AMS Collection Efficiency Summary

Aerodyne: Tim Onasch, Leah Williams, Sally Ng, Jesse Kroll, John Jayne, AchimTrimborn, Joel Kimmel, Donna Sueper, Doug Worsnop (Jenny McInnis, Erric

Beecher)

Boston College: Eben Cross, Paul Davidovits

NOAA: Brendan Matthew, Ann Middlebrook, Tim Bates, Trish Quinn

U. Manchester: James Allan

U. Colorado: Alex Huffman, Jose Jimenez

Environment Canada: Peter Liu

PNNL: Liz Alexander

…and many more AMS users!

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 particles 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

EL(dva) = Collection Efficiency for spherical particlesEs(dva) = Collection Efficiency for irregularly shaped particlesEb(dva) = Collection Efficiency due to particle bounce

Beam characterization and quantification:• Lens alignment• Spot pictures• Particle beam width probe• Light scattering module• Particle CE experiments

Lens Alignment

Spot Pictures

Particle Beam Width Probe

Huffman et al., 2005

Nearly all sampled aerosols strike our 3.8 mm vaporizer

Light Scattering Module

• LS – shows that particles that pass through the lens into the AMS can bounce off the vaporizer without vaporizing

NH4, H2SO4, HNO3 Phase Diagram

SOLID+

LIQUID

Ammonium Sulfate

Sulfuric acid

LIQUID SOLID

Matthews et al.

LIQUIDAmmonium

Nitrate

Particle CE Experiments

• Particle phase appears to control CE

AMS CE

• Ambient CE’s range from 45-100%– Factor of ~2

• Laboratory CE’s range from 10-100%– Factor of >5!

Eb(dva) ≥ EL(dva) >> Es(dva)

Research tasks0. Vaporizer Design:

– Modified vaporizer designs to increase quantification– Optimize trade-off between quantification and size-resolved information

1. Lens Development: – PM2.0 Lens developed and proven, but problems building consistently– Lino Gonzolez (ARI) is continuing our modeling efforts of the lens design to

improving both small and large particle transmissions2. AMS CE measurements:

– Investigations of particle bounce for mixed phase (complex) laboratory particles and as a function of particle size

– Particle manipulation through condensational growth3. Light Scattering Probe Development:

– Completed second generation LS module hardware with great success!– Work is continuing on LS data analysis software with goal of a ready to use

software being released this year

Porosity of the Vaporizer

1.0

0.8

0.6

0.4

0.2

0.0

Det

ectio

n R

atio

80% 62% 50%

Density of Vaporizer Tungsten

Factor ~ 1.8 X

• Significant work for little pay-off

1.0

0.8

0.6

0.4

0.2

0.0

Fra

ctio

n

806040200

Coating Thickness (nm)

Prompt Delayed Null Prompt+Delayed

• Coating thicknesses of ~70 nm (radius) required for near 100% CE with respect to particle bounce

273 nm AS core

Particle Coating Experiments

DOS coated AS particles

Coating Material Dependence? - No

1.0

0.8

0.6

0.4

0.2

0.0

Frac

tion

120100806040200

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 Dependence? - No

1.0

0.8

0.6

0.4

0.2

0.0

Frac

tion

100806040200DOP coating thickness (nm)

Prompt particlesDOP coating

PSL Core: 350 nm AS Core: 352 nm

Core Size Dependence? - Yes(DOP coating on AS particles)

1.0

0.8

0.6

0.4

0.2

0.0

Frac

tion

806040200

DOP Coating Thickness (nm)

Prompt Delayed Null Prompt+Delayed

Core = 273 nm Core = 352 nm Core = 442 nm

200

150

100

50

0

Coa

ting

thic

knes

s (n

m)

2000150010005000Core size (nm)

PM2.5 particles

Large liquid droplets

80x10-3

604020

0

Int.

Sig.

6x10-3420TOF (s)

25

0Tim

e H

ist

150Signal Hist

80x10-3

604020

0

Int.

Sig.

6x10-3420TOF (s)

100Ti

me

His

t

100Signal Hist

307 threshhold crossings 199 chopper cycles

•Liquid oil droplets “boil-off” at larger sizes and high temperatures (lowered temp. and did not observe)•Add mass for single chopper cycle and recover single particle mass (low number densities)•Shows that particles strike oven at least twice.

Vaporizer

500 nm DEHS particles

• Particle phase is important– Solid particles exhibiting CE’s 10-30% – Liquid droplets exhibiting CE’s ~100%

• Mixed phase particles have CE’s between solid and liquid droplets (CE ~ 10-100%)– Suggests that ambient particles are mixed phase most of

the time and/or likely not solid

• Particle size is important– Larger particles exhibit more bounce

Particle Coating Experiments

Light Scattering Module

Cross et al., ACP 2009

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

Single Particle CE Results 1

1.0

0.8

0.6

0.4

0.2

0.0

Num

ber F

ract

ion

00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00

Time of Day

Null Fraction Delayed Fraction Prompt Fraction

• diurnal trend (chemical composition related?)

• LS-SP CE trend vary by 20-40%

Single Particle CE Results 2

• size-dependent trend

• LS-SP CE trends vary by 20-40%

• LS-SP CE: Prompt 23%, Delayed 26%, Null 51%

PTOF Mass Distribution Results

• Particle bounce ‘spreads out’ PTOF mass to larger sizes (PTOF times)

• LS-SP accounts for between 85-100% of PTOF signal for detectable range

1.0

0.8

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0.0

PTO

F/M

S R

atio

00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00Time of Day

Org SO4 NO3

Integrated PTOF vs MS DIFF Signal

• PTOF sampling time << MS DIFF (OPEN/CLOSED)

• PTOF accounts for between 60-85% of MS Signal

MS DIFF Signals to Other Measurements

• AMS MS DIFF accounts for ~50% of ambient NR-PM1

The Numbers• LS-SP CE: Prompt 23%, Delayed 26%, Null 51%• LS-SP 85-100% of PTOF• PTOF 60-85% of MS DIFF• MS DIFF 50% of NR-PM1

• Infer LS-SP detects 40-50% of (detectable) NR-PM1, with Null particle vaporization events accounting for the majority of missing signal

• If a single CE value can be used for all AMS measured chemical species, then Null ~ Prompt particle chemistry

CE with LS-AMS, PILS, DMPS

• LS-SP CE (prompt+delayed/total) accounts for Sulfate phase-dependent CE (AMS/PILS SO4)

AMS vs DMPS and PILS

• Sulfate phase-dependent CE and Aerodynamic Lens TE accounts for both AMS/PILS SO4 comparison and AMS/DMPS PM1 mass comparison

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!

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

• Lens developements– Continue modeling of large and small particle lens designs– PM2 - Difficulty building consistent lenses, but soon!

• Vaporizer material and designs have been researched– Significant work for little pay-off

• 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 understand AMS CE better– Develop a potential technique for independently checking CE while in the field (won’t be

continuous)

• 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)– Analysis software in works and will be released this coming year– Best in situ measure of AMS CE

Zhang et al., 2006

Inorganic Fraction Composition Varies