Multiple-scattering effect on ozone retrieval from space-based differential absorption lidar measurements

Single-scattering and multiple-scattering lidar signals are calculated for a spaceborne differential absorption lidar system for global ozone measurements at the on and off wavelength pair at 305 and 315 nm. The effect of multiple scattering is found to be negligible on stratospheric and tropospheric ozone retrieval under background stratospheric aerosol. Under low-visibility conditions in the planetary boundary layer the presence of multiple scattering causes an overestimation in maritime aerosol and an underestimation in urban as well as in rural aerosol. This effect is also examined in three cirrus models. The multiple scattering does not permit accurate ozone retrieval within cirrus; however, below it the solution recovers somewhat with generally an underestimation depending on the type and density of cirrus. The effect of aerosol and Rayleigh extinction on the ozone retrieval is also discussed.     

Multiple-scattering lidar retrieval method: tests on Monte Carlo simulations and comparisons with in situ measurements

A multiple-field-of-view (MFOV) lidar measurement and solution technique has been developed to exploit the retrievable particle extinction and size information contained in the multiple-scattering contributions to aerosol lidar returns. We describe the proposed solution algorithm. The primary retrieved parameters are the extinction coefficient at the lidar wavelength and the effective particle diameter from which secondary products such as the extinction at other wavelengths and the liquid-water content (LWC) of liquid-phase clouds can be derived. The solutions are compared with true values in a series of Monte Carlo simulations and with in-cloud measurements. Good agreement is obtained for the simulations. For the field experiment, the retrieved effective droplet diameter and LWC for the available seven cases studied are on average 15% and 35% (worst case) smaller than the measured data, respectively. In the latter case, the analysis shows that the differences cannot be attributed solely to lidar inversion errors. Despite the limited penetration depth (150-300 m) of the lidar pulses, the results of the studied cases indicate that the retrieved lidar solutions remain statistically representative of measurements performed over the full cloud extent. Long-term MFOV lidar monitoring could thus become a practical and economical option for cloud statistical studies but more experimentation on more varied cloud conditions, especially for LWC, is still needed.     

Transient radiative transfer equation applied to oceanographic lidar

We estimate the optical signal for an oceanographic lidar from the one-dimensional transient (time-dependent) radiative transfer equation using the discrete ordinates method. An oceanographic lidar directs a pulsed blue or green laser into the ocean and measures the time-dependent backscattered light. A large number of parameters affect the performance of such a system. Here the optical signal that is available to the receiver is calculated, rather than the receiver output, to reduce the number of parameters. The effects of albedo of a uniform water column are investigated. The effects of a school of fish in the water are also investigated for various school depths, thicknesses, and densities. The attenuation of a lidar signal is found to be greater than the diffuse attenuation coefficient at low albedo and close to it at higher albedo. The presence of fish in the water is found to have a significant effect on the signal at low to moderate albedo, but not at high albedo.     

Stable near-end solution of the lidar equation for clear atmospheres

A stable variant of the near-end solution has been developed for inversion of lidar signals measured in clear atmospheres. The inversion is based on the use of reference values of the extinction coefficient obtained with a nephelometer at the lidar measurement site. The inversion method, based on a combination of the optical depth and boundary point solutions, is illustrated by simulated and experimental data.     

Multiple-scattering suppression by cross correlation

We describe a new method for characterizing particles in turbid media by cross correlating the scattered intensity fluctuations at two nearby points in the far field. The cross-correlation function selectively emphasizes single scattering over multiple scattering. The usual dynamic light-scattering capability of inferring particle size from decay rate is thus extended to samples that are so turbid as to be visually opaque. The method relies on single-scattering speckle being physically larger than multiple-scattering speckle. With a suitable optical geometry to select nearby points in the far field or equivalently slightly different scattering wave vectors (of the same magnitude), the multiple-scattering contribution to the cross-correlation function may be reduced and in some cases rendered insignificant. Experimental results demonstrating the feasibility of this approach are presented.     

Multiple-scattering resonance self-shielding factors in foils

A recently developed theoretical formalism for the calculation of multiple-scattering neutron self-shielding factors G(E) as a function of the incident neutron energy E is applied to some of the main resonances of Mn, W, Cu, Au and In. Tables of the self-shielding factors of these dosimetrical materials are presented for thin foils of different thicknesses. Two special cases are analysed separately: the narrow resonance and the interfering resonances from different isotopes of the same material.     

Experimental determination of a geometric form factor in a lidar equation for an inhomogeneous atmosphere

We experimentally determine a geometric form factor for an inhomogeneous atmosphere by using the polynomial regression method in a lidar equation.     

Transmission as an input boundary value for an analytical solution of a single-scatter lidar equation

Single-scatter lidar signals carry information on the spatial atmospheric backscatter coefficient, attenuated by the path-integrated extinction. Assuming that the relationship between the backscatter and the extinction is known, the inverted extinction profile and the path-integrated extinction are uniquely related to the input boundary value. The integrated extinction over a certain range interval is a measure of the optical transmission along that path. In reverse, for a given transmission over the path of interest, the input boundary value is uniquely defined. An analytical expression is derived that describes the input boundary condition for the inversion of the single-scatter lidar equation in terms of the transmission losses over the path of interest. The proposed method is useful in situations in which independent transmission measurements are carried out or in situations in which targets such as multiple cloud layers or beam stops are available in the lidar path. Equations for both the forward and the backward integration method are presented. Compared with the widely accepted inversion schemes that are based on single-point reference extinction values, the proposed method is less sensitive to noise.     

Multiple-scattering effects on static light-scattering optical structure factor measurements

We show that the extent and effect of multiple scattering on angularly resolved light-scattering intensity measurements, the optical structure factor, can be quantitatively described by a single parameter, the average number of scattering events along the scattering volume. This quantity is easily measured or calculated and hence provides a useful experimental indicator of multiple scattering, which is a hindrance to accurate structure factor measurements.     

Multiple-scattering suppression: cross correlation with tilted single-mode fibers

Multiple light scattering can be suppressed by slightly tilting two single-mode fibers viewing the same sample volume. The cross-correlation function of the two signals shows more or less contributions from single scattering, depending on the tilt angle. We show experimental results for polystyrene spheres at a scattering angle of 90 degrees . The measured size, intercept, and second cumulant for different tilt angles demonstrate the practicality of this technique. Both polarization components show multiple-scattering contributions, but only the parallel component contains single scattering.     

Experimental determination of the lidar overlap profile with Raman lidar

The range-dependent overlap between the laser beam and the receiver field of view of a lidar can be determined experimentally if a pure molecular backscatter signal is measured in addition to the usually observed elastic backscatter signal, which consists of a molecular component and a particle component. Two methods, the direct determination of the overlap profile and an iterative approach, are presented and applied to a lidar measurement. The measured overlap profile accounts for actual system alignment and for all system parameters that are not explicitly known, such as actual laser beam divergence and spatial intensity distribution of the laser light.     

Airborne Raman lidar

We designed and tested an airborne lidar system using Raman scattering to make simultaneous measurements of methane, water vapor, and temperature in a series of flights on a NASA-operated C-130 aircraft. We present the results for methane detection, which show that the instrument has the requisite sensitivity to atmospheric trace gases. Ultimately these measurements can be used to examine the transport of chemically processed air from within the polar vortex to mid-latitudinal regions and the exchange of stratospheric air between tropical and mid-latitudinal regions.     

Two-wavelength lidar inversion algorithm

Potter [Appl. Opt. 26, 1250 (1987)] has presented a method to determine profiles of the atmospheric aerosol extinction coefficients by use of a two-wavelength lidar with the assumptions of a constant value for the extinction-to-backscatter ratio for each wavelength and a constant value for the ratio between the two extinction coefficients at the two wavelengths. Triggered by this idea, Ackermann [Appl. Opt. 36, 5134 (1997)] expanded this method to consider lidar returns that are a composition of scattering by atmospheric aerosols and molecules, assuming that the molecular scattering is known. In both papers the method is based on the well-known solutions of Bernoulli's differential equation in an iterative scheme with an unknown boundary transmission condition. This boundary condition is less sensitive to noise than boundary extinction conditions. My main purpose is to critically consider the principle behind Potter's method, because it seems that there are several reasons why the number of solutions is not limited to one, as suggested by his original work.     

Comparison of an fe boltzmann temperature lidar with a na narrow-band lidar

The modeled performance of an Fe Boltzmann temperature lidar system is compared with existing Na narrow-band temperature techniques. The Fe Boltzmann technique employs mesospheric Fe as a fluorescence tracer and relies on the temperature dependence of the population difference of two closely spaced Fe transitions. The relative performance of the new technique is compared with an existing Na narrow-band temperature technique, and a link analysis is performed with measured data for both Na and Fe. It is shown that for currently available laser technology the two systems yield similar performance but the Fe system allows for the use of more broadband lasers.     

Raman-shifted eye-safe aerosol lidar

The design features of, and first observations from, a new elastic backscatter lidar system at a wavelength of 1543 nm are presented. The transmitter utilizes stimulated Raman scattering in high-pressure methane to convert fundamental Nd:YAG radiation by means of the 1st Stokes shift. The wavelength-converting gas cell features multipass operation and internal fans. Unlike previous lidar developments that used Raman scattering in methane, the pump beam is not focused in the present configuration. This feature prevents optical breakdown of the gas inside the cell. Additionally, the gas cell is injection seeded by a diode to improve conversion efficiency and beam quality. The receiver uses a 40.6-cm-diameter telescope and a 200-microm InGaAs avalanche photodiode. The system is capable of operating in a dual-wavelength mode (1064 and 1543 nm simultaneously) for comparison or in a completely eye-safe mode. The system is capable of transmitting an energy of more than 200 mJ/pulse at 10 Hz. Aerosol backscatter data from vertical and horizontal pointing periods are shown.     

Extinction-to-backscatter ratio of Asian dust observed with high-spectral-resolution lidar and Raman lidar

Extinction-to-backscatter ratio or lidar ratio is a key parameter in the issue of backscatter-lidar inversions. The lidar ratio of Asian dust was observed with a high-spectral-resolution lidar and a combined Raman elastic-backscatter lidar during the springs of 1998 and 1999. The measured values range from 42 to55 sr in most cases, with a mean of 51 sr. These values are significantly larger than those predicted by the Mie computations that incorporate measured Asian dust size distributions and a range of refractive index with a typical value of 1.55-0.005i. The enhancement of lidar ratio is mostly due to the nonsphericity of dust particles, as indicated by the T-matrix calculations for spheroid particles and a number of other theoretical studies. In addition, possible contamination of urban aerosols may also contribute somewhat in optically thin cases. Mie theory, although it can well describe spherical particle scattering, will not be sufficient to represent the scattering characteristics of irregular particles such as Asian dust, especially in directions larger than approximately 90 degrees when the size parameter is large.     

High-resolution random-modulation cw lidar

A high-resolution random-modulation continuous wave lidar for surface detection using a semiconductor laser diode is presented. The laser diode is intensity modulated with the pseudorandom binary sequence. Its enhanced resolution is achieved via interpolation and a novel front-end analog technique, lowering the requirement of the analog-to-digital converter sampling rate and the associated circuitry. Its mathematical model is presented, including the derivation of the signal-to-noise ratio and the distance standard deviation. Analytical and experimental results demonstrate its capability to achieve distance accuracy of less than 2?cm within 2.6?ms acquisition time, over distances ranging from 1 to 12?m. The laser diode emits 1.4?mW of optical power at a wavelength of 635?nm.