Jaron C. Hansen

Radical–Molecule Complexes: Changing Our Perspective on the Molecular Mechanisms of Radical–Molecule Reactions and their Impact on Atmospheric Chemistry

The strong binding energies for radical-molecule complexes allow these unique systems to influence both the photochemical and reaction pathways for both chemical and atmospherically important reactions. This Minireview summarizes the work that has been presented in the literature and also introduces new radical-molecule complexes that may play important roles in new chemistry.

Vascular Function and Short-Term Exposure to Fine Particulate Air Pollution

ABSTRACT Exposure to fine particulate air pollution has been implicated as a risk factor for cardiopulmonary disease and mortality. Proposed biological pathways imply that particle-induced pulmonary and systemic inflammation play a role in activating the vascular endothelium and altering vascular function. Potential effects of fine particulate pollution on vascular function are explored using controlled chamber exposure and uncontrolled ambient exposure. Research subjects included four panels with a total of 26 healthy nonsmoking young adults. On two study visits, at least 7 days apart, subjects spent 3 hr in a controlled-exposure chamber exposed to 150–200 μg/m3 of fine particles generated from coal or wood combustion and 3 hr in a clean room, with exposure and nonexposure periods alternated between visits. Baseline, postexposure, and post-clean room reactive hyperemia–peripheral arterial tonometry (RH-PAT) was conducted. A microvascular responsiveness index, defined as the log of the RH-PAT ratio, was calculated. There was no contemporaneous vascular response to the few hours of controlled exposure. Declines in vascular response were associated with elevated ambient exposures for the previous 2 days, especially for female subjects. Cumulative exposure to real-life fine particulate pollution may affect vascular function. More research is needed to determine the roles of age and gender, the effect of pollution sources, the importance of cumulative exposure over a few days versus a few hours, and the lag time between exposure and response. IMPLICATIONS This study found no contemporaneous vascular response in healthy young adults to exposure to a few hours of generated fine particulate matter in a controlled experimental setting. Two-day uncontrolled ambient fine particulate pollution at much lower concentrations was associated with small but significant changes in vascular function, primarily for female subjects only. Clearly, additional research is needed to establish the potential effect of cumulative exposure over a few days versus a few hours and the lag time between exposure and response.

The effects of water vapor on the CH3O2 self-reaction and reaction with HO2.

The gas phase reactions of CH3O2 + CH3O2, HO2 + HO2, and CH3O2 + HO2 in the presence of water vapor have been studied at temperatures between 263 and 303 K using laser flash photolysis coupled with UV time-resolved absorption detection at 220 and 260 nm. Water vapor concentrations were quantified using tunable diode laser spectroscopy operating in the mid-IR. The HO2 self-reaction rate constant is significantly enhanced by water vapor, consistent with what others have reported, whereas the CH3O2 self-reaction and the cross-reaction (CH3O2 + HO2) rate constants are nearly unaffected. The enhancement in the HO2 self-reaction rate coefficient occurs because of the formation of a strongly bound (6.9 kcal mol(-1)) HO2 x H2O complex during the reaction mechanism where the H2O acts as an energy chaperone. The nominal impact of water vapor on the CH3O2 self-reaction rate coefficient is consistent with recent high level ab initio calculations that predict a weakly bound CH3O2 x H2O complex (2.3 kcal mol(-1)). The smaller binding energy of the CH3O2 x H2O complex does not favor its formation and consequent participation in the methyl peroxy self-reaction mechanism.

Computational study on the existence of organic peroxy radical-water complexes (RO2.H2O).

The existence of a series of organic peroxy radical-water complexes [CH3O2.H2O (methyl peroxy); CH3CH2O2.H2O (ethyl peroxy); CH3C(O)O2.H2O (acetyl peroxy); CH3C(O)CH2O2.H2O (acetonyl peroxy); CH2(OH)O2.H2O (hydroxyl methyl peroxy); CH2(OH)CH2O2.H2O (2-hydroxy ethyl peroxy); CH2(F)O2.H2O (fluoro methyl peroxy); CH2(F)CH2O2.H2O (2-fluoro ethyl peroxy)] is evaluated using high level ab initio calculations. A wide range of binding energies is predicted for these complexes, in which the difference in binding energies can be explained by examination of the composition of the R group attached to the peroxy moiety. The general trend in binding energies has been determined to be as follows: fluorine approximately alkyl < carbonyl < alcohol. The weakest bound complex, CH3O2.H2O, is calculated to be bound by 2.3 kcal mol-1, and the strongest, the CH2(OH)O2.H2O complex, is bound by 5.1 kcal mol-1. The binding energy of the peroxy radical-water complexes which contain carbonyl and alcohol groups indicates that these complexes may perturb the kinetics and product branching ratios of reactions involving these complexes.

Computational study of isoprene hydroxyalkyl peroxy radical-water complexes (C5H8(OH)O2-H2O).

Herein we report an extensive ab initio study on the existence of eight beta-hydroxy isoprene peroxy radical-water complexes. Binding energies calculated at the MP2(full)/6-311++G(2d,2p)//CCSD(T)/6-311++G(d,p) level of theory range between 3.85 and 5.66 kcal mol(-1). The results of natural bond orbital calculations are presented to help rationalize complex formation. Atmospheric lifetimes, equilibrium constants, heats of formation, and the relative abundance of complexed to uncomplexed peroxy radicals are also reported and discussed.

Global warming potential assessment for CF3OCF=CF2

We have examined CF3OCF = CF2 regarding its reactivity toward OH radical, its infrared spectroscopic properties, its atmospheric lifetime, and its radiative forcing. From these we then determined the Global Warming Potentials (GWPs) for CF3OCF = CF2. The examination is completed using a combination of discharge flow coupled with mass spectrometer and resonance fluorescence (DF/MS/RF), Fourier transform infrared (FTIR) spectroscopy, ab initio molecular orbital calculation, and atmospheric and radiative transfer modeling. Mass spectral evidence suggests that both HF and CF3OCFC(O)F are products from the reaction of CF3OCF = CF2 with OH. The Arrhenius expression for CF3OCF = CF2 + OH is determined to be k1 = (6.41±0.82)×10−11 exp[(−868±40)/T] cm3 molecule−1 s−1 in the temperature range of 253–348 K. The atmospheric lifetime of CF3OCF = CF2 is estimated to be less than 5 days due to the OH attack. The calculated vibrational frequencies using ab initio molecular orbital calculations are in good agreement with FTIR experimental observation for the CF3OCF = CF2 molecule. Both C-O and C-F stretching modes in the CF3OCF = CF2 contribute to prominent absorption in the atmospheric window region. The absolute adjusted radiative forcing at the tropopause due to an increase in the concentration of CF3OCF = CF2 by one part per billion by volume (ppbv) is calculated to be 0.041 W m−2 ppbv−1. The Global Warming Potential for CF3OCF = CF2 is evaluated to be 0.004 for 100-year time horizon.

Rate constant for the reactions of CF3OCHFCF3 with OH and Cl

Abstract The kinetics of reactions of CF 3 OCHFCF 3 with hydroxyl radicals and chlorine atoms has been investigated using a discharged flow combined with both mass spectrometer and resonance fluorescence technique and using a relative rate technique, respectively, at 298 K. The rate constant for the reactions of CF 3 OCHFCF 3 with OH and Cl was determined to be k 1 =(4.98±1.64)×10 −15 cm 3 molecule −1 s −1 and k 2 =(3.1±2.5)×10 −14 cm 3 molecule −1 s −1 , respectively. On the basis of our kinetics measurements, the tropospheric lifetime of CF 3 OCHFCF 3 is calculated to be about 8 years, primarily due to reaction with the hydroxyl radicals in the troposphere.

Semicontinuous PM2.5 and PM10 mass and composition measurements in Lindon, Utah, during winter 2007.

Abstract The U.S. Environmental Protection Agency is promoting the development and application of sampling methods for the semicontinuous determination of fine particulate matter (PM2.5, particles with an aerodynamic diameter <2.5 µm) mass and chemical composition. Data obtained with these methods will significantly improve the understanding of the primary sources, chemical conversion processes, and meteorological atmospheric processes that lead to observed PM2.5 concentrations and will aid in the understanding of the etiology of PM2.5-related health effects. During January and February 2007, several semicontinuous particulate matter (PM) monitoring systems were compared at the Utah State Lindon Air Quality Sampling site. Semicontinuous monitors included instruments to measure total PM2.5 mass (filter dynamic measurement system [FDMS] tapered element oscillating microbalance [TEOM], GRIMM), nonvolatile PM2.5 mass (TEOM), sulfate and nitrate (two PM2.5 and one PM10 [PM with an aerodynamic diameter <10 µm] ionchromatographic-based samplers), and black carbon (aethalometer). PM10 semicontinuous mass measurements were made with GRIMM and TEOM instruments. These measurements were all made on a 1-hr average basis. Source apportionment analysis indicated that sources impacting the site were mainly urban sources and included mobile sources (gasoline and diesel) and residential burning of wood, with some elevated concentrations because of the effect of winter inversions. The FDMS TEOM and GRIMM instruments were in good agreement, but TEOM monitor measurements were low because of the presence of significant semi-volatile material. Semi-volatile mass was present dominantly in the PM2.5 mass.

Composition and secondary formation of fine particulate matter in the Salt Lake Valley: Winter 2009

Under the National Ambient Air Quality Standards (NAAQS), put in place as a result of the Clean Air Amendments of 1990, three regions in the state of Utah are in violation of the NAAQS for PM10 and PM2.5 (Salt Lake County, Ogden City, and Utah County). These regions are susceptible to strong inversions that can persist for days to weeks. This meteorology, coupled with the metropolitan nature of these regions, contributes to its violation of the NAAQS for PM during the winter. During January–February 2009, 1-hr averaged concentrations of PM10-2.5, PM2.5, NOx, NO2, NO, O3, CO, and NH3 were measured. Particulate-phase nitrate, nitrite, and sulfate and gas-phase HONO, HNO3, and SO2 were also measured on a 1-hr average basis. The results indicate that ammonium nitrate averages 40% of the total PM2.5 mass in the absence of inversions and up to 69% during strong inversions. Also, the formation of ammonium nitrate is nitric acid limited. Overall, the lower boundary layer in the Salt Lake Valley appears to be oxidant and volatile organic carbon (VOC) limited with respect to ozone formation. The most effective way to reduce ammonium nitrate secondary particle formation during the inversions period is to reduce NOx emissions. However, a decrease in NOx will increase ozone concentrations. A better definition of the complete ozone isopleths would better inform this decision. Implications: Monitoring of air pollution constituents in Salt Lake City, UT, during periods in which PM2.5 concentrations exceeded the NAAQS, reveals that secondary aerosol formation for this region is NOx limited. Therefore, NOx emissions should be targeted in order to reduce secondary particle formation and PM2.5. Data also indicate that the highest concentrations of sulfur dioxide are associated with winds from the north-northwest, the location of several small refineries.

A study of the kinetics and mechanisms involved in the atmospheric degradation of bromoform by atomic chlorine

Abstract Ab initio molecular orbital theory, in combination with FTIR spectroscopy, has been used to determine a second-order rate constant for the hydrogen abstraction reaction involving bromoform and chlorine atom. The rate constant for the reaction is determined to be 2.7±0.2×10 −13 cm 3 molecule −1 s −1 at 298 K. Ab initio molecular orbital theory calculations at the QCISD(T)/6-311G(d,p)//MP2/6-31G(d) level of theory predict the barrier for the hydrogen abstraction reaction to be 1.6 kcal mol −1 . It has also been determined that, in the presence of oxygen, the reaction will lead to a one-to-one conversion of bromoform into carbonyl dibromide.