Aerosol Depolarization Ratio

Roscoe is a new, more compact version of the NASA GSFC Cloud Physics Lidar that has flown on multiple NASA high altitude aircraft over the past two decades. While utilizing the same proven measurement technique of coupling a high repetition rate laser with photon-counting detection, Roscoe differs from CPL in two significant ways.  First, it is designed to simultaneously observe both upwards and downwards from the aircraft, to enable studies of stratospheric aerosols above flight altitude as well as below.  It is, essentially, two small CPL instruments in one package, one pointing nadir and one pointing zenith.  Second, it operates at only 1064 and 355 nm (not 532 nm) to satisfy eye-safety considerations for airborne operation.  Roscoe measures depolarization at both wavelengths to characterize the phase of the cloud and aerosol particles detected.

The NASA Langley airborne High-Spectral-Resolution Lidar – Generation 2 (HSRL-2) is used to characterize clouds and small particles in the atmosphere, called aerosols. From an airborne platform, the HSRL-2 instrument provides nadir-viewing profiles of aerosol and cloud optical and microphysical properties, which are used studies aerosol impacts on radiation, clouds, and air quality. HSRL-2 also provides measurements of the near-surface ocean, including depth-resolved subsurface backscatter and attenuation. HSRL-2 can also be configured to utilize the differential absorption (DIAL) technique for measuring profiles of ozone concentrations in addition to the above products.

The NASA Langley Airborne Differential Absorption Lidar (DIAL) system uses four lasers to make DIAL O3 profile measurements in the ultraviolet (UV) simultaneously with aerosol profile measurements in the visible and IR. Recent changes incorporate an additional laser and modifications to the receiver system that will provide aerosol backscatter, extinction, and depolarization profile measurements at three wavelengths (UV, visible, and NIR). For SEAC4RS, the DIAL instrument will include for the first time aerosol and cloud measurements implementing the High Spectral Resolution Lidar (HSRL) technique [Hair, 2008]. The modifications include integrating an additional 3-wavelength (355 nm, 532 nm, 1064 nm) narrowband laser and the receiver to make the following measurements; depolarization at all three wavelengths, aerosol/cloud backscatter and extinction at 532 nm via the HSRL technique, and aerosol/cloud backscatter at the 355 and 1064 nm via the standard backscatter lidar technique. Integration of the aerosol extinction profile at 532nm above and below the aircraft also provides aerosol optical depth (AOD) along the aircraft flight track.

The NASA Langley airborne High Spectral Resolution Lidar (HSRL) is used to characterize clouds and small particles in the atmosphere, called aerosols. From an airborne platform, the HSRL science team studies aerosol size, composition, distribution and movement.

The HSRL-1 instrument is an innovative technology that is similar to radar; however, with lidar, radio waves are replaced with laser light. Lidar allows researchers to see the vertical dimension of the atmosphere, and the advanced HSRL makes measurements that can even distinguish among different aerosol types and their sources. The HSRL technique takes advantage of the spectral distribution of the lidar return signal to discriminate aerosol and molecular signals and thereby measure aerosol extinction and backscatter independently.

The HSRL-1 instrument provides measurements of aerosol extinction at 532 nm and aerosol backscatter and depolarization at 532 and 1064 nm. The HSRL measurements of aerosol extinction, backscattering, and depolarization profiles are being used to:

1) characterize the spatial and vertical distributions of aerosols
2) quantify aerosol extinction and optical thickness contributed by various aerosol types
3) investigate aerosol variability near clouds
4) evaluate model simulations of aerosol transport
5) assess aerosol optical properties derived from a combination of surface, airborne, and satellite measurements.

The Aerosol Lidar system measures profiles of aerosol and/or cloud backscatter at 532 and 1064 nm and aerosol/cloud depolarization at 532 nm. Backscatter profiles at these two wavelengths provide information on the relative concentration and spatial distribution of aerosol/cloud particles. Comparison of aerosol/cloud backscatter at the two wavelengths provides some indication of particle size. Measurement of the depolarizing effect of the particles (that is, the degree to which the polarization of the backscattered light from the particles differs from the linear polarization of the transmitted laser light) provides an indication of particle phase.

The Aerosol Lidar is a piggy-back instrument on AROTEL lidar fielded by John Burris and Tom McGee of NASA Goddard Space Flight Center. The light source for the aerosol measurements is a Continuum 9050 Nd:YAG laser operating at 50 shots per second. The laser transmits approximately 600 mJ at 1064 nm, 250 mJ at 532 nm, and 350 mJ at 355 nm. AROTEL also employs an excimer laser transmitting at 308 nm and uses the molecular and Raman backscatter from the 355 and 308 beams to measure ozone and temperature. Backscattered light at all wavelengths is collected by a 16-inch diameter Newtonian telescope with a selectable field stop. In the aft optics assembly following the telescope and field stop, the UV signals are separated from the 532- and 1064-nm signals by a dichroic beam splitter. The UV signals are directed to the AROTEL receiver assembly and the 532- and 1064-nm signals are directed to the Aerosol Lidar receiver assembly. In the Aerosol Lidar receiver, a rotating shutter blocks the very strong near-range 532- and 1064-nm signals in order to reduce distortion in the relatively weaker signals from higher altitudes. The 532- and 1064-nm signals are separated by a dichroic beam splitter and the 532-nm signal is further separated into orthogonal polarization components using a polarizing beam cube. A computer-controlled half-wave plate in front of the polarizing beam cube is rotated so that the polarization of the 532 signals are parallel and perpendicular to the polarization of the transmitted laser pulses. The signals at both wavelengths and both 532-nm polarizations are transmitted to detectors at the Aerosol Lidar data acquisition rack via fiber optic cables. Each optical signal, the 1064-nm total backscatter and the 532-nm parallel and perpendicularly polarized backscatter, is directed to two separate detectors, with 10% going to one detector and 90% to the other, in order to more accurately measure the signals over their full dynamic range. The 532-nm returns are measured with photo-multiplier tubes and the 1064-nm returns are measured with avalanche photo-diodes. Because of the high optical signal levels, all data are acquired in analog mode, using 12-bit analog-to-digital converters. The instrument operates under both daytime and nighttime lighting conditions, with a slight degradation in data quality during the daytime.

The Raman Airborne Spectroscopic Lidar (RASL) consists of a 15W ultraviolet laser, a 24-inch (61-centimeter) diameter Dahl-Kirkham telescope, a custom receiver package, and a structure to mount these components inside an aircraft. Both the DC-8 at NASA Dryden and the P-3 at NASA/Wallops are aircrafts that could carry RASL. The system is unique because it requires the largest window ever put into either of these aircraft. A fused-silica window, diameter of 27 inches (68.6 centimeters) and 2.375 inches (6 centimeters) thick is needed to withstand the pressure and temperature differentials at a 50,000-foot (15.2-kilometer) altitude.

In June through August of 2007, RASL flew numerous times on board a King Air B-200 aircraft out of Bridgewater, VA, in support of the 2007 Water Vapor Validation Experiments (WAVES) campaign. The WAVES campaign was a series of field experiments to validate satellite measurements. RASL data, along with data from ground-based and balloon-borne instruments, were used to assess the CALIPSO and TES instruments and for studies of mesoscale water vapor variability. During the test flights, RASL produced the first-ever simultaneous measurements of tropospheric water vapor mixing ratio and aerosol extinction from an airborne platform.

This is a stratospheric lidar which is configured to fly on the NASA DC-8. It is a zenith viewing instrument, which makes vertical profile measurements of ozone, aerosols and temperature. Stratospheric ozone can be measured at solar zenith angles greater than ~30 degrees, while temperature and aerosols require SZA > 90 degrees. The SNR is maximized under dark coonditions. The measurement of Near-field water vapor measurements is being investigated and could be readily implemented. The instrument utilizes a XeCl excimer laser and a Nd-YAG laser to make DIAL, Raman DIAL, and backscatter measurements. A zenith viewing 16" telescope receives the lidar returns.