NASA - National Aeronautics and Space Administration

The CLS is flown on the ER-2 to conduct cloud radiation and severe storm field experiments. Designed to operate at high altitudes in order to obtain measurements above the highest clouds, the instrument provides the true height of cloud boundaries and the density structure of less dense clouds. The height structure of cirrus, cloud top density and multiple cloud layers may also be profiled. The system specifications are as follows:

Laser Type: Nd:YAG I,II
Wavelength: 1064, 532 nanometer
Pulse Energy: 90, 30 mJ
Pulse repetition frequency: 10 Hz
Beam width: 1 mrad

Diameter: 0.15 m
Beam width: 1.4 mrad
Polarization: vert. and horiz.

Sample rate: Measurements at 20 m intervals at 200 m/s aircraft speed
Range Resolution: 7.5 m
Number of Channels: 4
Samples per Channel: 3310
Record Capacity: 8 hours

Aerosols of size 0.05 µm to 5 µm are collected with Ames wire impactors. This instrument consists of 25 µm, 75 µm and 500 µm diameter palladium or gold wires on ring mounts exposed to air for up to 5 minutes. Smaller diameter wires utilize their higher collection efficiency for small particles. Alternately, the wires can be replaced by Formvar-coated glass rods to collect cloud particles of sizes up to 500 µm. The collectors are brought back to the laboratory for analysis of size, shape and elemental/chemical composition of the collected particles using optical and electron microscopy, energy-dispersive X-ray spectrometry and microchemical reaction spots on substrates sensitized with specific chemicals.

Improved time and space resolution of ice particle collections is achieved by simultaneous sampling with the continuous Formvar replicator. The prime utility of this instrument is to obtain direct measurements of ice and liquid (volatile) particle concentration, size (1µm < D < 500µm) and shape over the period of approximately 2 hours per flight with a spatial resolution on the order of 20 m (at aircraft speed of 200 m/s). This opens the possibility of obtaining horizontal and vertical gradients of these quantities in cirrus clouds and contrails. Analysis of particles replicated on the films takes place by optical microscopy, interference microscopy and electron microscopy. The phases of supercooled or supersaturated solution droplets can be inferred from whether or not particles shatter or splash on impact to give sharp edged fragments or splash characteristics of high impact speed and high Langmuir numbers (high kinetic-to-surface surface energy ratios).

The continuous flow diffusion chambers are oriented for vertical flow through an annular space. They are constructed of two cylindrical, thin, ebonized copper walls that are separated by approximately 1.1 cm. The walls of the CFDC are force-cooled either by circulating coolant through copper tubing coils surrounding the outer wall and inside the inner wall (laboratory CFDC) or by using these same coolant coils as evaporators for refrigeration compressor units (aircraft CFDC). In operation, the walls are coated with ice, achieved by flooding the chamber with water. An inlet manifold directs sample air containing aerosol particles into the center of a laminar flow field where the sample is surrounded on either side by particle-free sheath air (or N2). By varying the set temperatures of the two walls, the warm wall provides a vapor source to the cold wall so that water vapor and temperature fields are created. These fields and airflow determine the conditions of exposure for the aerosols during their typical 5 to 20 s residence time in the CFDC. Ice particles grow to relatively large sizes compared to aerosol particles and are distinguished from them using an optical particle counter (0.4 to 20 mm) at the base of the CFDC.

The aircraft CFDC transitions to a hydrphobic warm wall surface in the lower third of the device so that liquid water drops formed at RH>100% will evaporate, leaving only ice crystals as large particles. The only other physical differences between the two devices is the fact that the laboratory CFDC is approximately 50% longer, providing additional ice crystal growth time at ambient lab pressures and the laboratory device has associated equipment for aerosol generation and preconditioning.

An impactor is sometimes used following the optical counter to collect ice crystals onto specialized transmission electron microscope (TEM) grids for analysis of the residual particles. Calculations of air flow, temperature, and humidity are made assuming steady-state conditions (Rogers, 1988). The temperature and supersaturation range are determined by wall temperatures and air flow.