CAPS: 1998 Publications

We test the hypothesis that the barrier to a gas-phase radical-molecule reaction is controlled by an avoided curve crossing of ground and ionic states of the reactants and products. We focus on the competing role of orbital overlap and energy difference on the delocalization energy of the transition state, comparing the reactions OH + ethane, OH + propane, and OH + cyclopropane using experimental data and theoretical analysis. These reactions constitute a homologous series in which the spatial extent and energy of interacting orbitals change dramatically, providing for an examination of the relative importance of energy sind overlap on barrier height control. In addition, contrasting pictures of barrier height control, either by molecular properties or by bond properties of the reactants and products, are evaluated. Our kinetic data, obtained in a high-pressure flow system, cover a suppressed temperature range (180 - 360 K) in order to isolate the lowest barrier pathway. The results for ethane and propane are consistent with barrier height control by the singly occupied molecular orbital (SOMO) of the OH radical and the highest occupied molecular orbital (HOMO) of the molecule. These are the historically defined frontier orbitals. The results for cyclopropane, however, suggest that it is the interaction of the SOMO with the second highest occupied molecular orbitals (SHOMOs) which controls barrier height. The SHOMOs of cyclopropane are spatially extended relative to the HOMOs; at the transition state the interaction between OH and the SHOMOs of cyclopropane overwhelms the interaction between OH and the HOMOs of cyclopropane. We examine the competition between energy and overlap of two reacting species and present an alternative definition of the frontier orbitals not necessarily as the highest energy orbitals, but rather as the orbitals that delocalize to the greatest extent at the transition state. C1 Harvard Univ, Dept Chem & Chem Biol, Cambridge, MA 02138 USA.

We present a theory of radical-molecule abstraction reactions based on the crossing of reactant ground and ionic states at the transition state. By calculating the evolution of ground- and ionic-state energies as the reactants approach each other, we are able to specify the boundary conditions for an avoided curve crossing problem as the atom is transfered. The lower the ionic-state energy, the lower in energy the transition state will be. This drives strong correlations between barrier heights and the difference of ionicand ground-state energies. This theory successfully explains the evolution of barrier heights in a series of reactions involving alkanes and several radicals (OH, O, H, F, Cl, Br), in which barriers range from 0 to 10 kcal/mol. A perturbation treatment of the ionic- and ground-state energies improves the performance of the theory. We compare predicted curve-crossing heights with observed barriers for both our theory and for the covalent (singlet-triplet) curve-crossing theory. We also compare observed barriers with reaction enthalpy. Only the ionic curve-crossing theory can simultaneously explain both radical and molecule reactivity. C1 Harvard Univ, Dept Chem & Biol Chem, Cambridge, MA 02138 USA.

OH alkane reactions play a central role in atmospheric and combustion chemistry, and accurate modeling requires accurate knowledge of their rate constants over a wide temperature range. We present data for 10 OH radical - alkane reactions over the temperature range 300-400 K and then combine our data with other literature data to find optimally estimated Arrhenius parameters using a modified Arrhenius form consistent with transition state theory. This modified form explicitly accounts for the loose modes in the transition state that cause curvature in Arrhenius plots. It thus fits data over a wide temperature range in a realistic manner, allowing precise comparison of data sets covering different temperature ranges. By comparing our data with other published data, we establish that most of these rate constants are now known to better than 5% accuracy over the 300-400 K temperature range. C1 Harvard Univ, Dept Chem & Biol Chem, Cambridge, MA 02138 USA. SUNY Albany, Dept Earth Atmospher Sci, Albany, NY 12222 USA.

Ozone olefin reactions may be a significant source of OH in the urban atmosphere, but current evidence for OH production is indirect and contested. We report the first direct observation of OH radicals from the reaction of ozone with a series of olefins (ethene, isoprene, trans-2-butene and 2,3 dimethyl-2-butene) in 4-6 torr of nitrogen. Using LIF to directly observe the steady-state of OH produced by the initial ozone-olefin reaction and subsequently destroyed by the OH-olefin reaction, we are able to establish OH yields broadly consistent with indirect values. The identification of the OH is unequivocal, and there is no indication that it is produced by a secondary process. To support these observations, we present a complete ab-initio potential energy surface for the O-3-ethene reaction, extending from the reactants to available products. C1 Harvard Univ, Dept Chem, Cambridge, MA 02138 USA. SUNY Albany, Dept Earth Atmospher Sci, Albany, NY 12203 USA. SUNY Albany, Atmospher Sci Res Ctr, Albany, NY 12203 USA.

A computationally efficient and rigorous thermodynamic model that predicts the physical state and composition of inorganic atmospheric aerosol is presented. One of the main features of the model is the implementation of mutual deliquescence of multicomponent salt particles, which lowers the deliquescence point of the aerosol phase. The model is used to examine the behavior of four types of tropospheric aerosol (marine, urban, remote continental and non-urban continental), and the results are compared with the predictions of two other models currently in use. The results of all three models were generally in good agreement. Differences were found primarily in the mutual deliquescence humidity regions, where the new model predicted the existence of water, and the other two did not. Differences in the behavior (speciation and water absorbing properties) between the aerosol types are pointed out. The new model also needed considerably less CPU time, and always shows stability and robust convergence. C1 Univ Aegean, Dept Environm Sci, Mytilene, Greece. Univ Miami, Rosenstiel Sch Marine & Atmospher Sci, Div Marine & Atmospher Chem, Miami, FL 33149 USA. Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA.

An inorganic aerosol equilibrium model is used to investigate the response of inorganic particulate matter (PM) concentrations with respect to the precursor concentrations of sulfuric acid, ammonia, and nitric acid over a range of temperatures and relative humidities. Diagrams showing regions of PM response to precursor concentrations are generated,thus allowing the qualification of assumptions concerning the response of PM to sulfate and overall sensitivity to ammonia and nitric acid availability. The PM concentration level responds nonlinearly to sulfate and shows overall sensitivity to ammonia and nitric acid availability for specific atmospheric conditions and precursor concentrations. The generated diagrams are applied as a means of approximating the PM response to precursor concentrations for two urban polluted areas. In both cases, reductions in ammonia emissions have the most significant impact on the total PM level. However, such a reduction will result in significant increases in atmospheric acidity. C1 Carnegie Mellon Univ, Dept Chem Engn & Engn Publ Policy, Pittsburgh, PA 15213 USA.

Previous research on the direct effect of atmospheric aerosols on climate has estimated the average radiative forcing per unit sulfate mass, and has used this average to calculate the magnitude and spatial distribution of sulfate forcing. In this paper, we posit that radiative forcing is often a nonlinear function of sulfate mass concentration. In contrast to measures of average forcing, we introduce the concept of marginal forcing, which is defined as the change in radiative forcing for an incremental change in sulfate concentration. A multi-component, size-resolved aerosol box model is used, which couples an aerosol chemical equilibrium model with a model for calculating radiative forcing based on Mie theory. The results for a typical nonurban continental aerosol show that total aerosol mass and radiative forcing are nonlinear functions of sulfate concentration. This nonlinearity is mainly due to the chemical interaction of sulfate with volatile inorganic components of the aerosol (ammonium, nitrate, and water). As a result, the marginal forcing varies significantly as a function of sulfate concentration, from - 550 to + 20 W (g SO42-)(-1) at a relative humidity (RH) of 80%. Estimates of marginal forcing are strongly sensitive to RH. Absolute marginal forcing also decreases significantly with total nitrate concentration, increases with total ammonia concentration, and generally increases with temperature. We estimate hat the bias in assuming a constant average forcing may cause overestimates in local continental aerosol radiative forcing by up to 50%, and in the marginal forcing by a factor of two or more.This bias is greatest at intermediate sulfate concentration, high RH, high total nitrate concentration,low total ammonia concentration( greater than or equal to 2 mu g m(-3)), and low temperature. (C) 1998 Published by 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. Carnegie Mellon Univ, Dept Civil & Environm Engn, Pittsburgh, PA 15213 USA. Univ Aegean, Dept Environm Sci, GR-81100 Mytilene, Greece. Univ Miami, Rosenstiel Sch Marine & Atmospher Sci, Miami, FL 33149 USA.

Atmospheric aerosols have mixed chemical composition, with a variety of inorganic (e.g., sulfate, nitrate, ammonium, and sodium) and organic species often present in a single particle. In the present study, we investigate experimentally the cloud condensation nuclei (CCN) activation of submicron aerosol consisting of an inorganic core (e.g., ammonium sulfate) coated by an organic film, at typical atmospheric supersaturations. We use two types of organic coatings on the (NH4)(2)SO4 particles. The first is glutaric acid, a CCN active organic found in the atmosphere, and the second species is dioctylphthalate (DOP), a nonhygroscopic organic. The CCN activation of (NH4)(2)SO4-glutaric acid particles was measured at a supersaturation of 0.3%, for different inorganic core sizes and organic film thickness. We found that a coating of glutaric acid increases the CCN activation of an (NH4)(2)SO4 particle and that this behavior can be predicted by Kohler theory. The deviation from Kohler theory for the mixed aerosol was determined by comparing theoretical and experimental CCN activation diameters for the particles and was found to be within experimental error. A thick coating of DOP (at least 70% by mass) did not hinder the activation of (NH4)(2)SO4 particles at supersaturations of 0.5 and 1.0%. The values for the measured activation diameters for the DOP coated (NH4)(2)SO4 particles were within the experimental error determined by the pure inorganic experiments, indicating that DOP was most likely acting as inert mass during activation. C1 Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA.

Previous work (Ahn and Liu (1990) J. Aerosol. Sci. 21, 249-261; Brockmann (1981) Ph.D. Thesis, University of Minnesota; Rebours et al. (1992) J. Aerosol. Sci. 23, S189-S192; Stolzenburg (1988) Ph.D. thesis, University of Minnesota) has shown that for particles smaller than about 15 nm, pulse heights produced by the optical detector in a white-light ultrafine condensation nucleus counter (UCNC; Stolzenburg and McMurry (1991) Aerosol. Sci. 'Technol. 14, 48-65) decrease with initial particle size. We have previously reported on the use of pulse heights from this instrument to determine the concentrations of freshly nucleated atmospheric nanoparticles in the 3-4 nm diameter range (Weber et al. (1995) J. Atm. Sci. 52, 2242-2257) Weber et al. (1997) J. Geophys. Res. 102, 4375-4385). In this paper we report on the inversion of measured pulse-height distributions to obtain size distributions of particles in the 3-10 nm diameter range. Using methods developed by Stolzenburg (Stolzenburg (1988) Ph.D. Thesis, University of Minnesota) the effect of diffusional broadening is taken into account so as to obtain monodisperse kernel functions from measured pulse-height distributions produced by DMA-generated calibration aerosols in the 3-50 nm diameter range. These kernel functions are then used with the MICRON algorithm described by Wolfenbarger and Seinfeld (1990, J. Aerosol. Sci. 21, 227-247) to obtain size distributions of nanoparticle aerosols from measured pulse height distributions. Calculations were done to ensure that assumed pulse-height data generated from selected known size distributions can be inverted to recover the original size distribution. Results from these validation studies are discussed. Applications of the inversion algorithm to data acquired in studies of homogeneous nucleation in the atmosphere are also presented. Published by Elsevier Science Ltd. C1 Brookhaven Natl Lab, Environm Chem Div, Upton, NY 11973 USA. Aersol Dynam Inc, Albany, CA 94710 USA. Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA. Univ Minnesota, Dept Mech Engn, Particle Technol Lab, Minneapolis, MN 55455 USA.

Daily atmospheric concentrations of particulate oxalate measured at the Summit of the Greenland Ice Sheet are presented for the summers 1992 - 1995. We believe that four episodes of elevated concentrations are due to biomass burning plumes passing over the site. In at least two cases the source regions of the fires are located in northern Canada. Further characteristics of the aerosol are examined during one of these events. A large increase of particle number concentrations in the accumulation mode can be observed, while the increase is much more limited for total particle number, The suite of chemical species enriched in the aerosol includes typical biomass burning tracers like fine K, large concentrations of ammonium, particulate formate and acetate, as well as other organic species like glycolate. The size distributions of K, oxalate, and glycolate are skewed toward the accumulation mode and exhibit the very same shape as sulfate, suggesting internal mixing of these species in the same particles. Molar ratios S/K indicate incorporation of S during transport, most probably by production of sulfate, Concentrations of these species were measured in fog samples for radiative events that occurred during the plume passage. There is a good agreement in the relative variation of concentrations between the aerosol and fog for oxalate and glycolate, while the gas phase probably dominates incorporation in the fog droplets for acetate, formate, chloride, nitrate, and sulfate (incorporated as SO2, which is further oxidized), The complexity of the transfer of the organic acids from the atmosphere to fog is underlined. C1 Lab Glaciol & Geophys Environm, F-38402 St Martin Dheres, France. Georgia Inst Technol, Dept Civil & Environm Engn, Atlanta, GA 30332 USA. Carnegie Mellon Univ, Dept Civil & Environm Engn, Pittsburgh, PA 15213 USA. NOAA, Climate Monitoring & Diagnost Lab, Boulder, CO 80303 USA. Finnish Meteorol Inst, FIN-00810 Helsinki, Finland. Univ Wisconsin, Dept Geosci, Milwaukee, WI 53211 USA. Inst Nucl Sci, B-9000 Ghent, Belgium.

The isotopic composition and concentration of lead has been measured in fresh and slightly aged snow collected at Dye 3 in southern Greenland during one full year. The lead concentration displayed large variations ranging from 14-3016 pg/g in April (spring) to 3-6 pg/g in September (summer) while the isotopic ratios changed in regular manner during the year. The Pb-206/Pb-207 ratios were similar to 1.15 from spring to mid-summer snow, and increased in late summer to early autumn, reaching similar to 1.20 in winter. These isotopic data indicate that the lead in the autumn to winter snow originated in North America, while that in spring to mid-summer snow is from Eurasia. (C) 1998 Elsevier Science B.V. All rights reserved. C1 Curtin Univ Technol, Dept Appl Phys, Perth, WA 6845, Australia. CNRS, Lab Glaciol & Geophys Environm, F-38402 St Martin Dheres, France. Univ Grenoble 1, Inst Univ France, UFR Mecan, F-38041 Grenoble, France. Carnegie Mellon Univ, Dept Civil Engn, Pittsburgh, PA 15213 USA.

A mass balance for lead for the year 1989 in the South Coast Air Basin has inputs to the atmosphere of 600 +/- 190 kg/day and outputs of 580 +/- 160 kg/day, showing rough agreement. Stationary sources are responsible for only about 5% of the total lead emissions. The bulk of the lead is emitted from vehicles using leaded gasoline (37%) and unleaded gasoline (15%), as well as from resuspension of previously deposited lead on roads (43%). Over half of the total emitted lead deposits on roads and nearby soil, while about one-third is carried out of the basin by wind. A small amount, less than 10%, is deposited on surfaces throughout the basin. These percentages are approximately the same as those in a mass balance for the same region calculated for 1972, when lead emissions from leaded gasoline were about a factor of 70 greater than leaded gas emissions in 1989. When the lead emissions are used as inputs to a simple continuously stirred flow reactor model for the basin, reasonable agreement is obtained between calculated and measured concentrations. (C) 1998 Academic Press. C1 Carnegie Mellon Univ, Dept Civil & Environm Engn, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA.

Dry deposition flux and aerosol size distribution measurements were made concurrently aboard the RV Lake Guardian 19 km east of the Chicago shoreline during summer 1994 to assess atmospheric inputs of minor and trace elements to southern Lake Michigan. Size-segregated aerosol measurements were made over consecutive 12-h periods with Micro-Orifice and Noll Rotary impacters (MOI and NRI), and depositing-particulate collections to aerodynamically smooth airfoils were made over periods of 3-4 days. The combination of the MOI and NRI provided size-segregated particulate samples in 12 discrete intervals between 0.059 and, nominally, 100 mu m. The samples were analyzed for As, Ca, Mg, Se, Sb, V, and Zn by instrumental neutron activation analysis and for S by X-ray-fluorescence. Aerosol and deposition data for individual elemental constituents were fit with a chemical mass balance deposition (CMBD) model in which a set of particle-size-specific deposition velocities (V-d), best reconciling the deposition data, were determined by iterative (constrained) solution of a series of six linear equations using the Levenberg-Marquardt method. Under stable conditions and mean wind speed of 4.0 m s(-1), minimum V-d values for particles with physical diameters between 0.09 and 0.53 mu m averaged 0.006 +/- 0.005 cm s(-1), wherein uncertainties were determined by Monte Carlo analysis. This agrees favorably with Values determined by microscopy for which uncertainties were much larger and with those predicted by the Williams model for the same period. The results suggest that physically significant V-d values are obtainable from a constrained CMBD model. C1 Univ Maryland, Dept Chem & Biochem, College Pk, MD 20742 USA. Carnegie Mellon Univ, Dept Civil & Environm Engn, Pittsburgh, PA 15213 USA.

Dry deposition flux measurements to surrogate surfaces and airborne concentration measurements of Zn-containing, S-rich, and soil particles (analyzed by scanning electron microscopy) and Al, Ba, Dr, Ca, Cl, Cu, K, Mg, Mn, Na, Ti, and V (analyzed by neutron activation analysis) were made over southwestern Lake Michigan in July 1994 and January 1995 to determine atmospheric inputs of pollutants to the lake. Samples collected in the summer show that despite relatively tow airborne concentrations of particles with physical diameters >8 mu m, these particles account for the majority of the dry deposition mass flux. However, this sharp contrast is not found during January when particles with physical diameters of 4-8 mu m dominate both the airborne concentration and the flux. Dry deposition velocities (flux divided by airborne concentration) for particles are found to range from 0.0062 cm/s for 0.75-mu m particles to 5.4 cm/s far 24-mu m particles. C1 Carnegie Mellon Univ, Dept Civil & Environm Engn, Pittsburgh, PA 15213 USA. Univ Maryland, Dept Chem & Biochem, College Pk, MD 20742 USA.

Growth of hygroscopic aerosols near water surfaces is believed to enhance dry deposition rates, which are a strong function of particle size. Previous dry deposition models estimate hygroscopic growth by assuming equilibrium between aerosols and water vapor (Williams, R. M. Atmos. Environ. 1982, 16, 1933-1938). A model is presented here that combines the relative humidity profile above water surfaces with hygroscopic growth rates for (NH4)(2)SO4, assuming cases for a deliquescing and metastable aerosol. Model results show that particles greater than 0.1 mu m in diameter do not grow to their equilibrium size before depositing to a hypothetical water surface. As a consequence, equilibrium models overpredict the effects of hygroscopic growth on deposition velocities by as much as a factor of 5. In addition, model results suggest a significant difference in the deposition velocities of metastable and deliquescing aerosols. Based on measured (NH4)(2)SO4 size distributions, overall deposition velocities calculated from a thermodynamic equilibrium model, a mass transfer limited non-equilibrium model with a deliquescing aerosol, and a mass transfer limited non-equilibrium model with a metastable aerosol are 0.11, 0.055, and 0.040 cm/s, respectively. C1 Carnegie Mellon Univ, Dept Civil & Environm Engn, Pittsburgh, PA 15213 USA. NOAA, Climate Monitoring & Diagnost Lab, Boulder, CO 80303 USA. Brookhaven Natl Lab, Environm Chem Div, Upton, NY 11973 USA.