Dr John Saffell

Technical Director

Alphasense Ltd.

Great Notley, Essex UK

jrs@alphasense.com

SIG 2 COST Action TD1105 EuNetAir

Second Scientific Meeting Queens’ College Cambridge

December 2013

Electrochemical Gas Cells in

Air Quality Networks-

Solving the Problems

Who is Alphasense?

Private UK limited company, 10 minutes from Stansted

airport Started January 1997

65 people: 15 Technical, 40 manufacturing

Significant R&D investment, mostly with UK Universities

Markets: Industrial Safety and Air Quality gas sensors

Sensor technologies: electrochemical, metal oxides,

Optical particle counting, NDIR, PID, MEMS, spectroscopy

We make gas sensors and PM2.5/10 OPCs. That’s all.

Air Quality standards differ in USA and

Europe, but we want to resolve 10-20 ppb

with an error less than 20-50 ppb

USA Europe

Pollutant ppb Period ppb Limit EN standard

Ozone 75 8 hour average 120 Alert threshold EN 14625

NOx /NO2 50 Annual mean 210 Alert threshold EN 1421 1

NOx /NO2 100 1 hour mean 105 1 hr limit value EN 1421 1

SO2 500 3 hour mean 200 Alert threshold EN 1421 2

SO2 140 24 hour mean 140 1 hr limit value EN 1421 2

CO 35 ppm 1 hour average 8 ppm 8 hr limit value EN 14626

µg/m3

µg/m3

PM 10 150 24 hour average 40 Annual mean EN 12341

PM 2.5 15 Annual 25 Annual mean EN 14907

Most difficult targets to measure accurately: NO2 and PM 2.5

Electrochemical amperometric gas sensors Used in the AQMesh pods, MESSAGE and Heathrow projects

B series sensors for fixed site networks

A series sensors for mobile networks

PID for VOCs NO2 CO SO2

O3

NO

28 x 74 mm PCB, 2mA/ 3.6V power

32 mm dia

Low cost

Very low power

now- LM91000 smart chip

Inside a 4- electrode amperometric

electrochemical gas sensor

Working electrode analyte reaction: oxidation or reduction

Counter electrode balances the Working electrode reaction

Reference electrode sets the WE potential for selectivity

Auxiliary electrode corrects the zero current of the WE

Working Electrode(WE)

Reference ElectrodeelectrodeCounter Electrode

T Gas diffusion barrier

ElectrolyteReservoir

Wettingfilters

Auxiliary Electrode

Required sensor performance for

Air Quality networks

Three requirements-

ppb Sensitivity/ Limit of Detection

Long term zero and sensitivity Stability

Selectivity, removing interfering gases

Gas Sensor Limit of Detection (2 sd as the concentration approaches 0 ppb)

LoD (ppb) CO H2S NO NO2 O3 SO2

A series 20 5 80 15 5 15

B series 4 1 15 12 4 5

Mobile A series sensors have higher limit of detection,

but this is being improved with quieter electronics (esp. NO)

NO2-A4

0-200 ppb linearity test (raw data)

8/05/13 CO/NO2/O3 triple A4 ISB : NO2 calibration

250

260

270

280

290

300

310

47000 48000 49000 50000 51000 52000 53000 54000 55000

Time / s

Ou

tpu

t /

mV

0 ppb

20 ppb

40 ppb

60 ppb

80 ppb

100 ppb

120 ppb

140 ppb

160 ppb

180 ppb

200 ppb

Some examples of low level sensitivity H2S-B4

300

350

400

450

500

550

600

650

700

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

t (s)

Ou

tpu

t (m

V)

0 ppb

20 ppb

40 ppb

80 ppb

120 ppb

160 ppb

200 ppb

H2S-A4

240

260

280

300

320

340

360

380

400

420

440

460

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

t (s)

Ou

tpu

t (m

V)

0 ppb

20 ppb

40 ppb

80 ppb

120 ppb

160 ppb

200 ppb

H2S-B4

(for odour detection)

shows 1 ppb noise!

Data before smoothing

H2S-A4 has higher

noise- but still

5 ppb resolution

Data before smoothing

Cambridge laboratory measurements define noise at 1-2 ppb

after smoothing.

• Typical sensor sensitivities/LoD are < 5ppb (< 7mg/m3) for CO, 1-2 ppb (~2-4 mg/m3) for NO and NO2.

• SO2, O3 have comparable performance to NOx.

• Typical sensor t90 ~ 10-20s (determined by diffusion)

Noise characteristics:

Zero baseline stability and correction

Is the zero baseline stable over time? If not, then the calculated absolute

concentration using baseline correction will drift.

The zero baseline also changes with temperature. At low temperatures we

are fine, but above 25°C then the correction algorithm must be good.

How to correct?

• Using knowledge of atmospheric chemistry

• Using statistics and oversampling: used by some users

• Electrochemistry: the underpinning process of the sensors- our method

Electrochemical correction

of Zero current:

use the 4th electrode zero current temperature

dependence is very different

(x10, x100) for each sensor.

Follows Arrhenius

behaviour, so we can model

and predict.

SO2-B4

-10

0

10

20

30

40

50

-30 -20 -10 0 10 20 30 40 50

T (C)

Ou

tpu

t (n

A)

(re

fere

nc

e t

o 2

0°C

)

NO-B4

Working and Auxiliary Electrode

-1000

0

1000

2000

3000

4000

5000

-30 -20 -10 0 10 20 30 40 50

T (C)

Ou

tpu

t (n

A)

(re

fere

nc

e t

o 2

0°C

)

O3-B4

-100

0

100

200

300

400

500

-30 -20 -10 0 10 20 30 40 50

T (C)

Ou

tpu

t (n

A) (

refe

ren

ce to

20°

C)

Red: working electrode

Blue: auxiliary electrode

500nA FS

5000 nA FS

(-20 to+50C)

50 nA FS

O3-B4

NO-B4

SO2-B4

NO2-B4 zero current shows

reasonable stability over 500 days

±12 nA (equivalent to ±20 ppb)

-30

-20

-10

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350 400 450 500

Time (days)

Ze

ro c

urr

en

t (n

A)

Stability to 100 ppb ozone

Response to 100ppb O3

First test 30/07/2012

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

400

0 50 100 150 200 250 300 350 400 450 500

Time (days)

Se

ns

itiv

ity

(n

A/p

pm

)

O3-B4

SO2-B4

44

NO-B4

NO2-B4

NO sensor does not respond- good

O3 sensor is most sensitive, but loses sensitivity, then stabilises

SO2 sensor is the most stable sensor for O3

Stability to 350 ppb NO2

First test 30/07/2012

-1000

-800

-600

-400

-200

0

200

0 50 100 150 200 250 300 350 400 450 500

Time (days)

Se

ns

itiv

ity

(n

A/p

pm

)

NO-B4, CO-B4

NO2-B4

SO2-B4

O3-B4

Better stability after first 100 days, BUT: You need to stabilise in

the environment the sensors will be operating

Annual rh changes

NO2 and ozone

Difficult to separate: both are powerful oxidants

Separation Methods:

1 use multiple sensors: you cannot measure two

parameters with one sensor. We recommend 3 sensors.

2 Ensure the x-sensitivites are stable and use the small

ratio differences to deconvolve.

3 Use chemical filters to remove, for example the ozone

from NO2 sensor- so one summed sensor, one speciated

sensor.

Conclusions

• Low power, low cost electrochemical sensors can measure low ppb

concentrations of inorganic gases.

• Wireless air quality networks are not replacing AURNs, but are adding the

extra data for research, filling in fine detail, locating emission sources and

enticing the citizen to get involved.

• Our work is not done: better speciation, improving stability and modelling

sensors (both transport and chemistry/ electrochemistry) are our

continuing work.

• VOCs and PM are necessary partners in any complete air quality network.

Acknowledging our Air Quality network of

friends and colleagues • Wah On Ho, Mark Giles, Ronan Baron Alphasense Ltd.

• Rod Jones, Iq Mead, Vivien Bright Cambridge University Chemistry

• Michael Hitchman Innovative Coatings Ltd.

• Joe Stetter SPEC Sensors and KWJ Engineering Inc.

• Jeff Neasham Envirowatch Ltd. and Newcastle University

• Roger Riley, Stephen Earp, Dean Kavanaugh Geotech Instruments Ltd.

• Frank Dean, Mark Stockdale Ion Science Ltd.

• Nicolae Barsan Universitat Tubingen

• Paul Kaye University of Hertfordshire

• Paul Williams University of Manchester

• Mike Jerrett School of Public Health UC Berkeley

• Rex Britter, Rich Fletcher MIT Media Lab

• Jane Hodgkinson, Ralph Tatam Engineering, University of Cranfield

• Michele Penza ENEA

• CITISense FP7 NILU

• Julian Gardner, James Covington Dept Engineering, Warwick University

• Florin Udrea, Bill Milne Cambridge CMOS Sensors

Thank you and have a

Merry

Christmas

HM Queen Elizabeth II,

patroness of Queens’ College

Any questions?