Edward E. Hindman

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.

An integrated airborne particle-measuring facility and its preliminary use in atmospheric aerosol studies

Abstract An integrated airborne system for studying aerosol particles and their effects on the atmosphere is described. Particles from 0.01 to 30 μm in maximum dimensions, covering concentrations from 10 7 to 10 −6 cm −3 , can be measured and the measurements displayed in the aircraft. Particles from 5 to 100 μm are collected by impaction and their deliquescent nature and elemental compositions are determined in post-analysis. Also measured are the light scattering coefficient of the aerosol, Aitken nuclei concentrations, cloud condensation nuclei, and ice nuclei. Examples of data collected with this system over the Pacific Ocean, in the western and eastern regions of Washington State, and in the plume from a paper mill are presented. Differences in the particle size distributions and the nature of the particles from these different regions are apparent. From measurements obtained with the system in a rain-scavenged plume from a paper mill, values for the collection efficiencies of particles from 10 −2 to 5 μm in size have been deduced.

Cloud Condensation Nuclei from a Paper Mill. Part I: Measured Effects on Clouds

Abstract The effluents from a large Kraft-process paper mill were found to increase the concentration of large (0.2–2 μm diameter) and giant (>2 μm diameter) cloud condensation nuclei (CCN) above background concentrations; the concentrations of small (<0.2 μm diameter) CCN, however, were not increased significantly. Small, nonraining, warm cumulus clouds located in the plume of the paper mill were found to contain higher concentrations of droplets ≥30 μm diameter than similar clouds located outside of the plume; the clouds in both locations contained similar concentrations of droplets ≥5 μm diameter. Cloud droplet size distributions calculated using the CCN measurements are consistent with the measured droplet size distributions. It is concluded that the large and giant CCN from the paper mill increased the concentrations of droplets ≥30 μm diameter in clouds located in its plume. Furthermore, the clouds located in and out of the plume contained similar concentrations of droplets ≥5 μm diameter because th...

Cloud Condensation Nuclei from a Paper Mill. Part II: Calculated Effects on Rainfall

Abstract The paper mill at Port Townsend, Wash., is a source of large and giant condensation nuclei (CCN). These CCN cause the concentrations of droplets ≥30 μm in diameter to be higher in small, nonraining warm clouds located in the plume of the mill than in similar clouds unaffected by the plume. Calculations based on a model for nonsheared, warm cumulus clouds and a model for warm stratus clouds indicate that the higher concentrations of large droplets in the clouds in the plume should not cause any significant changes in the rainfall from these clouds. These results indicate that the large and giant CCN emitted by the mill are not by themselves responsible for the increased rainfall measured in the vicinity of the mill. The heat and moisture emitted by the mill, in combination with the CCN, may have been responsible for the increased rainfall.

Characteristics of Supercooled Liquid Water in Clouds at Mountaintop Sites in the Colorado Rockies

Abstract Observations and measurements were made of supercooled liquid water in clouds which enveloped high elevation sites in the Colorado Rocky Mountains for the winters of 1980/81 through 1983/84. The observations showed that liquid water was more frequent at the southern and northern Rocky sites than at the central sites. Eighty percent of the liquid water periods persisted 3 to 20 h at the northern Rocky site. This site was enveloped by supercooled liquid cloud 24% of the time during the months of December 1981 and January 1982. Average liquid water contents at the sites ranged between 0.14 to 0.23 g m−3; the maximum individual value was 0.60 g m−3. The measurements indicated that substantial amounts of liquid water were flowing over the Colorado Rockies at mountaintop heights.

Numerical Simulation of Ice Particle Growth in a Cloud of Supercooled Water Droplets

Abstract An empirical approximation is presented for estimating the growth of ice crystals by diffusion. The approximation is valid for computing crystal growth under conditions of constant temperature and water saturation. Crystal growth in these conditions has been realistically simulated for a period of 30 min in the range of temperatures from −1 to −30C. Modifications to the approximation are presented which permit it to he applied to varying temperature and varying saturation. The approximation for diffusion growth was coupled with an approximation for accretion growth to simulate ice particle growth by diffusion plus accretion. These calculations indicated that 1-μm ice crystals could grow to a size sufficient to collect supercooled droplets within a period of roughly 2 min regardless of the temperature and liquid water content conditions. The calculations also indicated that 1-cm hail could form from 1-μm ice crystals in roughly 27 main. The hail was calculated to form in a cloud of supercooled dro...

Cloud Condensation Nucleus Size Distributions and Their Effects on Cloud Droplet Size Distributions

Abstract Cloud droplet size distributions, calculated from cloud condensation nucleus (CCN) measurements made beneath non-raining, warm clouds in western and eastern Washington State, show reasonable agreement with droplet size distributions measured in the clouds. The results of both the measurements and the calculations show that high concentrations of large (0.2≤D≤2 µm) and giant (D≥2 µm) CCN lead to droplet size distributions which contain large droplets (D≥30 µm) only if low concentrations of small (0.06≲D<0.2 µm) CCN are present. If high concentrations of small CCN are present, the droplet size distributions contain few, if any, large droplets, regardless of the concentrations of giant CCN.

Laboratory Investigations of Cloud Nuclei from Combustion of Space Shuttle Propellant

Abstract Small quantities of solid rocket motor propellant, of the type to launch the Space Shuttle, were burned at ambient pressure in the laboratory to provide aerosol samples for characterization. A portion of each sample was injected into an isothermal cloud chamber and the remainder into a 770 l holding tank. Portable ice nucleus (IN) counters, filter devices for IN determinations and a cloud condensation nucleus (CCN) counter sampled from the tank. The measurements show that particles resulting from the combustion of the propellant are active IN (3.3 × 108 to 1.5 × 1011 g−1 active at −20°C). The portable counters and filters detected significantly fewer IN than the isothermal cloud chamber. The propellant aerosol is a prolific source of CCN that swamped the instrument.

Measurements of Cloud Nuclei in the Effluents from Launches of Liquid- and Solid-Fueled Rockets

Abstract Airborne measurements of cloud nuclei [cloud condensation nuclei (CCN) and ice nuclei (IN)] were made in the stabilized ground clouds resulting from the launches of a liquid-fueled ATLAS/Centaur rocket and a solid-fueled TITAN III rocket. Concentrations of CCN in both types of clouds were greater than ambient values for the ∼2 h duration of the measurements. The initial production of CCN active at 0.5% supersaturation in the ATLAS and TITAN clouds was equivalent to a 20 and 700 s emission, respectively, by the city of Denver, Colorado. Thereafter, the clouds continued to generate CCN at a rate of ∼1 cm−3 s−1. Concentrations of IN in the ATLAS cloud were greater than ambient values for only a short period after launch; the nuclei were probably from entrained launch pad and ground debris. The concentrations of IN in the TITAN cloud were mainly at or below ambient values (possibly due to the presence of high concentrations of HCI) until ∼2 h after launch when they increased substantially above ambie...

Particle generation, transport, and characterization at the First International Workshop on light absorption by aerosol particles

Aerosol particles with a wide range of light absorption properties were generated, transported, and characterized to permit meaningful intercomparisons of all the major types of light absorption instruments. The particles were generated in concentrations of ~1 mg m(-3) and 50 microg m(-3) for periods up to 2 h. The particle characteristics ranged from highly absorbing carbonaceous to nearly transparent ammonium sulfate, from submicron ammonium sulfate to supermicron Arizona road dust, and from spherical ammonium sulfate to chain-aggregate carbonaceous and irregular ambient particles.