Daniel O'Sullivan - Academia.edu (original) (raw)

Papers by Daniel O'Sullivan

Atmospheric Chemistry and Physics, Jul 17, 2019

Journal of Geophysical Research, May 20, 2008

Atmospheric Measurement Techniques, Apr 4, 2018

An ion-neutral chemical kinetic model is described and used to simulate the negative ion chemistr... more An ion-neutral chemical kinetic model is described and used to simulate the negative ion chemistry occurring within a mixed-reagent ion chemical ionization mass spectrometer (CIMS). The model objective was the establishment of a theoretical basis to understand ambient pressure (variable sample flow and reagent ion carrier gas flow rates), water vapor, ozone and oxides of nitrogen effects on ion cluster sensitivities for hydrogen peroxide (H 2 O 2), methyl peroxide (CH 3 OOH), formic acid (HFo) and acetic acid (HAc). The model development started with established atmospheric ion chemistry mechanisms, thermodynamic data and reaction rate coefficients. The chemical mechanism was augmented with additional reactions and their reaction rate coefficients specific to the analytes. Some existing reaction rate coefficients were modified to enable the model to match laboratory and field campaign determinations of ion cluster sensitivities as functions of CIMS sample flow rate and ambient humidity. Relative trends in predicted and observed sensitivities are compared as instrument specific factors preclude a direct calculation of instrument sensitivity as a function of sample pressure and humidity. Predicted sensitivity trends and experimental sensitivity trends suggested the model captured the reagent ion and cluster chemistry and reproduced trends in ion cluster sensitivity with sample flow and humidity observed with a CIMS instrument developed for atmospheric peroxide measurements (PCIMSs). The model was further used to investigate the potential for isobaric compounds as interferences in the measurement of the above species. For ambient O 3 mixing ratios more than 50 times those of H 2 O 2 , O − 3 (H 2 O) was predicted to be a significant isobaric interference to the measurement of H 2 O 2 using O − 2 (H 2 O 2) at m/z 66. O 3 and NO give rise to species and cluster ions, CO − 3 (H 2 O) and NO − 3 (H 2 O), respectively, which interfere in the measurement of CH 3 OOH using O − 2 (CH 3 OOH) at m/z 80. The CO − 3 (H 2 O) interference assumed one of its O atoms was 18 O and present in the cluster in proportion to its natural abundance. The model results indicated monitoring water vapor mixing ratio, m/z 78 for CO − 3 (H 2 O) and m/z 98 for isotopic CO − 3 (H 2 O) 2 can be used to determine when CO − 3 (H 2 O) interference is significant. Similarly, monitoring water vapor mixing ratio, m/z 62 for NO − 3 and m/z 98 for NO − 3 (H 2 O) 2 can be used to determine when NO − 3 (H 2 O) interference is significant.

AGU Fall Meeting Abstracts, Dec 1, 2002

Chromophoric dissolved organic matter (CDOM) is the optically active component of DOM in surface ... more Chromophoric dissolved organic matter (CDOM) is the optically active component of DOM in surface waters that undergoes chemical change upon absorption of sunlight. Photochemical peroxide generation is an important component of the photochemical oxidation of CDOM in natural waters as well as in redox cycling. However, little work has been done to relate the change in optical properties of CDOM

Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std. Z39.18 Public reporting burden for this col... more Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std. Z39.18 Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 5a. CONTRACT NUMBER a. REPORT 19a. NAME OF RESPONSIBLE PERSON 19b. TELEPHONE NUMBER

Atmospheric Chemistry and Physics, 2010

Phase B (INTEX-B), conducted over the eastern Pacific Ocean during spring 2006. Predicted trace g... more Phase B (INTEX-B), conducted over the eastern Pacific Ocean during spring 2006. Predicted trace gas and aerosol distributions over the Pacific are presented and discussed in terms of transport and source region contributions. Trace species distributions show a strong west (high) to east (low) gradient, with the bulk of the pollutant transport over the central Pacific occurring between ∼20 • N and 50 • N in the Published by Copernicus Publications on behalf of the European Geosciences Union. 2092 B. Adhikary et al.: Regional model analysis of aerosols and trace gases during INTEX-B 2-6 km altitude range. These distributions are evaluated in the eastern Pacific by comparison with the NASA DC-8 and NSF/NCAR C-130 airborne measurements along with observations from the Mt. Bachelor (MBO) surface site. Thirty different meteorological, trace gas and aerosol parameters are compared. In general the meteorological fields are better predicted than gas phase species, which in turn are better predicted than aerosol quantities. PAN is found to be significantly overpredicted over the eastern Pacific, which is attributed to uncertainties in the chemical reaction mechanisms used in current atmospheric chemistry models in general and to the specifically high PAN production in the SAPRC-99 mechanism used in the regional model. A systematic underprediction of the elevated sulfate layer in the eastern Pacific observed by the C-130 is another issue that is identified and discussed. Results from source region tagged CO simulations are used to estimate how the different source regions around the Pacific contribute to the trace gas species distributions. During this period the largest contributions were from China and from fires in South/Southeast and North Asia. For the C-130 flights, which operated off the coast of the Northwest US, the regional CO contributions range as follows: China (35%), South/Southeast Asia fires (35%), North America anthropogenic (20%), and North Asia fires (10%). The transport of pollution into the western US is studied at MBO and a variety of events with elevated Asian dust, and periods with contributions from China and fires from both Asia and North America are discussed. The role of heterogeneous chemistry on the composition over the eastern Pacific is also studied. The impacts of heterogeneous reactions at specific times can be significant, increasing sulfate and nitrate aerosol production and reducing gas phase nitric acid levels appreciably (∼50%).

The Journal of Physical Chemistry A, 2014

Absorption cross sections and quantum yields for NO2 production (Φ 2) are reported for gaseous me... more Absorption cross sections and quantum yields for NO2 production (Φ 2) are reported for gaseous methyl, ethyl, n-propyl and isopropyl nitrate at 294 K. Absorption cross sections in the wavelength range 240-320 nm agree well with prior determinations. NO2 quantum yields at photo-excitation wavelengths of 290, 295 and 315 nm are unity within experimental uncertainties for all the alkyl nitrates studied, and are independent of bath gas (N2) pressure for total sample pressures in the range 250-700 Torr. When averaged over all wavelengths and sample pressures, values of Φ 2 are 1.03  0.05 (methyl nitrate), 0.98  0.09 (ethyl nitrate), 1.01  0.04 (n-propyl nitrate) and 1.00  0.05 (isopropyl nitrate), with uncertainties corresponding to 1 standard deviation. Absorption cross sections for ethyl nitrate, isopropyl nitrate and two unsaturated dinitrate compounds, but-3-ene-1,2-diyl dinitrate and (Z)-but-2ene-1,4-diyl dinitrate in acetonitrile solution are compared to gas-phase values, and over the wavelength range 260-315 nm, the gas-phase values are well reproduced by dividing the liquid phase cross sections by 2.0,1.6, 1.7 and 2.2 respectively. Reasonable estimates of the gas-phase absorption cross sections for low volatility organic nitrates can therefore be obtained by halving the values for acetonitrile solutions. The quantum yield for NO2 formation from photoexcitation of but-3-ene-1,2-diyl dinitrate at 290 nm is significantly lower than those for the alkyl (mono) nitrates: a best estimate of Φ 2  0.25 is obtained from the experimental measurements.

Atmospheric Measurement Techniques Discussions, 2017

An ion-neutral chemical kinetic model is described and used to simulate the negative-ion chemistr... more An ion-neutral chemical kinetic model is described and used to simulate the negative-ion chemistry occurring within a mixed-reagent ion chemical ionization mass spectrometer (CIMS). The model objective was the establishment of a theoretical basis to understand ambient pressure (variable sample flow and reagent ion carrier gas flow rates), water vapor, ozone and oxides of nitrogen effects on ion-cluster sensitivities for hydrogen peroxide (H2O2), methyl peroxide (CH3OOH), formic acid (HFo) and acetic acid (HAc). The model development started with established atmospheric ion chemistry mechanisms, thermodynamic data and reaction rate coefficients. The chemical mechanism was augmented with additional reactions and their reaction rate coefficients specific to the analytes. Some existing reaction rate coefficients were modified to enable the model to match laboratory and field campaign determinations of ion-cluster sensi...

The distribution of Fe(II) in the euphotic waters of the equatorial Pacific Ocean was examined us... more The distribution of Fe(II) in the euphotic waters of the equatorial Pacific Ocean was examined using novel in situ sampling and analytical methodology. Dissolved Fe(II) was isolated and preconcentrated, at depth, using a ligand-exchange chromatographic technique and then determined spectrophotometrically. The distribution of Fe(II) in the upper 100 meters was both temporally and spatially variable but generally exhibited maxima near the surface and often at depths with higher concentrations of Chi a. The concentration of Fe(II) varied from below the detection limit of 0.12 nmol kg-1 to 0.53 nmol kg• 1. Exposure of equatorial Pacific seawater to simulated sunlight in the laboratory resulted in similar Fe(II) concentrations, suggesting that photochemical reduction may be an important source for Fe(II) in these waters. Field measurements indicate that Fe(II) may comprise a significant fraction of the Fe present in euphotic ocean waters, which typically contain less than 1.0 nmol kg-1 of dissolved Fe. The presence of detectable steady-state concentrations of Fe(II) is believed to be the result of faster rates of photochemical and(or) biological reduction of Fe(III), relative to the rates of Fe(II) oxidation by oxygen, peroxides and other oxidants.

We examine wet scavenging of soluble trace gases in storms observed during the Deep Convective Cl... more We examine wet scavenging of soluble trace gases in storms observed during the Deep Convective Clouds and Chemistry (DC3) field campaign. We conduct high‐resolution simulations with the Weather Research and Forecasting model with Chemistry (WRF‐Chem) of a severe storm in Oklahoma. The model represents well the storm location, size, and structure as compared with Next Generation Weather Radar reflectivity, and simulated CO transport is consistent with aircraft observations. Scavenging efficiencies (SEs) between inflow and outflow of soluble species are calculated from aircraft measurements and model simulations. Using a simple wet scavenging scheme, we simulate the SE of each soluble species within the error bars of the observations. The simulated SEs of all species except nitric acid (HNO₃) are highly sensitive to the values specified for the fractions retained in ice when cloud water freezes. To reproduce the observations, we must assume zero ice retention for formaldehyde (CH₂O) a...

Marine pollution bulletin, Jan 30, 2017

The influence of salinity, pH, temperature, and dissolved organic matter on the photolysis rate o... more The influence of salinity, pH, temperature, and dissolved organic matter on the photolysis rate of 2,4,6-trinitrotoluene (TNT) in marine, estuary, and laboratory-prepared waters was studied using a Suntest CPS+® solar simulator equipped with optical filters. TNT degradation rates were determined using HPLC analysis, and products were identified using LC/MS. Minimal or no TNT photolysis occurred under a 395-nm long pass filter, but under a 295-nm filter, first-order TNT degradation rate constants and apparent quantum yields increased with increasing salinity in both natural and artificial seawater. TNT rate constants increased slightly with increasing temperature (10 to 32°C) but did not change significantly with pH (6.4 to 8.1). The addition of dissolved organic matter (up to 5mg/L) to ultrapure water, artificial seawater, and natural seawater increased the TNT photolysis rate constant. Products formed by TNT photolysis in natural seawater were determined to be 2,4,6-trinitrobenzald...

Observations of hydrogen peroxide (H2O2) and methylhydroperoxide (CH3OOH) were made at Mt. Bachel... more Observations of hydrogen peroxide (H2O2) and methylhydroperoxide (CH3OOH) were made at Mt. Bachelor Observatory (MBO), Oregon, during the summer 2007. H2O2 and CH3OOH, primary HOx precursors, were measured in order to constrain the sources and sinks of mercury. Prior to constraining mercury observations, the relationship between peroxide levels and meteorological measurements at MBO were investigated. Ambient peroxides were collected using

Journal of Geophysical Research, 1999

Journal of Chemical & Engineering Data, 2011

Deep Sea Research Part II: Topical Studies in Oceanography, 1998

Aquatic Geochemistry, 2010

Production of fucoxanthin by diatoms has become an alternative research area due to their low cos... more Production of fucoxanthin by diatoms has become an alternative research area due to their low cost, convenience and diversity. The fucoxanthin content of Cylindrotheca closterium and its enhancement by altering the cultivation conditions via oxidative stress were investigated in this study. For this purpose, the extraction parameters were optimized and the highest fucoxanthin concentration (6.58 mg g-1) was achieved within 15.0 minutes at 40 o C. Then, this yield reached to 10.15 mg g-1 in the presence of NaOCl and Fe 2+. It is worth noting that, this is the first time that the effect of oxidative stress on fucoxanthin production in diatom has been studied according to our knowledge. Therefore, the results of this study and the discussion about the mechanisms can be a reference for the enrichment of fucoxanthin from other diatoms.