NO2 Measurements with Different ConvertersMolybdenum Converters vs. Specific NO2 Converters

Robert Gehrig; Christoph Hüglin, Beat SchwarzenbachEMPA Dübendorf SwitzerlandAir Pollution / Environmental Technology

Materials Science & Technology

Outline

NO2 measurement techniquesRequirements of EN 14‘211Results of parallel measurementsConclusions

Materials Science & Technology

NO2 Measurement TechniquesStandard technique: Measurement of NO with chemiluminescence

NO2 is reduced to NO by a converter

Heated metal converters, mostly molybdenum (Mo)+ efficient+ long lifetime+ reliable- not specific

NOx = NO + a·NO2 + b·HNO3+ c·NO3-+ d·PAN + e·NH3 + …

Alternative: photolytic converters

Materials Science & Technology

Principle: NO2 is reduced photolytically to NONO2 + hν NO + O hν: < 410nm O + O2 O3

NO + O3 NO2 + O2

NO2 + O2 + hν NO + O3

Light source: Xe-vapour lamp, metal halid lampe of Array of UV-LEDSpecific, but low converter efficiencyConverter efficiency needs to be calibrated at regular intervals

Photolytic converter

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„Blue Light Converter“ BLC

Cell volume: 17mlCell purge time at 1 l/min: 1sArray von UV-LED‘s λ ca. 380 - 410nmAir Quality Design, Inc., Wheatridge, CO, (USA)(formerly: Droplet Measurement Technologies, Inc., Boulder)Sale in Europa: meteorologie consult gmbh, D-Königstein (www.metcon.de)

Materials Science & Technology

NOx

Set-up of NOx-monitor with BLCMolybdenum converter replaced or bypassed, flow-controllcapillary after converter

Example: Thermo Electron Model 42C TL

R.C. PMT

Vor-kammer

Mo-Konverter

Ozonator

Pumpe

Sample

Filter

Trockner

NO2-Konverter

BLC

NO

Materials Science & Technology

BLC integrated in NOx monitor

Thermo Electron Model 42C TL

Thermo Electron Model 42i TL

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EN 14‘211; NOx measurements

ChemiluminescenceNo converter type specifiedConverter efficiency > 95%Interferences to other N-compounds: Only specified for NH3 (≤ 5,0 nmol/mol)

How to deal with specific measurements?

Materials Science & Technology

Sites with NO2 measurements with photolytic converters:

Specific NO2 measurements in NABEL

System Site Site type Altitude Measurement periods

Jungfraujoch (JUN) high alpine 3580 m from Sep 1991

Rigi-Seebodenalp (RIG) rural 1030 m Oct 2001 to Feb 2006Cranox1)

Rigi-Seebodenalp (RIG) rural 1030 m from Nov 2004

Payerne (PAY) rural 490 m from Feb 2006BLC

Zürich (ZUE) urban 410 m from Jan 2010

1) Cranox: Chemiluminescence monitor (CLD770AL ppt resp. CLD 89p) with photolytic converter(PLC 760 with Xe-vapour lamp or PLC 762 mit metal halid lamp), Eco Physics.

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Comparison Cranox – BLCNO2

NO

Daily means at Rigi-Seebodenalp

Daily means at Rigi-Seebodenalp

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NO2 (photolytic) at different sites

Materials Science & Technology

NO2 comparison

0

2

4

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1.05

5.05

9.05

1.06

5.06

9.06

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5.07

9.07

1.08

5.08

9.08

1.09

5.09

9.09

1.10

5.10

9.10

NO

2 (pp

b)

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1.05

5.05

9.05

1.06

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5.09

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5.10

9.10

NO2 (MC)NO2 (PC)Diff NO2

Payerne Rigi

Materials Science & Technology

0

1

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1.05

5.05

9.05

1.06

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5.07

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1.08

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9.09

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5.10

9.10

ppb

ZUEPAYRIG

ΔNO2 at different sites

daily means

0

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01.0

1.20

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01.0

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01.0

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01.0

6.20

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01.0

7.20

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01.0

8.20

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01.0

9.20

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01.1

0.20

10

ppb ZUE

PAYRIG

Materials Science & Technology

Mean values (Jan – Oct 2010)

SiteNO2

photolyticNO2

(Mo)ΔNO2

ΔNO2 in% of NO2 (Mo)

Rigi-Seebodenalp 2.2 ppb 4.1 ppb 1.9 ppb 45 %

Payerne 5.5 ppb 7.4 ppb 1.8 ppb 25 %

Zürich 14.7 ppb 16.4 ppb 1.7 ppb 10 %

Jungfraujoch 0.23 ppb 0.84 ppb 1) 0.61 ppb 1) 73 %1) Jungfaujoch: NO2 (Mo) is NOy – NO; ΔNO2 is NOy-NO-NO2; means 2005-2009

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ΔNO2 vs. Nitrate + HNO3Rigi- Seebodenalp

Payerne

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Conclusions

NO2 measurements with molybdenumconverters overestimate NO2 by 10-70%.ΔNO2 consists mostly of HNO3 and NO3

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Photolytic converters and new spectroscopicmethods allow for specific NO2 measurements.Choice of method depends on the aim of themeasurements.Clear specification of methods is needed for NO2data series.