Jingsong Zhang

Measurements of peroxy radicals using chemical amplification-cavity ringdown spectroscopy.

The peroxy radical chemical amplification (PERCA) method is combined with cavity ringdown spectroscopy(CRDS) to detect peroxy radicals (HO2 and RO2). In PERCA, HO2 and RO2 are first converted to NO2 via reactions with NO, and the OH and RO coproducts are recycled back to HO2 in subsequent reactions with CO and O2; the chain reactions of HO2 are repeated and amplify the level of NO2. The amplified NO2 is then monitored by CRDS, a sensitive absorption technique. The PERCA-CRDS method is calibrated using a HO2 radical source (0.5-3 ppbv), which is generated by thermal decomposition of H2O2 vapor (permeated from 2% H2O2 solution through a porous Teflon tubing) up to 600 degrees C. Using a 2-m long 6.35-mm o.d. Teflon tubing as the flow reactor and 2.5 ppmv NO and 2.5-10% vol/vol CO, the PERCA amplification factor or chain length, Delta[NO2]/([HO2]+[RO2]), is determined to be 150 +/- 50 (90% confidence limit) in this study. The peroxy radical detection sensitivity by PERCA-CRDS is estimated to be approximately 10 pptv/60 s (3sigma). Ambient measurements of the peroxy radicals are carried out at Riverside, California in 2007 to demonstrate the PERCA-CRDS technique.

Ultraviolet photodissociation dynamics of the benzyl radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled benzyl radical via the 4(2)B(2) electronically excited state is studied in the photolysis wavelength region of 228 to 270 nm using high-n Rydberg atom time-of-flight (HRTOF) and resonance enhanced multiphoton ionization (REMPI) techniques. In this wavelength region, H-atom photofragment yield (PFY) spectra are obtained using ethylbenzene and benzyl chloride as the precursors of benzyl radical, and they have a broad peak centered around 254 nm and are in a good agreement with the previous UV absorption spectra of benzyl. The H + C(7)H(6) product translational energy distributions, P(E(T))s, are derived from the H-atom TOF spectra. The P(E(T)) distributions peak near 5.5 kcal mol(-1), and the fraction of average translational energy in the total excess energy, , is ∼0.3. The P(E(T))s indicate the production of fulvenallene + H, which was suggested by recent theoretical studies. The H-atom product angular distribution is isotropic, with the anisotropy parameter β ≈ 0. The H/D product ratios from isotope labeling studies using C(6)H(5)CD(2) and C(6)D(5)CH(2) are reasonably close to the statistical H/D ratios, suggesting that the H/D atoms are scrambled in the photodissociation of benzyl. The dissociation mechanism is consistent with internal conversion of the electronically excited benzyl followed by unimolecular decomposition of the hot benzyl radical on the ground state.

Ultraviolet Photodissociation Dynamics of the 1-Propenyl Radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled 1-propenyl radical (CHCHCH3) were investigated at the photolysis wavelengths from 224 to 248 nm using high-n Rydberg atom time-of-flight (HRTOF) technique. The 1-propenyl radicals were produced from 193 nm photolysis of 1-chloropropene and 1-bromopropene precursors. The photofragment yield (PFY) spectra of the H atom product have a broad peak centered at 230 nm. The H + C3H4 product translational energy P(ET) distributions peak near ∼8 kcal/mol, and the fraction of average translational energy in the total available energy, ⟨fT⟩, is nearly a constant of ∼0.12 from 224 to 248 nm. The H atom product has an isotropic angular distribution with the anisotropy parameter β ≈ 0. Quasiclassical trajectory calculations were also carried out using an ab initio ground-state potential energy surface for dissociation of 1-propenyl at the excitation energy of 124 kcal/mol (230 nm). The calculated branching ratios are 60% to the methyl + acetylene products, 16% to H + propyne, 4% to H + allene, and 1% to H + cyclopropene. The experimental and calculated P(ET) distributions of the H + C3H4 products at 230 nm are in a qualitative agreement, suggesting that the H + propyne dissociation is the main H atom product channel. The calculated dissociation time scale on the ground electronic state is ∼1 ps, shorter than but close to the time scale of >10 ps for the overall UV photodissociation implied by the isotropic H atom product angular distribution. The UV photodissociation mechanism of 1-propenyl can be described as unimolecular decomposition of hot 1-propenyl radical on the ground electronic state following internal conversion from the electronically excited states of 1-propenyl.

Detection of Sulfur Dioxide by Cavity Ring-Down Spectroscopy

Sulfur dioxide (SO(2)) is a major air pollutant that can contribute to the production of particulate sulfate and increase the acidity in the environment. SO(2) is detected by cavity ring-down spectroscopy (CRDS) utilizing the SO(2) absorption in the 308 nm region. A ferrous sulfate scrubber and a sodium carbonate annular denuder are used to reduce background interferences and to obtain quantitative values of SO(2). The method is characterized using SO(2) standards in the laboratory and compared to a commercial pulsed fluorescence analyzer (PFA). A limit of detection of 3.5 ppb/10 s (S/N = 2) is demonstrated. Ambient measurements are attempted to demonstrate this technique.

Measurements of NOx, acyl peroxynitrates, and NOy with automatic interference corrections using a NO2 analyzer and gas phase titration

NO(2) analyzers are much more valuable if they can also measure NO since the two (NO+NO(2)=NO(x)) are often found together. NO can be quantitatively converted to NO(2) by reaction with ozone and subsequent thermal decomposition of the N(2)O(5) that may form from further oxidation. The conversion of NO, along with decomposition of N(2)O(5) and removal of the remaining unreacted ozone with a heated chamber, allows for quantitative determination of NO(x) using a NO(2) analyzer and the determination of decomposed acyl peroxynitrates. Ambient tests are performed to demonstrate these methods.

Observation of a New 2 Excited Electronic State of SF by Resonance-Enhanced Multiphoton Ionization Spectroscopy

The (2+1) resonance-enhanced multiphoton ionization (REMPI) spectrum of SF has been obtained in the single-photon wavelength region of 307-321 nm. Five vibronic bands were observed and assigned to the two-photon transitions from the ground state to a 2Σ Rydberg state. The term value Te, vibrational frequency, and the rotational constant of the 2Σ Rydberg state were determined. Another 2P state was observed near 312 nm.

Near-UV Photodissociation Dynamics of Thiomethoxy Radical via Ã2A1 State: H-atom Product Channel

Photodissociation dynamics of jet-cooled thiomethoxy radical (CH3S) via the A2A1 X2E transition is investigated near 352 nm. The H-atom product channel is observed directly for the first time by H-atom product yield spectrum and photofragment translational spectroscopy. The 2132 vibrational level of the A2A1 state dissociates to the H+H2CS products. The H+H2CS product translational energy release is modest and peaks around 33 kJ/mol; the H-atom angular distribution is isotropic. The dissociation mechanism is consistent with internal conversion of the excited A2A1 state to the X2E ground state and subsequent unimolecular dissociation on the ground state to the H+H2CS products.