1 2 Natural Anthropogenic 3 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 Ozone and alkene reaction 4 REACT DECOMPOSE NO
2, SO 2, H 2 O O 3 + ethylene (simplest alkene) Form CH 2 OO The
simplest CI O 3 + ethylene High energetic system Observed OH
yields: 10%-60% Understand the information from the stabilized CI
Direct CH 2 OO Kinetic info? 5 Motivation Generate CH 2 OO directly
without O 3 + ethylene reaction directCH 2 OO. Direct monitoring OH
formation from directCH 2 OO. Indirectly measure the kinetics info
of CH 2 OO 6 Direct CH 2 OO generation CH 2 I 2 + hv CH 2 I (351nm)
CH 2 I + O 2 CH 2 IOO (1) CH 2 I + O 2 CH 2 O + IO (2) Minor path
CH 2 I + O 2 I + CH 2 OO (3) Major path Direct CH 2 OO gives OH
signal, which can be detected using LP-LIF Welz, et, al. Science
Pump Out Thermocouple Photomultiplier Tube + Photon counting
Excimer laser, 351 nm 10 Hz, ~6 mW Nd: YAG laser +Dye laser, 282 nm
20kHz, < 1mW Laser Photolysis / Laser Induced Fluorescence Ar +
O 2 + CH 2 I 2 + HFA or SO 2 OH Signal 8 Experimental Conditions
Molecule Concentration (molecule cm -3 ) CH 2 I 2 (0 8)10 14 O2O2
(0 3)10 15 HFA(0 3)10 13 SO 2 (0 7)10 12 Total pressure (in Ar):
Torr Temperature: ~295K 9 Results 10 The data can be fitted using
empirical eq. OH signal = C1(e -k1t )+ C2(e -k1t - e -k2t )+C3(e
-k3t ) OH signal = C1(e -k1t )+ C2(e -k1t - e -k2t )+C3(e -k3t ) 11
k1: OH decay rate k2: OH formation rate Slow Fast CH 2 OO OH Other
OH + CH 2 I 2 H 2 O + CHI 2 12 Fast slow [CH 2 OO] = const[OH] OH
is at steady state The dependence of the rise time constant, K 2
The [CH 2 I 2 ] = 3.85 x10 14 molecules cm -3 ; P =50 Torr.
Expected rate of CH 2 I + O 2 products Rate const: (1.4 or 1.6) cm
3 molecule -1 s -1 Expected rate of CH 2 I + O 2 products Rate
const: (1.4 or 1.6) cm 3 molecule -1 s -1 Welz, et, al. Science The
amount of the fast OH signal (present at very short times) as a
fraction of the total OH signal as a function of [O 2 ]. 14 LIF
signals for OH as a function of time for three different
concentrations of HFA. The fitted decay constant K 1 for different
concentrations of HFA. Slope = rate constant of CH 2 OO with HFA.
(3.33 0.27) x cm 3 molecule -1 s -1 CH 2 OO + HFA Adduct 15 HFA =
hexafluoroacetone HFA does not react with OH CH 2 OO + SO 2 The
fitted decay constant, K 1, for different concentrations of SO 2
Slope = rate constant of CH 2 OO with SO 2 (3.53 0.29) x cm 3
molecule -1 s -1 The measured rate constants for the reaction of CH
2 OO with SO 2 as a function of the total pressure No pressure
dependence is evident over this pressure range 16 Summary [CH 2 OO]
= const[OH] use OH time profile to represent the CH 2 OO time
profile. Bimolecular reaction rate constants of CH 2 OO with
different molecules can be obtained. Rate constants of CH 2 OO +
HFA and CH 2 OO + SO 2 are obtained and the results agree with the
literature. Rate constant of CH 2 OO + SO 2 is pressure
independent. [CH 2 OO] = const[OH] use OH time profile to represent
the CH 2 OO time profile. Bimolecular reaction rate constants of CH
2 OO with different molecules can be obtained. Rate constants of CH
2 OO + HFA and CH 2 OO + SO 2 are obtained and the results agree
with the literature. Rate constant of CH 2 OO + SO 2 is pressure
independent. 17 Acknowledgement 18 Dr. Stanley Sander Dr. Kyle
Bayes Why signal increases in O3 study? O3 + hv(282nm) O1D + O2 O1D
+ CH2I2 CHI2 + OH 19 Increase CH 2 OO Scavenger concentration REACT
DECOMPOSE NO 2, SO 2, H 2 O How? Kinetic info? 20
