CAPS: 1990 - 1994 Publications

The dependence of cloud condensation nuclei (CCN) production on the marine dimethylsulfide (DMS) flux is modeled with a dynamic description of the gas, aerosol, and aqueous phase processes in a closed air parcel. The results support the conclusion reached in previous work with a steady state model that an approximately linear dependence exists between CCN concentration and DMS flux under typical remote marine conditions. This linearity does not hold for low DMS fluxes (the threshold is typically near 2.5 mu mol m(-2) day(-1)) because the seasalt particles heterogeneously convert the available SO2 to sulfate inhibiting the creation of new particles. The conditions under which this linear relationship holds are investigated by a series of sensitivity studies, focusing particular attention on the impact of the timing and frequency of cloud events. We consider the regimes of the model's semiempirical parameters, showing that the uncertainty associated with two such parameters, namely, the nucleation rate scaling factor and the sulfuric acid accommodation coefficient, is sufficient to change the predicted CCN production due to DMS from over 300 cm(-3) day(-1) to none. This sensitivity accounts for most of the range of results predicted by previous models of the DMS-CCN system. C1 CARNEGIE MELLON UNIV,DEPT ENGN & PUBL POLICY,PITTSBURGH,PA 15213. CALTECH,DEPT CHEM ENGN,PASADENA,CA 91125.

The relationship between the steady state cloud condensation nuclei (CCN) concentration and the dimethylsulfide (DMS) emission flux in remote marine regions is investigated by modeling the principal gas-, aerosol-, and aqueous-phase processes in the marine boundary layer (MBL). Results are in reasonable quantitative agreement with the available measurements of DMS, SO2, H2SO4, CCN, and condensation nuclei (CN) concentrations in remote marine regions of the globe and suggest that indeed DMS plays a major role in the particle dynamics of the MBL. For sufficiently low DMS fluxes practically all the SO2, produced by DMS photooxidation is predicted to be heterogeneously converted to sulfate in sea-salt aerosol particles. For DMS fluxes higher than approximately 2.5 mumole m-2 d-1 a linear relationship is found to exist between the CCN number concentration and the DMS flux. C1 CALTECH,DEPT CHEM ENGN,PASADENA,CA 91125.

Experiments were performed during June-July 1992 to determine the impact of dry deposition and fog deposition on surface snow chemical inventories. The fluxes of SO42-, NO3-, Cl-, MSA, Na, Ca, and Al were measured by collecting deposited fog on flat polyethylene plates. Dry deposition fluxes of SO42were measured using aerodynamic surfaces. Real-time concentrations of atmospheric particles greater than 0.5 um and greater than 0.01 um were measured using continuous monitors. Filter samplers were used to determine daily average atmospheric SO42- and MSA concentrations. Also, daily surface snow samples were taken and analyzed for SO42-, NO3-, Cl-, Na+, Ca2+, and NH4+. The real-time concentration data indicate that aerosol particles greater than 0.5 um are efficiently incorporated into fog droplets. Results also show that condensation nuclei (CN) are not as greatly affected by fog as the larger particles. Fog fluxes of SO42- and NO3- have similar values and are approximately 4 times greater than those of Cl-, an order of magnitude greater than those of MSA, Na, and Ca, and two orders of magnitude greater than those of Al. The fog deposition flux of SO42- appears to be much greater than the dry deposition flux, based on experimental data. This indicates that dry deposition has a negligible effect on surface snow SO42- concentrations on days when there is fog. Such a finding is consistent with significant increases in surface snow SO42-, NO3-, and NH4+ inventories seen after fog events. Cl- surface snow inventories are affected by fog but not as greatly. Variation in surface snow chemical inventories makes it difficult to obtain quantitative estimates of daily chemical fluxes. Surface snow Ca2+ and Na+ are relatively unaffected by post snowfall processes due to low atmospheric concentrations relative to the amount of material in fresh snow. Model results suggest that the fog fluxes have been underestimated by the current sampling technique. C1 CNRS,LAB GLACIOL & GEOPHYS ENVIRONNEMENT,F-38402 ST MARTIN DHERES,FRANCE. TSI INC,ST PAUL,MN 55126. UNIV NEW HAMPSHIRE,INST STUDY EARTH OCEANS & SPACE,DURHAM,NH 03824.

Sulfate and methanesulfonic acid (MSA) concentrations in aerosol, surface snow, and snowpit samples have been measured at two sites on the Greenland Ice Sheet. Seasonal variations of the concentrations observed for these chemical species in the atmosphere are reproduced in the surface snow and preserved in the snowpit sequence. The amplitude of the variations over a year are smaller in the snow than in the air, but the ratios of the concentrations are comparable. The seasonal variations for sulfate are different at the altitude of the Ice Sheet compared to those observed at sea level, with low concentrations in winter and short episodes of elevated concentrations in spring. In contrast, the variations in concentrations of MSA are similar to those measured at sea level, with a first sequence of elevated concentrations in spring and another one during summer, and a winter low resulting from low biogenic production. The ratio MSA/sulfate clearly indicates the influence of high-latitude sources for the summer maximum of MSA, but the large impact of anthropogenic sulfate precludes any conclusion for the spring maximum. The seasonal pattern observed for these species in a snowpit sampled according to stratigraphy indicates a deficit in the accumulation of winter snow at the summit of the Greenland Ice Sheet, in agreement with some direct observations. A deeper snowpit covering the years 1985-1992 indicates the consistency of the seasonal pattern for MSA over the years, which may be linked to transport and deposition processes. C1 CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213. UNIV NEW HAMPSHIRE,INST STUDY EARTH OCEANS & SPACE,GLACIER RES GRP,DURHAM,NH 03824.

During the third Soviet-American Gas and Aerosol (SAGA 3) expedition to the central Pacific in February to March 1990 we observed C1 - C5 nonmethane hydrocarbons (NMHCs) both in the atmosphere and in the ocean. Atmospheric NMHCs project strongly on two factors: those NMHCs with lifetimes over 1 week (ethane, ethyne, propane, cyclopropane) display north-south latitudinal gradients whose magnitudes are proportional to their photochemical loss rates, which is consistent with a northern hemispheric, continental source, while those NMHCs with lifetimes under 1 week (all alkenes and pentane) do not display latitudinal gradients but do vary in common in a more complicated manner, consistent with a heterogeneous, marine source. Data taken from a sea water equilibrator shows that alkene concentrations vary strongly with wind speed, supporting previous conclusions that sea-air evasion is a major loss term for mixed layer alkenes. A reasonable balance exists between sea-air fluxes and atmospheric column removal for some alkenes (ethene, propene, 1-butene, and 1-pentene), while the remainder are out of balance, with partial pressures insufficient to support atmospheric observations. The total NMHC sea-air flux is about 0.8 mumole m-2 d-1. Both our air and water concentrations fall near the low end of previously observed ranges. It is argued that some previous higher atmospheric observations may reflect systematic problems with canister sampling. During SAGA 3 the role of NMHCs in the remote atmospheric OH budget was secondary though important, accounting (in conjunction with NMHC oxidation products) for 10 to 20% of all OH removal during baseline periods and a substantially larger fraction during episodes of increased NMHC mixing ratios. C1 MIT,CTR GLOBAL CHANGE SCI,CAMBRIDGE,MA 02139.

A major purpose of the third joint Soviet-American Gases and Aerosols (SAGA 3) oceanographic cruise was to examine remote tropical marine O3 and photochemical cycles in detail. On leg 1, which took place between Hilo, Hawaii, and Pago-Pago, American Samoa, in February and March 1990, shipboard measurements were made Of O3, CO, CH4, nonmethane hydrocarbons (NMHC), NO, dimethyl sulfide (DMS), H2S, H2O2, organic peroxides, and total column O3. Postcruise analysis was performed for alkyl nitrates and a second set of nonmethane hydrocarbons. A latitudinal gradient in O3 was observed on SAGA 3, with O3 north of the intertropical convergence zone (ITCZ) at 15-20 parts per billion by volume (ppbv) and less than 12 ppbv south of the ITCZ but never less-than-or-equal-to 3 ppbv as observed on some previous equatorial Pacific cruises (Piotrowicz et al., 1986; Johnson et al., 1990). Total column O3 (230-250 Dobson units (DU)) measured from the Akademik Korolev was within 8% of the corresponding total ozone mapping spectrometer (TOMS) satellite observations and confirmed the equatorial Pacific as a low O3 region. In terms of number of constituents measured, SAGA 3 may be the most photochemically complete at-sea experiment to date. A one-dimensional photochemical model gives a self-consistent picture of O3-NO-CO-hydrocarbon interactions taking place during SAGA 3. At typical equatorial conditions, mean O3 is 10 ppbv with a 10-15% diurnal variation and maximum near sunrise. Measurements of O3, CO, CH4, NMHC, and H2O constrain model-calculated OH to 9 x 10(5) cm-3 for 10 ppbv O3 at the equator. For DMS (300-400 parts per trillion by volume (pptv)) this OH abundance requires a sea-to-air flux of 6-8 x 10(9) cm-2 s-1, which is within the uncertainty range of the flux deduced from SAGA 3 measurements of DMS in seawater (Bates et al., this issue). The concentrations of alkyl nitrates on SAGA 3 (5-15 pptv total alkyl nitrates) were up to 6 times higher than expected from currently accepted kinetics, suggesting a largely continental source for these species. However, maxima in isopropyl nitrate and bromoform near the equator (Atlas et al., this issue) as well as for nitric oxide (Torres and Thompson, this issue) may signify photochemical and biological sources of these species. C1 NATL CTR ATMOSPHER RES,BOULDER,CO 80307. NOAA,PACIFIC MARINE ENVIRONM LAB,SEATTLE,WA 98115. HARVARD UNIV,CAMBRIDGE,MA 02138. UNIV RHODE ISL,GRAD SCH OCEANOG,KINGSTON,RI 02882. UNIV NEW HAMPSHIRE,DURHAM,NH 03824. UNIV MIAMI,ROSENSTIEL SCH MARINE & ATMOSPHER SCI,MIAMI,FL 33149. MAIN GEOPHYS OBSERV,ST PETERSBURG,RUSSIA. NASA,GODDARDS SPACE FLIGHT CTR,WALLOPS FLIGHT FACIL,WALLOPS ISL,VA 23337. INST APPL PHYS,ATMOSPHER MONITORING LAB,MOSCOW,RUSSIA.

The size distribution of atmospheric secondary organic aerosol is simulated by a Lagrangian trajectory model that includes descriptions of gas-phase chemistry, inorganic and organic aerosol thermodynamics, condensation/evaporation of aerosol species, dry deposition and emission of primary gaseous and particulate pollutants. The model is applied to simulate the dynamics of aerosol size and composition along trajectories on 27-28 August 1987 during the Southern California Air Quality Study (SCAQS). The secondary organic aerosol material is predicted to condense almost exclusively on the submicron aerosol in agreement with the available measurements, and its size distribution for Claremont, CA, is predicted to be unimodal with a mass mean diameter of roughly 0.2 mum. The distributions of the various secondary organic aerosol species are predicted to be essentially the same. The secondary organic aerosol (SOA) size distribution is found to depend crucially on the mass and size distribution of primary aerosol on which the secondary species condense and on the surface accommodation coefficient of the condensable species. The SOA size distribution is predicted not to be significantly affected by diffusional dry deposition, sources and sinks of ammonia, emissions of VOC, and secondary aerosol yields from precursor hydrocarbons. A bimodal secondary organic aerosol size distribution is predicted only if the submicron primary dust particles reside mainly in the 0.5-1.0 mum diameter size range, or if the condensable species have a strong preference (an accommodation coefficient difference of two orders of magnitude) for the 0.51.0 mum diameter particles. The secondary organic aerosol distribution in Claremont is predicted to shift slightly towards the larger aerosol particles during the nighttime hours with its mass mean diameter peaking around midnight at 0.21 mum and having its minimum in early afternoon at 0. 18 mum. In coastal locations of the Los Angeles basin, secondary organic material exists in relatively smaller particles (mass mean diameter 0.16 mum) but in far inland locations it condenses on the available larger particles (mass mean diameter 0.23 mum). C1 UNIV DELAWARE,DEPT MECH ENGN,NEWARK,DE 19716.

The chemical composition of aerosol generated in the photooxidation of 1-octene was examined using infrared microscopy interfaced with a low pressure impactor. The low pressure impactor segregated the aerosol into eight size fractions and deposited the aerosol onto ZnSe impaction substrates. The ZnSe surfaces were transparent in the mid-infrared region and therefore allowed direct analysis of the aerosol, with no extraction, using infrared microscopy. Infrared spectra of the size segregated aerosol showed strong absorbances due to ketone, alcohol, carboxylic acid and organonitrate functional groups. Absorbance features were relatively independent of particle size, with the exception of the carboxylic acid absorbances, which were found only in the largest aerosol size fractions. Molar loadings for each of the groups were estimated, based on model compound calibration standards. The molar loadings indicate that most aerosol species are multifunctional, with an average of one ketone group per molecule, an alcohol group in two of every three molecules and an organonitrate group in one of every three molecules. C1 UNIV CALIF LOS ANGELES,DEPT CHEM ENGN,LOS ANGELES,CA 90024. CALTECH,DEPT CHEM ENGN,PASADENA,CA 91125.

DGASP was designed as an international effort to study the processes influencing chemical species that eventually reach the deep ice in the south of Greenland. These processes include long-range atmospheric transport, wet and dry deposition to the snow surface, and changes during aging of the snow. The program took place during August 1988-July 1989. Experimental work included sampling of aerosols and gases, collection of fresh and older surface snow, and sampling of snowpits. The various samples were analysed for chemical species that are tracers of specific source categories. Results of the program show that the southern Greenland Ice Sheet experiences very different airborne concentration patterns than sea-level arctic sites. Concentrations of SO42-, trace metals, and other species are episodic and peak in April, unlike the consistently high concentrations during January-April seen at lower elevations. Source regions influencing Dye 3 are variable, and include North America, western Europe, and the Arctic Basin. The last region is particularly important, as it may contain relatively high concentrations of chemical constituents from eastern Europe and western Asia that eventually reach Dye 3. The seasonal variations in airborne concentrations are generally also reflected in fresh snow. Similarly, these patterns are seen in snowpits, although some modifications to the glacial record are apparent. Other information on the extent of riming, aerosol/gas partitioning, and aerosol size distributions demonstrate the complexity of air-snow transfer processes, and illustrate the need for further research. C1 CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213. CARNEGIE MELLON UNIV,DEPT ENGN & PUBL POLICY,PITTSBURGH,PA 15213.

Chemical constituent concentrations in air and snow from the Dye 3 Gas and Aerosol Sampling Program show distinct seasonal patterns. These patterns are different from those observed at sea-level sites throughout the Arctic. Airborne SO42and several trace metals of crustal and anthropogenic origin show strong peaks in the spring, mostly in April, Some species also have secondary maxima in the fall. The spring peaks are attributed to transport over the Pole from Eurasian sources, as well as transport from eastern North America and western Europe. The fall peaks are attributed primarily to transport from North America, and less frequent transport from Europe. Airborne Be-7 and Pb-210 show strong peaks in both spring and fall, suggesting that vertical atmospheric mixing is favored during these two seasons. Several other airborne constituents peak at other times. For example, Na peaks in winter due to transport of seaspray from storms in ice-free oceanic areas, while MSA peaks in summer due to biogenic production in the oceans nearby. Many trace gases such as freons and other chlorine-containing species show roughly uniform concentrations throughout the year. CO and CH4 show weak peaks in February-March. Concentrations of chemical constituents in fresh snow at Dye 3 also show distinct seasonal patterns. SO42- and several trace metals show springtime maxima, consistent with the aerosol data. Na shows a winter maximum and MSA shows a summer maximum in the snow, also consistent with the aerosols. Be-7 and Pb-210 in the snow do not show any strong variation with season. Similarly, soot and total carbon in snow do not show strong variation. When used with dry deposition models, these air and snow concentration data suggest that dry deposition of submicron aerosol species has relatively minor influence on constituent levels in the snowpack at Dye 3 compared to wet deposition inputs(including scavenging by fog); crustal aerosol, on the other hand, may have a more significant input by dry deposition. Overall, the results suggest that gross seasonal patterns of some aerosol species are constistent in the air and in fresh snow, although individual episodes in the air are not always reflected in the snow. The differences in data reported here compared with data sets for sea-level arctic sites demonstrate the need for sampling programs on the Ice Sheet in order to properly interpret Greenland glacial record data. C1 CNRS,GLACIOL & GEOPHYS ENVIRONNEMENT LAB,F-38402 ST MARTIN DHERES,FRANCE. UNIV NEW HAMPSHIRE,INST STUDY EARTH OCEANS & SPACE,DURHAM,NH 03824. DESERT RES INST,CTR ATMOSPHER SCI,RENO,NV 89506. NOAA,CMDL,BOULDER,CO 80303. OREGON GRAD INST SCI & TECHNOL,BEAVERTON,OR 97006. CEA,CNRS,CTR FAIBLES RADIOACT,F-91198 GIF SUR YVETTE,FRANCE. UNIV PARIS 07,PHYSICOCHIM ATMOSPHERE LAB,F-75251 PARIS,FRANCE. NATL INST ENVIRONM RES,DK-4000 ROSKILDE,DENMARK. FINNISH METEOROL INST,SF-00810 HELSINKI,FINLAND.

Detailed examination of a two-week period in April 1989 during the Dye 3 Gas and Aerosol Sampling Program shows that episodes of relatively high concentration of certain chemical constituents occur at this time of year. Airborne concentrations of crustal metals such as Al and Ca can exceed 100 ng m(-3), while concentrations of SO42can exceed 1000 ng m(-3). Elevated concentrations of MSA, Be-7 and Pb-210 are also noted. Consideration of synoptic maps and backward air mass trajectories suggests that the episodes are due to transport from a variety of source regions, including Eurasia (transport over the Pole), North America and western Europe. In addition to elevated airborne concentrations, levels of these constituents in surface snow are high during April. However, it is difficult to develop quantitative relationships between concentrations in air and in snow due to the difficulty in measuring airborne concentrations at cloud-level; variations in scavenging by clouds may also be significant. It is concluded that the springtime maxima in airborne concentrations resulting from long-range transport from a variety of source regions are responsible for strong identifiable signals in ice cores and snowpits from this region. C1 CNRS,GLACIOL & GEOPHYS ENVIRONNEMENT LAB,F-38402 ST MARTIN DHERES,FRANCE. UNIV NEW HAMPSHIRE,INST STUDY EARTH OCEANS & SPACE,DURHAM,NH 03824. DESERT RES INST,CTR ATMOSPHER SCI,RENO,NV 89506. NOAA,CMDA,BOULDER,CO 80303. OREGON GRAD INST SCI & TECHNOL,BEAVERTON,OR 97006. CEA,CNRS,CTR FAIBLES RADIOACT,F-91198 GIF SUR YVETTE,FRANCE. UNIV PARIS 07,PHYSICOCHIM ATMOSPHERE LAB,F-75251 PARIS,FRANCE. NATL INST ENVIRONM RES,DK-4000 ROSKILDE,DENMARK. FINNISH METEOROL INST,SF-00810 HELSINKI,FINLAND.

Backward air mass trajectories for Dye 3, Greenland (elevation 2.5 km) show source regions that vary with season: the direction of greatest transport distance is from the southwest in fall, west in winter, and northwest in spring; the trajectories in summer do not show a strong preferred direction. Based on 5 d transit times, the trajectories in fall suggest the importance of North America as a potential source region, with occasional trajectories from western Europe, The trajectories in spring, especially in April, suggest Eurasia (transport over the Pole), eastern North America, and western Europe as potential source regions. Less transport of chemical constituents to Dye 3 is expected in summer when transport distances are shorter. Although some long-range transport to Greenland occurs in winter, the stability of the atmosphere over the ice sheet at this time of year is likely to limit the delivery of chemical constituents to the surface. Sources outside of these regions can also influence Dye 3 if transit times longer than 5 d are considered. These results are in contrast to trajectories reported by others for sea-level arctic locations such as Barrow, Alaska and Mould Bay, Canada, where transport over the Pole from Eurasia is responsible for high chemical species concentrations over much of the winter and early spring. Overall, the trajectories are consistent with aerosol chemical data for this time period at Dye 3 reported by several investigators, showing peak concentrations in spring and fall. C1 CNRS,GLACIOL & GEOPHYS ENVIRONNEMENT LAB,F-38402 ST MARTIN DHERES,FRANCE. ATMOSPHER ENVIRONM SERV,DOWNSVIEW M3H 5T4,ON,CANADA. UNIV NEVADA,DESERT RES INST,CTR ATMOSPHER SCI,RENO,NV 89506.

Concentrations of lead, cadmium, copper and zinc have been measured in a variety of samples of fresh or slightly aged snow collected at Dye 3, south Greenland, on a precipitation event basis from January to August 1989. Measured concentrations are found to be very variable from one snowfall to another, with high concentration peaks occurring in April and June. The four metals are shown to be mainly derived from anthropogenic sources, with the exception of Cu and Zn for some of the samples. The data obtained for several snow events are further discussed using 5 days backward air mass trajectories together with data for various other chemical species. C1 UNIV GRENOBLE 1,UFR MECAN,F-38041 GRENOBLE,FRANCE. CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213. RUSSIAN ACAD SCI,INST SPECT,TROITSK 142092,RUSSIA.

Cascade impactor samples were collected at Dye 3 on the south-central Greenland Ice Sheet during March 1989. The impactor was calibrated in the laboratory, and the resulting collection efficiency curves were used to derive the impactor response for use in a data inversion procedure. The impactor samples were chemically analysed by proton-induced X-ray emission (PIXE), and the chemical concentration data were used with the inversion procedure to generate smooth size distributions for 15 elements. Results show three distinct size distribution categories. The first category includes elements that mainly originate from gas to particle conversion, with a substantial fraction-from anthropogenic combustion (S, Pb, Zn, Br and Ni). These elements exhibit a unimodal size distribution with geometric mean aerodynamic diameter close to 0.6 mu m, although S and in shaw a weak second mode centered at about 2 mu m. Elements in the second category (Ti, Si, Fe, Mn, Ca, K) exhibit bimodal size distributions, with geometric mean diameters for the two modes of 0.6 and 2 mu m, respectively. These elements result from a variety of sources, including crustal erosion as well as combustion from natural and anthropopenic sources. For elements in the third category (Al, Cl, Na, Mg), most of the mass occurs in particle sizes above 1 mu m. Their size distribution is generally unimodal, with the geometric mean aerodynamic diameter around 2 mu m. These elements are most likely to be of crustal and/or marine origin. The best-fit size distributions were used with curves of dry deposition velocity vs aerodynamic particle diameter to estimate the overall dry deposition velocity expected from the entire distribution. The deposition velocities for S, Pb, Zn, Br and Ni are all very low, with values less than about 0.02 cm s(-1) if hygroscopic growth in the humid layer is neglected. For those other elements, deposition velocities are in the range 0.2-0.7 cm s(-1). For those distributions that are bimodal, the upper mode generally dominates deposition even when most of he airborne mass is associated with the lower modes, as in the case of S and Zn. C1 INST NUCL WETENSCHAPPEN,B-9000 GHENT,BELGIUM. CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213.

Airborne concentrations and size distributions of 15 elements over the Greenland Ice Sheet have been measured during a one-month period in March 1989. The concentrations are relatively uniform, varying by less than a factor of three for virtually all of the elements. Notable exceptions are Na and Cl which vary by more than an order of magnitude; these differences can probably be accounted for by the link with transport from the oceans surrounding Greenland, although a significant fraction of the Na is of crustal origin in some samples. The size distributions show strong peaks in the accumulation mode(0.4-1.0 mu m) or the coarse particle mode (1.0-2.5 mu m); some species show bimodal distributions with the presence of both modes. The aerosol chemistry and size distribution data are consistent with back trajectories and local weather conditions. For example, relatively high concentrations of Pb, Zn, Ni, Fe, and Mn in the accumulation mode during one of the runs are associated with trajectories from industrial regions of the Soviet Arctic. The elements Si, Al, Fe, K, Ca, Mn, and Ti in the coarse mode are believed to be dominated by crustal sources. However, some runs show the presence of an accumulation mode for most of these elements (with the exception of Al), suggestive of combustion sources. Overall, the results show that use of an impactor with several submicron size cuts combined with a suitable data inversion program can provide insights into the sources and transport of aerosols at remote locations such as the Greenland Ice Sheet. C1 CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213. FINNISH METEOROL INST,DEPT AIR QUAL,SF-00810 HELSINKI,FINLAND. INST NUCL WETTSCHAPPEN,B-9000 GHENT,BELGIUM.

As part of the DGASP program on the Greenland Ice Sheet, an investigation was conducted into the nature of ice particle formation processes that result in the formation of snow. Ice particle habits were determined using Formvar replicas of falling snow crystals. From these measurements an assessment of the primary growth process and altitude of formation was made. Results indicate that the scavenging of cloud water by falling ice particles, growth by accretion or riming, rarely occurs. However, when riming does occur, it is usually associated with warmer air masses from the south. The occurrence of riming was also observed to be dependent on the season, with a greater frequency occuring during warmer months. It was estimated that ice particle riming contributes less than 5% of the average annual water mass, but up to 30% of the deposition of some chemical species, deposited to the Greenland Ice Sheet at Dye 3. Ice particle habits indicate that they originate at higher altitudes above the ice cap in summer than in winter. Variations in the magnitude of ice particle riming, the elevation of origin of ice particles, the meteorology and the season of the year are all essential when interpreting snow chemistry and comparing snow and aerosol chemistry at Dye 3. C1 CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213.

Measurements in the Arctic troposphere over several years show that MSA concentrations in the atmospheric boundary layer, 0.08-6.1 parts per trillion (ppt, molar mixing ratio), are lower than those over mid-latitude oceans. The seasonal cycle of MSA at Alert, Canada (82.5 degrees N, 62.3 degrees W), has two peaks of 6 ppt in March-April and July-August and minima of 0.3 ppt for the rest of the year. At Dye 3 (65 degrees N, 44 degrees W) on the Greenland Ice Sheet, a similar seasonal MSA cycle is observed although the concentrations are much lower with a maximum of 1 ppt. Around Barrow, Alaska (71.3 degrees N, 156.8 degrees W), MSA is between 1.0 and 25 ppt in July, higher than 1.5+/-1.0 ppt in March-April. The mid-tropospheric MSA level of 0.6-1 ppt in the summer Arctic is much lower than about 6 ppt in the boundary layer. Al Alert, the ratio of MSA to non-sea-salt (nss) SO42ranges from 0.02 to 1.13 and is about 10 times higher in summer than in spring. The summer ratios are higher than found over mid-latitude regions and, when combined with reported sulfur isotope compositions from the Arctic, suggest that on average a significant fraction (about 16-23%) of Arctic summer boundary layer sulfur is marine biogenic. The measurements show that the summer Arctic boundary layer has a significantly higher MSA/nss-SO42- ratio than aloft. C1 UNIV NEW HAMPSHIRE,COMPLEX SYST RES CTR,INST STUDY EARTH OCEANS & SPACE,DURHAM,NH 03824. CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213.

Heterogeneous production of sulfate in an urban fog has been investigated using data collected during the SCAQS program in the Los Angeles area, for the period of 10-11 December 1987. Fog was observed during the night of 10 December and the early morning hours of 11 December near the coast of southern California. Measurements at several sites (Hawthorne, downtown Los Angeles, etc.) indicated a significant increase in sulfate concentration during the afternoon of 11 December. Trajectory analysis suggests that these high sulfate concentrations were associated with the arrival at the receptor sites of air parcels that passed through the fog layer the previous night. To quantify the contribution of aqueous-phase processes to the above sulfate levels, a detailed trajectory model was employed to simulate the gas-phase processes during that episode. The model, using the available information about SO2 and sulfate emissions, initial conditions and meteorology, successfully explained the sulfate levels in air parcels that did not pass through the fog layer, but underestimated by as much as 2.5 the sulfate levels of the trajectories through the fog. Sensitivity/uncertainty analysis indicated that the presence of sulfate beyond that attributable to gas-phase chemistry (around 10-mu-g m-3) cannot be attributed to uncertainties in the model parameters (e.g. initial conditions, emissions, mixing heights, deposition velocities). The episode was then simulated using a full psand aqueous-phase chemistry model and the analysis indicated that heterogeneous sulfate formation in fog droplets under the conditions of the episode can indeed explain the observed sulfate. C1 CALTECH,ENVIRONM QUAL LAB,PASADENA,CA 91125. AERO VIRONMENT INC,MONROVIA,CA 91016.

A Lagrangian trajectory model simulating the formation, transport and deposition of secondary organic aerosol is developed and applied to the Los Angeles area, for the air pollution episode of 27-28 August 1987. The predicted secondary organic aerosol on 28 August 1987 represents 15-22% of the measured particulate organic carbon at inland locations in the base case simulations, and 5-8% of that at coastal locations. A maximum secondary organic aerosol concentration of 6.8-mu-g m-3 is predicted for Claremont, CA, during this episode. On a daily average basis at Claremont about 46% of this secondary organic aerosol is predicted to be a result of the oxidation of non-toluene aromatics (xylenes, alkylbenzenes, etc.), 19% from toluene, 16% from biogenic hydrocarbons (alpha-pinene, beta-pinene, etc.), 15% from alkanes and 4% from alkenes. The major uncertainties in predicting secondary organic aerosol concentrations are the reactive organic gas emissions, the aerosol yields and the partitioning of the condensable gases between the two phases. Doubling the reactive organic gas (ROG) emissions results in an increase of the secondary organic aerosol predicted at Claremont by a factor of 2.3. Predicted secondary organic aerosol levels are less sensitive to changes in secondary organic aerosol deposition and NOx emissions than to ROG emissions. C1 CALTECH,DEPT ENVIRONM ENGN SCI,PASADENA,CA 91125.

The chemical composition of smog-chamber aerosol generated during the photo-oxidation of isoprene and beta-pinene was probed using infrared (i.r.) microscopy interfaced with a low pressure impactor. The low-pressure impactor employed ZnSe impaction surfaces which allowed direct analysis, with no extraction, using i.r. microscopy. The low detection limits of this technique, coupled with direct sample analysis, permitted the chemical analysis of aerosol generated at low to moderate hydrocarbon and ozone loadings. The quantitative analysis of the i.r. spectra is limited, in part, by the absence of calibration standards, however, it is clear that the biogenic aerosols generated in this work were liquid mixtures containing ketone, aldehyde, alcohol and organonitrate functional groups. Molar loadings of each of these functional groups were estimated for nine smog-chamber experiments. For aerosol formed in isoprene photo-oxidation, aldehyde and ketone groups dominated (1.8 groups per average molecule) while for aerosol formed in beta-pinene photo-oxidation, alcohols and ketones dominated (a combined 2.7 groups per average molecule). C1 UNIV CALIF LOS ANGELES,DEPT CHEM ENGN,LOS ANGELES,CA 90024. CALTECH,DEPT CHEM ENGN,PASADENA,CA 91125.

The resuspension rates of particulate chemical species have been estimated using aerodynamic surrogate surfaces (symmetrical airfoils) at a mixed deciduous forest site, and also using throughfall sampling at a loblolly pine forest site. The airfoils were placed above, within, and below the crown of the deciduous canopy to determine the vertical variation of deposition and resuspension. The resuspension rates of SO42-, C2O42-, and Ca from the airfoils are estimated to be on the order of 10(-6)-10(-5) s-1. The resuspension rates within the crown are only slightly less than those above the canopy. This is in contrast to the mean wind speed, which is much smaller within the crown than above the canopy. The smaller differences in the resuspension rate compared with differences in mean wind speed may be due to the effects of wind gusts which penetrate deep within the canopy. Based on the throughfall data, the resuspension rates for Ca, Mg, and Na are also in the range 10(-6)-10(-5) s-1. Overall, these results suggest that resuspension may need to be taken into account when particle deposition to vegetation is estimated. C1 CARNEGIE MELLON UNIV,DEPT MECH ENGN,PITTSBURGH,PA 15213. CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213. OAK RIDGE NATL LAB,OAK RIDGE,TN 37830.

A new model for particle deposition and bounceoff that combines current knowledge of turbulent bursts with the stochastic properties of turbulent fluctuations is presented. The model predictions for deposition velocities agree with experimental results in the literature for dimensionless particle relaxation time tau(p)+ > 2. For tau(p)+ > 10, most of the particles delivered to the edge of the viscous sublayer are able to deposit onto the surface due to their inertia; the deposition velocity approaches an asymptotic value because the process becomes limited by the rate or turbulent delivery to the viscous sublayer. Because of the penetration of turbulent fluctuations into the viscous sublayer, the minimum values of vertical velocities needed for particles to deposit onto the surface are smaller than those predicted by the free flight model. Most of the deposition occurs from those turbulent fluctuations at the upper tail of the distribution of the vertical component of air velocity. In addition to the deposition velocity the model is able to provide the distribution of particle velocities on reaching the surface which is used to compute the fraction of particle bounceoff. The model predictions for the fractions of rebound agree reasonably with the measured results from a wind tunnel experiment for tau(p)+ > 2. However, both the deposition velocity and the fraction of rebound are underestimated by the model for tau(p)+ < 2. Other mechanisms such as Brownian diffusion must be included in further revisions to this model in order to obtain satisfactory predictions for smaller values of tau(p)+. C1 CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213. CARNEGIE MELLON UNIV,DEPT MECH ENGN,PITTSBURGH,PA 15213.

The dynamics of particle rebound and resuspension were examined by using uranine particles, polymer microspheres, spores, and pollen in wind tunnel experiments. Particle diameters were 5-42 mum. Results show that both the fraction of rebound and the resuspension rate are strongly dependent on the free stream velocity, particle size, and relative humidity. The effects of relative humidity are more significant at lower windspeeds; a greater relative humidity appears to change the shape of the distribution of adhesion force. mainly affecting the lower range but not greatly affecting the upper end of the distribution. Resuspension rates decrease with time, essentially defining two regimes. The first regime lasts for < 1 min; after this time, the most easily resuspended particles have been removed, leaving only particles with much smaller resuspension rates for the second regime. At a windspeed of 6 m/s, the upper 20% of the distribution of turbulent fluctuations is responsible for approximately 65% of the particle resuspension. Once resuspended, the particles have trajectories which depend on characteristics of the turbulent airflow and not on the initial velocity of release from the surface. Overall, the data show th at resuspension is more sensitive to the type or particle than to the type of surface; particle shape and composition may be more important than particle size. C1 CARNEGIE MELLON UNIV,DEPT CIVIL ENGN,PITTSBURGH,PA 15213. CARNEGIE MELLON UNIV,DEPT MECH ENGN,PITTSBURGH,PA 15213.

A stochastic approach to the problem of incorporating variable emission events in indoor air quality models is proposed. A nonstationary Poisson process is used to account for the occurrence of range-top burner use. The combination of this emission event sequence with a simple one-compartment mass-balance model results in output that qualitatively agrees with measured concentrations in two occupied townhouses. Improved monitoring of stove usage times, gas flow rates and emission factors under field conditions would allow more effective estimation of model input parameters and more accurate prediction of concentration distributions.

Measurements of dry deposition were conducted to potted plants at Claremont, CA, during the Southern California Air Quality Study and to patches on an equestrian statue at Gettysburg National Military Park, PA. Simultaneous dry deposition measurements were also conducted on aerodynamically designed surrogate surfaces (symmetric airfoils) with predictable boundary layer characteristics at both locations. Species of interest include gaseous SO2 and particulate SO42-, NO3-, Ca, and Pb. The importance of each transport step in the deposition process was evaluated by combining the data from several types of surfaces with different surface characteristics. The significance of particle resuspension was determined by simultaneous sampling with different exposure times. The results suggest that the relative importance of aerodynamic, boundary layer, and surface resistances can vary greatly, depending on the depositing chemical species and the surface characteristics. Of particular interest is the highly variable surface resistance which is often the rate-limiting step in the overall process. Rates of resuspension for several surfaces including natural vegetation are in the range of 10(-6) to 10(-5) sec-1 for the aerosol chemical species considered. This implies that net dry deposition rates depend on the exposure times, which are not often considered in most ambient dry deposition studies. C1 ILLINOIS DEPT ENERGY & NAT RESOURCES,ATMOSPHER CHEM SECT,DIV STATE WATER SURVEY,CHAMPAIGN,IL 61820. US DEPT INTERIOR,DIV PRESERVAT ASSISTANCE,NATL PK SERV,WASHINGTON,DC 20240.

The epiphaniometer is a new instrument for continuous monitoring of the Fuchs surface of aerosol particles using the attachment of neutral radioactive Pb-211 atoms to the particles. Since the Pb-211 atoms have a half-life of 36.1 min, the ability of the epiphaniometer to detect short-term fluctuations in the Fuchs surface of the aerosol is limited. To overcome this drawback a general algorithm developed for the inversion of size distribution data from conventional aerosol instruments is adapted for the inversion of data from the epiphaniometer. This algorithm allows resolution of dramatic changes in the aerosol Fuchs surface occurring over a time scale of minutes. The method is applied to data collected during outdoor smog chamber experiments, with changes in aerosol concentration of several orders of magnitude occurring within a time period of 5 min. The inverted epiphaniometer data are in agreement with the corresponding data from a Scanning Electrical Mobility Spectrometer (SEMS) and an Optical Particle Counter.

In this work we prove that mixing of droplets with different pH that are individually in Henry's law equilibrium with the surrounding atmosphere always results in a bulk mixture that is supersaturated with weak acids like S(IV) and HCOOH and bases like NH3 with respect to the original atmosphere. The degree of supersaturation of the bulk liquid water sample for a particular species depends on its dissociation constant, on the initial pH of the bulk droplet mixture, and on the distribution of the pH and of the liquid water over the droplet spectrum. High supersaturations result only when the pH of the bulk droplet mixture exceeds the pK-alpha of the species, in which pH range large pH differences among droplets of different sizes lead to large deviations from Henry's law for the bulk mixture. The deviation is shown to depend on the ratio of the arithmetic mean to the harmonic mean of the hydrogen ion concentrations of the droplets with the liquid water content used as weighting factor in the calculation of the means. The theory developed can explain observed discrepancies from Henry's law in atmospheric samples and also other observed phenomena like the reported increase of pH values of bulk aqueous samples during storage. C1 CALTECH,ENVIRONM QUAL LAB,PASADENA,CA 91125.

Isoprene and beta-pinene, at concentration levels ranging from a few ppb to a few ppm, were reacted photochemically with NO(x) in the Caltech outdoor smog chamber facility. Aerosol formation from the isoprene photooxidation is found to be negligible even under extreme ambient conditions due to the relatively high vapor pressure of its products. Aerosol carbon yield from the beta-pinene photooxidation is as high as 8% and depends strongly on the initial HC/NO(x) ratio. The average vapor pressure of the beta-pinene aerosol is estimated to be 37 +/- 24 ppt at 31-degrees-C. Monoterpene photooxidation can be a significant source of secondary aerosol in rural environments and in urban areas with extended natural vegetation. C1 CALTECH,DEPT ENVIRONM ENGN SCI,PASADENA,CA 91125.

An extensive set of outdoor smog chamber experiments were carried out to study aerosol formation by two representative biogenic hydrocarbons; isoprene and beta-pinene. The hydrocarbons, at concentrations ranging from a few ppb to a few ppm, were photooxidized in the presence of NO(x). Isoprene was found to produce negligible aerosol at ambient conditions, whereas beta-pinene aerosol carbon yields were as high as 8%, depending strongly on the hydrocarbon to NO(x) ratio. Aerosol samples subjected to infrared absorption spectroscopy revealed that the dominant aerosol products for both isoprene and beta-pinene are organic nitrates, organic acids, as well as other carbonyls and hydroxy compounds. GCMS of the neutral fraction of the beta-pinene aerosol revealed nopinone and several other compounds with molecular weights ranging from 138-200 amu, indicating mainly mono- and dioxygenated products. The average vapor pressure of the beta-pinene aerosol was estimated to be 37 +/- 24 ppt at 31-degrees-C. Scanning electron micrographs showed that the particles consist of both liquid droplets and agglomerates of small (40-60 nm) solid particles. C1 CALTECH,DEPT ENVIRONM ENGN,PASADENA,CA 91125. UNIV CALIF LOS ANGELES,DEPT CHEM ENGN,LOS ANGELES,CA 90024. INST WATER RESOURCES & WATER POLLUT CONTROL EAWAG,CH-8600 DUBENDORF,SWITZERLAND. UNIV ZURICH,INST INORGAN CHEM,CH-8057 ZURICH,SWITZERLAND.