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 1 Goal To derive an expression for the relationship between NO mixing ratio and photon flux from chemiluminescence ID: 237672
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Slide1
AOSC 634Air Sampling and Analysis
NO, NOx, and NOy analysis via ChemiluminescenceSee Clough an Thrush, 1967Fehsenfeld et al. 1987.Copyright Brock et al. 1984; Dickerson 2015
1Slide2
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?2Slide3
NO + O3 Chemiluminescence
Clough and Thrush (1967)NO + O3 O2
+ NO
2
* (
2
B
1) 1aNO + O3 O2 + NO2 (2A1) 1bNO2* NO2 + hv 2NO2* + M NO2 + M* 3
3Slide4
To use chemiluminescence to detect NO.
4The 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.Slide5
Objective:
To use chemiluminescence to detect NO.5We will oxidize NO to NO
2
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 R
1 is the rate limiting step, then:I = PNO2 f1 f2I – emission intensity (photons/s)PNO2 – rate of production of NO2 (molecules/s) f1
–
fraction of NO
2
produced in excited state (
unitless
)
F
2
-
fraction of NO
2
emitting (photons/molecule).
How fast are NO
2
molecules produced?Slide6
To use chemiluminescence to detect NO.
6I = P
NO2
f
1
f
2How 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 cm
3
/s.
M is the molecular number density in in the chamber in molecules/cm
3
(
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 + O
3
kinetics.Slide7
To use chemiluminescence to detect NO.
7P
NO2
=
[
NO]
i f M (1 – e-t/t)t = V/f (in s)= [(k1a+k1b)[O3]M)]-1 (in s)The fraction produced in the excites state:f1 = k
1a/(
k
1a
+k
1b
)
k
1a
=
1.26x10
-12
exp (-2100/T) cm
3
s
-1
k
1b
= 2.0x10
-12
exp (
-1400
/T) cm
3
s
-1
f
1
= 0.63
exp
(-700/T)
= 0.06 at room temperature.Slide8
To use chemiluminescence to detect NO.
8P
NO2
=
[
NO]
i f M (1 – e-t/t)The fraction of NO2* emitting lightisf2 = k2/(k2+Mk3) For P > ~1 torr, Mk3 >>
k2
therefore
f
2
≈
k
2
/Mk
3
≈ 2.75
x
10
14
/M
Unitless
.
I = [NO]
I
x
f x
1.73
x
10
14
x
exp
(-700/T)Slide9
To use chemiluminescence to detect NO.
9 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/PThe 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.Slide10
To use chemiluminescence to detect NO.
10For maximum sensitivity to NO, you need the maximum pumping speed (weight and power) for the amount of ozone you can generate.Slide11
11Slide12
12Slide13
Aircraft use with other NOy species
13hv
PUMP
VENT