Calibration of Fast Response Differential Mobility Spectrometers

Jon Symonds

Cambustion Ltd, Cambridge, UK

Contents

• Introduction to fast response Differential Mobility Spectrometers (with reference to Cambustion DMS series)

• Data Processing and Data Inversion

• Size and Number Calibration of the Charging & Classification System

• Morphological Effects

• Calibration for Mass Measurement

• Sampling and Dilution Systems

• Traceability and Uncertainty

The Need for Fast Response•Aerosols can change rapidly, SMPS scan can take 2 minutes.•Fast response electrical mobility analysers:

� Electrical Aerosol Spectrometer, Tartu University / Airel Ltd� Cambustion DMS500� TSI EEPS� TSI FMPS� Cambustion DMS50

• DMS Series and EEPS especially aimed at measuring engine exhaust aerosols� Adoption of such systems by automotive researchers ⇒ “order of magnitude accuracy” no longer good enough

• But… all such instruments currently compromise on sensitivity and spectral resolution over SMPS systems

This paper uses the DMS series an example. Both these instruments are available with integrated sampling and dilution systems, so this paper considers the ‘whole picture’ of calibration.

DMS Series Principle of Operation

•Unipolar diffusion charger•Electrometer detection•Sizing by charge : drag ratio — electrical mobility•Similar principle applies to TSI EEPS and FMPS•DMS500: 10 Hz data, 200 ms time response, 5 nm to 1 µm or 2.5 µm •DMS50: 10 Hz data, 500 ms time response, 5 nm to 560 nm, 12 V operable

‘Fast Response Classification of Fine Aerosols with a Differential Mobility Spectrometer’; Reavell, K. Proc. AGM Aerosol Soc. UK. 2002

DMS Data Inversion

22 Electrometer Currents

Charging Model

Classifier Model

Transfer Function

Empirical Calibration

Calibrated Transfer Function

Least Squares Minimisation with

Smoothing

34/38/45 Channel Discrete Spectrum

Bayesian Algorithm

Multi-Lognormal Parameterisation

Measured Noise Base

Mass

Engine Air Flow

Particle Charging

• Unipolar Diffusion Charger

• Particles gain net, multiple, positive charge from corona discharge

• Bigger particles less mechanically mobile, but gain more charge

• Eventually, large particles become as electrically mobile as small particles: Mobility Inversion

• Inversion point moved to larger sizes by dropping pressure

• Cyclone important!

Modelled response to 100 nm NaCl particle with 1,2, 3 charges entering the classifier

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Detector #

Rel

ativ

e re

spon

se (a

.u.)

1 charge

2 charges

3 charges

Electrical Mobility of Particles in DMS500

10 100 1000 10000

Size (nm)

Ele

ctric

al M

obility

(a.

u.)

1 atm pressure

1/4 atm pressure

Instrument Transfer Function

Typical Adjusted Transfer Function

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10111213141516171819202122

1 10 100 1000 10000

Size (nm)

Det

ectio

n R

ing

Initially generated from Monte Carlo simulation. For a random particle of a given size:

•Random charge state is selected from a calculated probability distribution for that sized particle

•Entry point to the classifier randomly selected, and particle’s trajectory calculated to predict the landing detection ring and measured current.

•Repeated across all sizes for many particles →

•Empirically adjusted for every instrument during test using a linear transform ↓

Modelled Transfer Function

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10111213141516171819202122

1 10 100 1000 10000

Size (nm)

Det

ectio

n R

ing

Em

pirical Data

Size Calibration: PSL• Duke Scientific (now Thermo Scientific) Polystyrene Latex Spheres

• NIST Traceable, traceability provided by microscopy

• Large surfactant / impurity mode makes unsuitable for smaller sizes:

• Lognormal parameterisation used for ease of analysis & improved apparent spectral resolution

299nm PSL Spheres - Standard DMS500 Spectral Output

0.00E+00

2.00E+05

4.00E+05

6.00E+05

8.00E+05

1.00E+06

1.20E+06

1.00 10.0 100 1000

Dp (nm)

Standard

Lognormal

dN/dlogDp /cc

SURFACTANT PEAK PSL

PEAK

Regulator HEPA Collison Nebuliser Neutraliser

Diffusion Dryer

HEPA

DMS

spill

PSL suspension

air

Gain Calibration

Comparison of DMS500 and 3022 CPC

0.00E+00

5.00E+03

1.00E+04

1.50E+04

2.00E+04

2.50E+04

3.00E+04

3.50E+04

4.00E+04

4.50E+04

5.00E+04

0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00

Time / s

N/c

c

DMS

CPC

CPC PHOTOMETRIC MODE

CPC COUNT MODE

CPC not a primary standard in “photometric mode” at higher concentrations required by electrometer based fast response instruments.

Hence go back to methodology based on that recommended for CPC calibration; the use of a standard electrometer: Liu and Pui (1974). Useful primary standard if can be ensured that each particle is singly charged….

DMA / Electrometer-Based Calibration Setup

0.00E+00

5.00E+05

1.00E+06

1.50E+06

2.00E+06

2.50E+06

1.00 10.0 100 1000

Dp (nm)

dN/dlogDp /cc Ensure "cut" is made to RHS of spectrum to reduce the chances of larger, multiply charged particles passing the DMA. Weak solution used to reduce mean size of broadband aerosol. Tandem DMA used for very broad initial distributions (e.g. soot)

Cannot ensure single charge for largest particles, therefore rely on extrapolation of model for > 300 nm

Regulator HEPACollison

Nebuliser

Neutraliser (bipolar charger)

Diffusion

Dryer

HEPA

spillH2SO4(aq) or NaCl (aq)

air

Nucleation

ApparatusRegulator HEPA

3081 DMA

NeutraliserDMS

spill in

Filter in Faraday

cageMFCPump

Electrometer

Path for H2SO4

Dummy

Neutraliser

+1e

Regulator HEPACollison

Nebuliser

Neutraliser (bipolar charger)

Diffusion

Dryer

HEPA

spillH2SO4(aq) or NaCl (aq)

air

Nucleation

ApparatusRegulator HEPA

3081 DMA

NeutraliserDMS

spill in

Filter in Faraday

cageMFCPump

Electrometer

Path for H2SO4

Dummy

Neutraliser

+1e

0.00E+00

2.00E+04

4.00E+04

6.00E+04

8.00E+04

1.00E+05

1.20E+05

1.40E+05

1.60E+05

10 100 1000

Dp (nm)

Discrete

Lognormal

dN/dlogDp /cc

Lognormal narrower, ~same areas

Aerosol Sources

“Spherical Calibration” – GDI and Nucleation Mode

Soot Calibration – Diesel Accumulation Mode �Soot from Propane Flame (mini-Cast), 50 – 300 nm (later…)

300 – 1000 (or 2500) nm

50 – 300 nm

5 – 50 nm

Size Range

Polystyrene Latex Spheres (PSL)

Sodium Chloride

Sulphuric Acid

Aerosol

Nebuliser

Nebuliser

Nebuliser & Nucleation Rig (next slide)

Generation Method

No

Yes

Yes

Measure Gain?

Nucleation Source (H2SO4)

F – rotameter T – thermocouple

Size Spectral Density

0.00E+00

5.00E+06

1.00E+07

1.50E+07

2.00E+07

2.50E+07

1 10 100 1000

Dp (nm)

dN/dlogDp /cc

Acid

Soln.

In Nebuliser

Diffusion Dryer Heated Tube

HEPA

FF

Re-nucleation tube

HEPAHEPA

Residence Tube

Air

SecondaryDilution

Spill

T

T

Effect of Morphology (1)

What size is this?

Vehicle (steady state)

Feedgas

DPF

DMS Sample Head DMS 500

HR diluter OFF

4:1 Dilution Air

Heated Line @ 1 Atm

3080 DMA

Flow Meter

HEPA Filter

1 slpm

7 slpm

8 slpm

Vehicle (steady state)

Feedgas

DPF

DMS Sample Head DMS 500

HR diluter OFF

4:1 Dilution Air

Heated Line @ 1 Atm

3080 DMA

Flow Meter

HEPA Filter

1 slpm

7 slpm

8 slpm

Test apparatus

•DMS originally calibrated with spherical particles

•Compare DMA (mobility) sizing with DMS (electrical mobility) sizing for Diesel Agglomerates

Exhaust

Effect of Morphology (2)

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200 250 300 350

DMA Mean Diameter

DM

S M

ean

Dia

met

er

70 kph 4th Gear70 kph 5th Gear

Effect of Morphology (3)

DMS500 Mean Particle Charge

0

5

10

15

20

25

30

35

0 50 100 150 200 250 300 350 400 450 500

mobility diameter (nm)

mea

n ch

arge

NaCl / ejector pump

DEHS / peristaltic pump

Diesel, 4th Gear 70kph

Diesel, 5th Gear 70kph

Effect of Morphology (4)

•Differences observed with DMA cut soot under a “spherical calibration”:

•Solution is to empirically calibrate with soot for use with Diesel emissions. Only calibrate accumulation mode of lognormal “fit” with soot; use this output for “solid particle number”.

•Multiple charging / DMA size range / source particle size range makes this only workable up to ~ 300 nm (sufficient for most engine work)

-50.00%

0.00%

50.00%

100.00%

150.00%

200.00%

250.00%

10 60 110 160 210 260 310

Dp / nm

% d

iffer

eren

ce

Gain Difference (versus electrometer)

Size Difference (versus DMA)

Results after “soot calibration”

0.0E+00

5.0E+05

1.0E+06

1.5E+06

2.0E+06

2.5E+06

3.0E+06

3.5E+06

4.0E+06

4.5E+06

Trans

ients Idle

Trans

ients

Cold S

tart

War

m up

Fast I

dle

Fast I

dle40

00 rp

mTra

nsien

ts

"MOT"

N/c

c/s

Ave Acc N/cc CPC + VPR

Ave DMS Spherical Calibration

Ave Acc N/cc DMS Agglomerate Cal

mean DMS accumulation ~ CPC +9%mean DMS spherical cal ~ CPC +44%

Comparison of DMS with PMP system with Diesel soot; with and without “soot” calibration.

However , correlation for Gasoline Direct Injection (GDI) works best for original “spherical” calibration

⇒⇒⇒⇒ Need separate calibrations for GDI and Diesel soot in instruments with corona chargers.

0.E+00

1.E+11

2.E+11

3.E+11

4.E+11

5.E+11

6.E+11

0 20 40 60 80 100 120 140 160 180 200

Time (s)

Par

ticle

Num

ber/

Sec

ond

Particle Number from DMS500

Particle Number from PMP

Calibration Artefacts: Multiple charging�For a given size, each possible charge state produces a mobility response on the rings; data inversion deconvolutes this to give particle size spectrum

�Response to monodisperse (DMA “cut”) salt aerosol below:

�Inversion problem hardest in 20 – 50 nm region, beyond that effect is blurred by mobilities becoming closer together with increasing mean charge.

� For work requiring high accuracy in this region with narrow aerosols (e.g. gas turbine studies), need careful “micro-calibration” at many sizes in this region to avoid multiple peaks transferring to spectrum

Size Spectral Density

0.00E+00

5.00E+04

1.00E+05

1.50E+05

2.00E+05

2.50E+05

1 10 100 1000

Dp (nm)

dN/dlogDp /cc

Inversion

DMS Response to 30 nm aerosol

0.0E+00

5.0E+01

1.0E+02

1.5E+02

2.0E+02

2.5E+02

5 6 7 8 9 10 11 12 13 14

Detection Electrometer #

Cur

rent

/ fA

+2 e charges+1 e charge

Relating Size to Mass: The CPMA

Classifies by Charge:Mass ratio, as DMA does for Charge:Drag ratio. Opposing electrical and centrifugal forcesDevelopment of APM (K. Ehara et al.), but with inner and outer electrode rotating at different speeds, to create a stablefield and higher throughput of particles (Reavell & Rushton)

Measuring GDI Particle Mass (1)

1. Start with standard PSL particles of known size & density to calibrate system

2. Select size of particles with DMA (“size band-pass filter”)

3. All particles leaving DMA are charged

4. Measure number of particles (Condensation Particle Counter)

5. Select mass of particles with CPMA (“mass band-pass filter”)

6. Measure number of particles leaving CPMA (with 2nd CPC)

7. Ratio CPC readings to get penetration whilst varying CPMA voltage → transfer function

8. Peak voltage gives peak mass. System is now therefore calibrated with PSL

9. Repeat at the same size points with engine exhaust, peak in transfer function gives particle mass at that size.

97 nm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

-60 -40 -20 0 20

Voltage / V

Penetration

N

Modelled TF

( )1222 /ln rrr

neVm

cc

cc ω

=

Measuring GDI Particle Mass (2)

• Plot mass versus DMA cut size

• Gradient on log-log plot gives “fractal density factor” relating diameter to mass (would be 3 for spherical particles), Df = 2.65 for these GDI particles

• Particles get less dense as they get bigger, due to “open structure”• Equivalent measurements show Df = 2.3 for Diesel

• Therefore GDI particles’ structure less “open” than Diesel

• Probably due to “infill” by volatile material

0

100

200

300

400

500

600

700

800

900

0 50 100 150 200 250

Particle diameter (nm)

Effe

ctiv

e de

nsity

(kg

m-3)M = 1.72 × 10-24 • D p

2.65

1E-20

1E-19

1E-18

1E-17

10 100 1000

Particle Diameter (nm)

Par

ticle

Mas

s (k

g)

10-20

10-19

10-18

10-17

Density of particles emitted from a gasoline direct injection engine, J. Symonds, P. Price, P. Williams and R. Stone, ETH Conference on Nanoparticles, 2008

Mass Calibration: Diesel Particles

0.001

0.01

0.1

1

0.001 0.01 0.1 1

Tunnel Mass Conc by Filter or Exhaust Mass Conc by DPF / µµµµg / cc

Tun

nel o

r E

xhau

st M

ass

Con

c by

DM

S /

µµ µµg

/cc

Production vehicle DPF weights

CVS filter paper measurements

Dyno engine: prototype cal DPF weights

DMS500 Sampling & Dilution System

Required for direct engine exhaust sampling:

•Primary dilution stops condensation

•Secondary dilution reduces required cleaning

Primary Dilution

1000nm Cyclone

Raw Sample Flow

Dilution FlowAnnular type primary diluter

Dilute Sample Flow

Critical flow restrictor: 4:1 pressure drop

Raw Sample Flow (RSF) = Dilute Sample flow (DSF) – Dilution Flow (DF)

Dilution Factor (DF) = DSF/RSF = DSF/(DSF – DF)

∴ Need accurate co-calibration (in series) of both flow meters: @ DF = 5, 2% difference in flow ⇒ 8% gain error

Sample Line Losses

Sample Line Losses Fit “Turbulent Model” given in Hinds even for laminar flow!

Sample Line Efficiency: Diesel Exhaust Particles (R e = 1600, P = 1 bar, T = 383 K, ∆∆∆∆ l = 4 m, i.d. = 4.7 mm)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

10 100Particle diameter nm

Pe

netr

atio

n

ExperimentalTurbulentLaminar

Effect of Sample Line Losses on Typical Diesel Spec trum

0.00E+00

1.00E+07

2.00E+07

3.00E+07

4.00E+07

5.00E+07

6.00E+07

7.00E+07

1.00 10.0 100 1000

Dp (nm)

No Losses

2.5 m sample line

8 m sample line

dN/dlogDp /cc

Symonds, Olfert & Reavell, 2007

Kumar, Fennell, Symonds & Britter, 2008

Rotating Disc Diluter Calibration•Losses thought to be mainly particle diffusion to disc pocket walls:

� Worse for small particles

� Worse for higher dilution ratios (disc slower → more time to diffuse)

•Technically possible to apply size dependent correction, but would be impractical for every instrument.

•Broadband NaCl aerosol used to calibrate and check, size similar to engine soot…

Cambustion Rotating Disc Diluter Calibration

y = 0.9215x1.0488

R2 = 0.9978

10

100

1000

10 100 1000

Theoretical DF

Act

ual D

F (

75 n

m N

aCl)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 20 40 60 80 100 120

Size / nm

Effi

cien

cy

Efficiency of DMS diluter at DF=100

New R83 mean correction efficiency, applied to DMSdiluterOriginal absolute min efficiency proposal for Reg 83

+5%

-30%

+5%

-20%

New R83 relative diluter acceptability limits, applied to DMS diluter

Traceability• Size (PSL)

� Electron Microscopy � Length scale (possibly other nanospheres…)

• Size (DMA)� Physical characteristics of DMA, but ultimately final check from PSL sizing (as in ISO15900:2009)

� Electron Microscopy• Length scale

• Number� Electrometer

� Known current source (e.g. Keithley 5156)• Known voltage source

� Josephson Junction Standard (relates frequency to voltage)- Caesium Standard

• Known resistance� Quantum Hall Effect Standard

� Mass flow meter� Piston Prover

• Length scale• Clock

�Caesium standard• Pressure / temperature standards

• Primary dilution traceable to mass flow meters• Secondary dilution calibration only requires instrument to be linear in gain

Uncertainty Estimates

Size from DMA / PSL ~ Coefficient of variance of 5%, 95% CI = ±10% (assume ‘2k’)

Gain (no dilution) ~ CoV of 10%, 95% CI = ±20%

Gain (secondary dilution) ~ CoV of 10% (classifier) + CV of 10% (diluter) ~ CoV of 14% (assume independent), or 95% CI = ± 28%

Gain (primary & secondary dilution) ~ CoV of 10% (classifier) + CoV of 10% (2nd diluter) + CoV of 8% (1st diluter, assuming 2% error in flows) ~ CoV 16%, or 95% CI = 32%

+ sample line losses…

In practice, generally much better agreement with “PMP” systems for particle number concentrations than this is achieved. Of course, these systems are also subject to their own uncertainties…

Mass Uncertainty = size CoV of 5% × 3 (each dimension is not independent) ~ 15%, + gain CoV of 10%, so CoV is at least 18% (95% CI ~ 36%) even with no dilution.

Summary of DMS Series Calibration

•Size Calibration:�Ultimately traceable to PSL spheres

� Directly� via DMA using NaCl, H2SO4 or Soot aerosols

•Number Calibration�H2SO4 or NaCl or Soot particles charged and size selected with DMA

�Concentration measured with electrometer and mass flow meter

•Morphological Effects�Need soot calibration for Diesel engines, up to 40% error in number concentration if not used

•Dilution and Sampling Systems�Effect on any measurement system can be significant & sobering…..

Acknowledgements‘Team DMS R&D’:Kingsley Reavell, Chris Nickolaus, Tim Hands, Nick Collings, Mark Rushton, Andy Livesey, Andrew Ellison & James Burrell (Cambustion)

‘Team DMS Calibration’:Justin Hunt & Joe Evans (Cambustion)

CPMA Research: Jason Olfert (Universities of Cambridge & Alberta)

CPMA Data from GDI engine: Philip Price, Richard Stone (Oxford University), Paul Williams (Manchester University)

Sample Line Losses: Prashant Kumar (University of Surrey), Paul Fennell (Imperial College), Rex Britter (University of Cambridge), Jason Olfert

CambustionJ6 The Paddocks

347 Cherry Hinton RoadCambridgeCB1 8DH

United Kingdomwww.cambustion.comjps@cambustion.com

+44 1223 210250