Nimmi C. P. Sharma

An inexpensive active optical remote sensing instrument for assessing aerosol distributions.

Air quality studies on a broad variety of topics from health impacts to source/sink analyses, require information on the distributions of atmospheric aerosols over both altitude and time. An inexpensive, simple to implement, ground-based optical remote sensing technique has been developed to assess aerosol distributions. The technique, called CLidar (Charge Coupled Device Camera Light Detection and Ranging), provides aerosol altitude profiles over time. In the CLidar technique a relatively low-power laser transmits light vertically into the atmosphere. The transmitted laser light scatters off of air molecules, clouds, and aerosols. The entire beam from ground to zenith is imaged using a CCD camera and wide-angle (100 degree) optics which are a few hundred meters from the laser. The CLidar technique is optimized for low altitude (boundary layer and lower troposphere) measurements where most aerosols are found and where many other profiling techniques face difficulties. Currently the technique is limited to nighttime measurements. Using the CLidar technique aerosols may be mapped over both altitude and time. The instrumentation required is portable and can easily be moved to locations of interest (e.g. downwind from factories or power plants, near highways). This paper describes the CLidar technique, implementation and data analysis and offers specifics for users wishing to apply the technique for aerosol profiles. Implications Air quality studies require information on atmospheric aerosols over both altitude and time. A new inexpensive, simple, ground-based optical remote sensing technique assesses aerosol distributions. In the technique, called CLidar, a laser transmits light vertically into the nighttime atmosphere. The laser light scatters off of air molecules, clouds, and aerosols and the entire beam is imaged using a CCD camera and wide-angle optics. CLidar is optimized for boundary layer measurements where most aerosols are found. The instrumentation is portable and can easily be moved to locations of interest (e.g., downwind from factories or power plants, near highways).

A Lidar Collaboratory Data Management System

A data management system has been developed for the Connecticut State University (CSU) Lidar Collaboratory to facilitate user authentication, scheduling of remote lidar instrumentation control sessions, storage and retrieval of lidar datasets and generation of new data products. In addition to providing for efficient archival and retrieval of lidar data products, a major design goal of the data management system is to support collaborative, multidisciplinary, atmospheric sciences research projects. In this paper, we describe the framework of the CSU Lidar Collaboratory data management system and how the system interacts with the data acquisition and data analysis software.

Data Analyis System Design for Lidar Experimentation

A Web-based Lidar Experimentation and Data Analysis System (LEDAS) was developed, with support from a National Science Foundation award, to support resource sharing of lidar equipment, datasets and data analysis routines and collaboration between members of the Connecticut State University System (CSUS) Lidar Collaboratory. The system allows users at different geographical locations to conduct remote sensing research and education over the Web through remote access and control of a single shared lidar system and Web-based data analysis. Users need not have any specialized instrumentation or software at their institutions, thereby making real remote sensing research available to students and faculty from institutions which may not have the internal budgets for such facilities. An original structure providing basic functionality was developed and implemented. This paper describes the second generation data analysis system which provides significant new enhancements and capabilities.

Aerosol Layer Discrimination using Laser Radar and Genetic Algorithms

A technique has been developed to retrieve the height of the top of the aerosol layer from Micro Pulse Lidar (MPL) datasets. The technique combines first derivative estimates of normalized relative backscatter profiles with genetic algorithm refinements. The genetic algorithm is used to explore the gradient profiles to produce temporally coherent results. I. INTRODUCTION The distribution of aerosols over altitude and time has important effects on air quality, pollution,radiative forcing and climate, and rainfall patterns. Studies of aerosols also provide important information on atmospheric dynamics and transport. Thus atmospheric studies often require information on aerosol quantities and distributions. One important parameter for these studies is aerosol layer height. Lidar measurements of atmospheric backscatter may be used to track the height of the aerosol-rich layer over time. In this study, aerosol distribution data were obtained using the Connecticut State University (CSU) Lidar Collaboratorys Micro Pulse Lidar (MPL) system. The MPL is used to provide aerosol profiles for a variety of applications including air quality assessment and pollution control, climate modeling and studies of local atmospheric dynamics (1), (2), (3), (4). The CSU Lidar Collaboratory MPL is a Type 4 System from Sigma Space Corporation which monitors elastic backscatter at 527 nm. The system is eyesafe and thus may be operated autonomously. The MPL is a useful tool for boundary layer studies as it is capable of providing data in both daytime and nighttime conditions. Laser light pulses at 2500 Hz are transmitted vertically out of a beam-expansion telescope and the resulting backscatter is detected by a photon counting avalalanche photodiode. Detected intensity provides informa- tion on aerosol optical properties while timing of the scattered pulse return provides the altitude of the scatterer. Data used for this study were measured with an altitude resolution of 15 meters and a time interval of one minute. The data were range-corrected and also corrected for instrument artifacts and calibrations. The resulting datasets consist of time, altitude and normalized relative backscatter (NRB) signal intensity. These data are represented as images in which altitude is plotted on the vertical axis and time on the horizontal axis. The pixel value at each image pixel represents the NRB signal intensity. Examples of NRB datasets collected at New

Using a CCD camera lidar system for detection of Asian dust

During intense spring and early summer storms, substantial volumes of dust from east Asian desert regions are lofted over the continent and transported by prevailing winds across the Pacific Ocean. The phenomenon has wide reaching effects including long range nutrient and sediment transport as well as radiative forcing. Mauna Loa Observatory (MLO) is an atmospheric baseline station in Hawaii at an altitude of 3397-m.a.s.l.. MLO’s CCD Camera Lidar (CLidar) has fine near-ground altitude resolution, which makes it a useful system for Asian dust detection, especially at high altitude sites such as MLO. A 20-Watt, 532-nm Nd:YAG laser was vertically transmitted into the atmosphere above MLO. The side-scatter from atmospheric constituents, such as clouds, aerosols, and air molecules was detected by a wide-angle CCD camera situated 139-m from the laser. The obtained signal was range-normalized using a molecular scattering model and corrected for transmission with a column-averaged aerosol phase function derived from MLO-based AERONET photometer measurements. In several of the resulting aerosol extinction profiles, notable aerosol layers were observed near altitude ranges in which Asian dust is typically transported by prevailing winds. Corresponding relative humidity measurements made by nearby radiosondes were examined to differentiate aerosol scattering from cloud scattering. To further examine layers exhibiting both aerosol extinction peaks and relative humidity levels below that of tenuous ice clouds, back trajectories were conducted using NOAA’s Hybrid Single Particle Lagrangian Integrated Trajectory model. Several layers from 2008 and 2009 were traced back to East Asian deserts.

Using a bistatic camera lidar to profile aerosols influenced by a local source of pollution

A wide-angle CCD camera based bistatic lidar (CLidar) is used to monitor aerosol profiles in the atmosphere of The Bahamas. A 2-Watt CW laser beam ranging from ground to zenith is captured in a single image by a camera fitted with a fisheye lens which is placed at a different location from the laser. Scattering altitude is determined simply from the geometry of the CLidar in contrast to monostatic lidar which requires expensive electronics to measure the time of flight of the returned signal. Each image contains both molecular and aerosol single angle scattering. A cloud free image is used to normalize the signal intensity to a model of molecular scattering at a region free of aerosol layer. Then molecular portion is subtracted to retrieve aerosol side scattering. An aerosol phase function was assumed to convert side scatter to aerosol extinction. Corrections due to transmission effects are then iteratively calculated until convergence is reached. Aerosol extinction drops off sharply above 1 km indicating the planetary boundary level which agrees well with the relative humidity measurements obtained from the radiosonde data of Nassau airport observation. Additionally, aerosols originated from the smoke of a charcoal grill operating near experimental site were efficiently detected near ground levels. Aerosol extinction at 20 m above sea level is 0.085 km-1 during grilling compared to 0.03 km-1 during no grilling. Excellent altitude resolution of the CLidar at the ground levels allows its use for in-situ environmental characterization without the overlap effects faced when using traditional lidar.

Lidar observations of long range dust transport over Mauna Loa Observatory

A bistatic CCD camera lidar (CLidar) was used at the National Oceanic and Atmospheric Administration’s Mauna Loa Observatory (MLO) to map aerosol light scattering. Laser light from a 532 nm, Nd:YAG laser was vertically transmitted into the atmosphere and the scatter off clouds, aerosols and air molecules was detected using a CCD camera with wide angle optics and a laser line filter. The intensity of each CCD camera pixel imaging the beam was normalized to a molecular scattering model in an aerosol free region for subtraction of molecular scattering. Aerosol extinction was derived using a column average aerosol phase function derived from AERONET sun photometer measurements at MLO. The CLidar design allows measurements of aerosol scattering all the way to the ground without an overlap correction. MLO, at 3397 m.a.s.l., typically receives free tropospheric air. During spring months, prevailing winds can occasionally transport dust from Asian sources with high dust activity over MLO. Aerosol scattering measurements were taken by the CLidar during spring months at MLO and revealed extinction peaks at mid-range altitudes. Back trajectories of air parcels from MLO at the altitudes of these peaks were conducted using NOAA’s Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model and it was found that they passed over regions of Eastern Asia known as sources of high dust activity. Relative humidity data from radiosondes and the NOAA stratospheric lidar’s water vapor channel were examined to differentiate aerosol scattering from tenuous cloud scattering. This paper presents aerosol extinction data with observations of Asian dust as measured by the CLidar during spring months at MLO.

Identifying Asian dust with digital imaging of atmospheric laser light scatter

Digital imaging of atmospheric scatter of laser light was used to detect aerosol layers. A CCD camera with wide angle optics and laser line filter was located far (100 m or more) from a vertically pointing 532 nm laser, and imaged the entire laser beam at once. The data image of each exposure was used in conjunction with aerosol phase functions derived from AERONET radiometry data to derive aerosol extinction profiles as a function of altitude and time. These profiles combined with radiosonde data and HYSPLIT atmospheric back-trajectories were used to identify Asian dust aerosol layers.

Software environments for atmospheric lidar remote sensing

Research and training in atmospheric lidar remote sensing requires highly versatile software environments for both experimentation and analysis. Experimentation and analysis may be conducted either directly on site at the location of the lidar equipment and/or institutional computer center or remotely from a distant location. The requirements for lidar software environments depend not only on type of user access (remote or onsite) but also on the nature of the teaching or research missions they support and the characteristics of the lidar systems for which they are used. The software environments discussed in this paper have been used to support lidar aerosol studies in settings ranging from urban locations to a remote atmospheric baseline station. Experiments and data analysis studies have been conducted for two different ground-based lidar systems, a monostatic Micro Pulse Lidar system and a bistatic imaging lidar system.

Wide-Angle Imaging Lidar for High-Resolution Near-Ground Aerosol Studies

The development of an imaging lidar system allows high resolution (sub-meter) near-ground atmospheric aerosol measurements and enables aerosol light scattering measurements all the way to the ground without requiring an overlap correction function.