topic # 6 … how does carbonaceous particle composition, shape, and size affect optical properties...
TRANSCRIPT
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TOPIC # 6 … How does carbonaceous particle composition, shape, and size affect optical properties in the air and when sampled on a filter?
How might optical properties of particles in the air differ from those collected on a filter?
How might filter transmittance and reflectance change during heating as particle morphology and composition change?
Why might optical transmission and reflectance give different pyrolysis corrections?
Kirk A. Fuller, AOR ProgramUniversity of Alabama in Huntsville
National Space Science and Technology Center
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OVERVIEW
• Filter-based measurements of absorption
• Some pitfalls of measurements on filter deposits
• Effects of aggregation
• Effects of internal mixing
• Summary and suggested needs
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Some particles of interest
Aq. Sulfateswater2nd’ry OC
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National Aerosol-Climate Interactions Programhttp://www-c4.ucsd.edu/NACIP/
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(Mass-) Specific absorption cross section:
V
C
rticlesmass of pa
C
mass
C absabsabs
of Cabot Corp.’s Monarch 61 carbon black in air is not 9.68 m2/gram
densityflux incident
)(scattered aborbedflux radiant total
Absorption (scattering) cross section:
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Filter methods and associated problems:
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z
I0
I
Idealized absorption spectroscopy:
(blank)
sample)with (
I
IT
0I
IT
)(m N )(m b
) exp(- I z) b exp(- I I3-
molecules2
absabs
abs0abs0
or
Particlesin the single scattering limit:
)(m N )(m b
) exp(- I z) b exp(- I I3-
particles2
absscaext
0ext0
CC
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Rp ,Tp Rf ,Tf
…TpTf
1 - RpRf
Use diffuse, not directional, quantities
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Effects of particle spacing: Coherence
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g. scatterincleinterparti from
influence no is there if even s,meaningles is expression lexponentia The
EE TT
1.6 /2C N then /g,m 5 :aerosol theof extinction specific-Mass
,cm 2.2 sample of Area
µg/m 4 :ionconcentrat cAtmospheri
g/cm 1.8 :density material Aerosol
filter a through uteliters/min 21.7 @ hours 24over Collection
).b is C (N
order first to - xx-1 eexpansion theesapproximat
that thisispy spectroscoLambert -Beerin assumption lfundamenta The
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More rigorously, the electric field transmitted by a slab of noninteracting particles is
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Rather than voltage outputs related to abs. by
the measurement actually relates to extinction as
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21
2,1
/d
d< >
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Optical properties may be altered by:
(1) multiple scattering in the deposit/substrate system,
(2) alteration of absorption and scattering cross sections by
electromagnetic coupling between particles,
(3) electromagnetic coupling of particles to filter surfaces,
(4) optical coherence between particles with separations
comparable to the wavelength of the interrogating radiation,
(5) induced alignment of nonspherical particles along filter
surfaces,
(6) shape distortion of liquid droplets, and
(7) reactions among different chemical species, especially over
extended sampling times.
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• effects of aggregation• effects of mixing
Courtesy of National Park Service
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Pos
itio
n-av
erag
ed
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Rg = 0.10 m 0.80 m mass median diameter
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Mechanisms for making EC hygroscopic?
+ SO2 + H2O
H2SO4?
XDo these make EC polar?
of GC in H2O is about 10 m2/g, but 5-7 m2/g in air.
Is changed from its value in air by embedding in a filter deposit? Seems very likely if deposit is aqueous:Enhancement would be even greater if in (NH4)2SO4 sol’n
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A. More photoacoustic studies
B. More Trans/Nephelometer measurements• Long-path Trans a la IMPROVE• Short/folded path Trans to study humidification effects in photoacoustic work
A + B to improve filter-based measurements
Improved filter measurements for automated, real-time,reduced-cost monitoring of EC.
C. Optical models that better account for internal mixing, morphology, and filter artifacts.
What is needed?
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D. IMPROVE-type analysis of EC for chem-based parameterization of soot optics ?
E. Increased use of Raman spectroscopy, along with IR spectroscopy
• Include this in studies of thermal evolution
F. Critical review of reference material by all authors and reviewers
G. Specific absorption of Porter, Stout, Coffee, other important light-absorbing OCs
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Topic #6 Report / Research Strategy
Science team selected from RFP
Invite climate community participation
Invite combustion science community
Lab measurements on well-characterized particles
• Generated by investigators
• Provided by NIST
Theoretical analysis+
Characterize aging of soot
Include satellite remote sensing
Intercomparison of lab measurements & theoretical models
Targeted source and downwind measurement and analysis
Collocated measurements (e.g., @ IMPROVE and EPA sites)+
Method intercomparisons at selected super site
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Products and clients:
Standardized fast/cheap/good measurement of EC for climate and visibility communities
Baseline for EC to assist TOA methods for climate, visibilityand health communities
Caveat: Make everything as simple as possible, but no simpler:Protocols may require complementary measurements suchas vibrational spectroscopy, optical particle counters, etc.