Transcript ACN_OhioSt2007_v2.ppt

Kinetics measurements of HO2 and
RO2 self and cross reactions using
infrared kinetic spectroscopy (IRKS)
A.C. Noell, L. S. Alconcel, D.J. Robichaud, M. Okumura, S. P.
Sander*
*Jet Propulsion Laboratory
Department of Chemistry, California Institute of Technology
Pasadena, CA, USA
62nd International Symposium on Molecular Spectroscopy
June 18th, 2007
Peroxy Radicals in the Atmosphere
RH + OH  RO2 +H2O
O2
Urban Air, High NOx
Clean Air, low NOx
RO2 +NO 
 NO2 +RO
RO2 +HO2 
 ROOH+O2
•The central
reaction leading
to tropospheric
O3 production
•Hydroperoxides are
radical reservoirs,
i.e. they can remove
O3 forming species
•May be important in
secondary organic
aerosol formation
Reactions of Peroxy Radicals
HO2 +HO2  HOOH + O2
kHOOH
RO2 +HO2  ROOH + O2
kROOH
RO2 +RO2 
 Products
RO2 self reaction has a radical product
branch
RO2 +RO2 
 RO + RO +O2
kRO
 ROH +R(O)H+O2
kR(O)H
 ROOR + O2
kROOR
RO = kRO /(kRO +k R(O)H +k ROOR )
RO complicates the kinetics of RO2 and HO2
kHOOH
HO2 +HO2 
 HOOH+O2
RO2 +HO2  ROOH + O2
kROOH
RO2 +RO2 
 RO + RO +O2
kRO
 ROH +R(O)H+O2
kR(O)H
kROOR

 ROOR + O2
RO +O2

 HO2 R(O)H
kHO2
RO determines the amount of secondary HO2
100
12
80
x10
Concentration / (molec/cc x10
12
)
Secondary HO2 in the self rxn of C2H5O2
2.0
1.5
1.0
0.5
60
0 10 20 30 40 50
ms
40
C2H5O2
HO2, alpha = 0.3
HO2, alpha = 0.6
20
0
0
20
40
ms
60
Modeled w/ FACSIMILE
No previous direct measurements of
RO at atmospheric conditions
•CH3O2
• 7 previous studies, T 223 - 573 K, P 50 - 760 Torr
•C2H5O2
• 3 previous studies, T 298 – 373 K, P 6.5 – 700 Torr
•i-C3H7O2
• 2 previous studies, T 302, 333, 373 K, P 700 Torr
• All but one study were based on end product analysis
using a variation of GC/MS or FTIR
Previous techniques lacked sensitivity
and specificity
• UV
•Not sensitive
enough to detect
small amounts of
radical product
from self reaction
4
2
•Unable to detect
direct radical
products
Abs cross section / cm x10
• GC/MS and FTIR
Specificity
-18
Sensitivity
HO2
CH3O2
C2H5O2
3
2
1
200 220 240 260 280
wavelength / nm
• UV absorption was used to
monitor both HO2 and RO2.
Objective: Measure RO for the C2H5O2 (EtO2)
self reaction “directly”
EtO2 +EtO2 
 EtO + EtO +O2
kRO
 EtOH +CH3CHO+O2
kR(O)H
EtO +O2

 HO2 CH3CHO
kHO2
• Simultaneous measurement of
RO and kinetics
• Use Infrared Kinetic Spectroscopy (IRKS)
technique
The IRKS method uses NIR spectroscopy
to detect HO2 with high sensitivity
a. HO2 alone is detected by NIR diode laser wavelength
modulation spectroscopy
HO2 2f 21 spectrum
0.2
signal (V)
•2v1 Q(9) band is used
•High sensitivity: Detection
limit ~ 5 x 1011 (molec/cc).
•Able to detect secondary
HO2 from EtO2 self rxn.
6638.2 cm-1, 3
0.1
0.0
33. 5
-0.1
-0.2
~ 0.05 cm-1
b. EtO2 (or other RO2) is detected by UV absorption, λ = 250
nm, minimizes HO2 interference
3
DeSain et al. J. Mole Spec 219 (2003) 163–169
Experimental Apparatus
6.8 MHz
current modulator
NIR
UV
PD
computer
diode
laser
D2 lamp
2 x Freq,
phase shifter
InGaAs FM signal
detector
Herriott cell
Monochromator
Demodulated
signal
exit
exit
Excimer laser, 308 nm
gas entrance
0.22
1
0.1
0
0.0
00
20
10
40
20
ms
ms
6030 80
Absorbance/ 1x10
HO2 self rxn
Secondary
HO2
IR Data
Fit
Secondary HO2
-3
x10
-3
1.0
0.5
0.7
0.0
-0.5
0.6
-1.0
6
0.5
5
0.4
4
0.33
IR signal/ (V x10 )
-3
IR signal/ (V )
x10
-3
RO measured by fitting secondary HO2
4
2
0
-2
-4
60
UV Data
Fit
EtO2
50
40
30
20
10
0
0
20
EtO2 + EtO2 at 273 K and 50 Torr
40
ms
60
80
RO is lower than previous measurements
This Study
Nikki et al
Wallington et al
Anastasi et al
Branching Fraction
0.7
0.6
0.5
0.4
0.3
0.2
3.4
3.6
3.8
4.0
-1
-3
1/T (K ) x10
4.2
4.4
 = 0.32 ± 0.13 with no T dependence from 221 – 296 K
HO2 + EtO2 kinetics are sensitive to
8.0
High EtO2 High HO2
7.5
kEtOOH / 1x10
-12
3
-1
cm molec s
-1
alpha = 0.32
alpha = 0.60
7.0
6.5
6.0
2
4
6 8
2
1
[HO2]0/[EtO2]0
4
RO
Comparison of results
Self
reaction
RO, 298K
RO
T dep
CH3O21
Range:
0.28-0.43
573K ~ 0.82
223K ~ 0.10
Only 1 study below
298K
EtO22
Range:
0.57- 0.68
373K ~ 0.72
No studies below 298K
EtO2
current
0.32± 0.13
No T dep over 221 –
296 K
i-C3H7O22
Range:
0.58 - 0.65
373K = 0.74
1Tyndall
et al. J. Geophys. Res.106 D11 (2001) 12157-12182
2Lightfoot
et al. Atmos. Env. 26 A No 10 (1992) 1805-1961
Theory has not provided a simple
mechanism for the RO2 self reaction
Russell mechanism
2 R2CH-O + O2
Tetroxide transition state leading to stable products has
not been found
4Ghigo
et al. J. Chem. Phys. 118 No. 23 (2003) 10575-10583
Summary and Future Work
•
RO for the EtO2 self reaction was measured directly by
observing the time dependent absorption of HO2 in the
near IR using the IRKS technique
•
The temperature dependence of RO was measured for
the first time over the range 221-296 K
•
RO for EtO2 disagrees with the end product studies on the
same system
•
Theory does not currently provide a good explanation for
the stable product channel that this work measures as the
dominant one
•
Investigations of the MeO2 system
References and Acknowledgements
•
Thanks to
– Dave Natzic
– Lance Christensen
– The Okumura and Sander groups
Funding
NASA Upper Atmosphere Research and Tropospheric
Chemistry Programs
NASA Graduate Student Researchers Program Fellowship