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Quantitative Scanning Electron Microscopy Methods to
Characterize Ambient Air PM2.5
D. V. Martello, R. R. Anderson, C. M. White - NETLG. S. Casuccio, S. F. Schlaegle, - R. J. Lee Group, Inc.
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Different Missions
• EPA – Protect the public health
• DOE – Assist the private sector in supplying clean, abundant, and affordable energy
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PM2.5 Regulatory Process
• 1997 National Ambient Air Quality Standards (NAAQS) for PM2.5 based on “health effects”− Mean annual concentration < 15 µg/m3
− Maximum concentration < 65µg/ m3
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Why Is DOE Concerned About PM2.5?
• Coal-based power systems contribute to PM2.5
− Primary particles:
• Ultra-fine fly-ash (Spherical Alumino-silicates,SAS), carbon soot
− Gaseous precursors:
• SO2, SO3, NOx
• React with NH3 in the atmosphere to form ammonium sulfate and ammonium nitrate particles
• SIPs will likely restrict emissions from coal power plants.
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Ambient PM Monitoring and CharacterizationCurrent Project Portfolio
Upper Ohio River Valley Project (UORVP)
Southeastern Aerosol Research and Characterization Program (SEARCH)
Bravo Project
Great Smoky MountainsProject (GSMP)
Aerosol Research InhalationEpidemiology Study (ARIES)
Stuebenville Comprehensive Air Monitoring Project (SCAMP)
DOE-NETL Primary Funding
DOE-NETL Cost-sharing
NETL-PGH In-houseMonitoring Station
CMU/EPA “Supersite”
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Regional Ambient PM Monitoring Sites
Coal-fired power plants
1
2
UORVP Sites- Lawrenceville
- Holbrook
- Satellites
1
2
SCAMP Sites- Primary
- Satellites
1
1
- NETL In-house site
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Project Objective
• Measure the degree that coal-fired utilities contribute to the primary fine-particulate matter load in ambient air.
• Use Spherical Alumino-Silicates (SAS) as the measure of coal-fired utilities’ contribution to the primary fine-particulate matter load in ambient air.
Experimental Approach
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Experimental Objectives
• Quantitatively validate the SEM methods.
− Assess comparability of polycarbonate filters to FRM teflon filters.
• Improve methods to account for most particle species on the filters.
− Identify particle species including SAS, sulfates, and carbonaceous.
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Secondary Electron Image
SAS Particle
Carbon-rich Particle
Sulfur Droplet1 µm
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Experimental – Sample Collection
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Experimental – Sample Collection
• Andersen RAAS 400 Speciation Sampler
• Four flow channels
− One at 16.7 L/m, teflon filter for FRM comparison
− Two at 7 L/m, polycarbonate filters for SEM
− One at 16.7 L/m, ‘quartz’ filter for EC/OC
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Experimental – Sample Collection
• Channel configurations
− 1: FRM teflon filter
− 2: Carbon denuder, Pd coated polycarbonate filter, Nylasorb filter
− 3: Carbon denuder, uncoated polycarbonate filter, Nylasorb filter
− 4: Carbon denuder, ‘quartz’ filter, carbon filter
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Experimental – Sample Collection
• Pre-weigh and post-weigh filters.
• Sample SEM flow channels at 7 L/m found to produce good particle dispersion at typical 15µg/m3 ambient air loading.
• Sample ambient air for 24 hours.
• Analyze SEM filters ASAP after sampling or store particle filters under controlled conditions.
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Comparison of Gravimetric Filter Loading Variation:
RAAS Polycarbonate versus FRM and RAAS Teflon versus FRM
Filter Type
Number of Filters
Avg. abs. difference,%
Standard Deviation
Polycarbonate 46 9.6 6.8
Teflon 163 5.6 6.8
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Scanning Electron Microscopy (SEM)
• SEM analysis can provide both morphology and composition information for use in individual particle species determination.
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Scanning Electron Microscopy (SEM)
• Image information
− Secondary Electron (SE): Morphology
− Backscattered Electron (BSE): Chemistry, Morphology
• Composition information
− X-ray (EDX/EDS): Chemistry
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Secondary Electron Image
SAS Particle
Carbon-rich Particle
Sulfur Droplet1 µm
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Backscattered Electron Image
SAS Particle
Carbon-rich Particle
Sulfur Droplet1 µm
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SEM Technique Validation
• Test for uniform particle dispersion over the filter surface.
• Test for reasonable particle size distributions within measured fields.
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Experimental – SEM Analysis
• Mount filters with double-sided silver tape; no filter post sampling coating with carbon or metals
• SEM conditions: 15kV; stable beam current; fixed WD
• SE image fields at 1000x; center to perimeter of filter wedge analyzed; particle ID threshold monitored
• Reference spectra used; 7 sec spectra acquisition
• Pd coated filters for carbon particle identification− C:Pd EDX ratio used for ID particle threshold
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Section Location Perim. offilter
Center offilter
Area Analyzed 0.0234 mm2 0.0288 mm2
Comparison of Filter Area required to Obtain 1000 Particle Images from filter Center and filter
Perimeter
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Perim.Number
%
CenterNumber
%
Perim.Volume
%
CenterVolume
%Particle TypeSAS 1.8 1.4 4.1 4.3C-rich 85.5 79.6 69.7 68.8Crustal 8.6 14.4 21.3 23.8S-rich 4.1 4.6 4.9 3.1
Comparison of Particle Species from filter Center and filter Perimeter, Excluding
Ammonium Sulfate “Blobs”
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23KX 0.2 µm 33KX 0.2 µm
Secondary Electron ImagesSAS
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Secondary Electron ImagesCarbon Chain Agglomerates
15KX 0.5 µm 1 µm7500X
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16KX 0.5 µm 22KX 0.2 µm
Secondary Electron ImagesCrustal
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Secondary Electron ImagesSulfur Deposits
15KX 0.5 µm 0.2 µm27KX
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Secondary Electron ImagesPlant Material
19KX 0.2 µm 22KX 0.2 µm
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Distributions - Spherical ParticlesAll Quads, Aspect < 1.5, - 2198 Particles
0
50
100
150
200
250
300
350
400
0.20
0.44
0.68
0.92
1.16
1.40
1.64
1.88
2.12
2.36
2.60
2.84
3.08
3.32
3.56
3.80
Average Diameter, um
# o
f P
arti
cles
All Quads, Aspect <1.5, Si>0, 948 Particles
0
50
100
150
200
250
0.20
0.45
0.69
0.94
1.18
1.43
1.67
1.92
2.16
2.41
2.65
2.90
3.14
3.39
3.63
3.88
Average Diameter, um
# of
Par
ticle
s
All Quads, Aspect < 1.5, Si=0, S>0,1065 Particles
0
50
100
150
200
250
300
0.20
0.39
0.59
0.78
0.97
1.16
1.36
1.55
1.74
1.94
2.13
2.32
2.51
2.71
2.90
3.09
3.28
Average Diameter, um
# of
Par
ticle
s
All Quads, Aspect < 1.5, Si=0, S=0,185 Particles
05
101520253035404550
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
1.40
1.50
Average Diameter, um
# o
f P
artic
les
From “Quantitative Scanning Electron Microscopy of Ambient Air 2.5µm Particles Using”, Air Quality II Conference, McLean, Virginia, September 19-21, 2000.
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Non-spherical ParticlesAll Quads, Aspect >= 1.5, Si>0,
2166 Particles
0
100
200
300
400
500
600
0.20
0.50
0.81
1.11
1.41
1.71
2.02
2.32
2.62
2.93
3.23
3.53
3.83
4.14
4.44
4.74
Average Diameter, um
# of
Par
ticle
s
All Quads, Aspect >= 1.5, Si=0, S>0,2533 Particles
0
100
200
300
400
500
600
700
0.20
0.56
0.91
1.27
1.62
1.98
2.33
2.69
3.04
3.40
3.75
4.11
4.46
Average Diameter, um
# of
Par
ticle
s
All Quads, Aspect >= 1.5, Si=0, S=0,382 Particles
0
50
100
150
200
250
300
0.20
0.39
0.59
0.78
0.97
1.16
1.36
1.55
1.74
1.94
2.13
2.32
2.51
2.71
2.90
3.09
3.28
Average Diameter, um
# of
Par
ticle
s
All Quads, Aspect >= 1.5, - 5081 Particles
0100200300400500600700800900
1000
0.20
0.53
0.85
1.18
1.51
1.84
2.16
2.49
2.82
3.14
3.47
3.80
4.12
4.45
4.78
Average Diameter, um
# of
Par
ticle
s
From “Quantitative Scanning Electron Microscopy of Ambient Air 2.5µm Particles Using”, Air Quality II Conference, McLean, Virginia, September 19-21, 2000.
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Sample Start Date
24 Hour FRM ug/m3
SEM Ammonium
Sulfate
SEM Carbon
Particles
SEM All Crustal Particle Types
SEM Spherical Alumino-Silicates
SEM Crustal
SEM Ca/S Rich
SEM Fe Rich
Spheres
SEM Metal Oxide
Particles
SEM Misc.
Particles
08/06/00 20.8 9.77 5.97 5.06 0.87 3.79 0.30 0.06 0.02 0.03 08/08/00 23.4 13.95 4.93 4.51 0.24 3.67 0.51 0.00 0.01 0.07 08/10/00 15.7 7.94 6.34 1.37 0.03 0.92 0.00 0.09 0.30 0.03 08/12/00 7.6 2.08 3.57 1.95 0.09 1.10 0.07 0.09 0.39 0.21 08/14/00 17.8 9.07 6.20 2.53 0.08 2.04 0.03 0.22 0.15 0.01 08/16/00 9.5 4.76 2.72 2.02 0.26 0.60 0.67 0.00 0.37 0.13 08/18/00 14.0 8.17 3.66 2.17 0.07 1.17 0.41 0.22 0.24 0.07
Analysis ResultsFRM and SEM particle class data
normalized to µg/m3
FRM and SEM particle class data normalized to µg/m3 (SEM particle class volume fraction x daily FRM). SEM data normalized to IC ammonium sulfate analysis. From “Quantitative Analysis of Ambient Air 2.5µm Particles Using Scanning Electron Microscopy”, 11th International Conference on Coal Science, San Francisco, Oct. 2001.
ACS-222 / DVM’ / ST-10 / 8-28-2001
Carbon Values
0.000
1.000
2.000
3.000
4.000
5.000
6.000
7.000
8.000
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00
R&P 5400 Total Carbon ug/m3
CC
SEM
Car
bon
ug/m
3R&P Carbon versus SEM Carbon
From “Quantitative Analysis of Ambient Air 2.5µm Particles Using Scanning Electron Microscopy”, 11th International Conference on Coal Science, San Francisco, Oct. 2001.
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Conclusions
• SEM sampling methodology is reasonably comparable to the FRM, based on ug/m3 filter loading.
• The SEM technique provides satisfactory results to continue the method development.
• Although the methodology may be subject to improvement, it is of sufficient quality to apply it to the comparative measurement of SAS particles in a more comprehensive study.
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Future Work
• Correlate the SEM results with bulk analyses of companion filters.
• Analyze more filters to develop statistics on distributional analysis of particle classes.
• Use the palladium coating as an internal standard for quantifying the particle species.
• Revisit uncoated filters (check validity of calculating carbon by difference).
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Acknowledgements
• Dr. Elias J. George – NETL Environmental Safety and Health Division
• Traci Lersch – R.J. Lee Group
• The other PM2.5 Sampling and Analysis team
members
• Mr. Thomas J. Feeley – Product Manager: Environmental and Water Resources, NETL
