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)

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base

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NH

4 NO

3 C

E ba

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on (N

H4 )

2 SO

4 G

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data

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AM

S W

ater Mass Fraction

SOLID

LIQUID

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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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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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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

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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)