Kinetics measurements of HO 2 and RO 2 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 18 th, 2007

Urban Air, High NO x Clean Air, low NO x Peroxy Radicals in the Atmosphere The central reaction leading to tropospheric O 3 production Hydroperoxides are radical reservoirs, i.e. they can remove O 3 forming species May be important in secondary organic aerosol formation

Reactions of Peroxy Radicals

RO 2 self reaction has a radical product branch

complicates the kinetics of RO 2 and HO 2 determines the amount of secondary HO 2

Secondary HO 2 in the self rxn of C 2 H 5 O 2 Modeled w/ FACSIMILE

No previous direct measurements of at atmospheric conditions CH 3 O 2 7 previous studies, T K, P Torr C 2 H 5 O 2 3 previous studies, T 298 – 373 K, P 6.5 – 700 Torr i-C 3 H 7 O 2 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 absorption was used to monitor both HO 2 and RO 2. SpecificitySensitivity GC/MS and FTIR Unable to detect direct radical products UV Not sensitive enough to detect small amounts of radical product from self reaction

Objective: Measure for the C 2 H 5 O 2 (EtO 2 ) self reaction “directly” Simultaneous measurement of and kinetics Use Infrared Kinetic Spectroscopy (IRKS) technique

The IRKS method uses NIR spectroscopy to detect HO 2 with high sensitivity a. HO 2 alone is detected by NIR diode laser wavelength modulation spectroscopy b. EtO 2 (or other RO 2 ) is detected by UV absorption, λ = 250 nm, minimizes HO 2 interference 2v 1 Q(9) band is used High sensitivity: Detection limit ~ 5 x (molec/cc). Able to detect secondary HO 2 from EtO 2 self rxn cm -1, 3 ~ 0.05 cm -1 3 DeSain et al. J. Mole Spec 219 (2003) 163–169

Experimental Apparatus 6.8 MHz current modulator Demodulated signal Excimer laser, 308 nm D 2 lamp diode laser Mono- chromator computer 2 x Freq, phase shifter FM signal Herriott cell gas entrance exit PD UV NIR InGaAs detector

EtO 2 + EtO 2 at 273 K and 50 Torr measured by fitting secondary HO 2

is lower than previous measurements = 0.32 ± 0.13 with no T dependence from 221 – 296 K

HO 2 + EtO 2 kinetics are sensitive to

Comparison of results Self reaction CH 3 O 2 1 Range: K ~ K ~ 0.10 Only 1 study below 298K EtO 2 2 Range: K ~ 0.72 No studies below 298K EtO 2 current 0.32± 0.13No T dep over 221 – 296 K i-C 3 H 7 O 2 2 Range: K = Tyndall et al. J. Geophys. Res.106 D11 (2001) Lightfoot et al. Atmos. Env. 26 A No 10 (1992)

Theory has not provided a simple mechanism for the RO 2 self reaction Russell mechanism 4 Ghigo et al. J. Chem. Phys. 118 No. 23 (2003) R 2 CH-O + O 2 Tetroxide transition state leading to stable products has not been found

Summary and Future Work for the EtO 2 self reaction was measured directly by observing the time dependent absorption of HO 2 in the near IR using the IRKS technique The temperature dependence of was measured for the first time over the range K for EtO 2 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 MeO 2 system

References and Acknowledgements Thanks to –Dave Natzic –Lance Christensen –The Okumura and Sander groups NASA Upper Atmosphere Research and Tropospheric Chemistry Programs NASA Graduate Student Researchers Program Fellowship Funding