Atmospheric Optical Depth Research Articles

Operational Atmospheric Correction of Landsat TM Data

The recent algorithms developed for biophysical variables assessment require accurate surface reflectance measurements. This article describes algorithms used for atmospheric correction of Landsat Thematic Mapper (TM) data. Atmospheric corrections include Rayleigh scattering, gaseous absorption, and aerosol scattering in three visible channels (480 nm, 560 nm, and 660 nm), and the near-infrared channel (830 nm). Atmospheric constituents such as water vapor and ozone are extracted from climatology data sets, while aerosol optical depths (AODs) are derived from the TM scene itself by adopting the dark target approach. The dark target pixels are identified, and their reflectances in the visible channels are estimated using TM Channel 7 (2.1 μm). Atmospheric transmittance and aerosol optical depth are derived for 16 grid points equally distributed over the scene, then interpolated to match the TM spatial resolution. This technique considerably reduces the computing time without decreasing the accuracy. These algorithms were tested using 11 TM scenes over a wide variety of sites, including forest, crop, and semiarid areas. The AOD in the blue, green, and red channels retrieved using the dark target technique was validated using sunphotometer measurements. The absolute error associated with AOD assessment was less than 0.15. A statistical analysis was also conducted to evaluate the atmospheric correction method. Based on data from the FIFE (First ISLSCP Field Experiment) experiment, the absolute error between ground measurements and TM reflectance was less than 0.015 in the visible channels, and less than 0.08 in the near-infrared channel.

Fast radiative transfer model for simulation of infrared atmospheric sounding interferometer radiances

A fast radiative transfer model has been developed for prelaunch simulation studies of Infrared Atmospheric Sounding Interferometer (IASI) data and for the exploitation of IASI radiances within the framework of a numerical weather prediction variational analysis scheme. The model uses profile-dependent predictors to parameterize the atmospheric optical depths and is fast enough to cope with the processing of observations in near real time and with the several thousands of transmittance calculations required to simulate radiances from a full range of atmospheric conditions. The development of the model has involved the selection of a training set of atmospheric profiles, the production of a line-by-line transmittance database, the selection of optimal predictors for the gases considered in the study, and the production of regression coefficients for the fast transmittance scheme. The model fit to the line-by-line radiances shows that it can reproduce the line-by-line radiances to a degree of accuracy that is at or below the instrumental noise.

The interrelationship of atmospheric correction of reflectances and surface BRDF retrieval: a sensitivity study

This paper systematically studies the interrelationship between surface bidirectional reflectance distribution function (BRDF) retrieval and atmospheric correction. The study uses the atmospheric correction scheme of the Moderate Resolution Imaging Spectroradiometer (MODIS) and angular sampling expected for MODIS and the Multiangle Imaging SpectroRadiometer (MISR) for different land cover types and optical depths of aerosols. The results show the following two points. 1) Even for a nonturbid atmosphere, the assumption of a Lambertian surface in atmospheric correction causes relative errors in the retrieved surface reflectances that average from 2 to 7% in the red and near-infrared bands, with worst cases showing errors of up to about 15% for turbid conditions. Consequently, it is necessary for improved accuracy to consider surface anisotropy in atmospheric correction. 2) Surface BRDF retrieval and atmospheric correction can be coupled in a converging iteration loop that improves the quality of atmospheric correction and subsequent BRDF retrievals. For example, performing two steps of the iteration loop is already sufficient to obtain mean relative errors of less than 1% in the retrieved surface reflectances even for an atmospheric aerosol optical depth of 0.4. As BRDF retrieval accuracies improve, so do bihemispherical albedo retrieval accuracies, with mean relative errors being 1-5% when using a Lambertian assumption and less than 1% after two iteration steps.

Mount Mazama eruption: Calendrical age verified and atmospheric impact assessed

Geochemical identification of Mount Mazama ash in the Greenland Ice Sheet Project 2 (GISP2) ice core gives a calendrical age of 7627 ± 150 cal yr B.P. (5677 ± 150 B.C.) for the eruption, thus providing a more accurate early Holocene stratigraphic time line than previously available. The GISP2 record of volcanically derived sulfate suggests a total stratospheric aerosol loading between 88 and 224 Mt spread over an ∼6 yr period following the eruption of Mount Mazama. Taking into account the likelihood of some tropospheric aerosol transport to Greenland, realistic estimates of the resulting atmospheric optical depth range from 0.6 to 1.5. These values may have produced a temperature depression of ∼0.6 to 0.7 °C at mid to high northern latitudes for 1–3 yr after the eruption. These results indicate that the 5677 B.C. eruption of Mount Mazama was one of the most climatically significant volcanic events of the Holocene in the Northern Hemisphere. We also calculate a maximum stratospheric Cl − release of 8.1 Mt by the eruption, which may have led to substantial stratospheric ozone depletion.

Measurements of nighttime atmospheric optical depth preliminary data from a mountain site in southern Italy

A grating spectrometer equipped with a photomultiplier was used to measure atmospheric optical depth with the moon as an extraterrestrial light source. Measurements are presented in the window regions in the spectral range 0.32–0.64 μm, with a resolution of about 3 nm. The Langley method was applied to estimate atmospheric optical depth. Measurements have been performed at a mountain site in southern Italy, at 1247 m above sea level in the context of the Gamma-ray Air–Cherenkov Multi Telescope experiment.

Causes of atmospheric blur: comment on Atmospheric scattering effect on spatial resolution of imaging systems

A paper by Ben Dor [J. Opt. Soc. Am. A14, 1329 (1997)] concludes that the blur we measured in our experiments was not atmospherically scattered light and that our theoretical model is incorrect because it violates the rules of linearity. Their work is based in part on “lack of raw data” in one of our experimental papers [J. Opt. Soc. Am A12, 970 (1995)]. We present here the raw data measured in the experiments in question, which show clearly the measured atmospherically scattered light. Similar raw data has also been published elsewhere regarding other experiments. We also clarify some rules of linear systems that justify our conceptual approach, which is shown to be similar to that of turbulence modulation transfer function. A review of several dozen experiments and analyses by other investigators all over the world that directly contradict the Ben Dor et al. results and conclusions is presented. The well-known significance of aerosol blur in imaging through the atmosphere from satellites is discussed, and pictorial examples of satellite imagery are shown for different atmospheric optical depths. It is noted that atmospheric point-spread-function analyses in the remote-sensing literature generally neglect turbulence blur altogether and deal with aerosol blur only, which is often called the adjacency effect, and that such phenomena are well supported by many different types of experiments and many different Monte Carlo simulations for many different aerosol and instrumentation parameter situations. The Monte Carlo simulation results of Ben Dor et al. are shown also to contradict everyday reality such as the solar aureole. This wealth of literature by others strongly contradicts the results of Ben Dor et al. and confirms our conclusion that forward scatter of light by aerosols is indeed a significant source of blur in imaging through the atmosphere, especially if atmospheric optical depth is on the order of unity or more. This can be confirmed, too, by any observer looking through binoculars at the moon and surrounding moonlight even on a clear night. A broad system engineering approach involving both aerosol and turbulence blur is called for.

Simulation of solar cells utilization on the surface of Mars

Results concerning the simulation of solar cells operation on Mars surface are reported in this article. PV arrays based on silicon and gallium arsenide solar cells are analyzed under the assumption of both ideal and present state-of-the-art technology. Measurements performed by the Viking Landers are used as inputs. They refer to ambient temperature, wind speed and the optical depth of the Martian atmosphere. The incoming global solar radiation flux (direct, diffuse and ground reflected) is computed by using a model previously proposed by the author. A thermodynamic model of solar cell is developed. Simulations were performed for both Viking Landers 1 and 2 sites during all the four Martian seasons. Information about the optimum voltage and cell temperature is provided as well as details about the electric power supply and cell efficiency. Different sorts of stationary and tracking solar cells arrays are studied. The PV efficiency is obviously higher than that obtained under terrestrial conditions due to the lower ambient temperature on Mars. One confirms that the low operating temperatures will reduce the advantage in efficiency of GaAs solar cells over silicon. The strategy to be chosen for maximum power supply mainly depends on the atmospheric optical depth.

Equivalence of pyrheliometric and monochromatic aerosol optical depths at a single key wavelength

The atmospheric aerosol optical depth (AOD) weighted over the solar spectrum is equal to the monochromatic AOD at a certain wavelength. This key wavelength is ~0.7 mum, which is only slightly influenced by air mass and aerosol content. On the basis of this result, simple relations are proposed to predict monochromatic AOD from pyrheliometric data and vice versa. The accuracy achieved is close to ?0.01 units of AOD at ~0.7 mum, estimated from simultaneous sunphotometer data. The precision required for the estimation of the precipitable water-vapor content is approximately ?0.5 cm.

Effects of atmospheric optical depth on data acquired in the visible and near infrared bands of landsat TM

The effects of atmospheric optical depth on the satellite image characteristics have been studied using a Landsat TM image coupled with an atmospheric model so as to simulate images at the top of the atmosphere (TOA) for different atmospheric aerosol contents. The statistical properties of the simulated TOA image differs from that of the original image and aerosol content influences significantly the classification of surface features using satellite sensor images. With the increase of optical depth standard deviation of the image decreases appreciably in the two bands. In both the bands, the immediate effect of atmospheric scattering is to reduce the contrast between different surface types. For a given aerosol content, the different surface features are not equally affected. In TM band 1, scattering shifts the gray levels of the image towards higher values for soil, vegetation and water whereas, in TM band 4, gray levels are shifted towards lower value for soil and vegetation and towards higher values for water.

A method for spaceborne synthetic remote sensing of atmospheric aerosol optical depth and vegetation reflectance

Spaceborne synthetic remote sensing of atmospheric aerosol optical depth and vegetation reflectance is very significant, but it remains to be a question unresolved yet. Based on the property of vegetation reflectance spectra from near ultra-violet to near infrared and the sensitivity of outgoing radiance to vegetation reflectance and atmospheric aerosol optical depth, a new method for spaceborne synthetic remote sensing of the reflectance and the depth is proposed, and an iteration-correlation inversion algorithm is developed in this paper. According to numerical experiment, effects of radiance error, error in aerosol imaginary index and vegetation medium inhomogeneity on retrieved result are analyzed. Inversion results show that the effect of error in aerosol imaginary index is very important. As the error of aerosol imaginary index is within 0.01, standard errors of aerosol optical depth and vegetation reflectance solutions for 14 spectral channels from 410 nm to 900 nm are respectively less than 0.063 and 0.063 and 0.023. And as the radiance error is within 2%, the standard errors are less than 0.023 and 0.0056.

Investigation of the vertical profile of the aerosol scattering in the atmosphere by multichannel data from space station MIR

An approach is suggested and applied to determine the atmospheric optical depth parameters from spectral solar irradiance data provided by the multichannel trace spectrometric system “Spectrum 256”. The system “Spectrum 256” has been designed by scientists from the Solar-Terrestrial Influences Laboratory at the Bulgarian Academy of Sciences and has been operated onboard the MIR space station since 1988. By scanning the atmosphere with the spectrometer pointed directly at the Sun as the MIR station passed from the terminator zone to the sunset zone, the distribution of the atmospheric optical depth components in a vertical column of the Earth's troposphere in the layer between 2 km and 14 km was determined. The molecular (Rayleigh) scattering was modeled and the aerosol scattering model parameters were evaluated. The selective absorption due to gaseous components and water vapor in the 475–810 nm spectral range was determined, taking into account the influence of the optical transmission characteristics of the hatches of the MIR station.

Simultaneous remote sensing of aerosol optical depth and surface reflectivity—Principle and numerical simulation

Based on radiative-transfer model calculation, it is found that except for very heavy loading of atmospheric aerosols, the spectral reflectance observed by space-borne sensors could be expressed as the function of aerosol optical depth and surface spectral reflectances in some simple forms. On the other hand, the surface spectral reflectances for different surface types have their respective characteristic patterns, which will be used to relate spectral reflectance of one wavelength to others. In this paper, we will discuss the feasibility of simultaneous remote sensing of atmospheric aerosol spectral optical depth and surface spectral reflectances with space-borne spectral sensor. Preliminary numerical simulation showed that it is possible, at least for some simple cases.

Air mass parameterization and coastal aerosol modeling

The United States Navy is increasingly concerned with operations in coastal environments. One key threat to ships is the easily launched high precision anti-ship weaponry readily available in the world-arms market. Fleet units operating in either the open ocean or in coastal regions must be able to determine the standoff ranges at which they can detect and/or track such sophisticated weaponry. The Navy is presently developing and utilizing infrared (IR) technology for the detection and identification of such threats. Atmospheric aerosol extinction, refraction, turbulence and infrared sea and terrain background models in LOWTRAN and MODTRAN are presently inadequate for representing IR propagation close to the ocean surface and in coastal environments. This requires accurate models to describe the effects of low level ocean and coastal aerosols to be incorporated into LOWTRAN and MODTRAN for determining the performance of EO systems used for detecting these low-altitude targets.Not much is known concerning the coastal air mass characteristics required for advanced aerosol modeling. Since the diverse air mass transitions that frequent coastal environments may have profound effects upon aerosol properties and EO propagation, a good assessment of the NAM/NOVAM air mass parameter for coastal applications is essential. The NAM/NOVAM dN/dr aerosol size distribution model consists of three log normal distributions, the first component being scaled proportionate to the square of the air mass parameter (AMP). Therefore, when using NAM/NOVAM for calculating extinction coefficients and optical depths in the MBL, the AMP is the most critical input parameter and is, of course, the most difficult to obtain. Several techniques are being evaluated or utilized to obtain this parameter including air mass trajectory analysis, radon, condensation nuclei, and multiple wavelength nephelometer measurements. All of these techniques are in-situ measurements for a given geographic location. When considering the complexity and variability of the air mass characteristics within a coastal environment, an alternate remote sensing technique for determining the AMP must be determined for NAM/NOVAM to be applicable for coastal application. The purpose of this paper is to present an alternative approach for determining the surface AMP from satellite upwelling radiance measurements, LOWTRAN, and the NOVAM aerosol model.Using satellite detected upwelling radiance measurements, multiple wavelength satellites, such as the NOAA -14 AVHRR, can be used to determine the total atmospheric aerosol optical depth (ODSat) and its geographical variability. Utilizing this satellite OD data base and the LOWTRAN calculated OD (ODLOWTRAN) for tropospheric aerosol scatter above the marine boundary layer (MBL), the contribution to the total aerosol ODSat by the boundary layer (ODMBL) for a geographic location can be determined by subtracting these two values, e.g., ODMBL = ODSat - ODLOWTRAN. The AMP associated with this OD (ODMBL) can then be related to the NAM/NOVAM aerosol model by exercising NAM/NOVAM as a function of AMP until the NAM/NOVAM calculated OD (ODNOVAM) agrees with that required from the satellite observation for the MBL. The resulting AMP represents that required as an input to NAM/NOVAM for predicting the IR propagation characteristics within the MBL as a function of wavelength for a specific geographic location and time.

Retrieving land surface reflectances using the ATSR‐2: A theoretical study

The measurement of absolute land reflectance from Earth‐orbiting satellites requires the separation of the signal due to the reflection of solar radiation by the surface from that due to scattering by the atmosphere. Thus for accurate but generally applicable quasi‐operational retrieval techniques it is necessary to derive atmospheric parameters, such as the aerosol type and loading, simultaneously with the surface reflectances. The assumption of large‐scale horizontal homogeneity across an image, or the presence of very low reflectance pixels within an image, is avoided, since these are not necessarily applicable in the general case. In this paper, results of simulated retrievals using the visible and near‐infrared channels of the ATSR‐2 satellite radiometer are presented on the basis of techniques which exploit the dual look and other features of the instrument. The pixel‐based retrieval scheme comprises a fast representation of atmospheric scattering and a simple surface reflectance approximation. The effect of the atmospheric approximation on the accuracy of retrieval of the surface reflectance and atmospheric optical depth is tested by using a full multiple‐scattering radiative transfer model with simpler surface model. The effect on retrieval accuracy of the surface model is tested using a full surface vegetation canopy reflectance model with inclusion of the hot‐spot effect, combined with a successive orders of scattering atmospheric radiative transfer model, taken to second order. The results indicate that the scheme could provide accurate retrieval of surface reflectances even where no extraneous information on the surface bidirectional reflectance or on the atmospheric optical thickness is available. The scheme is also applicable to the AATSR instrument to be launched on the ENVISAT (Environmental Satellite) and, more widely, to other satellite radiometers with multiple views of the Earth's surface.

Spectral solar radiation measurements and turbidity: Comparative studies within a tropical and a sub-arctic environment

The effects of atmospheric turbidity due to aerosols on the direct solar irradiance at the surface at a low latitude (Ile-Ife (7°30′N, 4°31′E), south-western Nigeria) and a high latitude station (Abisko (68°21′N, 18°49′E), northern Sweden) before and after the Pinatubo eruption are compared. The Pinatubo aerosols increased the total atmospheric optical depths by a factor between 2 and 4 at Abisko and also had a drastic effect on spectral distribution of the optical depths. Under intense harmattan conditions, the atmospheric turbidity at Ile-Ife is about 7 times higher than that in the Arctic region, even with the significant effect of the Pinatubo volcanic eruption on the latter. The application of the Ångström formula for approximating the aerosol spectral extinction is discussed for the cases concerned.

Atmospheric optical depth and water vapour effects on the angular characteristics of surface reflectance in NOAA AVHRR

The effects of atmospheric optical depth and water vapour content on the bidirectional surface reflectance in channel 1 (visible) and channel 2 (near-infrared) of NOAA AVHRR have been analysed using a coupled surface atmosphere reflectance model. Two different cases of surface : (i) bare soil, and (ii) vegetation cover have been considered. In the case of bare soil, both the amplitude and angular distribution of the bidirectional reflectance of the surface are modified at satellite altitude due to scattering caused by atmospheric molecules and aerosols in the two channels and thereby, the directional properties of the surface are smoothed. Whereas, in the case of lawn, in channel 1, the angular variation of surface reflectance is enlarged together with a large augmentation in reflectance amplitude, and in channel 2, a small reduction in amplitude as well as a variation in angular distribution of reflectances are caused due to scattering particularly over large viewing angles and thereby, the directional variations are smoothed. In channel 1, atmospheric scattering reduces the contrast between the soil and vegetation and is very much significant for medium to high aerosol loadings. Atmospheric water vapour reduces the amplitude of the surface bidirectional reflectance without introducing any significant changes in angular distribution of the surface reflectance for both bare soil and vegetation canopy in channel 2.

Nephelometer derived and directly measured aerosol optical depth of the atmospheric boundary layer

The aerosol optical depth of the atmospheric boundary layer was determined both from direct solar irradiance measurements and from vertical extrapolation of ground-based nephelometry, during a period with cloudless skies and high aerosol mass loadings in the Netherlands. The vertical profile of the aerosol was obtained from lidar measurements. From humidity controlled nephelometry at the ground and humidity profiles from soundings, the scattering aerosol extinction as a function of height was assessed. Integration of the extinction over the aerosol layer gave the aerosol optical depth of the atmospheric boundary layer. This optical depth at the narrow band of the nephelometer was translated to a spectrally integrated value, assuming an Angstrom wavelength exponent of 1.5, a typical value for The Netherlands. It was found that scattering by the boundary layer aerosol contributed on average 80% to the total atmospheric aerosol optical depth. The uncertainty in this value is estimated to be of the order of 13%. Ammonium nitrate dominated the light scattering. This is an anthropogenic aerosol component. The radiative forcing caused by the light scattering of the anthropogenic aerosol was calculated assuming an upward scattered fraction of 0.3. An average value of − 12 W m −2 was found (with an estimated uncertainty of 20%). This corresponds to an absolute increase in the planetary albedo of 0.03, which is equivalent to a 15% increase in the local planetary albedo of 0.2.

Aerosol optical depth over Canada and the link with synoptic air mass types

Aerosol optical depth measurements acquired through the Canadian sunphotometer network were statistically analyzed for the 1987–1992 period in order to investigate spatial and temporal commonalities between the member stations. Four stations were chosen to yield a spatially representative sampling of atmospheric optical conditions across Canada (East Coast, eastern continental, western continental, and West Coast). All the stations were located in rural, local pollution free areas. The results of aerosol optical depth measurements showed significant differences between eastern and western Canadian stations. The effect of the Pinatubo volcanic eruption was clearly seen in the measurements acquired at Sable Island, Nova Scotia. Air mass relationships for the four stations sampled demonstrated the relevance of applying air mass classification criteria to the analysis and discrimination of atmospheric optical depth. Knowledge of the seasonal trend combined with information concerning air mass type enables a coarse a priori estimation of aerosol optical depth in the absence of traditional optical data. Synoptical air mass analysis facilitates the understanding of the mechanisms involved in the seasonal variations of aerosol optical depth and yields useful information about the atmospheric optical state. The relevance of the synoptical air mass approach was demonstrated in one particular case: a seasonal aerosol optical depth trend for Arctic air masses was observed for the three sunphotometer stations which regularly experience this type of air mass.

Impact of Pinatubo aerosols on the seasonal trends of global, direct and diffuse irradiance in two northern mid-latitude sites

In June 1991, Mt Pinatubo's eruption in the Philippines ejected a staggering 20 million metric tons of SO 2 into the stratosphere which resulted in an aerosol cloud covering most of the Earth within a few months after the eruption. In this article we illustrate how the seasonal trends of global, direct and diffuse solar radiation were modified by the eruption in two mid-latitude sites in Europe and the U.S., totalling about 12 yr of hourly data. A spectacular increase in the diffuse fraction of solar radiation as opposed to a decrease in direct radiation which extended from late 1991 to mid 1993 was observed in both sites and for clear sky conditions. Global radiation was not altered significantly from these data sets. Sunphotometer measurements in one of the sites show the volcanic aerosols tripled the total atmospheric aerosol optical depth at 1 μm and approximately doubled the aerosol optical depth at 0.5 μm.

Sun photometry using photodiode open circuit voltage measurements

A simple technique is explored in which the open circuit voltage of a photodiode is measured using a simple digital multimeter to monitor the solar irradiance and atmospheric optical depths in three spectral bands. The advantage of this technique is that the photometer only consists of an optical interference filter, photodiode, and a measuring digital multimeter, while the electronics circuitry otherwise needed in a conventional photometer is avoided. This makes the system very simple, cost effective, and versatile for even nonprofessionals to handle.