Transcript Lecture 10 NOx Chemiluminescence

AOSC 634
Air Sampling and Analysis
NO, NOx, and NOy analysis
via Chemiluminescence
See Clough an Thrush, 1967
Fehsenfeld et al. 1987.
Copyright Brock et al. 1984; Dickerson 2015
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Goal
To derive an expression for the relationship between NO mixing
ratio and photon flux from chemiluminescence.
• How to measure NO.
• Sensitivity and detection limits.
• How to measure other NOx and NOy species.
• What is required to measure those fluxes?
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Published on 01 January 1967 on http://pubs.rsc.org | doi:10.1039/TF9676300915
NO + O3 Chemiluminescence
Clough and Thrush (1967)
NO + O3 O2 + NO2* (2B1)
NO + O3 O2 + NO2 (2A1)
NO2* NO2 + hv
NO2* + M  NO2 + M*
1a
1b
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To use chemiluminescence to detect NO.
The investigator wants to detect red or infrared
light. PMT’s that are red sensitive are more
expensive and need cooling.
Also wants to maximize the intensity of that light.
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Objective:
To use chemiluminescence to detect NO.
We will oxidize NO to NO2 in front of a sensor, a photomultiplier tube, PMT
and count the photons. The sensitivity is proportional to rate of photon
emission intensity, I.
If R1 is the rate limiting step, then:
I = PNO2 f1 f2
I – emission intensity (photons/s)
PNO2 – rate of production of NO2 (molecules/s)
f1 – fraction of NO2 produced in excited state (unitless)
F2 - fraction of NO2 emitting (photons/molecule).
How fast are NO2 molecules produced?
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To use chemiluminescence to detect NO.
I = PNO2 f1 f2
How fast are NO2 molecules produced inside the reaction chamber?
Assume reaction chamber is in steady state with a constant volume flow.
PNO2 = [NO]i f M (1 – e-t/t)
Where =
[NO]i is the initial (ambient) mixing ratio of NO.
F is the volume flow in for the T and P of the reaction chamber in cm3/s.
M is the molecular number density in in the chamber in molecules/cm3
(1 – e-t/t) is fraction of NO reacted within residence time t for chemical
lifetime t.
t (in s) is the volume of the reaction chamber V divided by the flow f.
t (in s) is determined by NO + O3 kinetics.
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To use chemiluminescence to detect NO.
PNO2 = [NO]i f M (1 – e-t/t)
t = V/f (in s)
t = [(k1a+k1b)[O3]M)]-1 (in s)
The fraction produced in the excites state:
f1 = k1a/(k1a+k1b)
k1a = 1.26x10-12 exp (-2100/T) cm3 s-1
k1b = 2.0x10-12 exp (-1400/T) cm3 s-1
f1 = 0.63 exp(-700/T)
= 0.06 at room temperature.
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To use chemiluminescence to detect NO.
PNO2 = [NO]i f M (1 – e-t/t)
The fraction of NO2* emitting lightis
f2 = k2/(k2+Mk3)
For P > ~1 torr, Mk3 >> k2
therefore
f2  k2/Mk3
 2.7x1014/M
Unitless.
I = [NO]I x f x 1.73x1014 x exp(-700/T)
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To use chemiluminescence to detect NO.
I = [NO]i x f x 1.73x1014 x exp(-700/T)
The flow in the reaction chamber, f, is the STP flow F divided
by pressure in the reaction chamber, P.
f = F/P
The figure of merit for a vacuum pump, F/P, also called the
pumping speed. Vacuum cleaners might move 10 (STP) L/s at 0.5
atm for a pumping speed of 20L/s. A mechanical roughing
vacuum pump might move 0.02 (STP) L/s at an internal pressure
of 0.01 atm for a pumping speed of 2.0 L/s. As the pressure falls,
pumping speed falls too. The pumping speed of a vacuum cleaner
is zero when the pressure drops to about 0.1 atm.
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To use chemiluminescence to detect NO.
For maximum sensitivity to NO, you need the
maximum pumping speed (weight and power) for
the amount of ozone you can generate.
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Aircraft use with other NOy species
hv
PUMP
VENT
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