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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  “direct”CH 2 OO.  Direct monitoring OH formation from “direct”CH 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