P. Campuzano-Jost

Kinetic and mechanistic studies of the OH-initiated oxidation of dimethylsulfide at low temperature - A reevaluation of the rate coefficient and branching ratio

Abstract The pulsed laser photolysis–pulsed laser induced fluorescence (PLP–PLIF) technique has been used to study the reactions of OH with DMS and DMS-d 6 . The effective rate coefficient for the reaction of OH with DMS-d 6 has been determined as a function of O 2 partial pressure at 600 Torr total pressure in N 2 /O 2 mixtures at 298 and 261 K and for both DMS and DMS-d 6 at 240 K. Currently recommended rates are based on an empirical fit to a two-channel mechanism. This work shows that at low temperatures the currently recommended expression underestimates both the effective rate coefficient for reaction together with the branching ratio between addition and abstraction.

Gas phase elemental mercury: A comparison of LIF detection techniques and study of the kinetics of reaction with the hydroxyl radical

Abstract We have examined the sensitivity of single and sequential two-photon laser-induced fluorescence (LIF) techniques for the detection of elemental mercury, Hg(0), in the gas phase. Single photon LIF involves excitation of the 6 3 P 1 –6 1 S 0 transition at 253.7xa0nm, followed by observation of resonance fluorescence. Sequential two-photon techniques follow the initial 6 3 P 1 –6 1 S 0 excitation with a second excitation step to either the 7 1 S 0 or 7 3 S 1 levels followed by observation of blue or red shifted fluorescence. We have examined four variants of these approaches which all exceed the sensitivity of single photon LIF. The most sensitive detection approach involves the initial 253.7xa0nm excitation followed by excitation of the 7 1 S 0 –6 3 P 1 transition at 407.8xa0nm. Fluorescence is observed on the 6 1 P 1 –6 1 S 0 transition at 184.9xa0nm. Using this approach, our limits of detection are 0.1xa0ngxa0m−3 with a 10xa0s integration time in air. We have also examined the effects of saturation, quenching and line-width on detection sensitivity. We have used the pulsed laser photolysis–pulsed laser-induced fluorescence (PLP-PLIF) technique to study the kinetics of the reaction of elemental mercury with the hydroxyl radical under atmospheric conditions at 298xa0K. We see no evidence for reaction and obtain an upper limit of 1.2×10−13xa0cm3xa0molecule−1xa0s−1 for the rate coefficient.

Kinetics of the OH‐initiated oxidation of isoprene

Absolute rate coefficients have been determined for the reaction OH + C5H8 (1) and two of its isotopomeric variants. Reaction (1) was studied as a function of temperature and pressure in N2, N2/O2 and He buffer gases. At room temperature, a rate coefficient, k1, of (8.56±0.26)*10−11 cm³ s−1 independent of pressure and buffer gas identity was obtained. An Arrhenius fit to data over the temperature range 250 – 340 K gave a negative activation energy of −666±150 cal/mol. HO2 production was inferred from observations of radical regeneration in the presence of NO. The absence of a kinetic isotope effect, together with the specificity of radical regeneration, indicates that reaction proceeds via an addition channel with no significant abstraction component. Our rate coefficient for reaction (1) is 15% lower than the value currently recommenced for atmospheric modeling.

Near real-time measurement of sea‑salt aerosol during the SEAS Campaign: Comparison of emission-based sodium detection with an aerosol volatility technique

Abstract The first deployment of an emission-based aerosol sodium detector (ASD), designed to chemically characterize marine aerosols on a near-real-time basis, is reported. Deployment occurred as part of the Shoreline Environment Aerosol Study (SEAS) from 16 April to 1 May 2000 at Bellows Air Force Base on the east side of Oahu, where the University of Hawaiis Department of Oceanography maintains a tower for aerosol measurements. The instrument was operated in size-unsegregated mode and measurements were made that included two extended continuous sampling periods, each of which lasted for 24 h. During this time, the ASD was compared with measurements that used aerosol volatility coupled with optical particle counting to infer sea-salt size distributions. A reasonable agreement was obtained between the instruments when sampling in clean air, suggesting that under these conditions both approaches can provide reliable sea-salt distributions. The combination of these measurements suggested that sea salt was...

Vibrational deactivation studies of OH X 2Π(v= 1–5) by N2 and O2

The deactivation kinetics of vibrationally excited OH X 2Π n (v n = 1–5) were studied using a pulsed laser photolysis-pulsed laser induced fluorescence technique. Temporal profiles of OH (v) were obtained by exciting off-diagonal (Δv n = n −1,−3) transitions in the A–X band of OH and monitoring the diagonal, blue shifted fluorescence. Photolysis of O3 at 266 nm was used to produce O1D which reacted rapidly with H2, CH4 and H2O to produce OH (v). Deactivation rate coefficients for OH (v n = 1–5) with O2 and N2 and for OH (v n = 2,1) with O3 were obtained. The deactivation rate coefficients show an exponential dependence on vibrational level for both O2 and N2, however O2 is much more efficient.

Kinetic and mechanistic studies of the recombination of OH with NO2: vibrational deactivation, isotopic scrambling and product isomer branching ratios.

The kinetics and mechanism of the three-body recombination of OH with NO2 were studied using a pulsed laser photolysis pulsed laser induced fluorescence technique. The rate coefficients for deactivation of vibrationally excited OH (v = 1-5) by NO2 were found to be independent of vibrational level with a value of (6.4 +/- 0.3) x 10(-11) cm3 molecule s (-1) at 298 K. The rate coefficient for reaction of 18OH with NO2 was measured and found to be much faster than for unlabeled OH with a zero pressure rate of 1 x 10(-11) cm3 molecule(-1) s(-1) at 298 K and 273 K. Observation of temporal profiles of 16OH and 18OH suggest that isotopic scrambling in the initially formed [H18ON16O2] complex is complete on the microsecond time scale of our experiments. The rate coefficient for reaction of unlabeled OH with NO2 was measured at 413 K in 400 Torr of He. Biexponential temporal profiles were obtained and are consistent with a 10 +/- 3% yield of the weakly bound HOONO isomer.

Real-time measurement of sodium in single aerosol particles by flame emission: laboratory characterization

A flame emission aerosol sodium detector (ASD) has been developed to study the distribution of seasalt in individual marine aerosol droplets. The instrument detects sodium via D-line emission in a fuel-rich, laminar, hydrogen/oxygen/nitrogen flame. Laboratory studies with synthetic monodisperse aerosols were carried out in order to characterize the sensitivity, precision, and linearity of the technique. Experiments were also carried out with aerosols generated from mixed salt solutions and seawater in order to determine whether ionic or other matrix effects lead to interference. The ASD has a linear response function for NaCl aerosol particles from 100 nm to 2.0μm in diameter. The precision of sodium mass measurements is on the order of ±3% standard error on replicate measurements, with a quantitative response to the sodium content of a single aerosol particle that is independent of the chemical composition of the particle, i.e. anions, cations, seawater. No interferences were found with major ions in seawater and common atmospheric aerosols. These experiments demonstrate a detection limit equivalent to a 100 nm diameter dry 100% NaCl aerosol. Copyright