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Improvements in Ambient Nitrogen Dioxide (NO2) Measurementsand the NEW T500U CAPS NO2 Analyser

Enviro Technology Services PlcOctober 2014

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History of NO2 Monitors

Chemiluminescence NO-NOx-NO2 monitors have been the standard technology for AQMS since the mid-1980s.

Although this technology was the only one sensitive enough to meet regulatory requirements, it presented some weaknesses.

Beckman 951 NO-NOx Monitor

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Weaknesses in “Chemi” Technology

• It does not measure NO2 directly. It measures NO and NOx and subtracts the two to get NO2.

• It does not measure NO and NOx simultaneously. That means you can get false positive or false negative NO2 readings.

• It uses a moly converter to convert NO2 into NO. This requires an additional Converter Efficiency (CE) test.

• It uses an ozone generator. This requires a drier, an ozone “cleanser”, and an ozone scrubber.

• It requires two calibrations. One calibration each is required for the NOx and NO channels.

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Weaknesses in “Chemi” Technology

• The ozone generator makes nitric acid. H2O + N2 + O2 + hƲ HNO3. This can cause negative drift.

• If there is ammonia in the sample, you make ammonium nitrate. Ammonium nitrate is a solid that can form in the reaction cell and cause negative drift.

• The reaction cell runs under a vacuum. This means a big, external pump.

• The exhaust of the reaction cell has both ozone and NO2. Both need to be scrubbed.

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“200” Series for Ambient NO2

• T200 Standard Chemiluminescence– LDL: 0.4 ppb; Zero Drift: 0.2 ppb/day; Span Drift: 0.5 ppb/day; 60s

rise/fall– Range: 0 – 50 ppb; 0 – 20,000 ppb– Main Advantage: Cost effective, widely used

• T200U Trace-level Chemiluminescence– LDL: 0.05 ppb; Zero Drift: 0.1 ppb/day; Span Drift: 0.5 ppb/day;

50s rise/fall– Range: 0 – 5 ppb; 0 – 2,000 ppb– Main Advantage: Lower detection limit, less drift, slightly faster

• T200UP Trace, photolytic Chemiluminescence– LDL: 0.1 ppb; Zero Drift: 0.1 ppb/day; Span Drift: 0.5 ppb/day; 50s

rise/fall– Range: 0 – 5 ppb; 0 – 2,000 ppb– Main Advantage: “True NO2” conversion

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TAPI and “Chemi” Technology

• TAPI has addressed each of these weaknesses over many years and has reduced them to the absolute minimum.

• Consequently, the 200 Series of NO-NOx-NO2 analyzers is the most popular in the world.

• Our 200 Series is suitable for almost all of the NO2 monitoring applications.

• But, the user base of chemi-NOx analyzers have been conditioned into spending a lot of time and effort making useful NO2 measurements.

• Those days are over…

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Model T500U• Cavity Attenuated Phase

Shift (CAPS) Technology• Direct Measurement• No Converter• No Ambient Interferences• Very Fast Response• Low Maintenance• Low Power Consumption• T-Series Platform Model T500U CAPS NO2 Analyser

(EQNA-0514-212)

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CAPS Primary Components

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Measurement Principle

cotJ - cotJ0 = (eab + esc) • (c/2pf)

(2pf/c)•(cotJ - cotJ0 ) = eab + esc (Rayleigh Scattering)

c = speed of lightf = modulation frequency (typically ~16 kHz) e = extinction coefficient

ab=absorptionsc=scattering

CotJ0 = (c/2pfL)(1-R) where L is the baselength

450nm light

/DI I0 = (eab + esc) L

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Measurement Principle

J

Absorbing Gas

• Shape of the Curve Not Important - Just Highly Repeatable

• Phase shift measured using highly accurate ‘quadrature’ techniques (picosecond resolution)

• Inexpensive Counters & Software

• Highly Linear and Repeatable

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Pneumatic Diagram

ZERO/SPANVALVE

SAMPLE/CALVALVE

OPTION, ZERO/SPAN VALVES042340000

NC

COMNO

NC

NO COM

CAPs NO2 INSTRUMENT CHASSIS

PUMP

EXHAUST GAS OUTLET

PERMAPUE DRYER059940200

Orifice Dia. 0.007"

SAMPLE GAS IN

NO

NC

COM

AZERO VALVEVA-59

SAMPLE FILTERFL-33

AZERO PATHFL-20 AND FL-3

SAMPPRESS

SAMPTEMP

CAPs OVEN

FL-3

LEDDETECTOR

ZERO

SPAN

PU-98

FLOW RESTRICTORFT-429

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Inside The T500U

A

B

C D

A – Oven Containing Optical Bench (Cavity)B – Auto Reference Assembly Containing Nafion DryerC – Relay PCA And Power supply AssembliesD – 12V DC Internal Pump

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Minimal InterferencesOptical absorbance band of interest in T500U CAPS @ 450nm

Absorption by Total Atmosphere (all gases present)

Courtesy of NOAA

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Minimal Interferences

Negligible concentrations of known atmospheric gases overlap the 450nm (+/- 5nm filter) band used in the T500U CAPS NO2 analyzer.

Iodine Monoxide (shown here), as well as Glyoxal and Methyl Glyoxal have similar absorbance bands, but are not typically present in ambient air and are a much weaker absorber in this range (~20%).

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Maintenance Schedule

• Zero / Span (as needed)• Change particle and AREF in-line filters (one

per year)• Change vacuum pump every (every two years)• Clean or replace mirrors (once every few

years)– Cleaning mirrors takes finesse, but does not require

recalibration– Replacing mirrors is simple, but requires cavity

recalibration with oscilloscope

– A third possible option would be to send out a complete, calibrated bench

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Ranges: 0-5 ppb to 0-1 ppm

Zero Noise: < 20 ppt

Span Noise: < 0.2% of reading + 20 ppt

LDL: 40 ppt

Rise and Fall Time < 30s to 95%

Zero Drift 0.1ppb/24hrs

Span Drift 0.5% of reading/24hrs

Sample Flow: 900 cc/min (+/- 10%)

Size: T-series Chassis with Internal pump

Weight: 33 lbs (15kg)

Power: 80W; 100-250 VAC (50-60Hz)

Bold = better than T200U

Specifications