Anthony J. Hynes

Our current understanding of major chemical and physical processes affecting mercury dynamics in the atmosphere and at the air-water/terrestrial interfaces

The predictions of atmospheric chemical models are limited by the accuracy of our understanding of the basic physical and chemical processes that underlie the models. In this work we review the current state of our knowledge of the chemical processes that transform atmospheric mercury species via gas and aqueous phase reactions and the physical processes of deposition. We concur with the conclusions of other recent reviews that our understanding of the basic chemistry that controls mercury is incomplete and the experimental data either limited or nonexistent. In spite of this recent experimental and theoretical studies of mercury reaction kinetics have clarified some issues. Observations in Polar Regions suggest that Hg0 can undergo fast oxidation in the presence of elevated levels of bromine compounds. Both experimental and theoretical studies suggest that the recombination of Hg0 with Br atoms is sufficiently fast to initiate this oxidation process. However there is a large uncertainty in the value of the rate coefficient for this recombination reaction and in the fate of the reaction product, HgBr. Most global mercury models incorporate reactions of Hg0 with OH and O3. Based on the most recent high level ab-initio calculations of the stability of HgO it appears that neither of these reactions is likely to play a significant role in mercury oxidation. The most important aqueous oxidation for Hg0 appears to be reaction with O3 however that there has only been one determination of the Hg + O3 reaction rate constant in the aqueous phase. Aqueous phase reduction of oxidized mercury via reaction with HO2 is the only significant reduction reaction in current models but now seems unlikely to be significant. Again this suggests that the chemistry controlling mercury transformation in current models requires significant modification.

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.

Relative quantum yields for O1D production in the photolysis of ozone between 301 and 336 nm: evidence for the participation of a spin-forbidden channel

Abstract Relative quantum yields for the formation of O1D from the photolysis of ozone have been measured between 301 and 336 nm. O1D was monitored indirectly, using laser induced fluorescence detection of vibrationally excited OH. The OH was produced by the reaction of O1D with H2. The observation of ‘blue-shifted’ laser induced fluorescence provided high detection sensitivity and eliminated probe laser interference effects. The results confirm that there is a significant quantum yield for O1D production at wavelengths longer than 320 nm. A measurable yield is observed out to 336 nm providing evidence for spin-forbidden production of O1D.

Kinetics of the vibrational deactivation of OH X 2II (v = 3, 2, 1) with hydrides and reduced sulfides

The deactivation kinetics of vibrationally excited OH X 2 II were studied using the pulsed laser photolysis–pulsed laser induced fluorescence technique. Photolysis of O 3 at 266 nm was used to produce O( 1 D) which reacted rapidly with a precursor to produce OH(v). Temporal profiles of OH(v) were obtained by exciting off-diagonal (Δv = −1) transitions in the A–X band of OH and monitoring the diagonal, blue-shifted fluorescence. Deactivation rate coefficients for OH (v = 3, 2, 1) with H 2 O and CH 4 and for OH (v = 2, 1) with NH 3 , CS 2 , (CH 3 ) 2 S, CH 3 SH and CH 3 Br were obtained. The mechanisms of deactivation and the relationship of the deactivation rate coefficients to the thermal association rate coefficients for OH complexes are discussed.

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 (v = 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 = −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 = 1–5) with O2 and N2 and for OH (v = 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.

Correlated photofragment product distributions in the photodissociation of NO2 at 212.8 nm

Abstract The 212.8 nm photolysis of NO2 has been studied using REMPI–TOF and LIF techniques. Photodissociation occurs via a parallel transition on a time scale which is rapid compared to molecular rotation, producing anisotropic TOF profiles. Analysis of state selected NO flight profiles gives the correlated photofragment product distributions and shows that the predominant photodissociation channel produces vibrationally excited NO in v=3 with O 1 D as the co-fragment. If NO is formed in a rovibrational level for which O 1 D production is energetically allowed then it is the exclusively produced co-fragment.

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

High-resolution absorption cross sections of formaldehyde in the 30285-32890 cm -1 (304-330 nm) spectral region

Absolute room temperature (294 ± 2 K) absorption cross sections for the Ã(1)A(2)-X̃(1)A(1) electronic transition of formaldehyde have been measured over the spectral range 30,285-32,890 cm(-1) (304-330 nm) using ultraviolet (UV) laser absorption spectroscopy. Accurate high-resolution absorption cross sections are essential for atmospheric monitoring and understanding the photochemistry of this important atmospheric compound. Absorption cross sections were obtained at an instrumental resolution better than 0.09 cm(-1), which is slightly broader than the Doppler width of a rotational line of formaldehyde at 300 K (∼0.07 cm(-1)) and so we were able to resolve all but the most closely spaced lines. Comparisons with previous data as well as with computer simulations have been made. Pressure broadening was studied for the collision partners He, O(2), N(2), and H(2)O and the resulting broadening parameters have been measured and increase with the strength of intermolecular interaction between formaldehyde and the collision partner. The pressure broadening coefficient for H(2)O is an order of magnitude larger than the coefficients for O(2) and N(2) and will contribute significantly to spectral line broadening in the lower atmosphere. Spectral data are made available as Supporting Information.