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AOSC 634Air Sampling and Analysis

NO, NOx, and NOy analysis via Chemiluminescence

See Clough an Thrush, 1967Fehsenfeld 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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NO + O3 ChemiluminescenceClough and Thrush (1967)

NO + O3 O2 + NO2* (2B1) 1a

NO + O3 O2 + NO2 (2A1) 1b

NO2* NO2 + hv 2

NO2* + M NO2 + M* 3

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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

≈ 2.75x1014/MUnitless.

= [ ]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