1
2
Anthropogenic
Natural
3
Ozone and alkene reaction
 Production of OH radical
 An important source of HOx
 The observed yields: 10% - 100%.
 Generate Criegee intermediate (CI)
 CI reacts with many important molecules in the
atmosphere
 A potential Secondary organic aerosol precursor
 OH production mechanism is not completely
established
 Lack of CI kinetics information
4
O3 + ethylene (simplest alkene)
Form CH2OO – The simplest CI
O3 + ethylene  High energetic system
Observed OH yields: 10%-60%
Understand the information from the
stabilized CI  Direct CH2OO
NO2, SO2, H2O
DECOMPOSE
REACT
 Kinetic info?
5
Motivation
Generate CH2OO directly without O3 +
ethylene reaction“direct”CH2OO.
Direct monitoring OH formation from
“direct”CH2OO.
Indirectly measure the kinetics info of CH2OO
6
“Direct” CH2OO generation
CH2I2 + hv → CH2I (351nm)
CH2I + O2 → CH2IOO (1)
CH2I + O2 → CH2O + IO (2) – Minor path
CH2I + O2 → I + CH2OO (3) – Major path
“Direct” CH2OO gives OH signal, which can be
detected using LP-LIF
7
Welz, et, al. Science 2012
Laser Photolysis / Laser Induced Fluorescence
Excimer laser, 351 nm
10 Hz, ~6 mW
Nd: YAG laser
+Dye laser, 282 nm
20kHz,
< 1mW
Photomultiplier Tube
+ Photon counting
OH Signal
8
• The data can be fitted using empirical eq.
• OH signal = C1(e-k1t)+ C2(e-k1t- e-k2t)+C3(e-k3t)
OH signal rel. intensity
4000
raw data
fitted data
3000
2000
k1: OH decay rate
k2: OH formation rate
1000
0
0.00
0.02
0.04
0.06
0.08
0.10
Time/s
11
OH + CH2I2  H2O + CHI2
Slow
Fast
slow
OH
[CH2OO] = const×[OH]
OH is at steady state
Other
4000
OH signal rel. intensity
CH2OO
Fast
raw data
fitted data
3000
2000
1000
0
0.00
0.02
0.04
0.06
0.08
0.10
Time/s
12
LIF signals for OH as a function of time
for three different concentrations of
HFA.
HFA = hexafluoroacetone
HFA does not react with OH
15
The fitted decay constant K1 for different
concentrations of HFA.
Slope = rate constant of CH2OO with HFA.
(3.33 ± 0.27) x 10-11 cm3 molecule-1 s-1
CH2OO + HFA  Adduct
No pressure dependence is
evident over this pressure range
The fitted decay constant, K1,
for different concentrations of
SO2
Slope = rate constant of
CH2OO with SO2
(3.53 ± 0.29) x 10-11 cm3
molecule-1 s-1
16
The measured rate constants for
the reaction of CH2OO with SO2 as
a function of the total pressure
H
CH2OO + SO2
 H
O
O
O S
O
Summary
 [CH2OO] = const×[OH] use OH time profile to
represent the CH2OO time profile.
Bimolecular reaction rate constants of CH2OO
with different molecules can be obtained.
Rate constants of CH2OO + HFA and
CH2OO + SO2 are obtained and the results agree
with the literature.
Rate constant of CH2OO + SO2 is pressure
independent.
17
Acknowledgement
Dr. Stanley Sander Dr. Kyle Bayes
18
Why signal increases in O3 study?
• O3 + hv(282nm)  O1D + O2
• O1D + CH2I2  CHI2 + OH
19
NO2, SO2, H2O
REACT
DECOMPOSE
 How?
 Kinetic info?
Increase CH2OO
Scavenger
concentration
20