Continuous Flow Diffusion Chamber Research Articles

Isolation of Ambient Particles of Known Critical Supersaturation: The Differential Activation Separator (DAS)

A field-deployable instrument has been developed that isolates from an ambient aerosol those particles that have critical supersaturations, Sc, within a narrow, user-specified, range. This Differential Activation Separator (DAS) consists of two continuous flow diffusion chambers housed within a single enclosure. Particles are introduced into the upstream chamber referred to as the CCN remover (CCNR) near the centerline between a warm, water-soaked, plate and a cool, continuously circulated, water bath. Those particles that activate at the resulting peak supersaturation, Sp, grow quickly and fall into the water bath. The remaining aerosol enters the second chamber referred to as the CCN separator (CCNS), which differs from the CCNR primarily in the use of a salt solution in the lower bath. The imposed temperature differential establishes an Sp slightly higher than that maintained in the upstream chamber, while the presence of a salt solution at the lower boundary results in a subsaturated region in roughly...

The Zurich Ice Nucleation Chamber (ZINC)-A New Instrument to Investigate Atmospheric Ice Formation

A new instrument to study ice nucleation, the Zurich Ice Nucleation Chamber (ZINC), has been constructed. It is a continuous flow diffusion chamber following the design by Rogers (1988) but has a flat parallel plate geometry. The instrument can operate at temperatures as low as 236 K with the current setup with ice supersaturations of up to 50%. The typical sample flow is 1 lpm with a total flow of 10 lpm using twice 4.5 lpm for sheath flows. FLUENT simulations were performed and are presented to discuss the flow, temperature, and humidity profiles within the main chamber. Activation experiments with silver iodide particles were used to validate the instrument against literature data. We report the onset of freezing for an activated fraction of 2% of all particles. The data exhibit an almost linear trend between 257 K (111.5% RHi) and 237 K (119% RHi) with very good agreement with literature data.

Microbiology and atmospheric processes: the role of biological particles in cloud physics

Abstract. As part of a series of papers on the sources, distribution and potential impact of biological particles in the atmosphere, this paper introduces and summarizes the potential role of biological particles in atmospheric clouds. Biological particles like bacteria or pollen may be active as both cloud condensation nuclei (CCN) and heterogeneous ice nuclei (IN) and thereby can contribute to the initial cloud formation stages and the development of precipitation through giant CCN and IN processes. The paper gives an introduction to aerosol-cloud processes involving CCN and IN in general and provides a short summary of previous laboratory, field and modelling work which investigated the CCN and IN activity of bacterial cells and pollen. Recent measurements of atmospheric ice nuclei with a continuous flow diffusion chamber (CFDC) and of the heterogeneous ice nucleation efficiency of bacterial cells are also briefly discussed. As a main result of this overview paper we conclude that a proper assessment of the impact of biological particles on tropospheric clouds needs new laboratory, field and modelling work on the abundance of biological particles in the atmosphere and their CCN and heterogeneous IN properties.

Ice nucleation of ammonia gas exposed montmorillonite mineral dust particles

Abstract. The ice nucleation characteristics of montmorillonite mineral dust aerosols with and without exposure to ammonia gas were measured at different atmospheric temperatures and relative humidities with a continuous flow diffusion chamber. The montmorillonite particles were exposed to pure (100%) and diluted ammonia gas (25 ppm) at room temperature in a stainless steel chamber. There was no significant change in the mineral dust particle size distribution due to the ammonia gas exposure. 100% pure ammonia gas exposure enhanced the ice nucleating fraction of montmorillonite mineral dust particles 3 to 8 times at 90% relative humidity with respect to water (RHw) and 5 to 8 times at 100% RHw for 120 min exposure time compared to unexposed montmorillonite within our experimental conditions. The percentages of active ice nuclei were 2 to 8 times higher at 90% RHw and 2 to 7 times higher at 100% RHw in 25 ppm ammonia exposed montmorillonite compared to unexposed montmorillonite. All montmorillonite particles are more efficient as ice nuclei with increasing relative humidities and decreasing temperatures. The activation temperature of montmorillonite exposed to 100% pure ammonia was 15°C higher than for unexposed montmorillonite particles at 90% RHw. In the 25 ppm ammonia exposed montmorillonite experiments, the activation temperature was 10°C warmer than unexposed montmorillonite at 90% RHw. Degassing does not reverse the ice nucleating ability of ammonia exposed montmorillonite mineral dust particles suggesting that the ammonia is chemically bound to the montmorillonite particle. This is the first experimental evidence that ammonia gas exposed montmorillonite mineral dust particles can enhance its activation as ice nuclei and that the activation can occur at temperatures warmer than –10°C where natural atmospheric ice nuclei are very scarce.

Examinations of ice formation processes in Florida cumuli using ice nuclei measurements of anvil ice crystal particle residues

A continuous flow diffusion chamber (CFDC) was used to measure ice formation by cloud particle residuals during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers‐Florida Area Cirrus Experiment. These measurements were directed toward determining the relative contributions of homogeneous nucleation, heterogeneous nucleation, and secondary ice formation processes to the concentrations of ice crystals in anvil cirrus formed from convection. The CFDC sampled residual particles remaining after evaporation of cloud particles initially collected by a counterflow virtual impactor. This allowed, for the first time, determination of the ice nucleation ability of particles that included the presumed nuclei for cloud‐ice formation. The approach proved successful for estimating concentrations of heterogeneous ice nuclei (IN) transported into anvil clouds, but experimental issues limited measurements of homogeneous freezing and, consequently, in determining the role of secondary ice formation. Results suggest agreement within a factor of 2–3 between CFDC heterogeneous IN concentrations and anvil ice crystal concentrations in the size range above ∼30 μm. IN concentrations also correlated with ice concentrations inferred from measurements by the FSSP (Forward Scattering Spectrometer Probe). However, measured IN concentrations were nearly two orders of magnitude lower than FSSP concentrations. This difference may have resulted from homogeneous freezing, secondary ice formation, or other unidentified ice formation processes that were not fully captured by the CFDC. The data suggest that heterogeneous nucleation played a smaller role than homogeneous nucleation in determining anvil ice crystal concentrations, except during periods of strong desert dust ingestion by cumuli. Nevertheless, heterogeneous nucleation may provide the source for larger ice crystals present in anvil regions.

Ice Nucleation Studies of Mineral Dust Particles with a New Continuous Flow Diffusion Chamber

A new continuous flow diffusion chamber (CFDC) has been designed and constructed to study the ice nucleation efficiency of natural and anthropogenic aerosol particles over a range of temperatures and supersaturations. The CFDC system at Dalhousie University, Canada is based on the design of (Rogers et al. 1988, 1994) at Colorado State University, USA. A steady airflow (2.83 lpm) composed of sheath flows and an aerosol flow passes through the annular gap of the diffusion chamber. The walls of the chamber are ice-covered and are held at different temperatures. Aerosol particles are injected into the center of the gap near the location of maximum supersaturation. Particles greater than 5 μm in aerodynamic diameter are removed with impactors before entry to the chamber. Ice crystals are identified with an optical particle counter at the outlet of the chamber. In this article we report on the ice nucleation results of two mineral dust particles of potential atmospheric relevance, kaolinite and montmorillonite....

Airborne measurements of tropospheric ice‐nucleating aerosol particles in the Arctic spring

Instrumented aircraft flights were made during field experiments in the Arctic Ocean, the NASA FIRE Arctic Cloud Experiment and SHEBA (Surface Heat Budget of the Arctic). Airborne measurements of ice nucleating aerosol particles (IN) used a continuous flow diffusion (CFD) chamber, covering −10° to −30°C and humidities from ice saturation to water supersaturation. During selected time periods, ice crystals that grew on ice nuclei in the chamber were sampled onto electron microscope (EM) grids for later examination of the nucleating particles. Samples of total aerosol (IN and non‐IN) were also collected for comparative analyses. Concentrations of IN ranged from zero to rare very high values (hundreds per liter at −25°C), making the frequency distribution of IN highly skewed: when accumulated as 10 s average concentrations (volume ∼0.17 L), 50% were zero. Additional evidence of few IN was seen in thin low‐level stratus clouds at −10° to −20°C that persisted for several days with low concentrations of ice crystals (∼0.1 L−1) and a few tenths g m−3 liquid water. On occasion, small regions of high IN concentrations (hundreds per liter) were detected near the surface. The EM analyses indicated that ice nuclei a few tenths micrometer in size contained crustal materials (primarily Si) and had widely varying morphology. Many IN particles produced weak or no X‐ray signatures, suggesting a dominant low‐molecular‐weight component not detected by the energy dispersive X‐ray (EDX) system, and were probably carbonaceous. In contrast, for the total aerosol, S or S and Si were dominant components, and few particles had no X‐ray signature.

A Continuous-Flow Diffusion Chamber for Airborne Measurements of Ice Nuclei

Abstract A continuous-flow thermal gradient diffusion chamber was developed for operating in an aircraft and detecting ice nucleating aerosol particles in real time. The chamber volume is the annular space between two vertically oriented concentric cylinders. The surfaces of the chamber are coated with ice and held at different temperatures, thus creating a vapor supersaturation. Upstream of the chamber, all particles in the sample air larger than 2-μm diameter are removed with inertial impactors. The air then flows vertically downward through the chamber, where ice crystals nucleate and grow on active ice nuclei to between ∼3- and 10-μm diameter in 3–10 s of residence time. At the outlet of the chamber, an optical particle counter detects all particles larger than ∼0.8 μm. Those particles larger than 3 μm are assumed to be the newly formed ice crystals and comprise the ice nucleus count. This paper describes the principles of operation, hardware and construction, data system, calibration, operational pro...

Measurements of ice nucleating aerosols during SUCCESS

Instruments on NASA's DC‐8 Airborne Laboratory were used to obtain measurements of heterogeneous ice nucleating aerosols (IN) and CN (condensation nuclei, particles >0.012 µm diameter) in background and aircraft‐affected air during the April–May 1996 SUCCESS project (Subsonic Aircraft: Contrail and Cloud Effects Special Study). IN were measured with a continuous flow diffusion chamber over a range of processing conditions from about −15 to −40°C and from ice saturation to ∼15% water supersaturation. IN concentrations ranged from <0.1 to ∼500 ℓ−1, being generally greater at colder temperatures and higher supersaturations. Within limited sample periods, IN concentration related strongly to sampling temperature and supersaturation, but overall, there was wide variability. IN and CN concentrations did not generally correlate. During penetrations of aircraft exhaust plumes, CN exhibited a very strong response, but there was no strong evidence that exhaust is a significant IN source for the temperature and supersaturation conditions of our measurements.

Isolating and identifying atmospheric ice-nucleating aerosols: a new technique

Laboratory studies examined two key aspects of the performance of a continuous-flow diffusion chamber (CFD) instrument that detects ice nuclei (IN) concentrations in air samples: separating IN from non-IN, and collecting IN aerosols to determine chemical composition. In the first study, submicron AgI IN particles were mixed in a sample stream with submicron non-IN salt particles, and the sample stream was processed in the CFD at −19°C and 23% supersaturation with respect to ice. Examination of the residual particles from crystals nucleated in the CFD confirmed that only AgI particles served as IN in the mixed stream. The second study applied this technique to separate and analyze IN and non-IN particles in a natural air sample. Energy-dispersive X-ray analyses (EDS) of the elemental composition of selected particles from the IN and non-IN fractions in ambient air showed chemical differences: Si and Ca were present in both, but S, Fe and K were also detected in the non-IN fraction.

Measurements of natural ice nuclei with a continuous flow diffusion chamber

Measurements of natural ice nuclei were made in winter continental airmasses with a continuous flow thermal gradient diffusion chamber (described in a separate paper). Over the range of temperatures −7°C to −20°C, the concentration of ice nuclei was closely related to ice supersaturation ( SS i) for humidities both below and above water saturation. Measurements below water saturation were interpreted as deposition nuclei with average concentrations (per liter) approximately 0.32 SS i(%) 0.81. Measurements were made up to 5% above water saturation and activated both deposition and condensation-freezing nuclei. The average concentration of condensation-freezing nuclei was 0.25 e −0.15 T(°C). Sample residence time in the chamber was probably too small to detect contact nuclei, unless the nucleating aerosols are extremely small. There was large variability in nucleus concentrations, as much as two orders of magnitude at −15°C. Comparisons are made between these ice nuclei measurements and aircraft observations of ice crystal concentrations in winter orographic clouds.

New Primary Ice-Nucleation Parameterizations in an Explicit Cloud Model

Abstract Two new primary ice-nucleation parameterizations are examined in the Regional Atmospheric Modeling System (RAMS) cloud model via sensitivity tests on a wintertime precipitation event in the Sierra Nevada region. A model combining the effects of deposition and condensation-freezing nucleation is formulated based on data obtained from continuous-flow diffusion chambers. The data indicate an exponential variation of ice-nuclei concentrations with ice supersaturation reasonably independent of temperatures between −7° and −20°C. Predicted ice concentrations from these measurements exceed values predicted by the widely used temperatures dependent Fletcher approximation by as much as one order of magnitude at temperatures warmer than −20°C. A contact-freezing nucleation model is also formulated based on laboratory data gathered by various authors using techniques that isolated this nucleation mode. Predicted contact nuclei concentrations based on the newer measurements are as much as three orders of mag...

Measurements of natural deposition and condensation-freezing ice nuclei with a continuous flow chamber

Ice nucleus concentrations in samples of city air have been determined by using a continuous flow diffusion chamber. The chamber has been set-up so that ice nuclei may be activated as they pass through regions of controlled supersaturation and temperature similar to those experienced in natural clouds. It has been found that two distinct modes of nucleation may be discerned by adjusting the flow rate through the chamber to permit a range of nucleus activation and crystal growth times. For short passage times, deposition nuclei may be activated both above and below water saturation. At longer growth times, and above water saturation, condensation-freezing nuclei have time to activate and grow. The total nucleus concentration detected is thus the sum of the two distinct types activated in these experiments. The resulting concentrations are considerably in excess of estimates obtained with a conventional filter technique and help to account for the discrepancies between observed ice nucleus and measured ice crystal concentrations in clouds.

Continentality of the South Florida Summertime CCN Aerosol

Abstract Measurements of cloud condensation nuclei (CCN) were obtained in the south Florida region as part of NOAA's Florida Area Cumulus Experiment (FACE). During the summer of 1975, CCN measurements were obtained near the bases of cumulus clouds by means of an airborne static diffusion chamber operating at 0.75% supersaturation. Concentrations were highly variable across the peninsula. The change in concentration from low values over the water to very high concentrations over land surfaces was abrupt. A continuous surface-based CCN monitoring program was carried out during the summer of 1976 with two continuous flow diffusion chambers, operating at 0.4 and 1.0% supersaturation. These confirmed the continental (high CCN) characteristics of the surface aerosol (overall mean 718 cm−3 at 0.49% and 1644 cm−3 at 1.0%). The concentration of CCN was found to vary as a function of wind speed, wind direction, and time of day. Analysis of these data suggested that the concentration of CCN was modulated by short-te...