Characterization of atmospheric aerosols from infrared measurements: simulations, testing, and applications

An inversion method for the characterization of atmospheric condensed phases from infrared (IR) spectra is described. The method is tested with both synthetic IR spectra and the spectra of particles that flow in a cryogenic flow tube. The method is applied to the IR spectra recorded by the Atmospheric Trace Molecule Spectroscopy instrument carried by the Space Shuttle during three missions in 1992, 1993, and 1994. The volume density and particle size distribution for sulfate aerosol are obtained as a function of altitude. The density and size distribution of ice particles in several cirrus clouds are also retrieved. The probable radius of the ice particles in the high-altitude (10-15-km) cirrus clouds is found to be approximately 6-7 microm.     

Airborne differential absorption lidar system for measurements of atmospheric water vapor and aerosols

An airborne differential absorption lidar (DIAL) system has been developed at the NASA Langley Research Center for remote measurements of atmospheric water vapor (H(2)O) and aerosols. A solid-state alexandrite laser with a 1-pm linewidth and > 99.85% spectral purity was used as the on-line transmitter. Solid-state avalanche photodiode detector technology has replaced photomultiplier tubes in the receiver system, providing an average increase by a factor of 1.5-2.5 in the signal-to-noise ratio of the H(2)O measurement. By incorporating advanced diagnostic and data-acquisition instrumentation into other subsystems, we achieved additional improvements in system operational reliability and measurement accuracy. Laboratory spectroscopic measurements of H(2)O absorption-line parameters were perfo med to reduce the uncertainties in our knowledge of the absorption cross sections. Line-center H(2)O absorption cross sections were determined, with errors of 3-6%, for more than 120 lines in the 720-nm region. Flight tests of the system were conducted during 1989-1991 on the NASA Wallops Flight Facility Electra aircraft, and extensive intercomparison measurements were performed with dew-point hygrometers and H(2)O radiosondes. The H(2)O distributions measured with the DIAL system differed by ≤ 10% from the profiles determined with the in situ probes in a variety of atmospheric conditions.     

Tunable acousto-optic spectral imager for atmospheric composition measurements in the visible spectral domain

We describe a new spectral imaging instrument using a TeO₂ acousto-optical tunable filter (AOTF) operating in the visible domain (450-900?nm). It allows for fast (～1 second), monochromatic (FWHM ranges from 0.6?nm at 450?nm to 3.5?nm at 800?nm) picture acquisition with good spatial resolution. This instrument was designed as a breadboard of the visible channel of a new satellite-borne atmospheric limb spectral imager, named the Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere (ALTIUS), that is currently being developed. We tested its remote sensing capabilities by observing the dense, turbulent plume exhausted by a waste incinerator stack at two wavelengths sensitive to NO₂. An average value of 6.0±0.4×10¹⁷ molecules?cm^-2 has been obtained for the NO₂ slant column density within the plume, close to the stack outlet. Although this result was obtained with a rather low accuracy, it demonstrates the potential of spectral imaging by using AOTFs in remote sensing.     

Monte Carlo approach to identification of the composition of stratospheric aerosols from infrared solar occultation measurements

We describe an inversion method for determining the composition, density, and size of stratospheric clouds and aerosols by satellite remote sensing. The method, which combines linear least-squares minimization and Monte Carlo techniques, is tested with pure synthetic IR spectra. The synthetic spectral data are constructed to mimic mid-IR spectra recorded by the Improved Limb Atmospheric Spectrometer (ILAS-I and ILAS-II) instruments, which operate in the solar occultation mode and record numerous polar stratospheric cloud events. The advantages and limitations of the proposed technique are discussed. In brief we find that stratospheric aerosol in the size range from 0.5 to 4.0 02114 microm can be retrieved to an accuracy of 30%. We also show that the chemical composition of common stratospheric aerosols can be determined, whereas identification of their phases from mid-IR satellite remote-sensing data alone appears to be questionable.     

A study on major inorganic ion composition of atmospheric aerosols at Tirupati

Atmospheric aerosol samples were collected from an urbanized area (Tirupati, South India) during the period April to September 2001 and were analyzed for major inorganic ions-F, Cl, NO(3,) SO(4), Na, K, Mg, Ca and NH(4) by employing the ion chromatograph. The average mass of the aerosol was found to be 55.64 microgram(-3) with a total water-soluble load (total anion+total cation) of 5.74 microgram(-3). Seasonal distribution of the aerosol mass and temporal variations of the ion concentrations present a clear trend of lowering atmospheric levels during the rainy season due to washout effect. Composition of the aerosols showed higher concentration of SO(4) followed by NO(3) and NH(4) and found to be influenced by local terrestrial sources. The presence of SO(4) and NO(3) may be due to re-suspension of soil particles (formation by heterogeneous oxidation). Ca, Mg and Cl are mainly soil derived ones. The presence of NH(4) may be attributed to the reaction of NH(3) vapors with acidic gases such as H(2)SO(4), HNO(3) and HCl or ammonia vapor may react or condense on an acidic particle surface of anthropogenic origin. Equivalent ratios of NH(4)/(NO(3)+SO(4)) varied between 0.62 and 0.74. It shows the aerosol to be slightly acidic due to the neutralization of basicity by SO(2) and NO(x).     

Multiple field of view lidar returns from atmospheric aerosols

As a laser pulse propagates into the atmosphere, it becomes broader in the lateral direction as a result of scattering by aerosols. The laser pulse may be described as the superposition of a central, unscattered component of reduced intensity and a surrounding scattered component. A multiple field of view lidar has been developed that makes simultaneous measurements of the backscattered power from the central pulse and multiply scattered power arising from the scattered component. Measurements from various types of atmospheric aerosols and precipitation are presented and compared with simulated returns. The results show how the multiply scattered signals are influenced by the distribution of the aerosols along the lidar path, the characteristic size of the aerosols, and the optical depth. It is shown that the multiple field of view lidar can provide meaningful, additional information about the aerosols that is not available from a conventional single field of view lidar.     

Objective algorithms for the retrieval of optical depths from ground-based measurements

Optical depth retrieval by means of Langley regression is complicated by cloud transits and other time-varying interferences. An algorithm is described that objectively selects data points from a continuous time series and performs the required regression. The performance of this algorithm is compared by a double-blind test with an analysis done subjectively. The limits to accuracy imposed by time-averaged data are discussed, and an additional iterative postprocessing algorithm is described that improves the accuracy of optical depth inferences made from data with time-averaging periods longer than 5 min. Such routine algorithms are required to provide intercomparable retrievals of optical depths from widely varying historical data sets and to support large networks of instruments such as the multifilter rotating shadow-band radiometer.     

Significance of multiple scattering from tropospheric aerosols for ground-based backscatter lidar measurements

The influence of multiple scattering on the retrieval of extinction coefficients of tropospheric aerosols from ground-based backscatter lidar measurements is numerically modeled. In a first step, lidar returns are computed by means of a Monte Carlo code for model atmospheres with different aerosol types and different extinction coefficient profiles. In so doing, synthetic lidar signals with and without multiple scattering can be simulated. In a second step, both types of signal are inverted by the most frequently used analytical solution, which, however, is based on the single-scatter assumption. From a comparison of the results, the error of the retrieved aerosol-extinction profiles can be quantitatively determined. It was found that the contribution of multiply scattered photons to the lidar signals is typically below 10% and never exceeds 20%. The relative errors of the retrieved aerosol-extinction profile in the planetary boundary layer are still smaller; they were determined to be less than 3% for all aerosol types, even for extinction coefficients as large as 3.9 km(-1). Thus, for ground-based lidar measurements and typical meteorological conditions, errors caused by neglecting multiple scattering are by far less significant than other errors in lidar data evaluation.     

Differences arising in the determination of the atmospheric extinction coefficient by transmission and target reflectance measurements

Measurements of the optical extinction at a wavelength of 1.06 microm have been made in water droplet clouds. The extinction coefficient has been measured in the laboratory using two different methods simultaneously. In the first, measurements of the transmitted signal attenuation over a known path length were used. In the second the extinction coefficient was derived from the two-way attenuation of the signal reflected from a target on the opposite side of the cloud from the laser source and detector. It is found that in general the two values of the coefficient derived differ considerably, and the magnitude of the difference depends on the cloud density, the target size, and the system's optical parameters. The difference is shown to originate in the off-axis forward scattering caused by the cloud droplets, and the implications of the results on the measurement of the atmospheric extinction by reflection (lidar) techniques are discussed.     

Reconstruction of refractive indices from spectral measurements of monodisperse aerosols

For the investigation of two-component aerosols, one needs to know the refractive indices of the two aerosol components. One problem is that they depend on temperature and pressure, so one needs for their determination a robust measurement instrument such as the FASP device, which can cope with rigid environmental conditions. In this article, we show that the FASP device is capable of measuring the needed refractive indices, if monodisperse aerosols of the pure components are provided. We determine the particle radii of the monodisperse aerosols needed for this task and investigate how accurate the measurements have to be in order to retrieve refractive indices in a sufficient quality, such that they are suitable for investigations of two-component aerosols.     

Shape-dependent regularization for the retrieval of atmospheric state parameter profiles

We present an adaptive regularization approach to retrieve vertical state parameter profiles from limb-sounding measurements with high accuracy. This is accomplished by introducing a dedicated regularization functional based on a reasonable assumption of the profile characteristics. The approach results in shape-dependent weighting during least-squares computations and relies on a Cholesky decomposition of a preselected L(T)L matrix. Our method is compared with established regularization functionals such as optimal estimation and Tikhonov with respect to errors and achievable height resolution. The results show an improved height resolution of the retrieved profiles together with a reduction of absolute and relative errors obtained by test-bed simulations.     

Tomographic retrieval of atmospheric parameters from infrared limb emission observations

Typical inversion of limb-sounding measurements assumes local horizontal homogeneity of the atmosphere. This simplification corresponds to spectral radiance errors that can exceed the noise level of a typical infrared instrument by a factor of 10 and causes errors in retrieved state parameters. To avoid these errors and to take the horizontal structure of the atmosphere into account, a two-dimensional (2D) tomographic sequential estimation approach is described. Application to temperature retrievals from simulated measurements yields typical retrieval errors of the order of 1 K, and a one-dimensional retrieval with the same synthetic measurements shows differences to the true values up to 10 K in regions with strong horizontal inhomogeneities. The horizontal resolution of the 2D retrieval is even better (up to 40 km) than the horizontal tangent point spacing.     

Error analysis for retrieval of urban atmospheric aerosol properties from downwelling infrared radiation spectra

The possibility of retrieval of urban aerosol physical properties from downwelling atmospheric infrared radiation spectra between 700 and 1400 cm(-1) with 0.24-cm(-1) spectral resolution, which can be obtained from the tropospheric infrared interferometric sounder developed by the Central Research Institute of Electric Power Industry, was estimated from error analysis of the least-squares fit method. The error analysis for retrieval of the aerosol extinction coefficient spectra in three atmospheric layers (boundary, free troposphere, and stratosphere) showed the retrievability only of the boundary layer. Based on this result, we propose the retrieval for particle number density of each aerosol component, which is one of the parameters for the aerosol size distribution function, using the boundary aerosol extinction coefficient spectra. We assume that aerosols in urban areas consist of three types of component, namely, water soluble, soot, and dustlike. Under this assumption, we estimated the error of the retrieved volume density for each aerosol component. For the estimation we used the least-squares fit of Mie-generated spectral extinction coefficients. The estimated error shows that the volume density of each aerosol component in an urban boundary layer is equivalent to the retrieval target. We also show that the aerosol properties can be retrieved with higher accuracy when the effects of multiple scattering by aerosols are included in the retrieval procedure.     

Simulations of the accuracy in retrieving stratospheric aerosol effective radius, composition, and loading from infrared spectral transmission measurements

We examine the extent to which three physical aerosol parameters--effective radius, composition (sulfate weight percent), and total volume-can be determined from infrared transmission spectra. Using simulated transmission data over the range 800-4750 cm(-1) (12.5-2.1 microm) and errors taken from the infrared spectral measurements of the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument, we use optimal estimation to recover these aerosol parameters. Uncertainties in these are examined as a function of the size, composition, and loading of stratospheric aerosols and of the spectral range employed. Using the entire spectral range above, we retrieve all three parameters with a precision to within 3% if the size distribution form is known. Additional errors result, however, from an uncertainty in the size distribution width. These are small (only a few percent) for composition and total volume but are substantial (as much as 50%) for effective radius. Errors also increase substantially when the spectral range is reduced. The retrieved effective radius can have an error of 100% or greater for typical stratospheric aerosol sizes when the spectral range is restricted to the lower wavenumber part of the range. For good accuracy in effective radius, the spectral range must extend beyond approximately 3000 cm(-1). Composition and total volume are less sensitive to the spectral range than effective radius. These simulations were carried out with modeled data to test the potential accuracy of stratospheric sulfate aerosol retrievals from the Atmospheric Chemistry Experiment (ACE). Because of the limitations that result from the use of simulated data, we have tested our retrieval algorithm using ATMOS spectra in different filter regions and present here the aerosol parameters obtained.     

HIAPER Pole-to-Pole Observations (HIPPO): fine-grained, global-scale measurements of climatically important atmospheric gases and aerosols

The HIAPER Pole-to-Pole Observations (HIPPO) programme has completed three of five planned aircraft transects spanning the Pacific from 85 °?N to 67 °?S, with vertical profiles every approximately 2.2 ° of latitude. Measurements include greenhouse gases, long-lived tracers, reactive species, O(2)/N(2) ratio, black carbon (BC), aerosols and CO(2) isotopes. Our goals are to address the problem of determining surface emissions, transport strength and patterns, and removal rates of atmospheric trace gases and aerosols at global scales and to provide strong tests of satellite data and global models. HIPPO data show dense pollution and BC at high altitudes over the Arctic, imprints of large N(2)O sources from tropical lands and convective storms, sources of pollution and biogenic CH(4) in the Arctic, and summertime uptake of CO(2) and sources for O(2) at high southern latitudes. Global chemical signatures of atmospheric transport are imaged, showing remarkably sharp horizontal gradients at air mass boundaries, weak vertical gradients and inverted profiles (maxima aloft) in both hemispheres. These features challenge satellite algorithms, global models and inversion analyses to derive surface fluxes. HIPPO data can play a crucial role in identifying and resolving questions of global sources, sinks and transport of atmospheric gases and aerosols.     

Spectroradiometer for ground-based atmospheric measurements related to remote sensing in the visible from a satellite

A spectroradiometer was developed for use on the ground and aboard ship. The instrument separates global and diffuse solar radiation with a rotating shading band. Calibration results are compared with those of other sun photometers. The instrument was also mounted on a stabilized platform during an experimental trial aboard ship on the Tyrrhenian Sea. Results are comparable with those obtained by handheld sun photometer.     

Polarized light scattering by aerosols in the marine atmospheric boundary layer

The intensity and polarization of light scattered from marine aerosols affect visibility and contrast in the marine atmospheric boundary layer (MABL). The polarization properties of scattered light in the MABL vary with size, refractive index, number distributions, and environmental conditions. Laboratory measurements were used to determine the characteristics and variability of the polarization of light scattered by aerosols similar to those in the MABL. Scattering from laboratory-generated sea-salt-containing (SSC) [NaCl, (NH(4))(2) SO(4), and seawater] components of marine aerosols was measured with a scanning polarization-modulated nephelometer. Mie theory with Gaussian and log normal size distributions of spheres was used to calculate the polarized light scattering from various aerosol composition models and from experimentally determined distributions of aerosols in the marine boundary layer. The modeling was verified by comparison with scattering from distilled water aerosols. The study suggests that polarimetric techniques can be used to enhance techniques for improving visibility and remote imaging for various aerosol types, Sun angles, and viewing conditions.     

Using shaded filter instruments for measurements of sky radiance: retrieval of the apparent sky-view factor from ultraviolet radiation measurements

There is a growing need for sky radiometric measurements that encapsulate spatial as well as temporal variability. Since the advent of fast data acquisition systems in the 1980s, recent studies have utilized radiation filter instruments deployed in various sky-shading platforms. One cost effective method provides azimuthally averaged sky radiance distribution data at time scales down to fractions of a minute. Successful operation of this scheme requires knowledge of the apparent sky-view factor of the deployed sensor--an artifact of the instrument input optics, instrument filter design, and the instrument shading device employed. We provide a methodology for a determination of the sensor sky-view factor by using the employed shading device and the sun as a light source. The effect of an incorrect determination of instrument sky-view factor is also analyzed in context of the measurements.     

Reduced accuracy in infrared measurements in the presence of industrial aerosols

Translated from Izmeritel'naya Tekhnika, No. 12, pp. 52–53, December, 1992.