Lecture 6
AOSC/CHEM 637
Atmospheric Chemistry
R. Dickerson
OUTLINE
KINETICS
Experimental Methods
Finlayson-Pitts Chapt. 5B.
Fast Flow Systems
Flash Photolysis
Relative Rate Constants
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Rule #1: Shoot for Pseudo-First Order Conditions
A + B → Prod.
If [B] >> [A]
k'  k[ B]
[A] t
 e -k[B]t 
[A ]0
[A] t
ln
 k[ B]t 
[A ]0
1
A 
k[B]
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[A] t
ln
 k' t 
[A ]0
ln [A] t  k[B]t  ln[ A]0
A plot of ln[A] vs. time yields a straight line
with a slope equal to –k[B]0 (if B>>A then the
initial and mean concentrations of B are
identical) with an intercept of the natural log of
the original concentration of reactant A, ln[A]0.
A major advantage is that only relative
concentrations of A are needed.
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[A] t
ln
 k' t 
[A ]0
ln [A] t  k[B]t  ln[ A]0
If A strongly absorbs radiation, then Beer’s
Law is followed and the absorbance (Abs.) is
proportional to the number density (concentration)
in molecules cm-3.
A major advantage is that only relative light
intensities are needed.
I
ln   Abs.  Nl
I0
( Abs)
ln
  k[B]t
( Abs
) 0 © R. R. Dickerson 2010
Copyright
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Fast Flow Systems
Maintain steady state conditions if possible.
Watch one reactant disappear.
Short residence times and high linear velocities (1.0-10 m/s).
Useful for reactive atoms and radicals such as •Cl and •OH.
Limited to low pressures (0.5 to 10 torr) for rapid diffusion and
uniform plug flow.
If pressure too high, radial diffusion too slow; if pressure too
low axial diffusion too fast.
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Fast-Flow Discharge System for Kinetics.
t
dx
F/A
Where t = time, dx is distance of red arrow, F is volume flow and A is cross-sectional area.
Alternate
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Reactive species can be produced in a microwave discharge
(plasma) system; for example:
Reactant
Source
OH
H2 → 2H
H + NO2 → OH + NO
O(3P)
N2 → 2N
N + NO → N2 + O
NO3
F2 → 2F
F + HNO3 → NO3 + HF
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Flash-Photolysis
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Flash-Photolysis System
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Cavity Ring Down Method
A pulse of light is injected into a cell with two parallel highlyreflective mirrors. The rate of loss of light is proportional to the
concentration of an absorbing gas in the cell. If the mirrors are
near perfect an effective path length of 1000’s of m can be
achieved. Two gases can be mixed into a cell and allowed to
react. Next we show an example of how to measure a species
of atmospheric interest with CRDS. See also Finlayson-Pitts
Section 5B.
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Example CRDS:
Measurement of NO2
• Modified and characterized a commercial Cavity RingDown Spectroscopy NO2 Analyzer
– Los Gatos Research (Mountain View, CA)
• Conducted an ambient intercomparison with a
chemiluminescence instrument provided by NOAA
– Photolysis converted NO2 to NO
• Results appeared in Castellanos et al. (2009, Review of
Scientific Instruments)
Copyright © R. R. Dickerson 2010
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CRDS Method
t
I (t )  I o exp  
 o 
11 1 
N    
c  o 
Copyright © R. R. Dickerson 2010
t
I (t )  I o exp  
 
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CRDS Method
• Empty Cavity:
• R = mirror reflectivity
• l = length of the cavity
• K = coefficient of miscellaneous
scattering or absorption
• c = speed of light
dI
c
t
  I [(1  R )  K ] I (t )  I o exp(  )
dt
l
0
• Cavity with Analyte:
dI
c
  I [(1  R )  Nl  K ]
dt
l
t
I (t )  I o exp(  )
11 1 
N    
c  o 

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NO2
Molecule
Absorption5
Cross Section
[cm2 molecule-1]
SO2
3x10-24
PAN
5x10-21
O3
2x10-23
H2O
1x10-27
Kirsme et al.,
JGR (1997)
• Single wavelength diode 405 nm laser
– Technique is insensitive to lamp fluctuations
• Highly reflective, low curvature mirrors result in ~ 1 km
path-lengths
– Highly sensitive
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GPT Calibration
GPT: Mixed a small flow
of NO-in-Nitrogen
with excess ozone
Calculated [NO2] from:
•
Monitored change in
ozone (equal to
[NO2])
•
From standard
dilution flow rates
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Summary of Performance Statistics
Detection
Limit
Respons
e Time
Power Internal
Dimensions
Draw Pressure
[pptv]
[s]
[W]
[torr]
WxHxD
[cm]
[kg]
CRDS Analyzer
60 (60 s)
18 (95%)
90
170
42.5 x 22 x
56
23
NOAA
Chemiluminescence
100 (60 s)
3 (95%)
1000
30
42 x 33 x
58.4
30
Photolysis
Thermo Electron
Corp. Model 42i
TL
75 (120 s)
60
300
200-450
42.5 x 21.9
x 58.4
25
Hot Mo
Weight
NO2 to
NO
The fixed internal pressure, low power draw, and compact size
of the CRDS instrument makes it ideal for aircraft use at
18
altitudes up to ~10 km.Copyright © R. R. Dickerson 2010
NO
NO2 by Chemi.
NO2 by CRDS
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y = 0.96x + 0.28
y = 0.93x - 0.61
R = 0.995
R = 0.982
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Reference
•
Castellanos, et al., Modification of a Commercial Cavity Ring-Down
Spectroscopy NO2 Detector for Enhanced Sensitivity, Review of Scientific
Instruments. 2009, 80, 113107.
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