MEASURING RATE CONSTANTS FOR REACTIONS OF THE SIMPLEST

MEASURING RATE CONSTANTS FOR REACTIONS OF THE SIMPLEST CRIEGEE INTERMEDIATE CH 2 OO BY MONITORING THE OH RADICAL YINGDI LIU, KYLE D BAYES AND STANLEY P. SANDER JET PROPULSION LABORATORY, CALIFORNIA INSTITUTE OF TECHNOLOGY, CA 1

2

Anthropogenic Natural 3

Ozone and alkene reaction Ø Production of OH radical Ø An important source of HOx Ø The observed yields: 10% - 100%. Ø Generate Criegee intermediate (CI) Ø CI reacts with many important molecules in the atmosphere Ø A potential Secondary organic aerosol precursor Ø OH production mechanism is not completely established Ø Lack of CI kinetics information 4

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 OH DECOMPOSE NO 2, SO 2, H 2 O REACT Ø Kinetic info? 5

Motivation ØGenerate CH 2 OO directly without O 3 + ethylene reaction “direct”CH 2 OO. ØDirect monitoring OH formation from “direct”CH 2 OO. ØIndirectly measure the kinetics info of CH 2 OO 6

“Direct” CH 2 OO generation CH 2 I 2 + hv → CH 2 I (351 nm) 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 7 Welz, et, al. Science 2012

Laser Photolysis / Laser Induced Fluorescence Excimer laser, 351 nm 10 Hz, ~6 m. W Pu m p Nd: YAG laser +Dye laser, 282 nm 20 k. Hz, < 1 m. W Ou t e l p u o oc m T r e h 2 Ar + +O + CH 2 HF 2 I A 2 or SO • Advantages: High repetition rate dye laser gives OH profile for each photolysis pulse. Photomultiplier Tube + Photon counting • High sensitivity. OH Signal 8

• The data can be fitted using empirical eq. • OH signal = C 1(e-k 1 t)+ C 2(e-k 1 t- e-k 2 t)+C 3(e-k 3 t) k 1: OH decay rate k 2: OH formation rate 11

OH + CH 2 I 2 H 2 O + CHI 2 Slow CH 2 OO Fast OH Other Fast slow [CH 2 OO] = const×[OH] OH is at steady state 12

Increase [HFA] LIF signals for OH as a function of time for three different concentrations of HFA = hexafluoroacetone HFA does not react with OH 15 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 10 -11 cm 3 molecule-1 s-1 CH 2 OO + HFA Adduct

No pressure dependence is evident over this pressure range 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 10 -11 cm 3 molecule-1 s-1 16 The measured rate constants for the reaction of CH 2 OO with SO 2 as a function of the total pressure CH 2 OO + SO 2

Summary Ø [CH 2 OO] = const×[OH] use OH time profile to represent the CH 2 OO time profile. ØBimolecular reaction rate constants of CH 2 OO with different molecules can be obtained. ØRate constants of CH 2 OO + HFA and CH 2 OO + SO 2 are obtained and the results agree with the literature. ØRate constant of CH 2 OO + SO 2 is pressure independent. 17

Acknowledgement Dr. Stanley Sander Dr. Kyle Bayes Thank you~ 18

Why signal increases in O 3 study? • O 3 + hv(282 nm) O 1 D + O 2 • O 1 D + CH 2 I 2 CHI 2 + OH 19

OH NO 2, SO 2, H 2 O REACT DECOMPOSE Ø How? Ø Kinetic info? Increase CH 2 OO Scavenger concentration 20