CAPS: 1996 Publications

We report a new experimental method, reaction modulation spectroscopy, (RMS), that shows great promise in enabling the systematic analysis of complicated oxidation mechanisms over the full range of atmospheric pressures and temperatures. The method is a form of difference spectroscopy in which we employ FTIR absorption spectroscopy in a high-pressure flow system (HPFS) where a plume of reacting species is examined before any significant fraction reaches the tube wall. The onset of the reaction is modulated by modulating the flow of an initiating radical species over a period that is short compared to any associated with experimental drifts. Spectra obtained with the radical source on and off are ratioed, giving a transmittance spectrum showing only the effects of the reaction modulation. The system is in a steady state, so signal averaging over long periods (up to several days) may be employed, if necessary, to obtain a high signal-to-noise ratio. Because no bulk reagents are disturbed in the process, we observe extremely precise conservation of mass in the reaction plume. We illustrate the technique for the system OH + C3F6 (+ O-2, NOx,...) -> -> CF2O + CF3CFO where we observe nearly 100% conservation of odd-nitrogen species and roughly 90% conservation of carbon under conditions chosen to force the reaction to completion, with residual spectra consistent with unidentified minor products having cross sections similar to the observed aldehydes. C1 SUNY ALBANY,DEPT ATMOSPHER SCI,ALBANY,NY 12222.

Highly accurate reaction rate constants may be extracted from data obtained in well-developed (laminar or turbulent) diffusive flow over a wide range of experimental conditions. In many cases, data obtained in the core of such a flow may be treated as if the flow were one dimensional, leaving only a small correction for radial diffusion. This correction factor is derived analytically and presented in terms of easily observed quantities. The correction is robust; experimental and theoretical evidence is presented showing that it is insensitive to assumed axial symmetry and remains small so long as the interaction between the core and wall of a flowtube falls within well-defined bounds. The major limitations to accuracy in our high-pressure flow kinetics system (HPFS) are discussed in conjunction with observations of bimolecular reaction rate constants made over a very wide range of experimental conditions for the reactions OH + ethane -> products and OH + cyclohexane -> products. In the current experiment, accuracy for hydroxyl radical reactions is limited to roughly 12% for pressures of nitrogen ranging from 2 to 600 Torr. The accuracy in the measured ratio of two hydroxyl radical reaction rate constants is roughly 6%. Accuracy is currently limited primarily by absolute velocity measurements and by radical detection sensitivity. Strategies for further improving the accuracy of the high-pressure flow (HPF) technique are discussed. C1 SUNY ALBANY,ATMOSPHER SCI RES CTR,ALBANY,NY 12222.

A Lagrangian radiation fog model is applied to a fog event at Summit, Greenland. The model simulates the formation and dissipation of fog. Included in the model are detailed gas and aqueous phase chemistry, and deposition of chemical species with fog droplets. Model predictions of the gas phase concentrations of H2O2, HCOOH, SO2, and HNO3 as well as the fog fluxes of S(VI), N(V), H2O2, and water are compared with measurements. The predicted fluxes of S(VI), N(V), H2O2, and fog water generally agree with measured values. Model results show that heterogeneous SO2 oxidation contributes to approximately 40% of the flux of S(VI) for the modeled fog event, with the other 60% coming from preexisting sulfate aerosol. The deposition of N(V) with fog includes contributions from HNO3 and NO2 initially present in the air mass. HNO3 directly partitions into the aqueous phase to create N(V) and NO2 forms N(V) through reaction with OH and the nighttime chemistry set of reactions which involves N2O5 and water vapor. PAN contributes to N(V) by gas phase decomposition to NO2, and also by direct aqueous phase decomposition. The quantitative contributions from each path are uncertain since direct measurements of PAN and NO2 are not available for the fog event. The relative contributions are discussed based on realistic ranges of atmospheric concentrations. Model results suggest that in addition to the aqueous phase partitioning of the initial KNO3 present in the air mass, the gas phase decomposition of PAN and subsequent reactions of NO2 with OH as well as nighttime nitrate chemistry may play significant roles in depositing N(V) with fog. If a quasi-liquid layer exists on snow crystals, it is possible that the reactions taking place in fog droplets also occur to some extent in clouds as well as at the snow surface. C1 CARNEGIE MELLON UNIV,DEPT CIVIL & ENVIRONM ENGN,PITTSBURGH,PA 15213. GEORGIA INST TECHNOL,SCH CIVIL & ENVIRONM ENGN,ATLANTA,GA 30332. CARNEGIE MELLON UNIV,DEPT CIVIL & ENVIRONM ENGN,PITTSBURGH,PA 15213. CARNEGIE MELLON UNIV,DEPT CHEM ENGN & ENGN & PUBL POLICY,PITTSBURGH,PA 15213. CRNS,LAB GLACIOL & GEOPHYS ENVIRONNEMENT,F-38402 ST MARTIN DHERES,FRANCE. UNIV NEW HAMPSHIRE,INST STUDY EARTH OCEANS & SPACE,DURHAM,NH 03824. CARNEGIE MELLON UNIV,DEPT MECH ENGN,PITTSBURGH,PA 15213.

This work investigates the dependence of soil permeability to air on sampling scale in near-surface unsaturated soils. A new dual-probe dynamic pressure technique was developed to measure permeability in situ over different length scales and different spatial orientations in the soil. Soils at three sites were studied using the new technique. Each soil was found to have higher horizontal than vertical permeability. Significant scale dependence of permeability was also observed at each site. Permeability increased by a factor of 20 as sampling scale increased from 0.1 to 2 m in a sand soil vegetated with dry grass, and by a factor of 15 as sampling scale increased from 0.1 to 3.5 m in a Sandy loam with mature Coast Live Oak trees (Quercus agrifolia). The results indicate that standard methods of permeability assessment can grossly underestimate advective transport of gas phase contaminants through soils. C1 UNIV CALIF BERKELEY,DEPT CIVIL & ENVIRONM ENGN,BERKELEY,CA 94720. UNIV CALIF BERKELEY,LAWRENCE BERKELEY LAB,INDOOR ENVIRONM PROGRAM,BERKELEY,CA 94720.

Particles and gases can deposit from the atmosphere to polar snow by several mechanisms. Dry deposition can be considered to consist of three steps: aerodynamic transport from the free atmosphere to the viscous sublayer near the surface, boundary layer transport across the sublayer, and interactions with the surface. The particle dry deposition mass flux is dominated by the largest particles present in a size distribution. Wet deposition includes in-cloud and below-cloud scavenging, where the former refers to uptake of particles during nucleation of cloudwater as well as scavenging of particles and gases by existing droplets and ice crystals. Of all the wet deposition mechanisms, nucleation scavenging is often the most important mechanism for particles in the polar regions. Finally, incorporation of particles and gases into fog droplets and subsequent settling of the fog to the snow surface can be an important removal process in regions of frequent fog. For Summit, Greenland, the total deposition of MSA, SO42-, Na+, K+, and Ca2+ during May 24-July 13, 1993 was dominated by wet deposition: this mechanism accounted for an average of 62% of the total deposition for these species. Fog and dry deposition accounted for 21% and 17% of the total, respectively. These results suggest that all three mechanisms may need to be considered when estimating total deposition of certain chemical species to polar snow. C1 Carnegie Mellon Univ, Dept Civil & Environm Engn, Pittsburgh, PA 15213 USA. Carnegie Mellon Univ, Dept Engn & Publ Policy, Pittsburgh, PA 15213 USA.

Thirty six snow samples collected from a 1.6 m snow pit in central Greenland, have been analysed for Pb, Cd, Zn, Cu and other species using ultraclean analytical procedures. They cover continuously a 2 year time period from summer 1992 to spring 1990, with sub-seasonal resolution. Pronounced seasonal variations of the concentrations are observed for all four heavy metals, with low values in winter, and much higher concentrations not only in spring but also in summer. The factors of variations are 75 for Pb, 31 for Cd, 22 for Zn and 48 for Cu. Estimates of the contributions from natural sources show that anthropogenic contributions are dominant for Pb, Cd and Zn while a significant fraction of Cu derives from rock and soil dust in part of the samples. Our data confirm that the high altitude central areas of the Greenland ice sheet remain isolated from the highly polluted air masses of the Arctic basin in winter. The enhanced concentrations observed in the summer layers are attributed to inputs from pin point sources in high latitude continental areas. Copyright (C) 1996 Elsevier Science B.V. C1 CNRS,LAB GLACIOL & GEOPHYS ENVIRONNEMENT,F-38402 ST MARTIN DHER,FRANCE. CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213. UNIV GRENOBLE 1,INST UNIV FRANCE,UFR MECAN,F-38041 GRENOBLE,FRANCE. UNIV INSTELLING ANTWERP,DEPT CHEM,B-2610 WILRIJK,BELGIUM.

The volatilization of volatile organic chemicals during domestic water usage can result in significant indoor air concentrations, and the subsequent inhalation of these contaminants is an important route of exposure. The magnitude of these exposures is highly dependent on the activities undertaken by the exposed individual, as well as the activities of other occupants of the home. The indoor air quality-exposure Model for the Analysis of Volatiles and Residential Indoor Air Quality (MARVIQ) was used to ascertain the impact of water-use activities on the potential contaminant dose to household members. Human time-activity patterns of various population groups were sampled for om the California Air Resources Board database, applying distributions of water-use occurrence and water-use duration to each activity based on survey results. Indoor air concentrations in a sample house and the resulting potential inhalation dose to the occupants were computed for different individuals and pairs of individuals to test for exposure and coexposure effects. The simulated daily exposure is well described by a simplified equation that is a function of the amount of time the individual spends in the shower the bath, and the bathroom; the total water usage in the home; and the function of time the individual is at home. These results can be used to identify high-risk populations, individuals, and households. The study also demonstrates the importance of further research on joint time-activity patterns in multiperson households for assessment of exposure and coexposure effects. C1 CARNEGIE MELLON UNIV,DEPT CIVIL & ENVIRONM ENGN,PITTSBURGH,PA 15213.

A small deposit area low pressure impactor (abbreviated to SDI) has been developed and tested. The device has been designed specifically to collect size-fractionated aerosol samples in remote locations for subsequent chemical analysis by PIXE. The SDI is a 12-stage, multinozzle device, but the deposit for each stage remains confined to an area with diameter less than 8 mm. It operates at a flow rate of 11 L/min and accepts the same, 25 mm diameter substrate rings as the PIXE International cascade impactor. The experimental cut-points for stages 12 through 1 are 8.50, 4.08, 2.68, 1.66, 1.06, 0.796, 0.591, 0.343, 0.231, 0.153, 0.086 and 0.045 mu m equivalent aerodynamic diameter. The SDI has been tested in (and employed for) size-fractionated aerosol sampling in the Finnish Arctic and at Summit in Greenland. The data show that the SDI gives results very similar to those obtained with the PIXE International impactor, but with detection limits that are much lower. This suggests that the SDI can be used with shorter sampling times or in areas where concentrations are smaller to obtain reliable size distribution data. The results also suggest that data for a greater number of elements can be obtained with the SDI. C1 FINNISH METEOROL INST,SF-00810 HELSINKI,FINLAND. LAB GLACIOL & GEOPHYS ENVIRONM,F-38402 ST MARTIN DHERES,FRANCE. CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15313. CARNEGIE MELLON UNIV,DEPT ENGN & PUBL POLICY,PITTSBURGH,PA 15313.