[1] Several small ice particles were observed to grow and evaporate in a quadrupole electrodynamic levitation cell at temperatures between À40 and À60°C and variable supersaturation. The particle mass was traced throughout each experiment by analysis of the electrical (''springpoint'') boundaries for stable levitation. Measured growth of each particle was compared with that calculated from a standard particle-growth model. The small particle sizes and long durations of the experiments provided great sensitivity to the particle-mean deposition coefficient ( m ). Suitable agreement between the measured and modeled growth histories was obtainable only upon using very small deposition coefficients: e.g., 0.0045 < m < 0.0075, with m $ 0.006 being the most-likely value for a representative particle growing at À50°C. This finding of such low deposition coefficients suggests that cirrus-cloud simulations need to account for the strong influence that the surface-kinetic resistance exerts on the vapor growth of ice at atmospherically low temperatures.

Models of ice crystal vapor growth require estimates of the deposition coefficient α when surface attachment kinetics limit growth and when ice crystal shape is predicted. Parametric models can be used to calculate α for faceted growth as long as characteristic supersaturation values are known. However, previously published measurements of are limited to temperatures higher than −40°C. Estimates of at temperatures between −40° and −70°C are provided here through reanalysis of vapor growth data. The estimated follow the same functional temperature dependence as data taken at higher temperatures. Polynomial fits to are used as inputs to a parameterization of α suitable for use in cloud models. Comparisons of the parameterization with wind tunnel data show that growth at liquid saturation and constant temperatures between −3° and −20°C can be modeled by ledge nucleation for larger (hundreds of micrometers) crystals; however, comparisons with free-fall chamber data at −7°C suggest that dislocation growth may be required to model the vapor growth of small crystals (~20 μm) at liquid saturation. The comparisons with free-fall chamber data also show that the parameterization can reproduce the measured pressure dependence of aspect-ratio evolution. Comparisons with a hexagonal growth model indicate that aspect-ratio evolution based on the theory of Chen and Lamb produces unrealistically fast column growth near −7°C that is mitigated if a theory based on faceted growth is used. This result indicates that the growth hypothesis used in habit-evolving microphysical models needs to be revised when deposition coefficients are predicted.

A levitation diffusion chamber designed to examine the mass growth from the vapor of small ice particles (diameter &lt; 100 μm) at ambient pressure (≃970 hPa) and low temperature (T &lt; −30°C) is presented. The diffusion chamber is unique in that charged ice particles are levitated by an opposing voltage on the lower copper plate with lateral stability provided by button quadrupole electrodes attached to the upper copper plate. The button electrodes are far from the ice particle growth region, allowing ice particles to grow free of substrate influences. Experiments have been conducted for temperatures from −30° to −35.7°C, ice supersaturations from 2.5% to 28.6%, and over growth times ranging from 5 to 15 min. The experiments indicate that mass varies nonlinearly in time and exhibits a dependence on initial particle radius and ice supersaturation in accord with expectations from theory. In contrast to expectations from spherical capacitance theory, the derived mass growth rates do not scale linearly with radius, and derived effective shape factors (capacitance normalized with radius) are approximately 0.5. Fitting the growth data with a theoretical model indicates that growth is limited by surface kinetics with deposition coefficients ranging from 0.003 to 0.02.

There are few measurements of the vapor growth of small ice crystals at temperatures below -30°C. Presented here are mass-growth measurements of heterogeneously and homogeneously frozen ice particles grown within an electrodynamic levitation diffusion chamber at temperatures between -44 and -30°C and supersaturations ( si) between 3 and 29%. These growth data are analyzed with two methods devised to estimate the deposition coefficient ( α) without the direct use of si. Measurements of si are typically uncertain, which has called past estimates of α into question. We find that the deposition coefficient ranges from 0.002 to unity and is scattered with temperature, as shown in prior measurements. The data collectively also show a relationship between α and si, with α rising (falling) with increasing si for homogeneously (heterogeneously) frozen ice. Analysis of the normalized mass growth rates reveals that heterogeneously-frozen crystals grow near the maximum rate at low si, but show increasingly inhibited (low α) growth at high si. Additionally, 7 of the 17 homogeneously frozen crystals cannot be modeled with faceted growth theory or constant α. These cases require the growth mode to transition from efficient to inefficient in time, leading to a large decline in α. Such transitions may be, in part, responsible for the inconsistency in prior measurements of α.

The kinetics of the reactions of CH, O and CD30 with NO have been studied using a discharge flow reactor. CH, O and CD30 were detected using laser-excited fluorescence (LEF) near 300 nm. Total rate coefficients for the reaction of CH, O with NO were measured as a function of temperature (220-473 K) and pressure (0.755.0 Torr) of He or Ar. Total rate coefficients for the CD30 + NO reaction were measured at ca. 294 K over the pressure range 0.75-5.0 Torr He. Using molecular-beam mass spectrometry, the CH30N0 yield of the CH30