CAPS: 1995 Publications

A simple model is presented to estimate atmospheric concentrations of chemical species that exist primarily as aerosols based on snow core/ice core chemistry at Summit, Greenland. The model considers the processes of snow, fog, and dry deposition. The deposition parameters for each of the processes are estimated for S0(4)(2-) and Ca2+ and are based on experiments conducted during the 1993 and 1994 summer field seasons. The seasonal mean atmospheric concentrations are estimated based on the deposition parameters and snow cores obtained during the field seasons. The ratios of the estimated seasonal mean airborne concentration divided by the measured mean concentration ((C) over bar(a,est)/(C) over bar(a,meas)) for SO42over the 1993 and 1994 field seasons are 0.85 and 0.95, respectively. The (C) over bar(a,est)/(C) over bar(a,meas) Ca2+ are 0.45 and 0.90 for the 1993 and 1994 field seasons. The uncertainties in the estimated atmospheric concentrations range from 30% to 40% and are due to variability in the input parameters. The model estimates the seasonal mean atmospheric SO42- and Ca2+ concentrations to within 15% and 55%, respectively. Although the model is not directly applied to ice cores, the application of the model to ice core chemical signals is briefly discussed. C1 CARNEGIE MELLON UNIV,DEPT CIVIL & ENVIRONM ENGN & ENGN & PUBL POLICY,PITTSBURGH,PA 15213. UNIV NEW HAMPSHIRE,INST STUDY EARTH OCEANS & SPACE,DURHAM,NH 03824. CARNEGIE MELLON UNIV,DEPT CHEM ENGN & ENGN & PUBL POLICY,PITTSBURGH,PA 15213. CRNS,LGGE,F-38402 ST MARTIN DHERES,FRANCE.

We evaluate, using a box model, the sensitivity of direct climate forcing by atmospheric aerosols for a ”global mean” aerosol that consists of fine and coarse modes to aerosol composition, aerosol size distribution, relative humidity (RH), aerosol mixing state (internal versus external mixture), deliquescence/crystallization hysteresis, and solar zenith angle. We also examine the dependence of aerosol upscatter fraction on aerosol size, solar zenith angle, and wavelength and the dependence of single scatter albedo on wavelength and aerosol composition. The single most important parameter in determining direct aerosol forcing is relative humidity, and the most important process is the increase of the aerosol mass as a result of water uptake. An increase of the relative humidity from 40 to 80% is estimated for the global mean aerosol considered to result in an increase of the radiative forcing by a factor of 2.1. Forcing is relatively insensitive to the fine mode diameter increase due to hygroscopic growth, as long as this mode remains inside the efficient scattering size region. The hysteresis/deliquescence region introduces additional uncertainty but, in general, errors less than 20% result by the use of the average of the two curves to predict forcing. For fine aerosol mode mean diameters in the 0.2-0.5 mu m range direct aerosol forcing is relatively insensitive (errors less than 20%) to variations of the mean diameter. Estimation of the coarse mode diameter within a factor of 2 is generally Sufficient for the estimation of the total aerosol radiative forcing within 20%. Moreover, the coarse mode, which represents the nonanthropogenic fraction of the aerosol, is estimated to contribute less than 10% of the total radiative forcing for all RHs of interest. Aerosol chemical composition is important to direct radiative forcing as it determines (1) water uptake with RH, and (2) optical properties. The effect of absorption by aerosol components on forcing is found to be significant even for single Scatter albedo values of omega=0.93-0.97. The absorbing aerosol component reduces the aerosol forcing from that in its absence by roughly 30% at 60% RH and 20% at 90% RH. The mixing state of the aerosol (internal versus external) for the particular aerosol considered here is found to be of secondary importance, While sulfate mass scattering efficiency (m(2) (g SO42-)(-1)) and the normalized sulfate forcing(W (g SO42-)(-1))increase strongly with RH, total mass scattering efficiency (m(2) g(-1))and normalized forcing (W g(-1)) are relatively insensitive to RH, wherein the mass of all species, including water, are accounted for. Following S. Nemesure et al. (Direct shortwave forcing of climate by;anthropogenic sulfate aerosol: sensitivity to particle size, composition, and relative humidity, submitted to Journal of Geophysical Research, 1995), we find that aerosol forcing achieves a maximum at a particular solar zenith angle, reflecting a balance between increasing upscatter fraction with increasing solar zenith angle and decreasing solar flux (from Rayleigh scattering) with increasing solar zenith angle. C1 CARNEGIE MELLON UNIV,DEPT ENGN & PUBL POLICY,PITTSBURGH,PA 15213. UNIV MIAMI,MAC RSMAS,MIAMI,FL 33149. CALTECH,DEPT CHEM ENGN,PASADENA,CA 91125.

Experiments were performed during the period May-July of 1993 at Summit, Greenland. Aerosol mass size distributions as well as daily average concentrations of several anionic and cationic species were measured. Dry deposition velocities for SO42- were estimated using surrogate surfaces (symmetric airfoils) as well as impactor data. Real-time concentrations of particles greater than 0.5 mu m and greater than 0.01 mu m were measured. Snow and fog samples from nearly all of the events occurring during the field season were collected. Filter sampler results indicate that SO42is the dominant aerosol anion species, with Na+, NH4+, and Ca2+ being the dominant cations, Impactor results indicate that MSA and SO42- have similar mass size distributions. Furthermore, MSA and SO42- have mass in both the accumulation and coarse modes. A limited number of samples for NH4+ indicate that it exists in the accumulation mode. Na, K, Mg, and Ca exist primarily in the coarse mode. Dry deposition velocities estimated from impactor samples and a theory for dry deposition to snow range from 0.017 cm/s +/- 0.011 cm/s for NH4+ to 0.110 cm/s +/- 0.021 cm/s for Ca.SO42- dry deposition velocity estimates using airfoils are in the range 0.023 cm/s to 0.062 cm/s, as much as 60% greater than values calculated using the airborne size distribution data. The rough agreement between the airfoil and impactor-estimated dry deposition velocities suggests that the airfoils may be used to approximate the dry deposition to the snow surface. Laser particle counter (LPC) results show that particles > 0.5 mu m in diameter efficiently serve as nuclei to form fog droplets. Condensation nuclei (CN) measurements indicate that particles < 0.5 mu m are not as greatly affected by fog. Furthermore, impactor measurements suggest that from 50% to 80% of the aerosol SO42- serves as nuclei for fog droplets. Snow deposition is the dominant mechanism transporting chemicals to the ice sheet. For NO3. a species that apparently exists primarily in the gas phase as HNO3(g), 93% of the seasonal inventory (mass of a deposited chemical species per unit area during the season) is due to snow deposition, which suggests efficient scavenging of HNO3(g) by snowflakes. The contribution of snow deposition to the seasonal inventories of aerosols ranges from 45% for MSA to 76% for NH4+. The contribution of fog to the seasonal inventories ranges from 13% for Na+ and Ca2+ to 25% and 32% for SO2-4 and MSA. The dry deposition contribution to the seasonal inventories of the aerosol species is as low as 5% for NH4+ and as high as 23% for MSA. The seasonal inventory estimations do not take into consideration the spatial variability caused by blowing and drifting snow. Overall, results indicate that snow deposition of chemical species is the dominant flux mechanism during the summer at Summit and that all three deposition processes should be considered when estimating atmospheric concentrations based on ice core chemical signals. C1 CRNS,LAB GLACIOL & GEOPHYS ENVIRONNEMENT,F-38402 ST MARTIN DHERES,FRANCE. CARNEGIE MELLON UNIV,DEPT CIVIL & ENVIRONM ENGN,PITTSBURGH,PA 15213. UNIV NEW HAMPSHIRE,INST STUDY EARTH OCEANS & SPACE,DURHAM,NH 03824. CARNEGIE MELLON UNIV,DEPT CHEM ENGN,PITTSBURGH,PA 15213. FINNISH METEOROL INST,DEPT AIR QUAL,SF-00810 HELSINKI,FINLAND. STATE UNIV GHENT,INST NUCL WETENSCHAPPEN,B-9000 GHENT,BELGIUM.

Anthropogenic emissions leading to atmospheric aerosols have increased dramatically over the past century. Airborne particles have been implicated in human health effects, visibility reduction in urban and regional areas, acidic deposition, and altering the earth's radiation balance. The atmosphere subjects aerosol particles to an array of transport and transformation processes that alter their size, number, and composition; the transformation processes include condensation and evaporation. homogeneous nucleation, coagulation, and chemical reactions. A major goal of our research has been to use first principles to gain a predictive understanding of the physical and chemical processes that govern the dynamics, size. and chemical composition of atmospheric aerosols. We review here the current state of our ability to model this atmospheric aerosol behavior. C1 UNIV DELAWARE,DEPT MECH ENGN,NEWARK,DE 19716. CARNEGIE MELLON UNIV,DEPT CHEM ENGN & ENGN & PUBL POLICY,PITTSBURGH,PA 15213. CALTECH,DEPT CHEM ENGN,PASADENA,CA 91125.

To study the importance of changes in atmospheric pressure on radon entry into houses, we have simultaneously measured the soil-gas entry into an experimental basement structure and the fluctuations in atmospheric pressure. Small amplitude (similar to 10 Pa), rapid (similar to 20 min) fluctuations in atmospheric pressure were an important driving force for soil-gas entry because 1) the characteristic time for the propagation of a pressure disturbance in the soil gas was similar to 2 min, and 2) the time-rate-of-change of these small fluctuations is often larger than that of the semi-diurnal oscillations. An analytical model has been derived for a structure with a subslab gravel layer based on a one-dimensional solution to the transient pressure diffusion equation. This model correctly predicts the temporal response of the measured soil-gas entry into the experimental structure , but underpredicts the amplitude. C1 UNIV CALIF BERKELEY,DEPT MECH ENGN,BERKELEY,CA 94720.

This paper assesses current knowledge about the dry deposition of particles with a locus on implications for Europe. General aspects of sources, physical characteristics and concentration data of particles in the atmosphere are considered. Current modelling approaches For calculating the dry deposition of small and coarse particles are reviewed. Shortcomings and uncertainties of these models are presented. Measurements of small- and large-particle deposition with various techniques are summarized together with their limitations. Finally, implications of scale aspects of particle dry deposition are treated with an emphasis on Europe, The overall conclusion is that knowledge on particle dry deposition is still insufficient, both experimentally and theoretically, undermining reliable estimates of deposition fluxes over Europe. C1 CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213. AEA CONSULTANCY SERV,DEPT ENVIRONM ASSESSMENT,DIDCOT OX11 0RA,OXON,ENGLAND.