CAPS: 2003 Publications

Review of factors controling the temperature dependence of atmospheric reactions.

We present results of recent experiments on the pressure broadening of pure rotational transitions of (H2O)-O-16 water in the 380-600 cm(-1) range by hydrogen and helium. The hydrogen coefficients vary between 0.033 and 0.076 cm(-1)/atm (average: 0.056 cm(-1)/atm) while the helium coefficients vary between 0.006 and 0.017 cm(-1)/atm (average: 0.013 cm(-1)/atm). While this average hydrogen-broadening coefficient matches the average nitrogen-broadening coefficient we observed in this region earlier, the range over which they vary is narrower for hydrogen than for nitrogen. We find that previously published complex Robert-Bonamy formalism calculations of the hydrogen coefficients are broadly consistent with these results, and that, to some extent, our experimental results can be predicted from HITRAN air widths. (C) 2003 Elsevier Ltd. All rights reserved. C1 Univ Maryland Baltimore Cty, Joint Ctr Earth Syst Technol, Baltimore, MD 21250 USA.

A series of modeling approaches for the description of the dynamic behavior of secondary organic aerosol (SOA) components and their interactions with inorganics is presented. The models employ a lumped species approach based on available smog chamber studies and the UNIquac Functional-group Activity Coefficient (UNIFAC) method to estimate SOA water absorption. The additional water due to SOA species can change the partitioning behavior of the semi-volatile inorganics. Primary organic particles significantly influence the SOA partitioning between gas and aerosol phases. The SOA size distribution predicted by a bulk equilibrium approach is biased toward smaller sizes compared with that of a fully dynamic model. An improved weighting scheme for the bulk equilibrium approach is proposed in this work and is shown to minimize this discrepancy. SOA is predicted to increase the total aerosol water in Southern California by 2-13% depending on conditions. However, the effect of SOA water absorption on aerosol nitrate is insignificant for all the cases studied in Southern California. (C) 2003 Elsevier Ltd. All rights reserved. C1 Carnegie Mellon Univ, Dept Chem Engn & Engn & Publ Policy, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA. Bayer Corp, Corp Engn Dept, Baytown, TX 77520 USA.

[1] Three-dimensional chemical transport models typically include a bulk description of aqueous-phase atmospheric chemistry. Previously, this bulk description has been shown to be often inadequate in predicting sulfate production. The pH of the bulk mixture does not adequately describe the pH of the typically heterogeneous droplet population found in clouds and fogs. This often leads to an inability of bulk models to predict sulfate production when pH-dependent production pathways are important. A more accurate size-resolved aqueous-phase chemistry model, however, has long been considered infeasible for incorporation in a three-dimensional chemical transport model because of high computational costs. Here we investigate the feasibility of adding a computationally efficient size-resolved aqueous-phase chemistry module (Variable Size Resolution Model (VSRM)) to a three-dimensional model (the latest version of the Comprehensive Air Quality Model with extensions (PMCAMx)). The VSRM treats mass transfer between the gas phase and the different droplet populations and executes bulk or two-section sizeresolved chemistry calculations in each step on the basis of the chemical environment of each computational cell. A fall air pollution episode in California's South Coast Air Basin is simulated, and model predictions are compared to observations. In an environment where clouds or fogs are present, the model without aqueous-phase chemistry severely underpredicts secondary sulfate formation. In cases where there is a high potential for sulfate production and widely varying composition across the droplet spectrum ( over the ocean and near the coast), there is a significant increase in sulfate production over bulk predictions with the activation of a size- resolved aqueous-phase chemistry module. Unfortunately, measurements were only available at inland sites, where the difference between bulk and size- resolved sulfate predictions was small. The effects of other uncertainties on sulfate production are also examined. For limited computational costs (5% overhead), the VSRM can be included in a three-dimensional chemical transport model. C1 Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA.

Cloud condensation nuclei (CCN) in the atmosphere are usually composed of multiple inorganic and organic chemical species. Determining the ability of these multicomponent particles to activate into cloud droplets is necessary for understanding and quantifying the effect of aerosols on cloud formation and properties. Internally mixed, multicomponent particles as well as particles consisting of a core coated with hexadecane were used in the present study. Laboratory experiments were performed using combinations of sodium chloride, ammonium sulfate, pinonic acid, pinic acid, norpinic acid, glutamic acid, leucine, and hexadecane. Activation diameters were determined combining a Tandem Differential Mobility Analyzer (TDMA) with a thermal diffusion Cloud Condensation Nucleus Counter (CCNC). Studies were performed at supersaturations of 0.3% and 1% with dry particle diameters ranging between 0.02 and 0.2 micrometers. The results were compared to a theory assuming additive behavior of the constituent species. This assumption was sufficient for the prediction of the CCN activation diameter of the mixed particles. C1 Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA.

The formation of secondary organic aerosols (SON is simulated for the Nashville/western Tennessee domain using three recent SOA modules incorporated into the three-dimensional air quality model, CMAQ. The Odum/Griffin et al. and CMU/STI modules represent SOA absorptive partitioning into a mixture of primary and secondary particulate organic compounds (OC), with some differences in the formulation of the absorption process and the selection of SOA species and their precursors. Empirical representations based on measured laboratory SOA yields are used for condensable organic products in both these modules. The AEC module simulates SOA absorption into organic and aqueous particulate phases, and a representation based on an explicit gas-phase mechanism is used in the AEC module. Predicted SOA concentrations can vary by a factor of 10 or more. In general, the gas-phase mechanistic approach predicts a higher yield of SOA than those based on laboratory yields. There exist some differences in the two empirical modules despite their similar basis on experimental data. All three modules predict a dominance of SOA of biogenic origin as compared to SOA of anthropogenic origin. The causes for differences among the three SOA modules include the representation of terpenes, the mechanistic versus empirical representation of SOA-forming reactions, the identities of SOA, and the parameters used in the gas/particle partitioning calculations. Two sensitivity studies show that formation of water-soluble SOA and temperature dependence may be areas of key uncertainties affecting current models. C1 Atmospher & Environm Res Inc, San Ramon, CA 94583 USA. CALTECH, Dept Chem Engn, Pasadena, CA 91125 USA. Duke Univ, Dept Civil & Environm Engn, Durham, NC 27708 USA. Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA.

Condensation of gases (H2SO4, NH3, HNO3, low vapor pressure organics, etc.) onto existing aerosols can account for a significant portion of fine particulate matter. Since aerosols affect human health, visibility, and climate change, it is important to be able to model condensation/evaporation accurately in order to predict how particle mass will change over time. (Atmos. Environ. 34 (2000) 2957) proposed adapting the trajectory-grid method, used for solving the transport equation, to solve the condensation/evaporation equation. Their preliminary results showed it to be fast and accurate in simple systems (one component, etc.). The approach of Chock and Winkler has been modified for implementation in both the Hybrid method (Atmos. Environ. 34 (2000) 3617) and the improved multicomponent aerosol dynamics model (MADM) (Aerosol Sci. Technol. 32 (2000) 482) The first improvement in MADM modifies the method for restricting the acidic flux, while the second reduces physically meaningless dry/wet oscillations in the aerosol phase by assuming the aerosol is metastable when these oscillations are present. Measurements in Claremont, CA in August of 1987 are used to evaluate the new method in a one-dimensional model and the October 1995 PM episode in the South Coast Air Basin in CA is used for evaluation in a three-dimensional chemical transport model (PMCAMx). The trajectory-grid method allows the use of a simple scheme for time step selection and provides at least a factor of two or three reduction in computational requirements compared to an ODE solver at the same level of accuracy. The improvements to MADM also have a significant effect on performance, providing over an order of magnitude reduction in computational requirements compared to the original MADM. In the three-dimensional chemical transport model, the Hybrid method of (Atmos. Environ. (2000) 3617) is applied which assumes the smallest particles are in equilibrium while the condensation/evaporation equation is solved for the larger ones. Combined with the improvements to MADM and trajectory-grid method, this Hybrid approach takes just three to four times the computational requirements of assuming bulk equilibrium for all particles, while providing more accurate predictions of the aerosol size distribution. (C) 2003 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. Ford Motor Co, Ford Res Lab, Pittsburgh, PA 15213 USA.

The continued development of the dynamic and hybrid approaches (Pilinis et al. 2000; Capaldo et al. 2000) for the simulation of atmospheric aerosol dynamics is discussed in this paper. A linear interpolation method is proposed for the mapping of the moving aerosol size/composition distribution onto a fixed size grid. The 3 aerosol modules are incorporated into a trajectory model that includes descriptions of gas-phase chemistry, secondary organic aerosol formation, vertical dispersion, dry deposition, and emissions. The 3 approaches are evaluated against measurements from the Southern California Air Quality Study (SCAQS). All 3 models predict the 4-6 h averaged PM2.5 (particulate matter with diameter less than or equal to 2.5 microns) and PM10 (particulate matter with diameter less than or equal to 10 microns) mass concentrations of the major aerosol species with errors <30%. For the aerosol size/composition distribution, however, the dynamic and hybrid models show better agreement with measurements than the equilibrium model. The hybrid model aerosol size distribution predictions are similar to the dynamic model results. The hybrid approach in this case combines accuracy with computational efficiency. The dynamic approach is the most accurate, but at a higher computational cost. Daily average PM2.5 aerosol species predicted by the aerosol models with 16, 8, and 4 size sections all give reasonable agreement with the measurements. All 3 aerosol models show consistent sensitivities of nitrate, sulfate, and total PM2.5 to the changes of NOx, VOCs, NH3, and primary sulfate emissions. C1 Carnegie Mellon Univ, Dept Chem Engn, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA.

Cofiring biomass with coal in existing power plants offers a relatively inexpensive and efficient option for increasing near-term biomass energy utilization. Potential benefits include reduced emissions of carbon dioxide, sulfur, and nitrogen oxides and development of biomass energy markets. To understand the economics of this strategy, we develop a model to calculate electricity and pollutant mitigation costs with explicit characterization of uncertainty in fuel and technology costs and variability in fuel properties. The model is first used to evaluate the plant-level economics of cofiring as a function of biomass cost. It is then integrated with state-specific coal consumption and biomass supply estimates to develop national supply curves for cofire electricity and carbon mitigation. A delivered cost of biomass below $15 per ton is required for cofire to be competitive with existing coal-based generation. Except at low biomass prices (less than $15 per ton), cofiring is unlikely to be competitive for NOchi or SOchi control, but it can provide comparatively inexpensive control of CO2 emissions: we estimate that emissions reductions of 100 Mt-CO2/year (a 5% reduction in electric-sector emissions) can be achieved at 25 +/- 20 $/tC. The 2-3 year time horizon for deployment-compared with 10-20 years for other CO2 mitigation options-makes cofiring particularly attractive. C1 Carnegie Mellon Univ, Dept Mech Engn, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA.

Critical deposit properties such as thermal conductivity and strength are thought to be strongly dependent on deposit microstructure. Image analysis techniques were applied to scanning electron microscopy images of ash deposit cross sections to measure quantitatively the microstructure of two ash deposits formed while firing sub-bituminous coal in a pilot-scale combustor under different temperature conditions. The measurements indicate that the high-temperature deposit has a more interconnected and coarser structure than the low-temperature deposit. Little spatial variability of the structural parameters was observed, except for a significant gradient in the contiguity (a measure of particle interconnectedness) across the high-temperature deposit. The measurements are compared to predictions of a ballistic deposition model. Both sampled and simulated deposits are coincident in terms of the measured structural parameters. The results suggest that temperature and composition can be used to predict the effects of coal quality and combustion conditions on the initial microstructure of an ash deposit. Higher-temperature conditions create a deposit with a more porous structure but whose particles are more interconnected because of deformation. C1 Carnegie Mellon Univ, Dept Mech Engn, Pittsburgh, PA 15213 USA. Cellom Inc, Pittsburgh, PA 15219 USA.

[1] Estimates of indirect aerosol radiative forcing have focused on increased sulfate aerosol mass concentrations caused by anthropogenic emissions of gas-phase sulfur dioxide, implicitly neglecting the impact of direct particulate emissions. Emissions of primary particles and gas-phase precursors have different effects on cloud condensation nuclei (CCN) concentrations as they impact CCN concentrations via different microphysical pathways. We present a theoretical analysis and evidence from a three-dimensional global model of aerosol microphysics to show that particulate emissions are more efficient per unit mass than gas-phase emissions at increasing CCN concentrations. Both analyses show that the few percent of anthropogenic sulfur emitted as particulate sulfate results in an increase in CCN concentrations comparable to that resulting from much larger emissions of gas-phase sulfur dioxide. Therefore, models should explicitly distinguish between the microphysical impacts of particulate and gas-phase emissions to accurately estimate the magnitude of the indirect effect of aerosols on climate. C1 Carnegie Mellon Univ, Dept Civil & Environm Engn, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA. CALTECH, Dept Chem Engn, Pasadena, CA 91125 USA. CALTECH, Dept Environm Sci & Engn, Pasadena, CA 91125 USA.

A unified tropospheric chemistry-aerosol model has been developed within the Goddard Institute for Space Studies general circulation model (GCM). The model includes a detailed simulation of tropospheric ozone-NOx-hydrocarbon chemistry as well as aerosols and aerosol precursors. Predicted aerosol species include sulfate, nitrate, ammonium, black carbon, primary organic carbon, and secondary organic carbon. The partitioning of ammonia and nitrate between gas and aerosol phases is determined by online thermodynamic equilibrium, and the formation of secondary organic aerosols is based on equilibrium partitioning and experimentally determined parameters. Two-way coupling between aerosols and chemistry provides consistent chemical fields for aerosol dynamics and aerosol mass for heterogeneous processes and calculations of gas-phase photolysis rates. Although the current version of the unified model does not include a prognostic treatment of mineral dust, we include its effects on photolysis and heterogeneous processes by using three-dimensional off-line fields. We also simulate sulfate and nitrate aerosols that are associated with mineral dust based on currently available chemical understanding. Considering both mineral dust uptake of HNO3 and wet scavenging of HNO3 on ice leads to closer agreement between predicted gas-phase HNO3 concentrations and measurements than in previous global chemical transport model simulations, especially in the middle to upper troposphere. As a result of the coupling between chemistry and aerosols, global burdens of both gas-phase and aerosol species are predicted to respond nonlinearly to changing emissions of NOx, NH3, and sulfur. C1 CALTECH, Div Engn & Appl Sci, Pasadena, CA 91125 USA. CALTECH, Dept Chem Engn, Pasadena, CA 91125 USA. Harvard Univ, Dept Earth & Planetary Sci, Cambridge, MA 02138 USA. Harvard Univ, Div Engn & Appl Sci, Cambridge, MA 02138 USA.