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Stratospheric Ozone Photochemistry

Near-Infrared Photochemistry of Atmospheric Nitrites Paul Wennberg, Coleen Roehl, Geoff Blake, and Sergey Nizkorodov California Institute of Technology Ross Salawitch, Geoff Toon Jet Propulsion Laboratory. Stratospheric Ozone Photochemistry. Courtesy NASA Goddard.

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Stratospheric Ozone Photochemistry

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  1. Near-Infrared Photochemistry of Atmospheric NitritesPaul Wennberg, Coleen Roehl, Geoff Blake, and Sergey NizkorodovCalifornia Institute of TechnologyRoss Salawitch, Geoff ToonJet Propulsion Laboratory

  2. Stratospheric Ozone Photochemistry Courtesy NASA Goddard

  3. HO2 + O3 OH + O2 + O2OH + O3 HO2 + O2O3 + O3 O2 + O2 + O2 Catalytic destruction of Ozone by HOx Wennberg et al., Science, 266, 398, 1997

  4. HOx Photochemistry O3 Sources: O3 + hν (< 314 nm)  O (1D) + O2 O (1D) + H2O  2 OH Sinks (Direct): OH + HO2 H2O + O2 Sinks (Indirect): OH + NO2 HONO2 OH + HONO2 H2O + O3 HO2 + NO2 HO2NO2 OH + HOONO2 H2O + O2 + NO2 HO2 OH NO, O3

  5. O2 Tropospheric O3 Production OH + CO  CO2 + HO2 HO2 + NO  NO2 + OH NO2 + hν (< 450 nm)  NO + O O + O2 O3 Net: CO + 2 O2 O3 + CO2 More O3 production Less O3 production Jaegle et al., J. Geophys. Res.,105, 3877-3892, 2000.

  6. The Color of Sunlight

  7. Peroxy Nitric Acid (HO2NO2) Donaldson et al. (1997) proposed that dissociative excitation of OH vibrational overtones in H2O2, HNO3, and HO2NO2 is an additional source of OH in the atmosphere Wennberg et al. (1999) found unknown photochemical source of OH in the mid-latitude stratosphere with photolysis > 650 nm and suggested HO2NO2 as the carrier Near IR solar flux is orders of magnitude higher than UV flux D. J. Donaldson et al., Geophys. Res. Lett.24, 2651 (1997) P. O. Wennberg et al., Geophys. Res. Lett.26, 1373 (1999)

  8. HO2NO2 + h  HO2 + NO2 HO2 + NO  OH + NO2 Approach Vibrational Dissociation Spectroscopy • IR-photodissociation • Conversion into OH • Detection of OH

  9. Experiment • Direct overtone pumping of CH / OH stretches in PAN / PNA / HOONO • Chemical conversion of photodecomposition products into OH radicals • LIF detection of OH in a single photon counting regime

  10. Action Spectra • Different relative band intensities in FTIR and action spectra • Dissociation quantum yields determined by comparing spectra • Initial internal energy responsible for dissociation below D0 • (21, 240 K)=14%

  11. Relative band intensities in action spectra of PNA are T-dependent • {diss(31) = 1}  Absolute photodissociation cross sections and quantum yields for other bands

  12. MkIV HO2NO2 Observations Intensity Residual (%) Frequency (cm-1)

  13. OH + NO2  ? • The Reaction of OH with NO2 is among the most important reactions in Earth’s atmosphere. By sequestering both HOx and NOx it essentially shuts down reactive photochemistry. • It is assumed by all models that the only product formed is nitric acid

  14. Part II. HOONO • Suspected intermediate of the OH + NO2 association reaction • Proposed intermediate of liquid phase reactions of peroxynitrite ion (ONOO-) • Observed in rare-gas matrices in 1991 • Cheng et al. J. Phys. Chem. 95, 2814 (1991) • Extensively studied by theory • At least three stable conformers • Bound by 19 kcal/mol • Never observed in the gas phase

  15. HOONO Atmospheric Significance Reaction Intermediates  HO2 + NO HO + NO2 +7 0 HOONO -19 (3 isomers) HONO2 -48

  16. Preparation Produce HOONO directly in the gas-phase H2 + μwave discharge  2 H H + NO2 OH + NO OH + NO2 + M  HNO3 + M OH + NO2 + M  HOONO + M Photofragment: HOONO + hν  OH + NO2 Detect OH by LIF

  17. Observed Spectra 21 • Stronger peaks assigned to HOONO 21overtones and combination bands • Assignment for weaker bands remains ambiguous • Intensities affected by photodissociation dynamics D0

  18. Observed HOONO Yield • HOONO lifetime unknown  lower limit • Different conditions  incomparable • Higher yield expected for upper troposphere

  19. Future Projects • Photochemistry of reaction intermediates • HOCO • HOOOCl • CH3OONO • Chemistry and kinetics of weakly-bound molecules • CH3OONO2 • CH3C(O)OONO2 • HOONO • HO2NO2 • UV photodissociation spectroscopy of atmospheric molecules • CH3OOH • HO2NO2

  20. Thanks • Funding by NASA and NSF • Support for Sergey Nizkorodov (just appointed assistant professor of chemistry UC-Irvine) from the Dreyfus foundation. • You for your attention!

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