NearInfrared Photochemistry of Atmospheric Nitrites Paul Wennberg Coleen
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
Catalytic destruction of Ozone HO 2 + O 3 OH + O 2 + O OH + O 3 HO 2 + O 2 O 3 + O 3 O 2 + O 2
HOx Photochemistry Sources: O 3 + hν (< 314 nm) O (1 D) + O 2 O (1 D) + H 2 O 2 OH Sinks (Direct): OH + HO 2 H 2 O + O 2 Sinks (Indirect): OH + NO 2 HONO 2 OH + HONO 2 H 2 O + O 3 HO 2 OH HO 2 + NO 2 HO 2 NO 2 OH + HOONO 2 H 2 O + O 2 + NO 2 NO, O 3
Tropospheric O 3 Production O OH + CO 2 + HO 2 + NO 2 + OH NO 2 + hν (< 450 nm) NO + O 2 O 3 Net: CO + 2 O 2 O 3 + CO 2 Le O re o M Jaegle et al. , J. Geophys. Res. , 105, 3877 -3892, 2000. 3 p d ro t uc ion ss O 3 pr od u cti on
The Color of Sunlight
Peroxy Nitric Acid (HO 2 NO 2) Donaldson et al. (1997) proposed that dissociative excitation of OH vibrational overtones in H 2 O 2, HNO 3, and HO 2 NO 2 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 HO 2 NO 2 as the carrier D. J. Donaldson et al. , Geophys. Res. Lett. 24, 2651 (1997) P. O. Wennberg et al. , Geophys. Res. Lett. 26, 1373 (1999) Near IR solar flux is orders of magnitude higher than UV flux
Approach Vibrational Dissociation Spectroscopy 1. IR-photodissociation 2. Conversion into OH 3. Detection of OH HO 2 NO 2 + h HO 2 + NO 2 HO 2 + NO OH + NO 2
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
Action Spectra (2 1, 240 K)=14% • Different relative band intensities in FTIR and action spectra • Dissociation quantum yields determined by comparing spectra • Initial internal energy responsible for dissociation below D 0
• 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
Intensity Residual (%) Mk. IV HO 2 NO 2 Observations Frequency (cm-1)
OH + NO 2 ? • The Reaction of OH with NO 2 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
Part II. HOONO • Suspected intermediate of the OH + NO 2 association reaction • Proposed intermediate of liquid phase reactions of peroxynitrite ion (ONOO-) • Observed in rare-gas matrices in 1991 ü 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
HOONO Atmospheric Significance Reaction Intermediates HO 2 + NO HO + NO 2 +7 0 HOONO -19 (3 isomers) HONO 2 HOONO Irreversible Removal of HOx and NOx No effect HONO 2 -48
Preparation Produce HOONO directly in the gas-phase H 2 + μwave discharge 2 H H + NO 2 OH + NO 2 + M HNO 3 + M OH + NO 2 + M HOONO + M Photofragment: HOONO + hν OH + NO 2 Detect OH by LIF
Observed Spectra 2 1 ü Stronger peaks assigned to HOONO 2 1 overtones and combination bands ü Assignment for weaker bands remains ambiguous D 0 ü Intensities affected by photodissociation dynamics
Observed HOONO Yield This work Burkholder (253 K, 20 Torr) et al. >5 3% <5% (after 300 ms) Dransfield et al. Hippler et al. < 10 % Unpublished • HOONO lifetime unknown lower limit • Different conditions incomparable • Higher yield expected for upper troposphere
Future Projects • Photochemistry of reaction intermediates – HOCO – HOOOCl – CH 3 OONO • Chemistry and kinetics of weakly-bound molecules – CH 3 OONO 2 – CH 3 C(O)OONO 2 – HOONO – HO 2 NO 2 • UV photodissociation spectroscopy of atmospheric molecules – CH 3 OOH – HO 2 NO 2
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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