New method for simultaneous gas and aerosol retrievals from space limb-scanning spectral observation of the atmosphere

This study concerns the development of a new inversion method for simultaneous gas and aerosol retrievals in the upper layers of the atmosphere from limb-viewing multiwavelength-transmission infrared measurements. In this method, concentrations of gas species such as O3, NO2, HNO3, N2O, CH4, and H2O, and spectral dependences of the aerosol extinction coefficient are retrieved simultaneously. When this is done, smoothness constraints on the desired spectral dependencies of the aerosol extinction coefficient are used as an a priori assumption. The method is used in the treating of synthetic transmission spectra of the Improved Limb Atmospheric Spectrometer, which is based on the solar occultation technique and was on board the Advanced Earth Observing Satellite. A set of numerical tests shows the efficiency of the method.     

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.     

Retrieval of aerosol profile variations from reflected radiation in the oxygen absorption a band

Through a Fredholm integral equation of the first kind, aerosol kernel functions relate the variations in radiance measured by satellites to the variations in the aerosol extinction profile and thus permit profile retrieval from radiance measurements by inversion of the set of radiative transfer equations for various spectral intervals. Previously [Appl. Opt. 36, 1328 (1997)] the kernel functions were evaluated for the red and near-infrared spectral regions outside molecular absorption bands. Here they are computed within the oxygen A band with 20-cm(-1) spectral resolution. It is shown that, even with such a relatively low spectral resolution, the new set of kernels is able to provide better information on and improved accuracy of the retrieved profile.     

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.     

Optimal eigenanalysis for the treatment of aerosols in the retrieval of atmospheric composition from transmission measurements

The separation of the individual contributions of aerosol and gases to the total attenuation of radiation through the atmosphere has been the subject of much scientific investigation since remote sensing experiments first began. We describe a new scheme to account for the spectral variation of the aerosol extinction in the inversion of transmission data from occultation measurements. Because the spectral variation of the aerosol extinction is generally unknown,the inversion problem is underdetermined and cannot be solved without a reduction in the number of unknowns in the set of equations used to describe the attenuation at each wavelength. This reduction can be accomplished by a variety of methods, including use of a priori information, the parameterization of the aerosol spectral attenuation, and the specification of the form of the aerosol size distribution. We have developed and implemented a parameterization scheme based on existing empirical and modeled information about the microphysical properties of aerosols. This scheme employs the eigenvectors from an extensive set of simulations to parameterize the aerosol extinction coefficient for incorporation into the inversion algorithm. We examine the accuracy of our method using data sets containing over 24,000 extinction spectra and compare it with that of another scheme that is currently implemented in the Polar Ozone and Aerosol Measurement (POAM) satellite experiment. In simulations using 80 wavelengths in the UV-visible-near-IR spectral range of the Stratospheric Aerosol and Gas Experiment III (SAGE) instrument, we show that, for our optimal parameterization, errors below 1% are observed in 80% of cases, whereas only approximately 20% of all cases are as accurate as this in a quadratic parameterization employing the logarithm of the wavelength.     

Iterative method for the inversion of multiwavelength lidar signals to determine aerosol size distribution

Two iterative methods of inverting lidar backscatter signals to determine altitude profiles of aerosol extinction and altitude-resolved aerosol size distribution (ASD) are presented. The first method is for inverting two-wavelength lidar signals in which the shape of the ASD is assumed to be of power-law type, and the second method is for inverting multiwavelength lidar signals without assuming any a priori analytical form of ASD. An arbitrary value of the aerosol extinction-to-backscatter ratio (S(1)) is assumed initially to invert the lidar signals, and the ASD determined by use of the spectral dependence of the retrieved aerosol extinction coefficients is used to improve the value of S(1) iteratively. The methods are tested for different forms of altitude-dependent ASD's by use of simulated lidar-backscatter-signal profiles. The effect of random noise on the lidar backscatter signals is also studied.     

Aerosol properties from spectral extinction and backscatter estimated by an inverse Monte Carlo method

The feasibility of using a generalized stochastic inversion methodology to estimate aerosol size distributions accurately by use of spectral extinction, backscatter data, or both is examined. The stochastic method used, inverse Monte Carlo (IMC), is verified with both simulated and experimental data from aerosols composed of spherical dielectrics with a known refractive index. Various levels of noise are superimposed on the data such that the effect of noise on the stability and results of inversion can be determined. Computational results show that the application of the IMC technique to inversion of spectral extinction or backscatter data or both can produce good estimates of aerosol size distributions. Specifically, for inversions for which both spectral extinction and backscatter data are used, the IMC technique was extremely accurate in determining particle size distributions well outside the wavelength range. Also, the IMC inversion results proved to be stable and accurate even when the data had significant noise, with a signal-to-noise ratio of 3.     

Retrieval of aerosol refractive index from extinction spectra with a damped harmonic-oscillator band model

A new method for the retrieval of the spectral refractive indices of micrometer-sized particles from infrared aerosol extinction spectra has been developed. With this method we use a classical damped harmonic-oscillator model of molecular absorption in conjunction with Mie scattering to model extinction spectra, which we then fit to the measurements using a numerical optimal estimation algorithm. The main advantage of this method over the more traditional Kramers-Kronig approach is that it allows the full complex refractive-index spectra, along with the parameters of the particle size distribution, to be retrieved from a single extinction spectrum. The retrieval scheme has been extensively characterized and has been found to provide refractive indices with a maximum uncertainty of approximately 10% (with a minimum of approximately 0.1%). Comparison of refractive indices calculated from measurements of a ternary solution of HNO3, H2SO4, and H2O with those published in J. Phys. Chem. A 104, 783 (2000) show similar differences as found by other authors.     

Alternative statistical methods for spectral data processing: applications to laser-induced breakdown spectroscopy of gaseous and aerosol systems

A new spectral data processing scheme based on the standard deviation of collected spectra is compared with the traditional ensemble-averaging of laser-induced breakdown spectroscopy (LIBS)-based spectral data for homogenous (i.e., pure gas phase) systems and with a LIBS-based traditional conditional spectral analysis scheme for non-homogenous (e.g., aerosol system) analyte systems under discrete particle loadings. The range of conditions enables quantitative assessment of the analytical approaches under carefully controlled experimental conditions. In the homogeneous system with gaseous carbon dioxide producing the carbon atomic emission signal, the standard deviation method provided a suitable metric that is directly proportional to the analyte signal and compares favorably with a traditional ensemble averaging scheme. In contrast, the applicability of the standard deviation method for analysis of non-homogenous analyte systems (e.g., aerosol systems) must be carefully considered. It was shown both experimentally and via Monte Carlo simulations that the standard deviation approach can produce an analyte response that is monotonic with analyte concentration up to a point at which the analyte signal starts to transition from a non-homogeneous system to a homogeneous systems (i.e., around a 50% sampling point for aerosol particles). In addition, the standard deviation spectrum is capable of revealing spectral locations of non-homogeneously dispersed analyte species without a priori knowledge.     

Droplet size spectra and water-vapor concentration of laboratory water clouds: inversion of Fourier transform infrared (500-5000 cm(-1)) optical-depth measurement

Infrared extinction optical depth (500-5000 cm(-1)) has been measured with a Fourier transform infrared spectrometer for clouds produced with an ultrasonic nebulizer. Direct measurement of the cloud droplet size spectra agree with size spectra retrieved from inversion of the extinction measurements. Both indicate that the range of droplet sizes is 1-14 mum. The retrieval was accomplished with an iterative algorithm that simultaneously obtains water-vapor concentration. The basis set of droplet extinction functions are computed once by using numerical integration of the Lorenz-Mie theory over narrow size bins, and a measured water-vapor extinction curve was used. Extinction and size spectra are measured and computed for both steady-state and dissipating clouds. It is demonstrated that anomalous diffraction theory produces relatively poor droplet size and synthetic extinction spectra and that extinction measurements are helpful in assessing the validity of various theories. Calculations of cloud liquid-water content from retrieved size distributions agree with a parameterization based on optical-depth measurements at a wave number of 906 cm(-1) for clouds that satisfy the size spectral range assumptions of the parameterization. Significance of droplet and vapor contribution to the total optical depth is used to evaluate the reliability of spectral inversions.     

Tungsten source integrated cavity output spectroscopy for the determination of ambient atmospheric extinction coefficient

Broadband integrated cavity output spectroscopy (ICOS) utilizing an incoherent tungsten lamp as a spectroscopic source is described. This novel approach has been termed W-ICOS. The technique has been applied to make quantitative measurements of Rayleigh scattering by carbon dioxide between 570 and 590 nm and to make measurements of aerosol and atmospheric extinction. Minimum detectable extinction coefficients (kext) made in a 94 cm optical cavity ranged between 3.4 and 35 Mm(-1) depending on the level of signal averaging employed. The level of sensitivity achieved should allow measurements on static gas samples and regular, quantitative measurements of the atmospheric extinction coefficient.     

In situ measurement of the infrared absorption and extinction of chemical and biologically derived aerosols using flow-through photoacoustics

In an effort to establish a more reliable set of optical cross sections for a variety of chemical and biological aerosol simulants, we have developed a flow-through photoacoustic system that is capable of measuring absolute, mass-normalized extinction and absorption cross sections. By employing a flow-through design we avoid issues associated with closed aerosol photoacoustic systems and improve sensitivity. Although the results shown here were obtained for the tunable CO2 laser waveband region, i.e., 9.20-10.80 microm, application to other wavelengths is easily achievable. The aerosols considered are categorized as biological, chemical, and inorganic in origin, i.e., Bacillus atrophaeus endospores, dimethicone silicone oil (SF-96 grade 50), and kaolin clay powder (alumina and silicate), respectively. Results compare well with spectral extinction measured previously by Fourier-transform infrared spectroscopy. Comparisons with Mie theory calculations based on previously published complex indices of refraction and measured size distributions are also presented.     

Retrieval of stratospheric aerosol size and composition information from solar infrared transmission spectra

Infrared transmission spectra were recorded by the Jet Propulsion Laboratory MkIV interferometer during flights aboard the NASA DC-8 aircraft as part of the Airborne Arctic Stratospheric Expedition II (AASE II) mission in the early months of 1992. In our research, we infer the properties of the stratospheric aerosols from these spectra. The instrument employs two different detectors, a HgCdTe photoconductor for 650-1850 cm(-1) and an InSb photodiode for 1850-5650 cm(-1), to simultaneously record the solar intensity throughout the mid-infrared. These spectra have been used to retrieve the concentrations of a large number of gases, including chlorofluorocarbons, NOy species, O3, and ozone-depleting gases. We demonstrate how the residual continua spectra, obtained after accounting for the absorbing gases, can be used to obtain information about the stratospheric aerosols. Infrared extinction spectra are calculated for a range of modeled aerosol size distributions and compositions with Mie theory and fitted to the measured residual spectra. By varying the size distribution parameters and sulfate weight percent, we obtain the microphysical properties of the aerosols that best fit the observations. The effective radius of the aerosols is found to be between 0.4 and 0.6 microm, consistent with that derived from a large number of instruments in this post-Pinatubo period. We demonstrate how different parts of the spectral range can be used to constrain the range of possible values of this size parameter and show how the broad spectral bandpass of the MkIV instrument presents a great advantage for retrieval ofboth aerosol size a nd composition over instruments with a more limited spectral range. The aerosol composition that provides the best fit to the measured spectra is a 70-75% sulfuric acid solution, in good agreement with that obtained from thermodynamic considerations.     

Vertical profiles of aerosol volume from high-spectral-resolution infrared transmission measurements. I. Methodology

The wavelength-dependent aerosol extinction in the 800-1250-cm(-1) region has been derived from ATMOS (atmospheric trace molecule spectroscopy) high-spectral-resolution IR transmission measurements. Using models of aerosol and cloud extinction, we have performed weighted nonlinear least-squares fitting to determine the aerosol-volume columns and vertical profiles of stratospheric sulfate aerosol and cirrus cloud volume. Modeled extinction by use of cold-temperature aerosol optical constants for a 70-80% sulfuric-acid-water solution shows good agreement with the measurements, and the derived aerosol volumes for a 1992 occultation are consistent with data from other experiments after the eruption of Mt. Pinatubo. The retrieved sulfuric acid aerosol-volume profiles are insensitive to the aerosol-size distribution and somewhat sensitive to the set of optical constants used. Data from the nonspherical cirrus extinction model agree well with a 1994 mid-latitude measurement indicating the presence of cirrus clouds at the tropopause.     

Atmospheric correction over case 2 waters with an iterative fitting algorithm: relative humidity effects

In algorithms for the atmospheric correction of visible and near-IR satellite observations of the Earth's surface, it is generally assumed that the spectral variation of aerosol optical depth is characterized by an Angstr?m power law or similar dependence. In an iterative fitting algorithm for atmospheric correction of ocean color imagery over case 2 waters, this assumption leads to an inability to retrieve the aerosol type and to the attribution to aerosol spectral variations of spectral effects actually caused by the water contents. An improvement to this algorithm is described in which the spectral variation of optical depth is calculated as a function of aerosol type and relative humidity, and an attempt is made to retrieve the relative humidity in addition to aerosol type. The aerosol is treated as a mixture of aerosol components (e.g., soot), rather than of aerosol types (e.g., urban). We demonstrate the improvement over the previous method by using simulated case 1 and case 2 sea-viewing wide field-of-view sensor data, although the retrieval of relative humidity was not successful.     

光腔光谱方法气溶胶消光系数比对测量

搭建了非相干宽带腔增强吸收光谱(IBBCEAS)实验装置,旨在实现大气气溶胶宽波段消光系数的在线测量。为评估IBBCEAS系统测量大气气溶胶消光系数的稳定性和准确性,联用单波长腔衰减相位移反照率监测仪(CAPS-ALB)在复旦大学环境科学楼开展比对测量研究。比对结果显示,两台装置在532nm波长处测量的气溶胶消光系数变化趋势基本一致,相关系数为0.974,说明IBBCEAS装置具有与通过计量检测认证的高精准度仪器相似的稳定性和可靠性。通过CAPS-ALB仪器的比对校准,IBBCEAS装置的测量平均偏差由7.08Mm^-1降低到2.4Mm^-1,测量准确性得到显著提升。另外发现,宽波段光谱测量对研究气溶胶消光系数的波长依赖性具有重要意义。     

Sensitivity of the lidar solution to errors of the aerosol backscatter-to-extinction ratio: influence of a monotonic change in the aerosol extinction coefficient

Unlike other errors in the lidar equation solution for the two-component atmosphere, the error of the measured aerosol extinction coefficient caused by inaccuracies in the assumed aerosol backscatter-to-extinction ratios significantly depends on the aerosol spatial inhomogeneity. In a slightly nonhomogeneous atmosphere, an incorrect value in the assumed aerosol backscatter-to-extinction ratio does not significantly corrupt the measurement result, whereas in an atmosphere with a large monotonic change of the aerosol extinction [e.g., in the lower troposphere], the incorrect value yields a large distortion of the retrieved extinction-coefficient profile. In the latter case, even the far-end solution can produce a large error in the retrieved extinction coefficient. The analytical formulas for the determination of the range errors, obtained for the Klett and the optical-depth solutions, show that these errors significantly depend on the method of the boundary-condition determination. Distortions of the retrieved aerosol extinction profiles are, in general, larger if the assumed aerosol backscatter-to-extinction ratio is underestimated in relation to the real value.     

Optical and physical properties of stratospheric aerosols from balloon measurements in the visible and near-infrared domains. III. Presence of aerosols in the middle stratosphere

The aerosol extinction measurements in the ultraviolet and visible wavelengths by the balloonborne spectrometer Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et NOx (SALOMON) show that aerosols are present in the middle stratosphere, above 25-km altitude. These observations are confirmed by the extinction measurements performed by a solar occultation radiometer. The balloonborne Laboratoire de Météorologie Dynamique (LMD) counter instrument also confirms the presence of aerosol around 30-km altitude, with an unrealistic excess of micronic particles assuming that only liquid sulfate aerosols are present. An unexpected spectral structure around 640-nm observed by SALOMON is also detectable in extinction measurements by the satellite instrument Stratospheric Aerosols and Gas Experiment III. This set of measurements could indicate that solid aerosols were detected at these altitude ranges. The amount of soot detected up to now in the lower stratosphere is too low to explain these measurements. Thus, the presence of interplanetary dust grains and micrometeorites may need to be invoked. Moreover, it seems that these grains fill the stratosphere in stratified layers.     

Tropospheric aerosol extinction coefficient profiles derived from scanning lidar measurements over Tsukuba, Japan, from 1990 to 1993

Mie scattering lidar was used to observe aerosol extinction coefficient profiles in the troposphere over Tsukuba (140 E, 36 N), Japan, for three years from March 1990 to February 1993, and data obtained in fair weather were analyzed. The lidar measurements were made by a vertical scanning mode to generate profiles of extinction coefficients from the lidar level to a 12-km altitude. The extinction coefficients were derived from the lidar signals using a two-component (air molecule and aerosol) lidar equation, in which the ratio of aerosol extinction to backscattering was assumed to be constant. Seasonal average profiles were derived from individual profiles. Three-year average profiles were also calculated and modeled using mathematical expressions. The model profile assumed (1) a constant extinction ratio in the atmospheric boundary layer (ABL), (2) an exponentially decreasing extinction ratio above the ABL, and (3) a constant extinction ratio in the upper troposphere where the extinction ratio can be defined as the ratio of the aerosol extinction coefficient to the air molecule extinction coefficient. The extinction ratios both in the ABL and in the upper troposphere and the scale height that was used to express the exponential decrease were used as three unknown parameters. Seasonal variation of optical thickness that was obtained by integrating extinction coefficients with height was also investigated.     

Radiometric calibration of SeaWiFS in the near infrared

The radiometric calibration of the Sea-Viewing Wide-Field-of-View Sensor (SeaWiFS) in the near infrared (band 8, centered on 865 nm) is evaluated by use of ground-based radiometer measurements of solar extinction and sky radiance in the Sun's principal plane at two sites, one located 13 km off Venice, Italy, and the other on the west coast of Lanai Island, Hawaii. The aerosol optical thickness determined from solar extinction is used in an iterative scheme to retrieve the pseudo aerosol phase function, i.e., the product of single-scattering albedo and phase function, in which sky radiance is corrected for multiple scattering effects. No assumption about the aerosol model is required. The aerosol parameters are the inputs into a radiation-transfer code used to compute the SeaWiFS radiance. The calibration method has a theoretical inaccuracy of plus or minus 2.0-3.6%, depending on the solar zenith angle and the SeaWiFS geometry. The major source of error is in the calibration of the ground-based radiometer operated in radiance mode, assumed to be accurate to +/- 2%. The establishment of strict criteria for atmospheric stability, angular geometry, and surface conditions resulted in selection of only 26 days for the analysis during 1999-2000 (Venice site) and 1998-2001 (Lanai site). For these days the measured level-1B radiance from the SeaWiFS Project Office was generally lower than the corresponding simulated radiance in band 8 by 7.0% on average, +/- 2.8%.