Alternative Approaches for PM2.5 Mapping: Visibility as a Surrogate

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Alternative Approaches for PM2.5
MappingVisibility as a Surrogate

• Stefan Falke
• AAAS Science and Engineering Fellow
• U.S. EPA - Office of Environmental Information
• Rudolf Husar
• Washington University in St. Louis
• Center for Air Pollution Impact and Trend
  Analysis (CAPITA)

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Current PM2.5 Monitoring Network
Monitors reporting in the AIRS database at least
6 months of PM2.5 data sampled every 3rd day from
1999 2000 (FRM).
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Extrapolation of PM2.5 using Visibility as a
SurrogateSplit this so it is reused as Intro
Summary
Limitations
Benefits of Visibility Surrogate

• Visibility observations have a very good uniform
  coverage across country
• Visibility observations made hourly or every 10
  minutes
• Visibility as surrogate adds physically based
  extrapolation around measured anchor points.

• Inadequate spatial coverage
• Inadequate temporal coverage standard is once
  every 3 days
• Realistic interpolation methods

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Visibility Extinction Coefficient Relationship
Visual range observations provide a good
indicator or air quality but have the
disadvantage of being inversely related to
aerosol concentrations. A more suitable measure
is haziness or extinction coefficient, bext.

• where K is the Koschmieder constant.
• bext babs bscat
• The extinction coefficient is in units of km-1
  and is proportional to the concentration of light
  scattering and absorbing aerosols and gases.
• The raw visibility data needs to be filtered to
  eliminate and correct for
• weather influences (fog, precipitation)
• high humidity
• visual range threshold

Fine particles are efficient at scattering light
and are strongly correlated with extinction
coefficients, after correcting for meteorological
conditions
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Visibility as a PM2.5 Surrogate
PM2.5 7.6 ?g/m3
PM2.5 21.7 ?g/m3
PM2.5 65.3 ?g/m3
Adapted from Malm, An Introduction to
Visibility http//webcam.srs.fs.fed.us/intropdf.h
tm. Correlation Falke .. PM 200 FBext
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Method for Aiding Mapping when PM2.5 monitors are
spatially sparse
Extinction Coefficient 1/r2 Interpolation
Fine Mass Concentrations 1/r2 Interpolation
Bext Fine Mass 1/r2 Interpolation
Bext Aided FM Fine Mass Bext
x Bext
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ASOS Visibility Network
There are over 900 ASOS sensors including over 70
duplicate monitors (black dots). By design the
ASOS network is uniformly distributed while the
PM25 FRM network is more concentrated in urban
areas.
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ASOS Visibility Measurements
Lens-to-lens 3.5 feet
Prior to 1994, visual range was recorded hourly
by human observations Human observations were
replaced with automated light scattering
instruments for of the Automated Surface
Observing System (ASOS) The ASOS sensor measures
the extinction coefficient as one-minute averages
projector
detector
photocell
Forward scatter ASOS visibility sensor
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Application of ASOS for PM2.5 Mapping

• Currently, available only at a quantized
  resolution of 18 binned ranges with a visual
  range upper bound of 10 miles, even though the
  instrument can provide meaningful data up to
  20-30 miles.
• However, in the near future, it is anticipated
  that ASOS data will be available at their full
  resolution on the web in real-time.
• Even at full resolution, they are of limited use
  in the western U.S.
• The application to real-time mapping (hourly or
  less) needs to be studied
• Application to small spatial scales (urban areas)
  also needs more evaluation
• ASOS data have great potential in aiding PM2.5
  mapping

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Difficulties in Mapping PM2.5
Limitations
Benefits of Visibility Surrogate

• Visibility observations have a very good uniform
  coverage across country
• Visibility observations made hourly or every 10
  minutes
• Visibility as surrogate adds physically based
  extrapolation around measured anchor points.

• Inadequate spatial coverage
• Inadequate temporal coverage standard is once
  every 3 days
• Realistic interpolation methods