CAPS: 2001 Publications

We present direct, pressure-dependent measurements of OH yields from gas-phase ozone-alkene reactions, using the Harvard HOx instrument to obtain sensitive, accurate, and precise OH concentrations. As in previous studies from our laboratory, steady-state [OH] is measured by laser-induced fluorescence (LIF); here, the accurate LIF calibration allows us to present absolute OH yields for the first time. To calibrate our original LIF system, we measure yields from ozone plus tetramethylethylene (TME) as a function of pressure. The pressure dependence agrees with our previous results, so our yield measurements are now of comparable accuracy to those from indirect studies. Prompt, low-pressure yields agree well with 1 atm yields measured over longer timescales, confirming that much of the OH arises from decomposition of stabilized carbonyl oxides. We also explore the pressure dependence of yields from ethene, isobutene, and isoprene. Yields from isobutene and isoprene are pressure-dependent, consistent with the formation of substituted carbonyl oxides. We observe no pressure dependence of the OH yield from ozone + ethene, finding instead a constant yield of 14%, in line with most 1 atm studies but in contrast with the results of another pressure-dependent study. C1 Harvard Univ, Dept Chem & Biol Chem, Cambridge, MA 02138 USA. Carnegie Mellon Univ, Dept Chem, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA. SUNY Albany, Dept Earth & Atmospher Sci, Albany, NY 12203 USA. SUNY Albany, Atmospher Sci Res Ctr, Albany, NY 12203 USA.

In a companion paper (Kroll, J. K.; Clarke, J. S.; Donahue, N. M.; Anderson, J. G.; Demerjian, K. L. J. Phys. Chem. A 2001, 105, 1554) we present direct measurements of hydroxyl radical (OH) yields for the gas-phase reaction of ozone with a number of symmetric alkenes. Yields are strongly pressure-dependent, contrary to the results of prior scavenger studies. Here we present a statistical-dynamical model of OH production from the reaction, utilizing RRKM/master equation calculations to determine the fate of the carbonyl oxide intermediate. This model agrees with our experimental results, in that both theory and observations indicate strongly pressure-dependent OH yields. Our calculations also suggest that ethene ozonolysis produces OH via a different channel than the substituted alkenes, though the identity of this channel is not clear. This channel may play a role in the ozonolysis of monosubstituted alkenes as well. Our time-dependent master equation calculations show that the discrepancy between OH yields measured in our direct study and those measured in prior scavenger studies may arise from differing experimental time scales; on short time scales, OH is formed only from the vibrationally excited carbonyl oxide intermediate, whereas on longer time scales OH formation from thermal dissociation may be significant. To demonstrate this we present time-dependent measurements of OH yields at 10 Torr and 100 Torr; yields begin increasing after hundreds of milliseconds, an effect which is much more pronounced at 100 Torr. These results are entirely consistent with theoretical predictions. In the atmosphere, the thermalized carbonyl oxide may be susceptible to bimolecular reactions which, if fast enough, could prevent dissociation to OH; however there is little experimental evidence that any such reactions are important. Thus we conclude that both mechanisms of OK formation (dissociation of vibrationally excited carbonyl oxide and dissociation of thermalized carbonyl oxide) are significant in the troposphere. C1 Harvard Univ, Dept Chem & Chem Biol, Cambridge, MA 02138 USA. SUNY Albany, Dept Earth & Atmospher Sci, Albany, NY 12203 USA. SUNY Albany, Atmospher Sci Res Ctr, Albany, NY 12203 USA. Carnegie Mellon Univ, Dept Chem, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA.

Radical-molecule reaction barriers are often the product of an avoided curve crossing between two states: the reactant ground state, which ultimately correlates with a product ionic state, and a reactant ionic state, which ultimately correlates with the product ground state. The energy, location, and loose-mode frequencies of the transition state are controlled by this interaction. The curve crossing itself is constrained by long-range Coulombic forces acting primarily on the ionic states as the reactants approach each other. The crossing height is in essence a geometric mean of the ionic surface heights; a low ionic state energy in either the reactants or the products will force a low reaction barrier. The crossing location is controlled primarily by any asymmetry in the reactant and product ionic heights, while the interaction distance of the transition state is controlled by a balance in gradients on the ground and ionic states. The frequencies related to translation of the separated reagents within the center of mass frame of reference are controlled by the same physics controlling the barrier height, as is the imaginary frequency associated with the reaction itself. This drives a tight correlation between barrier heights. transition state frequencies (and thus preexponential terms), and the imaginary frequency (and thus the tunneling term). This is demonstrated by analyzing a series of H atom transfers from a mainfold of alkanes to a mainfold of atoms. The Hammond postulate-reaction enthalpy controls transition state location-does not correspond to the mechanism controlling either barrier height or location, but rather appears to work in cases where ionization potential correlates with bond strengths. C1 Harvard Univ, Dept Chem & Biol Chem, Cambridge, MA 02138 USA.

We here consider the fundamental interactions which drive barrier evolution in atom transfer reactions. By applying the functional behavior predicted by second-order nondegenerate perturbation theory to a symmetric linear curve crossing model, we are able to derive a simple formula for two-state mixing in the near-field. It becomes readily apparent that the system property most critically responsible for governing the magnitude of coupling is the diabatic state-to-state overlap. In order to explore this parameter in depth, we outline a basic strategy for diabatic state construction in the near-field and use the imposed symmetry requirements and phase relations to derive a functional form which relates the state-to-state overlap to molecular orbital properties of the isolated reactants and the interatomic overlap matrix. Finally, we show how trends in barrier heights may be analyzed in the context of combined far-field and near-field effects and how these effects may be separated in order to provide insight into the underlying physics and broadly applicable mechanistic information. C1 Harvard Univ, Dept Chem & Biol Chem, Cambridge, MA 02138 USA.

The gas-phase reactions between HOchi and NOchi are critical in determining the chemical composition of both the troposphere and stratosphere. They dominate both interconversion among radical species and formation of stable reservoir species for both HOchi acid NOchi. In many cases, the rates of these reactions are known, but the products and mechanisms are less well understood. In particular, the distribution of products among available channels as a function of temperature and pressure is very uncertain for several crucial reactions. One important reaction is that of OH with NO2; some fraction of reactions may lead to an isomer of nitric acid, peroxynitrous acid (HOONO), though this species has not been observed in the gas phase. We present an investigation of that possibility. With reaction modulation FTIR spectroscopy in our high-pressure flow system, we are able to examine the behavior of various (HOchi + NO2 -> products) reactions with independent control over system temperature and pressure. Application of strict mass-balance in our wall-less reactor allows for quantitative analysis of reactant and product concentrations, even in those cases where the integrated bandwidths are unavailable. We examine the reactions HO2 + NO2 -> HOONO2 and OH + NO2 -> products. Each reaction proceeds through at least one vibrationally excited intermediate, and in each case there is a potential for the dynamics of those intermediates to produce unexpected behavior. In the case of HO2 + NO2, there is no evidence that a hydrogen atom transfer in the intermediate produces any HONO, even at low pressure. This is consistent with previous work. In the case of OH + NO2 there are almost certainly two intermediates, HOONO and HONO2, but we see no evidence for stable HOONO formation, even at 230 K. C1 Harvard Univ, Dept Chem & Biol Chem, Cambridge, MA 02138 USA.

The reaction of OH with NO2 is central to atmospheric chemistry, and its dynamics can be constrained by studying the kinetics of isotopically labeled (OH)-O-18 with NO2. This labeling opens an isotopic scrambling pathway in the reaction coordinate for nitric acid formation, providing experimental constraints on the high-pressure behavior of the reaction with data obtained at low pressures. This reaction, however, is complicated by the presence of a second product isomer, peroxynitrous acid (HOONO), which does not have a scrambling pathway. We present data for the reaction of (OH)-O-18 with NO2 at room temperature between 4 and 200 Torr. The reaction is rapid and independent of pressure. We also locate the H-atom isomerization transition state and show that the isomerization rate constant is at least an order of magnitude faster than adduct dissociation. These results allow us to accurately constrain the formation rate constant of HONO2, which is a factor of 5 slower than the observed OH removal rate constant at high pressure. We conclude that the difference is due to HOONO formation. Our conclusion is consistent with recent theoretical predictions of this branching, and also provides the only self-consistent reconciliation of the high-pressure data with the remainder of the experimental data set. C1 Harvard Univ, Dept Chem & Biol Chem, Cambridge, MA 02138 USA. Univ Calif Los Alamos Natl Lab, Los Alamos, NM 87545 USA.

The gas-phase reaction of ozone with alkenes is known to be a dark source of HOchi radicals (such as OH, H, and R) in the troposphere, though the reaction mechanism is currently under debate. It is understood that a key intermediate in the reaction is the carbonyl oxide, which is formed with an excess of vibrational energy, The branching ratios of the ozone-alkene reaction products (and thus HOchi yields) depend critically on the fate of this intermediate: it may undergo unimolecular reaction (forming either OH or dioxirane) or be collisionally stabilized by the bath gas. To investigate this competition between reaction and quenching, we present direct, pressure-dependent measurements of hydroxyl radical (OH) yields for a number of gas-phase ozone-alkene reactions. Experiments are carried out in a high-pressure flow system (HPFS) equipped to detect OH using laser-induced fluorescence (LIF), Hydroxyl radicals are measured in steady state, formed from the ozone-alkene reaction and lost to reaction with the alkene. Short reaction times (usually similar to 10 ms) ensure negligible interference from secondary and heterogeneous reactions. For all substituted alkenes covered in this study, low-pressure yields are large but decrease rapidly with pressure, resulting in yields at 1 atm which are significantly lower than current recommendations and indicating the important role of collisional stabilization in determining OH yield. The influence of alkene size and degree of substitution on pressure-dependent yield is consistent with the influence of collisional stabilization as well as the accepted reaction mechanism. C1 Harvard Univ, Dept Chem & Chem Biol, Cambridge, MA 02138 USA. SUNY Albany, Dept Earth & Atmospher Sci, Albany, NY 12203 USA. SUNY Albany, Atmospher Sci Res Ctr, Albany, NY 12203 USA.

Under many conditions size-resolved aqueous-phase chemistry models predict higher sulfate production rates than comparable bulk aqueous-phase models. However, there are special circumstances under which bulk and size-resolved models offer similar predictions. These special conditions include alkaline conditions (when there is a high ammonia to nitric acid ratio or a large amount of alkaline dust) or conditions under which the initial H2O2 concentration exceeds that Of SO2. Given that bulk models are less computationally-intensive than corresponding size-resolved models, a model equipped to combine the accuracy of the size-resolved code with the efficiency of the bulk method is proposed in this work. Bulk and two-section size-resolved approaches are combined into a single variable size-resolution model (VSRM) in an effort to combine both accuracy and computational speed. Depending on initial system conditions, bulk or size-resolved calculations are executed based on a set of semi-empirical rules. These rules were generated based on our understanding of the system and from the results of many model simulations for a range of input conditions. For the conditions examined here, on average, the VSRM sulfate predictions are within 3% of a six-section size-resolved model, but the VSRM is fifteen times faster. (C) 2001 Elsevier Science Ltd. All rights reserved. C1 Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA.

The semivolatile oxidation products (trans-norpinic acid, pinic acid, cis-pinonic acid, etc.) of the biogenic monoterpenes (alpha -pinene, beta -pinene, etc.) contribute to the atmospheric burden of particulate matter. Using the tandem differential mobility analysis (TDMA) technique evaporation rates of glutaric acid, trans-norpinic acid, and pinic acid particles were measured in a laminar flow reactor. The vapor pressure of glutaric acid was found to be log(p(glutaric)(0)/Pa) = -3510 K/T + 8.647 over the temperature range 290-300 K in good agreement with the values previously reported by Tao and McMurry (1989). The measured vapor pressure of trans-norpinic acid over the temperature range 290-312 K is log(p(norpinic)(0)/Pa) = -2196.9 K/T + 3.522, and the vapor pressure of pinic acid is log(p(pinic)(0)/ Pa) 5691.7 K/T+ 14.73 over the temperature range 290-323 K. The uncertainty on the reported vapor pressures is estimated to be approximately +/- 50%. The vapor pressure of cis-pinonic acid is estimated to be of the order of 7 x 10(-5) Pa at 296 K. C1 Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA. Univ Copenhagen, Dept Chem, DK-2100 Copenhagen, Denmark.

The partitioning of nitrate and ammonium between the gas and particulate phases is studied combining available equilibrium models and measurements taken in Mexico City during the 1997 IMADA-AVER held campaign. Based on this analysis, there are no significant differences in model predictions, but some discrepancies exist between predictions and observations. The inclusion of crustal elements in the modeling framework improves agreement of model predictions: for particulate nitrate against measurements by approximately 5%. Although some equilibrium aerosol models do not explicitly treat crustal elements, these species can be treated as equivalent concentrations of sodium. Atmospheric equilibrium models predict daily average PM2.5 nitrate concentrations within 20% of the IMADA-AVER measurements at the MER site. Six-hour average PM2.5 nitrate concentrations an predicted within 30-50% on average except for the afternoon sampling periods (12:00-18:00h). Investigating the possible sources of these discrepancies, it appears that a dynamic instead of an equilibrium approach is more suitable in reproducing aerosol behaviour during these afternoon periods. By applying the Multicomponent Aerosol Dynamic Model (MADM), model performance in predicting concentrations of particulate nitrate significantly improves during the afternoon periods. (C) 2001 Elsevier Science Ltd. All rights reserved. C1 Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA. Univ Nacl Autonoma Mexico, Fac Quim, Mexico City 04310, DF, Mexico.

This paper describes measurements of NOx emissions from one engine on a commercial towboat operating on the Upper Ohio River system around the Port of Pittsburgh. Continuous measurements were made over a one-week period to characterize emissions during normal operations. The average NOx emission factor is 70 +/- 4.2 kg of NOx pert of fuel, similar to that of larger marine engines. A vessel-specific duty cycle is derived to characterize the towboat's operations; more than 50% of the time the vessel engines are at idle. Although recently promulgated EPA regulations apply only to new marine engines, these data provide insight into inland-river operations, which can be used to evaluate these regulations within the inland river context. This vessel operates as a courier service, scheduling pickups and deliveries of singleor multiple-barge loads per customers' requests; as many as 30% of the 277 towboats in the Pittsburgh region operate in this fashion. The EPA-prescribed ISO E3 duty cycle does not accurately describe inland-river operations of this towboat: its application overestimates actual NOx emissions by 14%. Only 41% of this vessel's operations fall within the Not-To-Exceed Zone defined by the EPA regulations, which limits the effectiveness of this component of the regulations to limit emissions from vessels that operate in a similar fashion. C1 Carnegie Mellon Univ, Dept Mech Engn, Pittsburgh, PA 15213 USA.

This paper describes a technique developed to make in situ, time-resolved measurements of the effective thermal conductivity of ash deposits formed under conditions that closely replicate those found in the convective pass of a commercial boiler. Since ash deposit thermal conductivity is thought to be strongly dependent on deposit microstructure, the technique is designed to minimize the disturbance of the natural deposit microstructure. Traditional techniques for measuring deposit thermal conductivity generally do not preserve the sample microstructure. Experiments are described that demonstrate the technique, quantify the experimental uncertainty, and determine the thermal conductivity of highly porous, unsintered deposits. The average measured thermal conductivity of loose, unsintered deposits is 0.14 +/- 0.03 W m(-1) K-1, approximately midway between rational theoretical limits for deposit thermal conductivity. C1 Sandia Natl Labs, Combust Res Facil, Livermore, CA 94551 USA.

This paper describes an experimental study that examines the influence of sintering and microstructure on ash deposit thermal conductivity. The measurements are made using a technique developed to make in situ, time-resolved measurements of the effective thermal conductivity of ash deposits formed under conditions that closely replicate those found in the convective pass of a commercial boiler. The technique is designed to minimize the disturbance of the natural deposit microstructure. The initial stages of sintering and densification are accompanied by an increase in deposit thermal conductivity. Subsequent sintering continues to density the deposit, but has little effect on deposit thermal conductivity. SEM analyses indicate that sintering creates a layered deposit structure with a relatively unsintered innermost layer. We hypothesize that this unsintered layer largely determines the overall deposit thermal conductivity. A theoretical model that treats a deposit as a two-layered material predicts the observed trends in thermal conductivity. C1 Sandia Natl Labs, Combust Res Facil, Livermore, CA 94551 USA.

An on-line simulation of aerosol sulfate, nitrate, ammonium, and water in the Goddard Institute for Space Studies general circulation model (GCM II-prime) has been used to estimate direct aerosol radiative forcing for the years 1800, 2000, and 2100. This is the first direct forcing estimate based on the equilibrium water content of a changing SO42-NO3-NH4+ mixture and the first estimate of nitrate forcing based on a global model of nitrate aerosol, Present-day global and annual average anthropogenic direct forcing is estimated to be -0.95 and -0.19 W/m(2) for sulfate and nitrate, respectively. Simulations with a future emissions scenario indicate that nitrate forcing could increase to -1.28 W/m(2) by 2100, while sulfate forcing declines to -0.85 W/m(2). This result shows that future estimates of aerosol forcing based solely on predicted sulfate concentrations may be misleading and that the potential for significant concentrations of ammonium nitrate needs to be considered in estimates of future climate change. Calculated direct aerosol forcing is highly sensitive to the model treatment of water uptake. By calculating the equilibrium water content of a SO42-NH4+ aerosol mixture and the optical properties of the wet aerosol, we estimate a forcing that is almost 35% greater than that derived from correcting a low relative humidity scattering coefficient with an empirical f(RH) factor. The discrepancy stems from the failure of the empirical parameterization to adequately account for water uptake above about 90% relative humidity. These results suggest that water uptake above 90% RH may make a substantial contribution to average direct forcing, although subgrid-scale variability makes it difficult to represent humid areas in a GCM. C1 CALTECH, Dept Chem, Pasadena, CA 91125 USA. NASA, Goddard Inst Space Studies, New York, NY 10025 USA. Harvard Univ, Div Engn & Appl Sci, Cambridge, MA 02138 USA.

In this study, particle transport and deposition were studied in a wind tunnel by measuring both the airflow turbulence characteristics and deposition of monodisperse uranine particles of 2.0-4.5 mum diameter on smooth, horizontal surfaces. Turbulence characteristics behind a 2.54 cm high rectangular bar were investigated for free stream velocities ranging from 3.3 m/s to 15.3 m/s. The well-developed boundary layer thickness was approximately four times the height of the rectangular bar at a distance of about 55 cm from the bar. Results of the wind tunnel experiments show the complex nature of deposition in turbulent flows due to the interactions between particles and turbulence. In general, the particle deposition flux is larger in the near wake region than in the far wake region. The particle deposition flux is roughly independent of the dimensionless particle relaxation time when the relaxation time is less than one, but increases rapidly as the relaxation time increases above one. C1 Carnegie Mellon Univ, Dept Civil & Environm Engn, Pittsburgh, PA 15213 USA.