A compact light detection and ranging (LiDAR) system provides aerosols profile measurements by identifying the aerosol
scattering ratio as function of the altitude. The aerosol scattering ratios are used to obtain multiple aerosol intensive ratio
parameters known as backscatter color ratio, depolarization ratio and lidar ratio. The aerosol ratio parameters are known to
vary with aerosol type, size, and shape. Different methods in the literature are employed for detection and classification of
aerosol from the measurements. In this paper, a comprehensive review for aerosol detection methods is presented. In addition,
results of implemented methods of quantifying aerosols in the atmosphere on real data are compared and presented showing
how the backscatter color, depolarization and lidar ratios vary with presence of aerosols in the atmosphere.
The Langley Mobile Ozone Lidar (LMOL) is a compact mobile differential absorption lidar (DIAL) system that was developed at NASA Langley Research Center, Hampton, VA, USA to provide ozone, aerosol and cloud atmospheric measurements in a mobile trailer for ground-based atmospheric air quality campaigns. This lidar is part of the Tropospheric Ozone Lidar Network (TOLNet) currently made up of six other ozone lidars across the U.S and Canada. This lidar has been deployed to Denver, CO July 15-August 15, 2014 for the DISCOVER-AQ air quality campaign. Ozone and aerosol profiles were taken showing the influence of emissions from the Denver region. Results of ozone concentration, aerosol scattering ratio, boundary layer height and clouds will be presented with emphasis on regional air quality.
A compact mobile differential absorption lidar (DIAL) system has been developed at NASA Langley Research Center to provide ozone, aerosol and cloud atmospheric measurements in a mobile trailer for ground-based atmospheric ozone air quality campaigns. This lidar is integrated into the Tropospheric Ozone Lidar Network (TOLNet) currently made up of four other ozone lidars across the country. The lidar system consists of a UV and green laser transmitter, a telescope and an optical signal receiver with associated Licel photon counting and analog channels. The laser transmitter consists of a Q-switched Nd:YLF inter-cavity doubled laser pumping a Ce:LiCAF tunable UV laser with all the associated power and lidar control support units on a single system rack. The system has been configured to enable mobile operation from a trailer and was deployed to Denver, CO July 15-August 15, 2014 supporting the DISCOVER-AQ campaign. Ozone curtain plots and the resulting science are presented.
Knowledge of the concentration and distribution of atmospheric aerosols using both airborne lidar and satellite instruments is a field of active research. An aircraft based aerosol lidar has been used to study the distribution of atmospheric aerosols in the California Central Valley and eastern US coast. Concurrently, satellite aerosol retrievals, from the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument aboard the Terra and Aqua satellites, were take over the Central Valley. The MODIS Level 2 aerosol data product provides retrieved ambient aerosol optical properties (e.g., optical depth (AOD) and size distribution) globally over ocean and land at a spatial resolution of 10 km.
The Central Valley topography was overlaid with MODIS AOD (5x5 km2 resolution) and the aerosol scattering vertical profiles from a lidar flight. Backward air parcel trajectories for the lidar data show that air from the Pacific and northern part of the Central Valley converge confining the aerosols to the lower valley region and below the mixed layer. Below an altitude of 1 km, the lidar aerosol and MODIS AOD exhibit good agreement. Both data sets indicate a high presence of aerosols near Bakersfield and the Tehachapi Mountains. These and other results to be presented indicate that the majority of the aerosols are below the mixed layer such that the MODIS AOD should correspond well with surface measurements. Lidar measurements will help interpret satellite AOD retrievals so that one day they can be used on a routine basis for prediction of boundary layer aerosol pollution events.
High resolution (5x5 km2 horizontal resolution) retrievals of aerosol optical depth (AOD) from the MODerate Resolution Imaging Spectroradiometer (MODIS) instruments aboard NASA's Aqua and Terra satellite platforms have been examined. These data products have been compared to coincident, hourly measurements of ground-based PM-2.5 routinely obtained by the San Joaquin Valley Air Pollution Control District (SJV APCD) and California Air Resources Board (CARB) and to airborne light detection and ranging (lidar) aerosol scattering measurements obtained by NASA in July 2003 in San Joaquin Valley (SJV). Comparison of MODIS AOD to ground based PM-2.5 measurement shows significant improvement for the higher resolution MODIS AOD. Lidar aerosol scattering measurements correspond well to MODIS AOD during a variety of atmospheric conditions, and throughout the SJV. Future lidar measurements are proposed to establish a high resolution vertical link between satellite and ground-based measurements during the winter. With the data from these two episodes, we plan to characterize the horizontal, vertical, and temporal distribution of PM-2.5 in SJV and evaluate the need for future intensive ground-based measurement and modeling studies in SJV.
Diode-pumped, tunable Cr:LiSAF lasers are well suited for airborne water vapor differential absorption lidar application. Three types of diode-pumped, tunable, narrow- linewidth, injection seeded, Q-switched Cr:LiSAF lasers for high resolution atmosheric water vapor DIAL measurements in the wavelength range of 810-830 nm have been developed and investigated. By using a total internal reflection laser resonator configuration, efficient coupling of pump radiation from large diode arrays is achieved as also the ability to limit the temperature rise in the LiSAF crystal at high pump powers. The first is a high-energy Cr:LiSAF laser producing up to 25 mJ/pulse at 816 nm with a repetition rate of 1-10 Hz. A DFB diode laser locked to a water vapor absorption line using a photo-acoustic cell was employed to injection seed and tune the slave Cr:LiSAF laser. High spectral purity (<99%) and wavelength stability of 0.08 pm over a period of 10 hours were demonstrated. The second Cr:LiSAF laser is designed to operate at 100 Hz while producing up to 10 mJ/pulse with a much lesser pump power. The reduction in size and weight of this laser coupled with the increased average power leads to significant improvement in the DIAL performance over the first laser. The third diode-pumped Cr:LiSAF laser is an ultra compact laser producing up to 0.1 mJ/pulse at 1000 Hz. This laser is suitable for measuring water vapor profiles in the lower troposhere (3 to 5 km). The input-output and spectral performance of these lasers are presented.
An advanced compact differential absorption lidar detection system for atmospheric water vapor measurement is reported. This system interfaces the lidar receiver telescope to a personal computer and contains an advanced avalanche photodiode detector, signal conditioning circuit, 14-bit, 10 MHz digitizer and a microcontroller. The whole system was realized on one electronic card. Characterization results indicate low noise with reduced size, reduced mass and an extended measurement range over current lidar detection systems. The new system can be incorporated in spacecraft lidar systems. Simulated lidar return measurements were performed with the new system in order to obtain its minimum detectable signal limits.
NASA Langley has an active water vapor differential absorption lidar program taking measurements from both C-130 and ER-2 aircraft. A research effort has started to increase the signal-to-noise ratio in the DIAL receiver by 1) evaluating new very low noise avalanche photo didoes (APD), 2) designing an optics system that will focus the return light signal to the APD efficiently and 3) constructing a 10-MHz waveform digitizer board that will be small enough to be placed at the APD and telescope. With these advances we anticipate improving the signal-to-noise ratio by a factor of ten over the current receiver system.
A flashlamp pumped Ti:sapphire laser has been constructed which could be used to make atmospheric DIAL measurements of ozone from aircraft. A 9-mm diameter by 15-cm long rod is pumped by four flashlamps, two lamps fired in series at a time with 300 microsecond time separation between firings to produce the 'on' and 'off' line DIAL laser pulses. The laser cavity has tow arms, one lasing at 867-nm and the other at 897-nm. The Q-switched output is doubled and tripled with a LBO and BBO crystal respectively to achieve DIAL pulses at 289-nm and 299-nm. Line narrowing is achieved with the use of three SF-10 prisms. Such a system could be used on an unpiloted atmospheric vehicle.