1. Improvements in Ambient
Nitrogen Dioxide (NO2) Measurements
and the NEW T500U CAPS NO2 Analyser
Enviro Technology Services Plc
October 2014
www.et.co.uk

2. History of NO2 Monitors
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Chemiluminescence NO-NO
x-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

3. 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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4. 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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5. “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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6. 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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7. 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
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Model T500U CAPS NO2 Analyser
(EQNA-0514-212)

8. CAPS Primary Components
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9. Measurement Principle
cotJ - cotJ0 = (eab + esc) • (c/2pf)
(2pf/c)•(cotJ - cotJ0 ) = eab + esc (Rayleigh Scattering)
c = speed of light
f = modulation frequency (typically ~16 kHz)
e = extinction coefficient
ab=absorption
sc=scattering
CotJ0 = (c/2pfL)(1-R) where L is the baselength
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450nm light
DI/I0 = (eab + esc) L

10. Measurement Principle
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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

11. Pneumatic Diagram
SAMPLE/CAL
NO COM
NC
ZERO/SPAN
VALVE
VALVE
NO COM
NC
OPTION, ZERO/SPAN VALVES
042340000
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CAPs NO2 INSTRUMENT CHASSIS
PUMP
EXHAUST
GAS OUTLET
PERMAPUE DRYER
059940200
Orifice Dia.
0.007"
SAMPLE GAS
IN
NO
COM
NC
AZERO VALVE
VA-59
SAMPLE FILTER
FL-33
AZERO PATH
FL-20 AND FL-3
SAMP
PRESS
SAMP
TEMP
CAPs OVEN
FL-3
DETECTOR LED
ZERO
SPAN
PU-98
FLOW RESTRICTOR
FT-429

12. Inside The T500U
A
B
C D
A – Oven Containing Optical Bench (Cavity)
B – Auto Reference Assembly Containing Nafion Dryer
C – Relay PCA And Power supply Assemblies
D – 12V DC Internal Pump
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13. Minimal Interferences
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Optical absorbance band of
interest in T500U CAPS @
450nm
Absorption by Total
Atmosphere (all gases
present)
Courtesy of NOAA

14. Minimal Interferences
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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%).

15. 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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16. 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)
www.et.co.uk
Bold = better
than T200U
Specifications