Characterization of Aerosol Physical, Optical and Chemical Properties During the Big Bend Regional Aerosol and Visibility Observational Study (BRAVO)

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Characterization of Aerosol Physical, Optical and
Chemical Properties During the Big Bend Regional
Aerosol and Visibility Observational Study
(BRAVO) Jenny Hand Eli Sherman, Sonia
Kreidenweis, Jeff Collett, Jr., Taehyoung Lee,
Derek Day? and Bill Malm? Colorado State
University Atmospheric Science ?CIRA/National
Park Service Funding by National Park Service
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• OUTLINE
• Motivation for participating in BRAVO
• Chemical measurements and preliminary results
• Fine (PM2.5) and Coarse (PM10- PM2.5) species
• Size distribution measurements
• Experimental set-up and instrument calibration
• Alignment method retrieved refractive index and
  density
• Comparisons between chemical and physical
  properties
• Optical properties column and point measurements
• bsp (fine and coarse), ?aer, Ångstrom exponent
• Summary

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• BRAVO STUDY
• July - October 1999
• Big Bend NP has some of the poorest visibility of
  any monitored Class 1 area in the western
  U.S.
• Seasonal trends
• Sulfates highest in summer
• Organic carbon highest in spring
• Blowing soil highest in July (Saharan dust
  episodes)
• (Gebhart et al., 2000)
• Recent work in Grand Canyon NP demonstrated that
  discrepancies of up to 50 or more exist between
  measured and reconstructed extinction (Malm and
  Day, 2000)
• Particle absorption or coarse scattering?

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• Aerosol Chemistry Measurements
• PM2.5 composition
• CSU daily samples, on-site analyses of major
  ionic species and particle acidity
• IMPROVE daily samples major ionic species, plus
  soil, organic and elemental carbon
• PM10 composition
• IMPROVE daily samples major ionic species, plus
  soil, organic and elemental carbon ? Coarse
  composition (PM10 - PM2.5)
• Ionic species particle size distribution MOUDI
  samples
• Aethalometer- black carbon

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BRAVO PM2.5 Aerosol Acidity
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BRAVO Soil Composition
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• Aerosol Size Distribution Measurements
• Dry size distributions were measured continuously
  ranging from 0.05lt Dplt 20 µm
• Instruments
• TSI Differential Mobility Analyzer (DMA)
• 0.05 lt Dp lt 0.87 µm (21 bins)
• PMS Optical Particle Counter (OPC)
• 0.1 lt Dp lt 2 µm (8 bins)
• TSI Aerodynamic Particle Sizer 3320 (APS)
• 0.5 lt Dp lt 20 µm (51 bins)
• Pre-, during-, and post-study calibration were
  performed using PSL, ammonium sulfate and oleic
  acid.

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• Instrument Calibration
• Empirical equations determined from instrument
  calibration relate real refractive index to OPC
  channel diameter (Dopt ? Dp)
• Channel collection efficiencies were determined
• Effective density (?e) was related to APS channel
  diameter (Dae? Dp) by the following equation
• where

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Examples of Aligned and Unaligned DMA and OPC
Volume Distributions
Unaligned
Aligned
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Example of Combined Volume Distribution
BRAVO 991008
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BRAVO Volume Distributions
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• Comparisons between
• chemical and physical properties
• Refractive index and density retrieved from
  alignment method and calculated from chemical
  composition
• Total (PM10) reconstructed mass and M ?Vtot
  from size distributions, assuming X1.2
• MOUDI mass size distributions and volume
  distributions
• EC and aethalometer measurements

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• Accumulation Mode Parameters

Dgv
?g
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• Coarse Mode Parameters

Dgv
?g
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• Refractive Index and Density
• Real refractive index and effective density were
  retrieved from size distribution alignment method
• Values based on chemistry were calculated using a
  volume weighted method
• and
• Species included
• (NH4)2SO4 m 1.53, ? 1.76 g cm-3
• OC m 1.55, ? 1.4 g cm-3
• EC m 1.96 - 0.66i, ? 2.0 g cm-3
• NH4NO3 m 1.554, ? 1.725 g cm-3
• Soil SiO2, Al2O3, Fe2O3, CaO, TiO2 (IMPROVE)

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Aerosol Refractive Index and Density
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• Total Mass Comparisons
• PM10 total mass concentration
• M ?Vtot, assuming X 1.2

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MOUDI Mass and Volume Distributions
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• Calculations of Light Scattering Coefficient
  (bsp)
• bsp was calculated using combined volume
  distributions and converged values of refractive
  index
• Qsp is the Mie scattering efficiency assuming
  spherical particles.
• bsp was calculated for the accumulation and
  coarse particle modes

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BRAVO scattering distribution
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Comparisons of NPS and CSU Dry bsp
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Dry Mass Scattering Efficiency
Accumulation Mode
Coarse Mode
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• Calculation of Aerosol Optical Depth (?aer)
• USDA UV-B radiation monitoring program has a
  fully instrumented site approximately 30 miles
  from BRAVO site in Big Bend National Park
• YES visible Multi-Filter Rotating Shadowband
  Radiometer measures irradiance with seven
  wavelength channels 415, 500, 610, 665, 860, and
  940 nm (Bigelow et al., 1998)
• Rayleigh and ozone optical depths were removed
  from column measurements of total optical depth
• Clouds and high sun angle measurements were
  removed
• Point measurements of ?aer were determined by
  assuming a well-mixed layer and estimates of
  boundary layer heights

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Two days were chosen for comparison
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Aerosol Optical Depth at 500 nm
August 15, 1999
October 12, 1999
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• Ångstrom Wavelength Exponent (?)
• Calculated for both point and column measurements
  over the wavelength range from 415 nm - 860 nm
  (Eck et al., 1999 Reid et al., 1999)
• Two days were chosen for comparison,
  demonstrating very different aerosol physical,
  chemical and optical properties

Column
Point
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Ångstrom wavelength exponent (415 - 860 nm)
August 15, 1999
October 12, 1999
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Correlations between bsp and ?aer were found for
several days
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Correlations between bext and ?aer were found for
all months
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Correlations between ?CSU and ?UVB were found for
all months
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• Summary
• Sulfate was typically the major chemical species
  in the fine mode, although soil and OC were
  important during certain events
• Size distributions suggested that high coarse
  mode volume contributed significantly to total
  volume, especially during suspected Saharan dust
  events
• A new alignment method allowed for retrieving
  refractive index and effective density, in
  agreement with calculated values
• Calculated light scattering coefficients agreed
  well with measured values, and demonstrated
  periods when coarse scattering was important,
  often during suspected Saharan dust events

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• Summary, continued
• Time resolved sulfate measurements were observed
  to trend with light scattering coefficients,
  suggesting sulfate was the major contributor to
  visibility degradation during the study
• MOUDI mass distributions compared well with
  measured volume distributions
• Column and point measurements of aerosol optical
  depth were observed to be correlated for several
  days investigated
• Angstrom wavelength exponents agreed well between
  the two methods, and reflected the different
  aerosol types observed