Post on 20-Aug-2020
AMS Collection Efficiency Issues• SIZE:
– Aerodynamic lenses focuses particles onto the AMS vaporizer– Particle transmission loss through the lens system for large particles impacting
on orifice plates– Particle transmission loss for small particle through Brownian forces exerted
exiting the lens system causing the particles to miss the vaporizer– Large particles may not fully vaporize prior to ejection off of hot vaporizer
surface• SHAPE:
– Nonspherical particles are not as well focused as spherical liquid droplets and may miss vaporizer.
• PHASE: – Refractory materials are not measured (e.g. sea salt, crustal oxides, and soot)– Solid particles and mixed phase particles (solid+liquid) may bounce off the
vaporizer prior to full evaporation.
… an important ongoing quantitative issue
AMS Collection Efficiencies
Beam characterization and quantification:• Lens alignment• Spot pictures• Particle beam width probe• Light scattering module
Lens Alignment
Aerodynamic Inlet Focusing
Beam Width Probe
Huffman et al., 2005
Nearly all sampled aerosols strike our 3.8 mm vaporizer
Particle Bounce
• LS – shows that particles that pass through the lens into the AMS can bounce off the vaporizer without vaporizing
NEAQS 2004 – Gulf of MaineAMS Sulfate
Comparisons with other instruments
Particle Bouncedue to Particle Phase
Tim Onasch Brendan MatthewAnn Middlebrook
Eben CrossLeah WilliamsJenny McInnis
John Jayne
• Matthew, Onasch, Middlebrook, AS&T in press
CE vs NH4/SO41.0
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3.02.52.01.51.00.50.0NH4/SO4 Ratio
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AMS
Water M
ass Fraction
Ammonium Sulfate
Sulfuric acid
• Phase is important!
AMS vs PILS and DMPS
GoMACCS 2006 – Gulf of Mexico
NH4, H2SO4, HNO3 Phase
Diagram12
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Collection Efficien va ) (%)
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(NH
4 )2 S
O4 M
ass
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F)
CE
base
d on
NH
4 NO
3 C
E ba
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on (N
H4 )
2 SO
4 G
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aliz
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data
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3.02.52.01.51.00.50.0NH4/SO4 Ratio
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AM
S W
ater Mass Fraction
SOLID
LIQUID
LIQUID
SOLID+
LIQUID
Zhang et al., 2006
Inorganic Fraction Composition Varies
Ambient CE EstimatesInternal mixtures:• Aerosol dominated by inorganic phase to first order
(typically sulfate or nitrate)External mixtures:• Dependent upon phase of each mode
Size Dependence:• Lens transmission dependent upon size range of each mode
Recommend ALWAYS testing any CE hypothesis with instrumental comparisons!
Research tasks to address AMS Collection Efficiency Issues
• Lens Development: – Previously discussed– Elena de la Rosa Blanco (ARI) has taken over the reins from Dahai Tang
(MIT) in terms of modeling lens design and transmission– Clear plans for moving forward with respect to improving both small and large
particle transmissions• Light Scattering Probe Development:
– Cross et al. work on module development and fundamental measurement/understanding of particle transmission/loss
– Kimmel, Jayne, Onasch work on second generation LS module hardware– Cross, Onasch, and Sueper work on LS module analysis
• Vaporizer Design:– Modify vaporizer design to increase quantification– Optimize trade-off between quantification and size-resolved information
• Particle Manipulation through Condensational Growth: – Investigation of reduction of particle bounce through liquid coatings.
Light Scattering Module development for characterizing the
AMS Collection Efficiency
Eben CrossTim OnaschJoel Kimmel
Donna SueperJohn Jayne
Doug Worsnop
Schematic of LS-TOF-AMS
Particle Bounce Observations:3 types of events
• LS – shows that particles that pass through the lens into the AMS can bounce off the vaporizer without vaporizing or with partial vaporization
Prompt
DelayedParticle
Vaporization
PTOF Mass Distribution Results
• Particle bounce ‘spreads out’ PTOF mass distribution to higher sizes (PTOF times)
Single Particle CE Results3000
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logD
va
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1000Dva (nm)
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Deteciton R
atioMCMA2006 T1 Results
Total # particles Prompt Fraction Delayed Fraction Null Fraction Detected Particle Fraction
• Large number of ‘null’ events (~50%)
• Size effect – larger particles tend to bounce more prior to full evaporation
Attempts to Improve the AMS Collection Efficiency with Modifications to the Vaporizer
Eric Beecher, MITJenny McInnis, Cornell
Leah WilliamsTim Onasch
Achim TrimbornJohn Jayne
VAPORIZER CONFIGURATIONSVaporizer Geometry:
1. Flat2. Standard inverted cone3. Extension tube ending with inverted cone4. Mesh filled tube
Density of Tungsten Material:A. 80%B. 62%C. 50%
Surface Modifications: I. Chemical etchingII. Grit blasting
Challenge aerosol = Ammonium Nitrate and Sulfate
Particle Bounce: NH4NO3
• Vaporizer shape and density have no measurable effect on Ammonium Nitrate measurements• Data includes light scattering, single particle MS data, and average MS data compared with
CPC results
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ectio
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3 4 5 6 7 8 9100
2 3 4 5 6 7 8 91000
Dva Particle Size (nm)
NH4NO3Conical 80% Dense Vaporizer Conical 62% Dense Vaporizer Conical 50% Dense Vaporizer Flat 50% Dense Vaporizer
Experimental Data (NH4NO3, DEHS, NaNO3) 760 torr
CFD Model 760 torr
Particle Bounce: (NH4)2SO4
• Light scattering data for AS is reasonably in agreement with AN results• NOTE: the apparent differences with vaporizer densities is an artifact
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Particle Size (nm)
(NH4)2SO4 Light Scattering:Conical 80% Dense Vaporizer Conical 62% Dense Vaporizer Conical 50% Dense Vaporizer
Experimental Data (NH4NO3, DEHS, NaNO3) 760 torr
CFD Model 760 torr
Particle Bounce: (NH4)2SO4
• Single particle and average MS data for AS are lower due to particle bounce effects • Average MS data generally higher than single particle MS data• Results here are combined lens transmission and bounce effects…
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ectio
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3 4 5 6 7 8 9100
2 3 4 5 6 7 8 91000
Particle Size (nm)
(NH4)2SO4 Single Particle MS results:Conical 80% Dense Vaporizer Conical 62% Dense Vaporizer Conical 50% Dense Vaporizer Average MS results:Conical 80% Dense Vaporizer Conical 62% Dense Vaporizer Conical 50% Dense Vaporizer
Experimental Data (NH4NO3, DEHS, NaNO3) 760 torr
CFD Model 760 torr
Single Particle CE Results
• Vaporizer measurements are LS-QAMS results, whereas the ambient results are LS-TOFAMS – slightly different algorithms for analysis
• Observe same trend with particle size for both ambient and lab results
• Apparent trend with vaporizer density
3000
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dN/d
logD
va
1002 3 4 5 6 7 8 9
1000Dva (nm)
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Deteciton R
atioMCMA2006 T1 Results
Total # particles Prompt Fraction Delayed Fraction Null Fraction Detected Particle Fraction
Conical 80% Dense Vaporizer Conical 62% Dense Vaporizer Conical 50% Dense Vaporizer
Porosity of the Vaporizer
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80% 62% 50%
Density of Vaporizer Tungsten
Factor ~ 1.8 X
50% porous flat vaporizer
The same vaporizer after chemical etching in a potassium hydroxide and potassium ferricyanidesolution for five minutes
Chemical Etching Comparison
80% porous flat vaporizer
The same vaporizer afterbeing blasted by 300 gritalumina oxide – removes machining marks!
Grit-Blasting Comparison
Comparison of CE for Surface Treatments
• Surface etching does not appear to show a significant effect on CE (mass-based, anyway)• Grit blasted appears to show that a 80% dense flat now provides mass-based CE similar to 50% flat surface, but more work needs to be done on this technique
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Murakami's Etched, 50% Conical 50% Conical Grit Blasted, 80% Flat 50% FlatVaporizer Treatment
300 nm (NH4)2SO4 LS counts/CPC counts MS mass/CPC mass MS counts/CPC counts
Vaporizer Design Changes
• Tubing Extension Layouts:– Long Tube
– Short Tube
• Tungsten Mesh:– 0.07mm diameter wires
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Long Tube Extension Short Tube Extension Molyb. Mesh Standard 80% ConicalVaporizer Modification
300 nm (NH4)2SO4 LS counts/CPC counts MS counts/CPC counts MS mass/CPC mass
Comparison of CE for Vaporizer Modifications
• Long tube extension performed poorly• Short tube extension had same CE as unmodified vaporizer• Molybdenum mesh improves mass collection, needs more testing
Liquid Coatings to Reduce Particle Bounce
Sally Ng Eben Cross
Leah WilliamsJenny McInnisTim OnaschJohn Jayne
• Started by Ann and Brendan
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Coating Thickness (nm)
Prompt Delayed Null Prompt+Delayed
DOS coated AS particles
• Coating thicknesses of ~70 nm (radius) required for near 100% CE with respect to particle bounce
273 nm AS core
Dependence on coating substance? - NO
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DOP Coating Thickness (nm)
Prompt particlesCore: Ammonium sulfate
DOP coating (Core = 273 nm) DOP coating (Core = 352 nm)
Oleic acid coating (Core = 300nm)
Core Material Dependence1.0
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Prompt particlesDOP coating
PSL Core: 350 nm AS Core: 352 nm
Dependence on core size? - Yes(DOP coating on AS particles)
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DOP Coating Thickness (nm)
Prompt Delayed Null Prompt+Delayed
Core = 273 nm Core = 352 nm Core = 442 nm
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2000150010005000Core size (nm)
Dependence on core size?(DOP coating: AS particles)
Summary• AMS CE is mainly due to particle incomplete vaporization (i.e. bounce) and lens
transmission effects (impaction, diffusion, and focusing)– Must characterize AMS CE through instrument comparisons for each study!– Combining mass and size measurements provides the most robust AMS CE measurements
• LS module has provided important information on our CE issues and continues to be developed for more wide-spread applications
– Counting efficiency has been dramatically improved– Laser power is being increased for better small particle detection (current limit >200 nm)– May be best in situ measure of AMS CE
• Vaporizer material and designs are continuing to be researched– Tungsten density (80 to 50%) appears to have a factor of ~1.8 X increase in AS particle CE, but
more work needs to be done on consistency of these results and if this works as well for field work
– Grit blasting may show some promise, removing the machining marks (flat surfaces)– Mesh design also shows some promise for total PM measurements, though the PTOF
distributions signals are nearly useless
• Coating of particles with liquid oils shows CE’s ~ 100% are achievable with >70 nm of coatings for >200 nm particles
– Continue this work to develop a potential technique for independently checking CE while in the field (won’t be continuous)