Randolph D. Borys

Measurements of the concentration and composition of nuclei for cirrus formation

This article addresses the need for new data on indirect effects of natural and anthropogenic aerosol particles on atmospheric ice clouds. Simultaneous measurements of the concentration and composition of tropospheric aerosol particles capable of initiating ice in cold (cirrus) clouds are reported. Measurements support that cirrus formation occurs both by heterogeneous nucleation by insoluble particles and homogeneous (spontaneous) freezing of particles containing solutions. Heterogeneous ice nuclei concentrations in the cirrus regime depend on temperature, relative humidity, and the concentrations and physical and chemical properties of aerosol particles. The cirrus-active concentrations of heterogeneous nuclei measured in November over the western U.S. were <0.03 cm–3. Considering previous modeling studies, this result suggests a predominant potential impact of these nuclei on cirrus formed by slow, large-scale lifting or small cooling rates, including subvisual cirrus. The most common heterogeneous ice nuclei were identified as relatively pure mineral dusts and metallic particles, some of which may have origin through anthropogenic processes. Homogeneous freezing of large numbers of particles was detected above a critical relative humidity along with a simultaneous transition in nuclei composition toward that of the sulfate-dominated total aerosol population. The temperature and humidity conditions of the homogeneous nucleation transition were reasonably consistent with expectations based on previous theoretical and laboratory studies but were highly variable. The strong presence of certain organic pollutants was particularly noted to be associated with impedance of homogeneous freezing.

Storm Peak Laboratory: A Research, Teaching, and Service Facility for the Atmospheric Sciences

The Storm Peak Laboratory (SPL), operated by the Atmospheric Sciences Center of the Desert Research Institute, is now located in a newly constructed permanent building at elevation 3210 m (10 530 ft) above mean sea level in the northwestern Colorado Rocky Mountains. The laboratory provides a site for the conduct of basic and applied research in the atmospheric sciences, hands-on instruction in meteorology for students ranging from middle school through graduate school, and high-elevation atmospheric measurement programs for various scientific groups, agencies, and private companies. This article provides a background of the history of SPL, its past and current activities, and a description of the facilities and opportunities available at the laboratory.

Chemical constituents in the air and snow at Dye 3, Greenland—I. Seasonal variations

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 SO42− and several trace metals ofcrustal 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 7Be and 210Pb 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. 7Be and 210Pb 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.

The chemical fractionation of atmospheric aerosol as a result of snow crystal formation and growth

The relationships between the physical and chemical properties of mixed-phase clouds were investigated at Storm Peak Laboratory (3220m MSL) located near the continental divide in northwestern Colorado. Interstitial aerosol particles, cloud droplets and snow crystals were concurrently collected when the laboratory was enveloped by a precipitating cloud. All samples were analyzed for trace elements, soluble anions, electrical conductivity and acidity.The results show average trace constituent concentration ratios of cloud water to snow water range from 0.4 to 26. All but six of the 32 elements and ions measured had ratios greater than one. This result suggests a chemical species dependency of in-cloud aerosol particle scavenging processes. Evidence of a decrease of in-cloud aerosol particle scavenging efficiency by snow due to increases in aerosol concentration is also presented.Differences between the chemical composition of cloud water and snow water are manifested most strongly when snow crystals grow by vapor deposition. In-cloud scavenging efficiencies by snow crystals for most aerosol particle chemical species are dependent on the growth of the snow crystals by accretion of cloud droplets. This chemical fractionation of the atmospheric aerosol by snow crystal formation and growth should be most active where narrow, continental cloud droplet size distributions and low liquid water contents are prevalent, enhancing the probability of snow crystal growth by diffusion.

Influence of Cloud Condensation Nuclei on Orographic Snowfall

Abstract Pollution aerosols acting as cloud condensation nuclei (CCN) have the potential to alter warm rain clouds via the aerosol first and second indirect effects in which they modify the cloud droplet population, cloud lifetime and size, rainfall efficiency, and radiation balance from increased albedo. For constant liquid water content, an increase in CCN concentration (NCCN) tends to produce an increased concentration of droplets with smaller diameters. This reduces the collision and coalescence rate, and thus there is a local reduction in rainfall. While this process applies to warm clouds, it does not identically carry over to mixed-phase clouds in which crystal nucleation, crystal riming, crystal versus droplet fall speed, and collection efficiency play active roles in determining precipitation amount. Sulfate-based aerosols serve as very efficient cloud nuclei but are not effective as ice-forming nuclei. In clouds where precipitation formation is dominated by the ice phase, NCCN influences precipi...

Studies of ice nucleation by Arctic aerosol on AGASP-II

Aerosol particles were collected on filters for studies of their ability to nucleate ice during the second Arctic Gas and Aerosol Sampling Program (AGASP-II) in April, 1986. The ice nuclei (IN) samples were collected from an aircraft at altitudes ranging from the surface to the vicinity of the tropopause in Arctic locations over Alaska, northern Canada and Greenland. Samples of other components of the aerosol were collected and measurements were made of other properties of the aerosol coincident in time with the IN samples. The IN filters were exposed to water saturation in a dynamic developing chamber at −15° C and −25° C. Ice crystals grew on the IN and were counted on the filters at discrete time intervals during the exposure period to determine the rate of ice nucleation and the final concentration of (IN). Results show that Arctic haze aerosol, identified by pollutant signatures, had lower IN concentrations, a lower IN to total aerosol fraction and slower ice nucleation rates than aerosol which had a chemical signature more indicative of the remote unpolluted troposphere. These observations suggest that the Arctic haze aerosol does not efficiently form ice in the arctic troposphere. This may be a factor contributing to the long-range transport of Arctic haze.

Chemical constituents in the air and snow at Dye 3, Greenland—II. Analysis of episodes in April 1989

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 SO42− can exceed 1000 ng m−3. Elevated concentrations of MSA, 7Be and 210Pb 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.

Chemical and microphysical properties of marine stratiform cloud in the North Atlantic

The chemical and microphysical properties of marine stratiform cloud were measured at a ridgetop elevation of 992 m above mean sea level (AMSL) on Tenerife in the Canary Islands in the eastern North Atlantic during the summers of 1995 and 1996. The results show an inverse relationship between hourly-averaged cloud droplet diameter and droplet number concentration, which ranged from 116 to 1355 cm−3. Strong relationships were observed between droplet number and equivalent clear air concentrations of non-sea-salt sulfate, nitrate, and elemental carbon in the droplets. Droplet sizes inferred from radiances measured by satellite for clouds offshore and upwind agreed with droplet sizes derived for clouds over the mountain sampling site, and also with those measured in cloud 4–5 hours later. Estimated cloud short-wave radiative forcing was enhanced by 8% in radiative model studies of polluted versus clean clouds with droplet concentrations of 786 and 127 cm−3 and droplet effective radii of 6 and 10 μm, respectively.

Satellite Microphysical Retrievals for Land-Based Fog with Validation by Balloon Profiling

Abstract Digital data from the National Oceanic and Atmospheric Administration Advanced very High Resolution Radiometer (AVHRR) satellite instrument provides multispectral images in visible near-infrared and thermal infrared wave bands, which have been utilized to develop retrieval techniques for estimating the droplet effective radius and optical depth of land-based fog. The retrieval methods are based on multiple scattering calculations that simulate the increased near-infrared absorption by fog layers with increasing droplet size and liquid water path. The AVHRR thermal window channels are utilized to remove the effects of thermal emission in the near-infrared band. New instrumentation and field sampling methods have been developed for obtaining detailed vertical profiles of fog droplet size distributions and thermodynamic conditions in fog decks. The in situ measurements derived from the field observations were employed to test the satellite retrieval techniques. Intercomparison shows a close correspo...

Collecting Supercooled Cloud Droplets as a Function of Droplet Size

Abstract Supercooled cloud droplets were inertially impacted onto “cloud-sieves” at a mountaintop location. The large cross-sectional areas of the sieve meshes permitted grams of cloud water to be passively collected in minutes. Each sieve was constructed from specific diameter cylindrical strands and collected all cloud droplets larger than a critical size. Procedures are developed to produce liquid water content (LWC) and chemical composition values as a function of droplet-size interval. The sieve LWC measurements were compared with simultaneous LWC measurements obtained from a standard cloud droplet spectrometer. The sieve and spectrometer values were consistent for droplets between approximately 4 and 13 µm in diameter. The sieves overestimated the water contents of larger and smaller droplets in low LWC clouds (<0.1 gm−3). In high LWC clouds, the sieve LWC values for all droplet sizes closely approximated the spectrometer values. Sources of error were investigated. Rime “feathers” and frost grew on ...