Aircraft-based aerosol sampling in clouds is complicated by the generation of shatter artifact particles from aerodynamic or impaction breakup of cloud droplets and ice particles in and around the aerosol inlet. Aerodynamic breakup occurs when the Weber number of a droplet, which primarily depends on the droplet size and the magnitude of the relative motion of the droplet and the local air mass, exceeds a critical value. Impaction breakup of a droplet occurs when the droplet's impaction breakup parameter, K, which is a combination of Weber and Ohnesorge numbers, exceeds a critical value. Considering these two mechanisms, the critical breakup diameters are estimated for two aerosol inlets of different designs-a conventional forward-facing solid diffuser inlet (SDI) and a cross-flow sampling sub-micron aerosol inlet (SMAI). From numerical simulations, it is determined that cloud droplets of all sizes will experience impaction breakup in SDI, while only droplets larger than ∼16 µm will experience impaction breakup in SMAI. The relatively better in-cloud sampling performance of SMAI is because of its cone design that slows the flow just upstream of the sample tube. The slowing upstream flow, however, causes aerodynamic breakup of drops larger than ∼100 µm. The critical breakup diameters determined from analysis of field data largely validate numerical predictions. The cross-flow sampling design of SMAI is seen to ensure that shatter artifacts in the inlet are minimal even when there are a significant number of particles larger that the critical breakup size. The study results, thus, suggest that the SMAI design presents an effective approach to sample interstitial particles from aircraft.

Design of a new submicron aerosol inlet (SMAI) for airborne sampling of aerosol particles is introduced and its performance characteristics under a range of sampling conditions are presented. Analysis of inlet performance in clear-air and cloud systems shows that submicron aerosols are sampled representatively by the inlet, and in comparison with other types of inlets the SMAI has a relatively minor or nonexistent problem of droplet shatter contamination. The SMAI has a flow-through cone, with a perpendicular subsampling tube inside it. The cone acts as a virtual blunt body and decelerates the velocity directed toward a subsampling tube within the cone, resulting in reduced droplet impaction velocities and negligible artifact particle generation. The use of a perpendicular subsampling tube helps eliminate large shattered droplets from entering the sample volume, though it also results in lowering the aerosol sampling cut size. The SMAI sampling characteristics are determined from computational fluid dynamics simulations, and its cut size is calculated to be ~3 fj.m. In warm clouds, the shatter artifacts in the SMAI measurements are significantly less than that in a diffuser-type inlet, and shatter artifacts are only observed to increase when concentrations of drops larger than -100 ^m increase. In cold-cloud systems, shatter artifacts are significantly reduced with SMAI and some dependence of the inlet's performance on the shape of the ice particles is observed.

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Interaction of liquid cloud droplets and ice particles with aircraft aerosol inlets can result in the generation of a large number of secondary particles and contaminate aerosol measurements. Recent studies have shown that a sampler designed with a perpendicular subsampling tube located within a flow-through conduit (i.e., a flow-restriction inlet) was best suited for in-cloud sampling. Analysis of field data obtained from different flow-restriction inlets shows that their critical cloud droplet breakup diameters are strongly dependent on design details and operating conditions. Using computational fluid dynamics (CFD) simulations, in-cloud sampling performance of a selected inlet can be predicted reasonably accurately for known operating conditions. To understand the relation between inlet design parameters and its sampling performance, however, CFD calculations are impractical. Here, using a simple, representative one-dimensional velocity profile and a validated empirical droplet breakup criteria, a parametric study is conducted to understand the relationship between different inlet design features and operating conditions on its critical breakup diameters. The results of this study suggest that an optimal inlet for in-cloud aerosol sampling should have a combination of a restriction nozzle at the aft end of the flow-through conduit to minimize wall-impaction shatter artifacts and a blunt leading edge to minimize shatter artifact generation from the aerodynamic breakup of cloud droplets. Inlets for in-cloud aerosol sampling from aircraft will, therefore, differ significantly in design from those used for clear-air aerosol sampling.

Abstract. There is limited understanding of the role of aerosols in the formation and modification of clouds, partly due to inadequate data on such systems. Aircraft-based aerosol measurements in the presence of cloud particles have proven to be challenging because of the problem of cloud droplet/ice particle shatter and the generation of secondary artifact particles that contaminate aerosol samples. Recently, the design of a new aircraft inlet, called the Blunt-body Aerosol Sampler (BASE), which enables sampling of interstitial aerosol particles, was introduced. Numerical modeling results and laboratory test data suggested that the BASE inlet should sample interstitial particles with minimal shatter particle contamination. Here, the sampling performance of the inlet is established from aircraft-based measurements. Initial aircraft test results obtained during the PLOWS (Profiling of Winter Storms) campaign indicated two problems with the original BASE design: separated flows around the BASE at high altitudes and a significant shatter problem when sampling in drizzle. The test data were used to improve the accuracy of flow and particle trajectory modeling around the inlet, and the results from the improved flow model were used to guide design modifications of the BASE to overcome the problems identified in its initial deployment. The performance of the modified BASE was tested during the ICE-T (Ice in Clouds Experiment -Tropics) campaign, and the inlet was seen to provide near shatter-free measurements in a wide range of cloud conditions. The initial aircraft test results, design modifications, and the performance characteristics of the BASE relative to another interstitial inlet, the submicron aerosol inlet (SMAI), are presented.