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.     

Retrieval of atmospheric optical parameters from airborne flux measurements: application to the atmospheric correction of imagery

Methodologies that employ auxilliary flux data collected by upward- and downward-looking optical sensors to improve atmospheric corrections of airborne multispectral images are presented and evaluated. Such flux data often are collected in current airborne sensors to produce bidirectional reflectance factor (BRF) images and estimates of hemispherical-hemispherical reflectance. The fact that these images must then be corrected for atmospheric interference raises the question as to whether the auxilliary flux information can be employed to estimate some of the input parameters required by atmospheric correction models. Radiative transfer simulations are employed to demonstrate that the utilization of the downwelling and upwelling fluxes as a means of inferring intrinsic atmospheric optical information can be used to better characterize the local atmosphere and accordingly to improve the atmospheric corrections applied to the apparent BRF images.     

UV-visible absorption spectra of silicon CVD intermediates

Transient absorption in 394-435 nm wavelength range following 193 nm photolysis of disilane has been measured by using cavity ring-down spectroscopy (CRDS). A broad and continuum absorption band was observed. Time profiles of the absorption measured at several wavelengths were similar and found to have at least two components. The decaying part of the absorption can be attributed to Si(H₂)Si based on the kinetic consideration and available information from the literature. The absorption was also measured in the hot wire CVD (HW-CVD) of SiH₄. A broad and continuum band was observed.     

Development of Fourier transform spectrometry for UV-visible differential optical absorption spectroscopy measurements of tropospheric minor constituents

Concentration measurements of trace gases in the atmosphere require the use of highly sensitive and precise techniques. The UV-visible differential optical absorption spectroscopy technique is one that is heavily used for tropospheric measurements. To assess the advantages and drawbacks of using a Fourier transform spectrometer, we built a differential optical absorption spectroscopy optical setup based on a Bruker IFS 120M spectrometer. The characteristics and the capabilities of this setup have been studied and compared with those of the more conventional grating-based instruments. Two of the main advantages of the Fourier transform spectrometer are (1) the existence of a reproducible and precise wave-number scale, which greatly simplifies the algorithms used to analyze the atmospheric spectra, and (2) the possibility of recording large spectral regions at relatively high resolution, enabling the simultaneous detection of numerous chemical species with better discriminating properties. The main drawback, on the other hand, is due to the fact that a Fourier transform spectrometer is a scanning device for which the scanning time is small compared with the total measurement time. It does not have the signal integration capabilities of the CCD or photodiode array-based grating spectrographs. The Fourier transform spectrometer therefore needs fairly large amounts of light and is limited to short to medium absorption path lengths when working in the UV.     

Concurrent multiaxis differential optical absorption spectroscopy system for the measurement of tropospheric nitrogen dioxide

The development of a new concurrent multiaxis (CMAX) sky viewing spectrometer to monitor rapidly changing urban concentrations of nitrogen dioxide is detailed. The CMAX differential optical absorption spectroscopy (DOAS) technique involves simultaneous spectral imaging of the zenith and off-axis measurements of spatially resolved scattered sunlight. Trace-gas amounts are retrieved from the measured spectra using the established DOAS technique. The potential of the CMAX DOAS technique to derive information on rapidly changing concentrations and the spatial distribution of NO2 in an urban environment is demonstrated. Three example data sets are presented from measurements during 2004 of tropospheric NO2 over Leicester, UK (52.62 degrees N, 1.12 degrees W). The data demonstrate the current capabilities and future potential of the CMAX DOAS method in terms of the ability to measure real-time spatially disaggregated urban NO2.     

Inherent optical properties of the ocean: retrieval of the absorption coefficient of chromophoric dissolved organic matter from airborne laser spectral fluorescence measurements

The absorption coefficient of chromophoric dissolved organic matter (CDOM) at 355 nm has been retrieved from airborne laser-induced and water Raman-normalized CDOM fluorescence. Four combined airborne and ship field experiments have demonstrated that (1) the airborne CDOM fluorescence-to--water Raman ratio is linearly related to concurrent quinine-sulfate-standardized CDOM shipboard fluorescence measurements over a wide range of water masses (coastal to blue water); (2) the vicarious calibration of the airborne fluorosensor in units traceable to a fluorescence standard can be established and then maintained over an extended time period by tungsten lamp calibration; (3) the vicariously calibrated airborne CDOM fluorescence-to-water Raman ratio can be directly applied to previously developed shipboard fluorescence-to-absorption algorithms to retrieve CDOM absorption; and (4) the retrieval is not significantly affected by long-path multiple scattering, differences in attenuation at the excitation and emission wavelengths, or measurement in the 180° backscatter configuration. Airborne CDOM absorption measurements will find immediate application to (a) forward and inverse modeling of oceanic water-leaving radiance and (b) validation of satellite-retrieved products such as CDOM absorption.     

Polarized radiance distribution measurements of skylight. I. System description and characterization

A new system to measure the natural skylight polarized radiance distribution has been developed. The system is based on a fish-eye lens, CCD camera system, and filter changer. With this system sequences of images can be combined to determine the linear polarization components of the incident light field. Calibration steps to determine the system 's polarization characteristics are described. Comparisons of the radiance measurements of this system and a simple pointing radiometer were made in the field and agreed within 10 % for measurements at 560 and 670 nm and 25 % at 860 nm. Polarization tests were done in the laboratory. The accuracy of the intensity measurements is estimated to be 10 %, while the accuracy of measurements of elements of the Mueller matrix are estimated to be 2 %.     

吸收式制冷机组专利技术发展综述

吸收式制冷技术是一种常见制冷方式,吸收式制冷机组由于其环保、可利用低品位热能、变负荷容易、运行方便等特点,从上世纪初起就得到了广泛应用,在国内外市场中一直占有重要位置,几十年来,其发展也经历了跌宕起伏的过程。本文从吸收式制冷专利技术角度阐述了近30年来该领域的技术发展趋势,为该领域技术的进一步发展提供参考。     

Retrieval of aerosol properties from combined multiwavelength lidar and sunphotometer measurements

Simulation studies were carried out with regard to the feasibility of using combined observations from sunphotometer (SPM) and lidar for microphysical characterization of aerosol particles, i.e., the retrieval of effective radius, volume, and surface-area concentrations. It was shown that for single, homogeneous aerosol layers, the aerosol parameters can be retrieved with an average accuracy of 30% for a wide range of particle size distributions. Based on the simulations, an instrument combination consisting of a lidar that measures particle backscattering at 355 and 1574 nm, and a SPM that measures at three to four channels in the range from 340 to 1020 nm is a promising tool for aerosol characterization. The inversion algorithm has been tested for a set of experimental data. The comparison with the particle size distribution parameters, measured with in situ instrumentation at the lidar site, showed good agreement.     

Retrieval of phytoplankton size from bio-optical measurements: theory and applications

The absorption coefficient of a substance distributed as discrete particles in suspension is less than that of the same material dissolved uniformly in a medium—a phenomenon commonly referred to as the flattening effect. The decrease in the absorption coefficient owing to flattening effect depends on the concentration of the absorbing pigment inside the particle, the specific absorption coefficient of the pigment within the particle, and on the diameter of the particle, if the particles are assumed to be spherical. For phytoplankton cells in the ocean, with diameters ranging from less than 1 µm to more than 100 µm, the flattening effect is variable, and sometimes pronounced, as has been well documented in the literature. Here, we demonstrate how the in vivo absorption coefficient of phytoplankton cells per unit concentration of its major pigment, chlorophyll a, can be used to determine the average cell size of the phytoplankton population. Sensitivity analyses are carried out to evaluate the errors in the estimated diameter owing to potential errors in the model assumptions. Cell sizes computed for field samples using the model are compared qualitatively with indirect estimates of size classes derived from high performance liquid chromatography data. Also, the results are compared quantitatively against measurements of cell size in laboratory cultures. The method developed is easy-to-apply as an operational tool for in situ observations, and has the potential for application to remote sensing of ocean colour data.     

Effect of aerosol microphysical properties on polarization of skylight: sensitivity study and measurements

We analyze the sensitivity of the degree of linear polarization in the Sun's principal plane as a function of aerosol microphysical parameters: the real and imaginary parts of the refractive index, the median radius and geometric standard deviation of the bimodal size distribution (both fine and coarse modes), and the relative number weight of the fine mode at a wavelength of 675 nm. We use Mie theory for single-scattering simulations and the doubling-adding method with the inclusion of polarization for multiple scattering. It is shown that the behavior of the degree of linear polarization is highly sensitive to both the small mode of the bimodal size distribution and the real part of the refractive index of aerosols, as well as to the aerosol optical thickness; whereas not all parameters influence the polarization equally. A classification of the importance of the input parameters is given. This sensitivity study is applied to an analysis of ground-based polarization measurements. For the passive remote sensing of microphysical and optical properties of aerosols, a ground-based spectral polarization measuring system was built, which aims to measure the Stokes parameters I, Q, and U in the visible (from 410 to 789 nm) and near-infrared (from 674 to 995 nm) spectral range with a spectral resolution of 7 nm in the visible and 2.4 nm in the near infrared. We compare polarization measurements taken with radiative transfer simulations under both clear- and hazy-sky conditions in an urban area (Cabauw, The Netherlands, 51.58 degrees N, 4.56 degrees E). Conclusions about the microphysical properties of aerosol are drawn from the comparison.     

Remote measurements of vertical profiles of atmospheric constituents with a UV-visible ranging spectrometer

A study of the feasibility of retrieving vertical profiles of atmospheric constituents with a new UV-visible ranging spectrometer recently described by R. L. Jones [Optical Methods in Atmospheric Chemistry, U. Platt and H. I. Schiff, eds., Proc. Soc. Photo-Opt. Instrum. Eng. 1715, 393 (1992)] is presented. This instrument resembles a lidar, in that pulses of UV-visible radiation are transmitted vertically upward and backscattered to receiving optics. However, the pulse is a broadband source, and the receiving optics includes a two-dimensional CCD array that allows a series of absorption spectra to be recorded, each corresponding to a different altitude. This allows the simultaneous measurement of the vertical profiles of such atmospheric constituents as O(3), H(2)O, and NO(2) in the troposphere and lower stratosphere. Formal retrieval theory has been used to model the retrieval of vertical profiles with this instrument, demonstrating that it should be possible to obtain profiles at accuracies better than 30% and resolution better than 3 km up to altitudes of 12-15 km. The way in which the measurement error, flash-lamp pulse length, CCD recording interval, and mixing-ratio profile each affect the accuracy and the vertical resolution of the retrieved profile has also been investigated.     

Comparison of airborne lidar measurements with 420 kHz echo-sounder measurements of zooplankton

Airborne lidar has the potential to survey large areas quickly and at a low cost per kilometer along a survey line. For this reason, we investigated the performance of an airborne lidar for surveys of zooplankton. In particular, we compared the lidar returns with echo-sounder measurements of zooplankton in Prince William Sound, Alaska. Data from eight regions of the Sound were compared, and the correlation between the two methods was 0.78. To obtain this level of agreement, a threshold was applied to the lidar return to remove the effects of scattering from phytoplankton.     

Water vapour foreign-continuum absorption in near-infrared windows from laboratory measurements

For a long time, it has been believed that atmospheric absorption of radiation within wavelength regions of relatively high infrared transmittance (so-called 'windows') was dominated by the water vapour self-continuum, that is, spectrally smooth absorption caused by H(2)O--H(2)O pair interaction. Absorption due to the foreign continuum (i.e. caused mostly by H(2)O--N(2) bimolecular absorption in the Earth's atmosphere) was considered to be negligible in the windows. We report new retrievals of the water vapour foreign continuum from high-resolution laboratory measurements at temperatures between 350 and 430?K in four near-infrared windows between 1.1 and 5?μm (9000-2000?cm(-1)). Our results indicate that the foreign continuum in these windows has a very weak temperature dependence and is typically between one and two orders of magnitude stronger than that given in representations of the continuum currently used in many climate and weather prediction models. This indicates that absorption owing to the foreign continuum may be comparable to the self-continuum under atmospheric conditions in the investigated windows. The calculated global-average clear-sky atmospheric absorption of solar radiation is increased by approximately 0.46?W?m(-2) (or 0.6% of the total clear-sky absorption) by using these new measurements when compared with calculations applying the widely used MTCKD (Mlawer-Tobin-Clough-Kneizys-Davies) foreign-continuum model.     

Determination of the concentration of single-walled carbon nanotubes in aqueous dispersions using UV-visible absorption spectroscopy

Stable, homogeneous, aqueous dispersions of single-walled carbon nanotubes (SWNTs) are prepared by nonspecific physical adsorption of surfactants enhanced by sonication. Upon centrifugation, supernatant and precipitate phases are obtained. The initial weights of the SWNTs and the surfactant are divided between these two phases, and the respective SWNT concentration in each phase is unknown. The focus of this work is on the determination of the true concentration of raw, exfoliated HiPCO SWNTs in the supernatant phase. A UV-visible absorption-based approach is suggested for a direct measurement of the SWNT and the surfactant concentration in the supernatant. UV-visible absorbance spectra of SWNTs-surfactant dispersions and surfactants alone reveal that the intensity of a certain peak, attributed to the pi-plasmon resonance absorption, is unaffected by the presence of most surfactants. A calibration plot is then made by monitoring the intensity of the peak as a function of the true concentration of the exfoliated SWNTs. Thus, we are able to determine the unknown concentration of surfactant-dispersed HiPCO SWNTs in the supernatant solution, simply by measuring its optical absorbance. Moreover, we can now calculate the surfactant efficiency in dispersing SWNTs. Cryogenic-transmission electron microscopy and thermogravimetric analysis techniques are used for the characterization of these dispersions and to complement the UV-visible measurements.     

Retrieval of atmospheric profiles from satellite sounder measurements by use of the discrepancy principle

It is known that an infrared or a microwave remote-sensing equation is an integral equation of the first kind. As a result, it is ill-posed, the solution is unstable, and difficulties arise in its retrieval. To make the solution stable, either an a priori error covariance matrix or a smoothing factor gamma is necessary as a constraint. However, if the error covariance matrix is not known or if it is estimated incorrectly, the solution will be suboptimal. The smoothing factor gamma depends greatly on the observations, the observation error, the spectral coverage of channels, and the initial state or the first guess of the atmospheric profile. It is difficult to determine this factor properly during the retrieval procedure, so the factor is usually chosen empirically. We have developed a discrepancy principle (DP) to determine the gamma in an objective way. An approach is formulated for achieving an optimal solution for the atmospheric profile together with the gamma from satellite sounder observations. The DP method was applied to actual Geostationary Operational Environment Satellite (GOES-8) sounder data at the Southern Great Plains Cloud and Radiation Testbed site. Results show that the DP method yields a 21.7% improvement for low-level temperature and a 23.9% improvement for total precipitable water (TPW) retrievals compared with the traditional minimum-information method. The DP method is also compared with the Marquardt-Levenberg algorithm used in current operational GOES data processing. Results of the comparison show significant improvement, 6.5% for TPW and 11% for low-level water-vapor retrievals, in results obtained with the DP method compared with the Marquardt-Levenberg approach.     

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.     

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.