with polarized scattering)

•   During spring, the
    polarization index is
    high over Central
    America and the
    adjacent oceans.

•   In the late fall, the
    polarization index is
    low throughout
    North America.
             Surface Visibility over North America

• The surface extinction coefficient, Bext, over North America is
  relatively low compared to other continents.
• The Bext contours are based on 7500 surface visibility stations.
        Surface Visibility over North America

       The size of the yellow rectangle is proportional to Bext at each station.

• Over Central America, Bext is highest in the spring season.
• Over the Eastern US, Bext is highest in the summer.
            Summer Visibility Trends, 1980 – 1995
           Since the 1980s the Bext has decreased throughout the US.

                                                    Over the Eastern US, the
• Thehighest regional Bext                          decline during 1980-195
values cover the Eastern US                         was 10-15%
       Seasonal Pattern and Long-Term Trend of Bext

•   The Bext has a sharp spring    •   The spring peak occurs
    peak throughout C. America.        regularly every year
•   The noon Bext values are not   •   However, the peak spring Bext
    influenced by humidity.            varies from one year to another.
                        Bext Pattern in 1998
• Data from multiple
  stations show that
  Bext in 1998 was 2-
  4 times higher than
  in previous years

 • During 1998, the
   daily pattern of Bext
   varied significantly
   from one station to
      Fire Locations

    In 1998 the number of
    fire locations was much
    higher then in 1997

TOMS Aerosol Index

•     The excess AI in 1998
      compared to 1997
      shows the pattern of
      smoke due to the
      Central American
•     The fires and the
      smoke occur mainly at
      lower elevations.
      Smoke from the Central American Fires

Based on SeaWiFS and
other satellite imagery,
thick smoke was seen
over southern Mexico,
Guatemala and
Honduras and adjacent
oceans throughout the
1998 spring season.
3D SeaWiFS May 14, 1998
             Smoke Transport over North America
                              (SeaWiFS, TOMS and Bext)

On May 15 the smoke plume was swiftly transported northward along the Mississippi Valley.
By May 16 the smoke plume reached Hudson Bay in Canada.
The smoke was also observed at the the surface visibility stations.
                     Surface Bext-Ozone Comparison

•   Surface Bext 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 over the surrounding areas
    Approximate PM Concentration in Central American Cities

• Urban centers in Central America are PM ‘hot spots’.
• PM10 levels are in the range of the EPA-WHO standard.
• High TSP levels indicate that much of the urban PM is composed of coarse
                      Summary and Conclusions
•   The aerosol levels in North America are relatively low compared to other continents.
•   Over the Eastern US the highest Bext occurs during the summer season and has been
    declining by 10-15% since 1980.
•   During spring, the AOT, TOMS and POLDER index and Bext are all high over
    Central America and the adjacent oceans.
•   The spring peak over Central America occurs regularly every year but varies in
    magnitude from one year to another.
•   Biomass smoke from forest and agricultural fires is the main contributor to the
    springtime aerosol peak over the lower-lying areas of Central America.
•   In 1998 the fires and the smoke were 2-4 times higher then in previous years. In May
    1998, the Central American smoke plume covered much of eastern North America
    from Mexico to Hudson Bay.
•   Urban centers in Central America are PM ‘hot spots’ dominated by coarse particles.

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