Wavelength Dependence of Backscatter by use of Aerosol Microphysics and Lidar Data Sets: Application to 2.1- mum Wavelength for Space-Based and Airborne Lidars

An aerosol microphysics dataset was used to model backscatter in the 0.35-11-mum wavelength range, with the results validated by comparison with measured cw and pulsed lidar backscatter obtained during two NASA-sponsored airborne field experiments. Different atmospheric features were encountered, with aerosol backscatter ranging over 4 orders of magnitude. Modeled conversion functions were used to convert existing lidar backscatter datasets to 2.1 mum. Resulting statistical distribution shows the midtropospheric aerosol backscatter background mode of beta(2.1) to be between ~3.0 x 10(-10) and ~1.3 x 10(-9) m(-1) sr(-1), ~10-20 times higher than that for beta(9.1); and a beta(2.1) boundary layer mode of ~1.0 x 10(-7) to ~1.3 x 10(-6) m(-1) sr(-1), ~3-5 times higher than beta(9.1).     

Signal processing and calibration of continuous-wave focused CO(2) Doppler lidars for atmospheric backscatter measurement

Two continuous-wave (CW) focused CO(2) Doppler lidars (9.1 and 10.6 μm) were developed for airborne in situ aerosol backscatter measurements. The complex path of reliably calibrating these systems, with different signal processors, for accurate derivation of atmospheric backscatter coefficients is documented. Lidar calibration for absolute backscatter measurement for both lidars is based on range response over the lidar sample volume, not solely at focus. Both lidars were calibrated with a new technique using well-characterized aerosols as radiometric standard targets and related to conventional hard-target calibration. A digital signal processor (DSP), a surface acoustic wave spectrum analyzer, and manually tuned spectrum analyzer signal analyzers were used. The DSP signals were analyzed with an innovative method of correcting for systematic noise fluctuation; the noise statistics exhibit the chi-square distribution predicted by theory. System parametric studies and detailed calibration improved the accuracy of conversion from the measured signal-to-noise ratio to absolute backscatter. The minimum backscatter sensitivity is ~3 × 10(-12) m(-1) sr(-1) at 9.1 μm and ~9 × 10(-12) m(-1) sr(-1) at 10.6 μm. Sample measurements are shown for a flight over the remote Pacific Ocean in 1990 as part of the NASA Global Backscatter Experiment (GLOBE) survey missions, the first time to our knowledge that 9.1-10.6-μm lidar intercomparisons were made. Measurements at 9.1 μm, a potential wavelength for space-based lidar remote-sensing applications, are to our knowledge the first based on the rare isotope (12)C (18)O(2) gas.     

Aerosol lidar intercomparison in the framework of the EARLINET project. 1. Instruments

In the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET), 19 aerosol lidar systems from 11 European countries were compared. Aerosol extinction or backscatter coefficient profiles were measured by at least two systems for each comparison. Aerosol extinction coefficients were derived from Raman lidar measurements in the UV (351 or 355 nm), and aerosol backscatter profiles were calculated from pure elastic backscatter measurements at 351 or 355, 532, or 1064 nm. The results were compared for height ranges with high and low aerosol content. Some systems were additionally compared with sunphotometers and starphotometers. Predefined maximum deviations were used for quality control of the results. Lidar systems with results outside those limits could not meet the quality assurance criterion. The algorithms for deriving aerosol backscatter profiles from elastic lidar measurements were tested separately, and the results are described in Part 2 of this series of papers [Appl. Opt. 43, 977-989 (2004)]. In the end, all systems were quality assured, although some had to be modified to improve their performance. Typical deviations between aerosol backscatter profiles were 10% in the planetary boundary layer and 0.1 x 10(-6) m(-1) sr(-1) in the free troposphere.     

Variational method for the retrieval of the optical thickness and the backscatter coefficient from multiangle lidar profiles

A variational method for retrieving the aerosol optical thickness and backscatter coefficient profiles from multiangle lidar measurements is presented and discussed. A monostatic single-wavelength low-energy lidar system was operated at different zenith angles during the Indian Ocean Experiment (INDOEX) campaign in 1999 to characterize the aerosol plumes in the Indian monsoon. The variational method was applied to lidar data to retrieve profiles of optical thickness and the backscatter coefficient for nighttime and daytime measurements. Results are obtained with an uncertainty of 10% below 3 km (nighttime) and 2.8 km (daytime) and a bias of less than 0.01. During daytime the retrieval of optical parameters is indeed limited to a lower altitude owing to the sky background signal and the atmospheric inhomogeneity. In both cases the total aerosol optical thickness is consistent (+/- 10%) with the integrated value derived from sunphotometer measurements. Backscatter-to-extinction ratios estimated in different regions by two distinct methods compared well, which proves the capability of the method to assess optical measurements and account for the altitude dependence of the phase function.     

Measurement of the lidar ratio for atmospheric aerosols with a 180 degrees backscatter nephelometer

Laser radar (lidar) can be used to estimate atmospheric extinction coefficients that are due to aerosols if the ratio between optical extinction and 180 degrees backscatter (the lidar ratio) at the laser wavelength is known or if Raman or high spectral resolution data are available. Most lidar instruments, however, do not have Raman or high spectral resolution capability, which makes knowledge of the lidar ratio essential. We have modified an integrating nephelometer, which measures the scattering component of light extinction, by addition of a backward-pointing laser light source such that the detected light corresponds to integrated scattering over 176-178 degrees at a common lidar wavelength of 532 nm. Mie calculations indicate that the detected quantity is an excellent proxy for 180 degrees backscatter. When combined with existing techniques for measuring total scattering and absorption by particles, the new device permits a direct determination of the lidar ratio. A four-point calibration, run by filling the enclosed sample volume with particle-free gases of a known scattering coefficient, indicates a linear response and calibration reproducibility to within 4%. The instrument has a detection limit of 1.5 x 10(-7) m(-1) sr(-1) (~10% of Rayleigh scattering by air at STP) for a 5-min average and is suitable for ground and mobile/airborne surveys. Initial field measurements yielded a lidar ratio of ~20 for marine aerosols and ~60-70 for continental aerosols, with an uncertainty of ~20%.     

Airborne CO(2) coherent lidar for measurements of atmospheric aerosol and cloud backscatter

An airborne CO(2) coherent lidar has been developed and flown on over 30 flights of the NASA DC-8 research aircraft to obtain aerosol and cloud backscatter and extinction data at a wavelength near 9μm. Designed to operate in either zenith- or nadir-directed modes, the lidar can be used to measure vertical profiles of backscatter throughout the vertical extent of the troposphere and the lower stratosphere. Backscatter measurements in absolute units are obtained through a hard-target calibration methodology. The use of coherent detection results in high sensitivity and narrow field of view, the latter property greatly reducing multiple-scattering effects. Aerosol backscatter profile intercomparisons with other airborne and ground-based CO(2) lidars were conducted during instrument checkout flights over the NASA Ames Research Center before extended depolyment over the Pacific Ocean. Selected results from data taken during the flights over the Pacific Ocean are presented, emphasizing intercom arisons with backscatter profile data obtained at 1.06 μm with a NASA Goddard Space Flight Center Nd:YAG lidar on the same flights.     

Particle backscatter and extinction profiling with the spaceborne high-spectral-resolution Doppler lidar ALADIN: methodology and simulations

The European Space Agency will launch the Atmospheric Laser Doppler Instrument (ALADIN) for global wind profile observations in the near future. The potential of ALADIN to measure the optical properties of aerosol and cirrus, as well, is investigated based on simulations. A comprehensive data analysis scheme is developed that includes (a) the correction of Doppler-shifted particle backscatter interference in the molecular backscatter channels (cross-talk effect), (b) a procedure that allows us to check the quality of the cross-talk correction, and (c) the procedures for the independent retrieval of profiles of the volume extinction and backscatter coefficients of particles considering the height-dependent ALADIN signal resolution. The error analysis shows that the particle backscatter and extinction coefficients, and the corresponding extinction-to-backscatter ratio (lidar ratio), can be obtained with an overall (systematic+statistical) error of 10%-15%, 15%-30%, and 20%-35%, respectively, in tropospheric aerosol and dust layers with extinction values from 50 to 200 Mm(-1); 700-shot averaging (50 km horizontal resolution) is required. Vertical signal resolution is 500 m in the lower troposphere and 1000 m in the free troposphere. In cirrus characterized by extinction coefficients of 200 Mm(-1) and an optical depth of >0.2, backscatter coefficients, optical depth, and column lidar ratios can be obtained with 25%-35% relative uncertainty and a horizontal resolution of 10 km (140 shots). In the stratosphere, only the backscatter coefficient of aerosol layers and polar stratospheric clouds can be retrieved with an acceptable uncertainty of 15%-30%. Vertical resolution is 2000 m.     

Aerosol and cloud backscatter at 1.06, 1.54, and 0.53 mum by airborne hard-target-calibrated Nd:YAG /methane Raman lidar

A lidar instrument was developed to make simultaneous measurements at three distinct wavelengths in the visible and near infrared at 0.532, 1.064, and 1.54 mum with high cross-sectional calibration accuracy. Aerosol and cloud backscatter cross sections were acquired during November and December 1989 and May and June 1990 by the NASA DC-8 aircraft as part of the Global Backscatter Experiment. The instrument, methodology, and measurement results are described. A Nd:YAG laser produced 1.064- and 0.532-mum energy. The 1.54-mum transmitted pulse was generated by Raman-shifted downconversion of the 1.064-mum pulse through a Raman cell pressured with methane gas. The lidar could be pointed in the nadir or zenith direction from the aircraft. A hard-target-based calibration procedure was used to obtain the ratio of the system calibration between the three wavelengths, and the absolute calibration was referenced to the 0.532-mum lidar molecular backscatter cross section for the clearest scattering regions. From the relative wavelength calibration, the aerosol backscatter cross sections at the longer wavelengths are resolved for values as small as 1% of the molecular cross section. Backscatter measurement accuracies are better than 10(-9) (m sr)(-1) at 1.064 and 1.54 mum. Results from the Pacific Ocean region of the multiwavelength backscatter dependence are presented. Results show extensive structure and variation for the aerosol cross sections. The range of observed aerosol cross section is over 4 orders of magnitude, from less than 10(-9) (m sr)(-1) to greater than 10(-5) (m sr)(-1).     

Instrument for long-path spectral extinction measurements in air: application to sizing of airborne particles

A novel instrument that is capable of taking spectral extinction measurements over long optical paths (approximately 1-100 m) in the UV, visible, and IR ranges is described. The instrument is fully automated, and the extinction spectrum is acquired in almost real time (approximately 5-10 s) with a resolution of ~3 nm. Its sensitivity and accuracy were estimated by tests carried out in a clean room that showed that, for optical paths between 50 and 100 m, the extinction coefficient can be detected at levels of ~10(-5) m(-1). Tests carried out on calibrated latex particles showed that, when it was combined with an appropriate inversion method, the technique could be profitably applied to characterize airborne particulate distributions. By carrying out measurements over optical paths of ~100 m, the instrument is also capable of detecting extinction coefficients that are due to aerosol concentrations well below the limits imposed by the European Economic Community for atmospheric pollution (150 mug/m(3)). Scaled over optical paths of ~10 m, the limit imposed for particle emissions from industrial plants (10 mg/m(3)) can also be detected sensitively.     

Possibility of Hard-Target Lidar Detection of a Biogenic Volatile Organic Compound, nu-Pinene Gas, Over Forest Areas

The absorption spectrum of alpha-pinene gas, a biogenic volatile organic compound, was directly measured with a pulsed mid-infrared laser. The maximum absorption wavelength was found to be ~3.42 mum, and an absorption cross section of 4.8 x 10(-23) m(2) molec(-1) was obtained. A simple theoretical calculation with the measured spectral data showed that several hundreds of parts in 10(12) (ppt) of alpha-pinene gas in forest-mountain areas over a range of ~10 km were detectable by a long-path-averaged hard-target absorption lidar. Requirements for system development were also discussed.     

Lidar aerosol backscatter cross sections in the 2-νm near-infrared wavelength region

Lidar backscatter cross-sectional measurements at 1.064, 0.532, and 1.54 μm were acquired during November 1989 and May-June 1990 around the Pacific region by the NASA DC-8 aircraft as part of the Global Backscatter Experiment. The primary motivation for the Global Backscatter Experiment was the study of lidar backscatter cross sections for the development of a spaceborne wind-sensing lidar. Direct backscatter measurements obtained by the NASA Goddard Space Flight Center visible and infrared lidar are compared with backscatter cross sections calculated from aerosol size distributions obtained by particle counters. Results for one flight with pronounced aerosol layers in the upper troposphere southeast of Japan are presented. Because 2-μm region wavelengths are possible candidates for a spaceborne wind-sensing lidar, the visible and infrared lidar backscatter cross sections at 1.064, 0.532, and 1.54 μm are extrapolated to the 2-μm region. The extrapolated 2-μm cross sections are compared with lidar measurements at 9 μm. A significant range in the ratio of 2-9-μm backscatter cross sections is found, but a large number of points concentrate near ratios of three to ten. A large number of 1.064- and 1.54-μm cross sections were binned to provide an estimate of backscatter for various percentiles for the flight. The ratio of the 50-percentile backscatter values at 1.064 and 1.54 μm suggest a λ(-1.9) to λ(-3.0) wavelength dependence of aerosol backscatter cross section in the near infrared for the observational case.     

Upper tropospheric temperature measurements with the use of a Raman lidar

Upper tropospheric temperature profiles were measured with the NASA Goddard Space Flight Center scanning Raman lidar five months after the eruption of Mt. Pinatubo. To derive temperatures in regions of high aerosol content, the aerosol transmission is calculated for the Raman N(2) return signals under cloud-free conditions. The lidar-derived aerosol backscattering ratio and an estimate of the aerosol extinction-to-backscatter ratio were used to compute the aerosol transmission. With a model reference temperature at 25 km, temperature profiles with a root-mean-square difference between the lidar and radiosonde temperatures of <2 K were obtained over an altitude range of 5-10 km for a 10-min integrated measurement with 300-m resolution.     

Tropospheric ozone differential-absorption lidar using stimulated Raman scattering in carbon dioxide

A UV ozone differential-absorption lidar (DIAL) utilizing a Nd:YAG laser and a single Raman cell filled with carbon dioxide (CO(2)) is designed, developed, and evaluated. The generated wavelengths are 276, 287, and 299 nm, comprising the first to third Stokes lines of the stimulated Raman scattering technique. The correction terms originated from the aerosol extinction, the backscatter, and the absorption by other gases are estimated using a model atmosphere. The experimental results demonstrate that the emitted output energies were 13 mJ/pulse at 276 nm and 287 nm and 5 mJ/pulse at 299 nm, with pump energy of 91 mJ/pulse and a CO(2) pressure of 0.7 MPa. The three Stokes lines account for 44.0% of the available energy. The use of argon or helium as a buffer gas in the Raman cell was also investigated, but this leads to a dramatic decrease in the third Stokes line, which makes this wavelength practically unusable. Our observations confirmed that 30 min of integration were sufficient to observe ozone concentration profiles up to 10 km. Aerosol extinction and backscatter correction are estimated and applied. The aerosol backscatter correction profile using 287 and 299 nm as reference wavelengths is compared with that using 355 nm. The estimated statistical error is less than 5% at 1.5 km and 10% at 2.6 km. Comparisons with the operational carbon-iodine type chemical ozonesondes demonstrate 20% overestimation of the ozone profiles by the DIAL technique.     

Multiwavelength lidar observations of the decay phase of the stratospheric aerosol layer produced by the eruption of mount pinatubo in june 1991

A small three-wavelength (355-, 532-, and 1064-nm) lidar system at NASA Langley Research Center in Hampton, Virginia, has been used since 1992 to make measurements on stratospheric aerosols. The data have been processed to study the decay rate of the stratospheric aerosol layer formed after the eruption of Mount Pinatubo in 1991 and its modulation, the aerosol effective radius, and the column mass loading. The stratospheric aerosol decay curves show annual and biennial cycles as well as short-term changes. At 532 nm, the decay time constant was 302 days for the period from February 1992 to August 1994 and had increased to 645 days for the period from September 1994 to December 1997. By 1996 the integrated stratospheric aerosol backscatter had fallen to levels (7.7 x 10(-5) sr(-1) at 532 nm) close to those seen in 1979 and 1989-1991. This decreasing trend was still continuing in 1997, showing no evidence for any anthropogenic contribution to the stratospheric aerosol.     

Theory of the double-edge molecular technique for Doppler lidar wind measurement

The theory of the double-edge lidar technique for measuring the wind with molecular backscatter is described. Two high-spectral-resolution edge filters are located in the wings of the Rayleigh-Brillouin profile. This doubles the signal change per unit Doppler shift, the sensitivity, and improves measurement accuracy relative to the single-edge technique by nearly a factor of 2. The use of a crossover region where the sensitivity of a molecular- and an aerosol-based measurement is equal is described. Use of this region desensitizes the molecular measurement to the effects of aerosol scattering over a velocity range of +/-100 m/s. We give methods for correcting short-term, shot-to-shot, frequency jitter and drift with a laser reference frequency measurement and methods for long-term frequency correction with a servo control system. The effects of Rayleigh-Brillouin scattering on the measurement are shown to be significant and are included in the analysis. Simulations for a conical scanning satellite-based lidar at 355 nm show an accuracy of 2-3 m/s for altitudes of 2-15 km for a 1-km vertical resolution, a satellite altitude of 400 km, and a 200 km x 200 km spatial resolution.     

NASA multipurpose airborne DIAL system and measurements of ozone and aerosol profiles

An airborne differential absorption lidar (DIAL) system has been developed for the remote measurement of gas and aerosol profiles in the troposphere and lower stratosphere. The multipurpose DIAL system can operate from 280 to 1064 nm for measurements of ozone, sulfur dioxide, nitrogen dioxide, water vapor, temperature,pressure, and aerosol backscattering. The laser transmitter consists of two narrow linewidth Nd: YAG pumped dye lasers with automatic wavelength control. The DIAL wavelengths are transmitted with a 00-,usec temporal separation to reduce receiver system complexity. A coaxial receiver system is used to collect and optically separate the DIAL and aerosol lidar returns. Photomultiplier tubes detect the backscattered laser returns after optical filtering, and the analog signals from three tubes are digitized and stored on high-speed magnetic tape. Real-time gas concentration profiles or aerosol backscatter distributions are calculated and displayed for experiment control. Operational parameters for the airborne DIAL system are presented for measurements of ozone, water vapor, and aerosols in the 290-, 720-, and 600-nm wavelength regions, respectively. The first ozone profile measurements from an aircraft using the DIAL technique are discussed in this paper. Comparisons between DIAL and in situ ozone measurements show agreement to within +/-5 ppbv in the lower troposphere. Lidar aerosol data obtained simultaneously with DIAL ozone measurements are presented for a flight over Virginia and the Chesapeake Bay. DIAL system performance for profiling ozone in a tropopause folding experiment is evaluated, and the applications of the DIAL system to regional and global-scale tropospheric investigations are discussed.     

Characteristics of aerosol optical properties in pollution and Asian dust episodes over Beijing, China

Aerosol optical properties were continuously measured with the National Institute for Environmental Studies (NIES) compact Raman lidar over Beijing, China, from 15 to 31 December 2007. The results indicated that in a moderate pollution episode, the averaged aerosol extinction below 1 km height was 0.39+/-0.15 km(-1) and the lidar ratio was 60.8+/-13.5 sr; in heavy pollution episode, they were 1.97+/-0.91 km(-1) and 43.7+/-8.3 sr; in an Asian dust episode, they were 0.33+/-0.11 km(-1) and 38.3+/-9.8 sr. The total depolarization ratio was mostly below 10% in the pollution episode, whereas it was larger than 20% in the Asian dust episode. The distinct characteristics of aerosol optical properties in moderate and heavy pollution episodes were attributed to the difference in air mass trajectory and the ambient atmospheric conditions such as relative humidity.     

Precision stabilization of the optical frequency in a large ring laser gyroscope

Pressure-induced fractional changes of 10(-7) in the geometry of a large He-Ne ring laser gyroscope induce backscatter phase changes and thus a fractional pulling of the Sagnac frequency of ~5 x 10(-3). To counter this, the optical frequency was stabilized against an iodine-stabilized laser with a high-finesse Fabry-Perot interferometer and piezoelectric control of the ring perimeter. This scheme, although limited in principle by residual geometric asymmetry and in practice by low beam powers (10 pW), stabilized the perimeter to 2.4 nm (6 x 10(-10) or 300 kHz for the optical frequency) and the Sagnac frequency to 100 parts per million over several days.     

Aerosol lidar intercomparison in the framework of the EARLINET project. 3. Raman lidar algorithm for aerosol extinction, backscatter, and lidar ratio

An intercomparison of the algorithms used to retrieve aerosol extinction and backscatter starting from Raman lidar signals has been performed by 11 groups of lidar scientists involved in the European Aerosol Research Lidar Network (EARLINET). This intercomparison is part of an extended quality assurance program performed on aerosol lidars in the EARLINET. Lidar instruments and aerosol backscatter algorithms were tested separately. The Raman lidar algorithms were tested by use of synthetic lidar data, simulated at 355, 532, 386, and 607 nm, with realistic experimental and atmospheric conditions taken into account. The intercomparison demonstrates that the data-handling procedures used by all the lidar groups provide satisfactory results. Extinction profiles show mean deviations from the correct solution within 10% in the planetary boundary layer (PBL), and backscatter profiles, retrieved by use of algorithms based on the combined Raman elastic-backscatter lidar technique, show mean deviations from solutions within 20% up to 2 km. The intercomparison was also carried out for the lidar ratio and produced profiles that show a mean deviation from the solution within 20% in the PBL. The mean value of this parameter was also calculated within a lofted aerosol layer at higher altitudes that is representative of typical layers related to special events such as Saharan dust outbreaks, forest fires, and volcanic eruptions. Here deviations were within 15%.     

Airborne detection of oceanic turbidity cell structure using depth-resolved laser-induced water Raman backscatter

Airborne depth-resolved laser-induced sea-water Raman-backscatter waveforms have been obtained along a flight line extending westward from a point approximately 30 km seaward of Assateague Island to a point where the beach was intersected at latitude 38.1 degrees N and longitude 75.2 degrees W. Pulses from a 337.1-nm nitrogen laser were repetitively transmitted vertically downward into the water column. The laser-induced water Raman backscatter pulse at 381-nm wavelength was depth (or time) resolved into forty bins having widths of -25 cm each. When converted to along-track profiles, the waveforms reveal cells of decreased Raman backscatter superimposed on an overall trend of monotonically decreasing water column optical transmission. This airborne lidar technique shows potential for (1) rapid, quantitative, synoptic study of the homogeneity of the oceanic water column and (2) measurement of the horizontal spatial distribution of the optical transmission of the upper mixed layer of the ocean. A multiple convolution model of a Gaussian transmitted pulse, Gaussian sea surface height, and slope probability density, together with an exponential-decay water-column impulse response, is shown to qualitatively account for the observed pulse shape.