CAPS: 2000 Publications

Theories of barrier height control in radical-molecule reactions must be tested against data spanning a wide range in reactivity, by a method for separating multiple, correlated terms in the theories. Here we present an analysis technique designed to reveal reactant properties controlling reactivity and rate constant measurements for an extensive series of reactions where that control is very much in doubt. The measurements were made with a new high-pressure flow experiment designed specifically to facilitate the study of multiple radicals. The derivative technique consists of graphically analyzing partial derivatives of modeled barrier heights, using measured barriers and reactant properties. We use this technique to uncover the governing parameters for hydrogen atom abstraction reactions, which are dominated by an essentially ionic excited state of the reactants. More generally, multiple excited states contribute to barrier formation. with different states dominating for different classes of reactions. The new experimental apparatus is a significantly more flexible (and much smaller) version of our original high-pressure flow system. In this case, we use hydrogen atoms as the attacking radical, enabling a study of hydrogen atom addition to alkenes, where reactivity may he controlled by ionic states, singlet-triplet splittings, reaction enthalpy, or a combination of these factors. By using hydrogen atoms, we eliminate potentially confounding influences on the ground state, and by selecting a series of alkenes and haloalkenes to systematically vary ionization potential, singlet-triplet splittings, and Jr-electron density, we lay the foundation for an extensive study of barrier height control for this reaction class. The data presented here include the first temperature-dependent measurements for 9 of the 13 reactions studied. C1 Harvard Univ, Dept Chem & Chem Biol, Cambridge, MA 02138 USA.

In order to identify the underlying factors determining barrier heights when hydrogen atoms add to alkenes, we present a theoretical framework isolating the fundamental quantum-chemical properties involved and enabling evaluation of the relative influence of each property. This approach describes the control of these barriers and motivates a series of experimental measurements as a rigorous test. A two-state avoided curve crossing model provides the essential description. but only when multiple excited states are combined to yield a mixed state of dual covalent-ionic character. We show that variations in mixed-state energy drive the evolution in barrier heights, and that by selecting a set of test reactions with diverse energetic and overlap interactions, one may discover which of several excited states dominates this evolution. Results from the experimental test show conclusively that it is variation in the lowest ionic-state energy, and not variations in either singlet-triplet splitting or reaction enthalpy that drive barrier height evolution over die series of H + alkene addition reactions. Combining this result with our earlier results for H-atom abstraction reactions, we have demonstrated that barrier heights of essentially all radical-molecule reactions with electrophilic radicals are controlled by the excited ionic states formed by the transfer of an electron from the molecule to the radical. C1 Harvard Univ, Dept Chem & Chem Biol, Cambridge, MA 02138 USA.

Deliquescence and hygroscopic growth measurements were performed for four internally mixed aerosol mixtures: NaCl-glutaric acid, NaCl-pinonic acid, (NH4)(2)SO4-glutaric acid, and (NH4)(2)SO4-pinonic acid with varying organic mass fractions (0, 0.2, 0.5, 0,8, and 1.0). No effect on the deliquescence relative humidity of the salts was observed for any of the organic mixtures tested. The NaCl-organic mixed aerosols deliquesced at a relative humidity (DRH) 75 +/- 1% and the (NH4)(2)SO4-organic aerosol at 79 +/- 1% independent of organic mass fraction. The growth factors at RH = 85 +/- 1%, G(85%), were also measured for the different aerosol mixtures. There was an observed decrease in 0(85%) with increasing mass fraction of the organic. Measured 0(85%) for the mixtures can be approximated as a first step with the assumption that the species absorb water independently. Overall, the organic portion was observed to enhance the water uptake of the (NH4)(2)SO4organic aerosol systems by as much as a factor of 2-3 for particles consisting of 80% organic acids. The NaCl-organic mixtures presented evidence of positive and negative interaction depending on organic mass fraction, ranging from a 40% decrease to an 20% increase in water uptake as compared to that by the inorganic fraction alone. C1 Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA.

The evaporation of submicrometer ammonium nitrate (NH4NO3) aerosol coated with an organic film was measured in order to determine the effect of the film on mass transfer rate and equilibration time of the semi-volatile inorganic. Ammonium nitrate(NH4NO3) particles 100-200 nm in diameter were coated with dioctyl phthalate (DOP) and allowed to evaporate in a constant temperature laminar Row reactor. Evaporation rates for the organic-coated particles were compared to pure ammonium nitrate evaporation at 22 and 27 degrees C. A decrease, up to 50%, in NH4NO3 evaporation rate due to the presence of the DOP film was observed. The decrease in evaporation due to the DOP can be described mathematically at 22 degrees C by a decrease in the accommodation coefficient for NH4NO3, alpha(NH4No3), from 0.4 (the pure NH4NO3 value) to 0.25 for the DOP-coated NH4NO3. Similarly, at 27 degrees C, a decrease in alpha(NH4NO3) from 0.3 for the pure NH4NO3 to 0.25 for the DOP-coated particles was estimated. The decrease in evaporation rates can also be explained by a decrease in NH4NO3 effective diffusivity. The implications to NH4NO3 formation and evaporation in the atmosphere are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved. C1 Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA.

The potential impact of ship emissions on concentrations of nitrogen oxides and reactive nitrogen compounds in the marine boundary layer is assessed using a global chemical transport model. The model predicts significant enhancements of these compounds over large regions, especially over the northern midlatitude oceans. This result is consistent with a recently published study, though the impacts predicted here are more widespread and the peak enhancements are not as large. However, comparisons of model results with recent measurements over the central North Atlantic Ocean do not provide support for these model predictions. While one cannot completely overlook the possibility that emissions of nitrogen oxides from ships may be overestimated, our analysis suggests that there may be a gap in our understanding of the chemical evolution of ship plumes as they mix into the background atmosphere in the marine boundary layer. On a related note, it is also possible that the overestimate of the impacts of ships on nitrogen oxides in the marine boundary layer by global models is due to the lack of parameterized representations of plume dynamics and chemistry in these models. C1 Duke Univ, Nicholas Sch Environm, Durham, NC 27708 USA. NOAA, Geophys Fluid Dynam Lab, Princeton, NJ 08542 USA. Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA. Michigan Technol Univ, Dept Civil & Environm Engn, Houghton, MI 49931 USA. NOAA, Aeron Lab, Boulder, CO 80303 USA. Univ Colorado, CIRES, Boulder, CO 80303 USA.

Dynamic mass transfer methods have been developed to better describe the interaction of the aerosol population with semi-volatile species such as nitrate, ammonia, and chloride. Unfortunately, these dynamic methods are computationally expensive. Assumptions are often made to reduce the computational cost of explicit dynamic calculations, including instantaneous equilibrium and/or use of bulk-aerosol composition. A novel approach to the modeling of the mass transfer of semi-volatile species is presented. A hybrid method is developed that utilizes equilibrium assumptions for the fine aerosol mode (particles with diameters less than 1 mu m) and the dynamic approach for the coarse aerosol mode. A comparison among three methods (equilibrium, dynamic, and hybrid) is made for varying conditions of aerosol acidity, dry and wet particles, and marine and urban environments. Results show that the hybrid method maintains most of the predictive ability of the dynamic approach and is 50 times more computationally efficient for our test scenario. Sensitivity of the hybrid method to the equilibrium cut-off diameter and to the frequency of the evaluation of the equilibrium portion of the aerosol distribution is also discussed. (C) 2000 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 Aegean, Dept Environm Sci, GR-81100 Mytilene, Greece.

A Multicomponent Aerosol Dynamics Model (MADM) capable of solving the condensation/evaporation equation of atmospheric aerosols is presented. Condensable species may be organic and/or inorganic. For the inorganic constituents the equilibrium model ISORROPIA is used to predict the physical state of the particle, i.e., whether the aerosol is liquid or solid. The mass transfer equations for the fluxes for solid atmospheric particles are developed. MADM is able to simulate aerosol deliquescence, crystallization, solid to solid phase transitions, and acidity transitions. Aerosols of different sizes can be in different physical states (solid, liquid, or partially solid and partially liquid). Novel constraints on the electroneutrality of the species Bur between the gas and aerosol phases are presented for both liquid and solid aerosols, These constraints aid in the stability of the algorithm, yet still allow changes in aerosol acidity. As an example, MADM is used to predict the dynamic response of marine aerosol entering an urban area. C1 Univ Aegean, Dept Environm Sci, GR-81100 Mytilene, Greece. Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA. CALTECH, Dept Chem Engn, Pasadena, CA 91125 USA. Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA.

The hygroscopic nature of atmospheric aerosol has generally been associated with its inorganic fraction. In this study, a group contribution method is used to predict the water absorption of secondary organic aerosol (SOA). Compared against growth measurements of mixed inorganic-organic particles, this method appears to provide a first-order approximation in predicting SOA water absorption. The growth of common SOA species is predicted to be significantly less than common atmospheric inorganic salts such as (NH4)(2)SO4 and NaCl. Using this group contribution method as a tool in predicting SOA water absorption, an integrated modeling approach is developed combining available SOA and inorganic aerosol models to predict overall aerosol behavior. The effect of SOA on water absorption and nitrate partitioning between the gas and aerosol phases is determined. On average, it appears that SOA accounts for approximately 7% of total aerosol water and increases aerosol nitrate concentrations by approximately 10%. At high relative humidity (greater than or equal to 85%) and low SOA mass fractions (<20% of total PM2.5), the role of SOA in nitrate partitioning and its contribution to total aerosol water is negligible. However, the water absorption of SOA appears to be less sensitive to changes in relative humidity than that of inorganic species, and thus at low relative humidity (similar to 50%) and high SOA mass fraction concentrations (similar to 30% of total PM2.5), SOA is predicted to account for approximately 20% of total aerosol water and a 50% increase in aerosol nitrate concentrations. These findings could improve the results of modeling studies where aerosol nitrate has often been underpredicted. C1 Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA.

With the aid of three atmospheric aerosol equilibrium models, we quantify the effect of metastable equilibrium states (efflorescence branch) in comparison to stable (deliquescence branch) on the partitioning of total nitrate between the gas and aerosol phases. On average, efflorescence branch concentrations of aerosol nitrate are 11% greater than those of the deliquescence branch at low aerosol nitrate concentrations ( < 8 mu g m(-3)), whereas for higher aerosol nitrate concentrations ( > 8 mu g m(-3)), deliquescence branch concentrations are 3% greater. In the low aerosol nitrate range, approximately 40% of the time deliquescence and efflorescence branch concentrations of aerosol nitrate have differences greater than 20% implicating the importance of considering both branches of aerosol behavior in this region. The largest differences between the two equilibrium states occur at several sets of conditions: at temperatures above 295 K and mid-range rh (60%), at mid-range temperatures (290-300 K) and low rh ( < 40%), and for sulfate-to-aerosol nitrate molar ratios of less than 0.5 and greater than 1 at low rh ( < 40%). In these two regions; average differences of 1-2 mu g m(-3) between deliquescence and efflorescence branch concentrations of aerosol nitrate are estimated. The potential existence of efflorescence branch aerosols in Southern California, where pollutant levels are high, appears to have a small effect on total nitrate partitioning. However, for areas characterized by moderate-to-low pollutant levels such as the Northeastern US, a significantly larger effect is predicted. The implications of these findings for modeling studies are discussed. (C) 1999 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.

This paper describes a novel in-line sensor that measures the magnitude and direction of gas flow in a tribe. The sensor possesses a unique set of performance characteristics: low detection limit, little resistance to flow: and directional sensitivity. The sensor consists of two hot wire anemometers mounted in a U-shaped tube. Differences in the signals between the two hot wires under low velocity conditions are used to determine the direction of the pow. Calibration curves of flow rate versus measured velocity are used to determine the magnitude of the flow. The sensor has applications in systems that are characterized by naturally driven oscillating flows . [S0098-2202(00)02701-2]. C1 Carnegie Mellon Univ, Dept Mech Engn, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA. EO Lawrence Berkeley Natl Lab, Div Energy & Environm, Berkeley, CA 94720 USA.

Soiling on the walls of limestone buildings can be washed off when the surface erodes due to rain impingement. In this study, the delivery of rain to the 42-story Cathedral of Learning in Pittsburgh, Pennsylvania, represented by a 30 m x 30 m x 160 m rectangular block, was modeled using the RNG K-epsilon model for turbulence and Lagrangian trajectory calculations for individual rain drops. Local Effect Factors (LEF) for the rectangular block compared well with earlier work in the literature. LEFs increased with wind speed, raindrop size, and height along the block. Wind speed, direction, and rain intensity were measured continuously over a seven-week period and provided input parameters for modeling rain fluxes to the Cathedral of Learning. Model results suggested that sections of the building receiving larger amounts of rain corresponded to white areas, indicating that rain fluxes have a significant effect on the soiling patterns. Intermediate wind speeds (2.5 and 5 ms(-1)) resulted in high rain fluxes. Although less frequent, high wind speeds also resulted in high rain fluxes. Much of the rain was delivered to the block as 1.25 and 2.5 mm drops with 5 mm drops having a smaller effect. Consideration of wind incidence angles other than 0 degrees was shown to be important for future modeling efforts. (C) 2000 Elsevier Science Ltd. All rights reserved. C1 Carnegie Mellon Univ, Dept Civil & Environm Engn, Pittsburgh, PA 15213 USA. Natl Ctr Preservat Technol & Training, Natchitoches, LA 71497 USA.

Soiling of limestone caused by air pollution has been studied at the Cathedral of Learning on the University of Pittsburgh campus. The Cathedral was constructed in the 1930s during a period of heavy pollution in Pittsburgh, PA. Archival photographs show that the building became soiled while it was still under construction. Reductions in air pollutant concentrations began in the late 1940s and 1950s and have continued to the present day. Concurrent with decreasing pollution, soiled areas of the stone have been slowly washed by rain, leaving a white, eroded surface. The patterns of white areas in archival photographs of the building are consistent with computer modeling of rain impingement showing greater wash off rates at higher elevations and on the corners of the building. Winds during the rainstorms are predominantly from the quadrant SW to NW at this location, and wind speeds as well as rain intensities are greater wh en winds a re from this quadrant as compared with other quadrants; the sides of the building facing these directions are much less soiled than the opposing sides. Overall, these results suggest that rain washing of soiled areas on buildings occurs over a period of decades, in contrast to the process of soiling that occurs much more rapidly. C1 Carnegie Mellon Univ, Pittsburgh, PA 15213 USA. Natl Ctr Preservat Technol & Training, Natchitoches, LA 71497 USA.