Aerosol Optical Thickness Retrieval Research Articles

Angular and seasonal variation of spectral surface reflectance ratios: implications for the remote sensing of aerosol over land

We obtain valuable information on the angular and seasonal variability of surface reflectance using a hand-held spectrometer from a light aircraft. The data is used to test a procedure that allows us to estimate visible surface reflectance from the longer wavelength 2.1 /spl mu/m channel (mid-IR). Estimating or avoiding surface reflectance in the visible is a vital first step in most algorithms that retrieve aerosol optical thickness over land targets. The data indicate that specular reflection found when viewing targets from the forward direction can severely corrupt the relationships between the visible and 2.1 /spl mu/m reflectance that were derived from nadir data. There is a month by month variation in the ratios between the visible and the mid-IR, weakly correlated to the Normalized Difference Vegetation Index (NDVI). If specular reflection is not avoided, the errors resulting from estimating surface reflectance from the mid-IR exceed the acceptable limit of /spl Delta//spl rho//spl sim/0.01 in roughly 40% of the cases, using the current algorithm. This is reduced to 25% of the cases if specular reflection is avoided.

Comparison between satellite retrieved aerosol optical thickness and results obtained from the LOTOS model

Aerosols affect the Earth radiation balance by absorbing and scattering solar radiation and changing the albedo and the lifetime of clouds. Aerosols also play a role in health-related problems. The aerosol lifetime varies from a few days 10 about a week and their quite strong but rather Iocalised sources produce highly variable aerosol concentrations in space and time. Satellite remote sensing can provide the spatial and temporal resolution to monitor such variations, However, usually only total column-integrated aerosol properties are obtained that are useful for, e.g., climate applications. For other purposes, the satellite data interpretation requires the use of a transport model. Therefore sulphate and nitrate aerosols are introduced in the LOTOS model (Builtjes, 1992) and preliminary results are used to interpret the relative contribution of these aerosol types to the observed aerosol optical thickness (AOT)

Comparison of model‐estimated and measured diffuse downward irradiance at surface in cloud‐free skies

Diffuse downward shortwave irradiance at the surface arises from the scattering of radiation by molecules and aerosol particles. Recently, we reported that pyranometer‐measured diffuse solar irradiance in cloud‐free atmospheres is overestimated by radiative transfer models at two low‐altitude sites by an amount that exceeds modeling and measurement uncertainties but is correctly estimated within these uncertainties at two high‐altitude sites [Halthore et al., 1998]. Here we explore this phenomenon in detail, with more cases and improved uncertainty analysis, confirming that the excess in modeled diffuse irradiance cannot be explained by uncertainties in measurements or aerosol‐scattering properties that are input into the radiative transfer models or by errors in multiple‐scattering schemes. The phenomenon is observed for all comparisons with data obtained intermittently over a 5‐year period at the low‐altitude sites. Model computations also exceed radiometer‐measured sky radiance along the solar almucantar. Despite the inconsistencies between measured and modeled diffuse irradiance, atmospheric transmittance models correctly compute direct normal solar irradiance at all sites. These results indicate that at low altitudes a continuum atmospheric absorption process accounting for 0.022±0.01 in vertical optical thickness at 550 nm, corresponding to 4±2% absorptance, may need to be included in radiative transfer models and in models that retrieve aerosol optical thickness from extinction measurements. This is a substantial excess absorption with major implications for climate modeling and weather forecasting. In remote sensing studies, neglect of this excess absorption would lead to substantial errors in satellite sensor calibration and satellite inferred top‐of‐atmosphere flux. An agent or process for this absorption has not yet been identified.

Comparisons of LASE, aircraft, and satellite measurements of aerosol optical properties and water vapor during TARFOX

We examine aerosol extinction and optical thickness from the Lidar Atmospheric Sensing Experiment (LASE), the in situ nephelometer and absorption photometer on the University of Washington C‐131A aircraft, and the NASA Ames Airborne Tracking Sun Photometer (AATS‐6) on the C‐131 A measured during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) over the east coast of the United States in July 1996. On July 17 and 24 the LASE profiles of aerosol extinction and aerosol optical thickness (AOT) had a bias difference of 0.0055 km−1 (10%) and a root‐mean‐square difference of 0.026 km−1 (42%) when compared to corresponding profiles derived from the airborne in situ data when the nephelometer measurements are adjusted to ambient relative humidities. Larger differences for two other days were associated with much smaller aerosol optical thicknesses (July 20) and differences in the locations sampled by the two aircraft (July 26). LASE profiles of AOT are about 10% higher than those derived from the airborne Sun photometer, which in turn are about 10–15% higher than those derived from the airborne in situ measurements. These differences are generally within the error estimates of the various measurements. The LASE measurements of AOT generally agree with AOT derived from both the Along‐Track and Scanning Radiometer 2 (ATSR 2) sensor flown on the European Remote Sensing Satellite 2 (ERS‐2) and from the Moderate‐Resolution Imaging Spectroradiometer (MODIS) airborne simulator (MAS) which flew with LASE on the NASA ER‐2 aircraft. Effective particle sizes derived from the MAS data indicate that the LASE retrievals of AOT are valid for effective particle radii less than 0.4 μm. Variations in the relative humidity derived from the LASE water vapor measurements on July 26 are found to be highly correlated with variations in the effective particle size derived from the MAS.

Comparison of aerosol optical thickness retrieval from spectroradiometer measurements and from two radiative transfer models

The spectral values of the aerosol optical thickness τ a λ in the 400–670 nm band have been determined from 500 solar direct irradiance spectra at normal incidence registered at Valencia (Spain) in the period from July 1993 to March 1997. The τ a λ values obtained from experimental measurements have been compared with the boundary layer aerosol models implemented in the radiative transfer codes ZD-LOA and LOWTRAN 7. For the ZD-LOA code, the continental and maritime models have been considered and for the LOWTRAN 7 code the rural, maritime, urban and tropospheric models have been used. The obtained results show that the aerosol model that best represents the average turbidity of the boundary layer for the urban area of Valencia (Spain) is the continental model when the ZD-LOA code is used and the urban model when the LOWTRAN 7 code is used.

Path radiance technique for retrieving aerosol optical thickness over land

The key issue in retrieving aerosol optical thickness over land from shortwave satellite radiances is to identify and separate the signal due to scattering by a largely transparent aerosol layer from the noise due to reflection by the background surface, where the signal is relatively uniform compared to the highly inhomogeneous surface contribution. Sensitivity studies in aerosol optical thickness retrievals reveal that the apparent reflectance at the top of the atmosphere is very susceptible to the surface reflectance, especially when aerosol optical thickness is small. Uncertainties associated with surface reflectance estimation can greatly amplify the error of the aerosol optical thickness retrieval. To reduce these uncertainties, we have developed a “path radiance” method to retrieve aerosol optical thickness over land by extending the traditional technique that uses the “dark object” approach to extract the aerosol signal. This method uses the signature of the correlation of visible and middle‐IR reflectance at the surface and couples the correlation with the atmospheric effect. We have applied this method to a Landsat TM (Thematic Mapper) image acquired over the Oklahoma southern Great Plains site of the Department of Energy Atmospheric Radiation Measurement (ARM) program on September 27, 1997, a very clear day (aerosol optical thickness of 0.07 at 0.5 μm) during the first Landsat Intensive Observation Period. The retrieved mean aerosol optical thickness for TM band 1 at 0.49 μm and band 3 at 0.66 μm agree very well with the ground‐based Sun photometer measurements at the ARM site. The ability to retrieve small aerosol optical thickness makes this path radiance technique promising. More importantly, the path radiance is relatively insensitive to surface inhomogeneity. The retrieved mean path radiances in reflectance units have very small standard deviations for both TM blue and red bands. This small variability of path radiance further supports the current aerosol retrieval method.

Retrieval and monitoring of aerosol optical thickness over an urban area by spaceborne and ground-based remote sensing

We used an instrumental synergy of both ground-based (sunphotometer) and spaceborne [POLDER (polarization and directionality of the Earth's reflectances) and Meteosat] passive remote-sensing devices to determine the aerosol optical thickness over the suburban area of Thessaloniki, Greece, from April 1996 to June 1997. The POLDER spaceborne instrument measures the degree of polarization of the solar radiance reflected by the Earth-atmosphere system. Aerosol optical thickness (AOT) retrieval needs an accurate estimate of the contribution of the ground surface to the top of atmosphere's polarized radiance. We tested existing surface reflectance models and fitted their parameters to find the best model for the Thessaloniki area. The model was constrained and validated by use of independent data sets of coincident sunphotometer and POLDER measurements. The comparison indicated that the urban AOT over Thessaloniki was retrieved by the POLDER instrument with an accuracy of +/-0.05. From analysis of Meteosat data we found that approximately 40% of the days with high AOT (>0.18) are associated with African dust transport events, all observed in the period March-July. Excluding dust events, the 15-month AOT averages 0.12 +/- 0.04. During the 15-month period that the study was conducted, we observed aerosol pollution peaks with an AOT of >0.24 on 15 of the 164 days on which measurements were possible.

Retrieval of aerosol optical thickness by airborne sun photometer measurements during LACE'98

Retrieval of aerosol optical thickness by airborne sun photometer measurements during LACE'98

RETRIEVAL OF AEROSOL OPTICAL THICKNESS BY MEANS OF THE LEAST-MEDIAN-SQUARES ROBUST ALGORITHM

Atmospheric aerosols play an active role in the atmospheric radiative energy transfer by interacting with the solar energy through light scattering and/or absorption. Such an interaction has the potential to counter balance or enhance the warming caused by the greenhouse gases (e.g. carbon dioxide, water vapor, and methane). The interactions would depend on a variety of factors such as the microphysics and chemistry of aerosol particles, and the environmental conditions. To model the energy transfer processes accurately, several aerosol parameters such as aerosol optical thickness (AOT), single scattering albedo, scattering phase function, refractive index, and chemical composition of aerosols have to be known. In this report, a robust method for aerosol optical thickness is presented. The method is based on the least median squares (LMS) regression technique. The LMS-based technique is fundamentally different from the traditional least-squares (LS) technique commonly used today to retrieve AOT. The LMS technique was found to resist influential outliers and sustain the impacts of outliers much better than the LS in our application. The outlier-resistance property is an important design consideration for an automatic AOT retrieval algorithm. We demonstrated the strength of the new technique by using the shortwave irradiance measurements taken by one of the Multi-filter Rotating Shadow-band Radiometers installed at the Southern Great Plains in Oklahoma, U.S.A., one of the three sites operated by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program. The data cover a time period from September 1995 to May 1997. The LMS-retrieved mean top-of-atmosphere solar irradiance values (i.e. I 0) were stable based on the one-tailed Student’s t tests. It was also found that the difference between the 75th and 25th percentiles (a non-parametric robust estimate of variation) of I 0 over the 2-year period was 0.2 for the 499-nm channel, and 0.05 for the 860-nm channel. The monthly averaged AOT values over the 2-year period were from 0.05 to 0.30 for the 499-nm channel and from 0.03 to 0.15 for the 860-nm channels. AOT values peaked between May and September and reached minima in between November and January. The synoptic flow from the North might have contributed to the low level of AOT values during the wintertime, while the southerly flow, traversing through more industrial areas before reaching the site, could have contributed to the influx of particles during the summertime.

Validation of the first algorithm applied for deriving the aerosol properties over the ocean using the POLDER/ADEOS measurements

Global map of aerosol parameters (optical thickness, Angstrom exponent, and refractive index) are derived from the POLDER instrument on board the ADEOS-1 platform. This paper focuses on aerosol optical thickness and Angstrom exponent retrievals that are based on visible and near infrared back scattering measurements. Assessment of the retrieval quality is achieved by means of comparison with AERONET sunphotometer data. The results show that the POLDER measurements can be used to distinguish several aerosol types from their Angstrom coefficients, in addition to a precise estimate of the aerosol optical thickness.

Retrieval of aerosol optical thickness and size distribution over ocean from the MODIS airborne simulator during TARFOX

Radiation and in‐situ measurements collected during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) are used to test the method for remote sensing of aerosol properties and loading from the MODIS instrument. MODIS, a Moderate Resolution Imaging Spectroradiometer, will be launched in 1999 aboard the first EOS (Earth Observing System). Following the MODIS procedure [Tanré et al, 1997], the spectral radiance at the top of the atmosphere (TOA) measured over the ocean in a wide spectral range (0.55–2.13 μm) is used to derive the aerosol optical thickness (proportional to the aerosol total loading) and the aerosol size distribution (integrated over the vertical column) of the ambient (undisturbed) aerosol by comparing measured radiances with values in look‐up table (LUT). The LUT includes the gas‐phase oxidation accumulation mode, cloud‐phase accumulation mode, and a coarse mode that represents maritime particles (salt) and dust. In each inversion, one accumulation and one coarse mode can be retrieved. The inversion retrieves the ratio of the contribution to the optical thicknesses of the two particle modes and the mean particle size that best fits the measurements. This algorithm is successfully applied to the data sets acquired during TARFOX. The MODIS airborne simulator (MAS) aboard the NASA ER‐2 aircraft flew several times during the experiment above the University of Washington C‐131A research aircraft on which the six‐channel Ames Airborne Tracking Sun Photometer (AATS‐6) was mounted. It flew also above surface‐based Sun photometers. Optical thicknesses (at λ = 550 nm) as well as the spectral dependence from the various data sets compare very well.

Remote sensing of smoke from MODIS airborne simulator during the SCAR‐B experiment

MODIS (moderate resolution imaging spectroradiometer) airborne simulator (MAS) data acquired during the SCAR‐B (Smoke, Clouds, and Radiation‐Brazil) experiment provide a test bed to evaluate aerosol retrievals for MODIS measurements over land. The SCAR‐B MAS data covered forest and cerrado regions with varying smoke concentration as a result of fires in August–September, 1995. Excellent agreement is obtained by comparing the retrieved aerosol optical thickness from the MAS data with AERONET (Aerosol Robotic Network) ground‐based Sun photometer observations. The evaluation of MODIS aerosol algorithm was performed for a range of smoke optical thickness from 0.1 to 2 at 0.66 μm wavelength over cerrado and forest sites. All the retrieved values of optical thickness are found within the anticipated retrieval errors of Δτ= ±0.05±0.2τ. This confirms for smoke aerosol the validity of the MODIS algorithms and the use of the new dynamic aerosol models in this algorithm (the algorithm was already validated previously for urban/industrial aerosol). The retrieved optical thickness is found to be very sensitive to the assumed value of the single‐scattering albedo but not sensitive to the aerosol refractive index (real part). The single‐scattering albedo of 0.90 suggested in the aerosol model yields the best results. Test of the MODIS use of a grid box of 10×10 km in the aerosol algorithm shows that the standard deviation of the retrieved aerosol should generally not be more than 10% of the average value for a large range of optical thickness values from 0.2 to 2.0, close and far from fires.

Aerosol optical thickness monitoring in the mediterranean

The Mediterranean atmosphere is often affected by large plumes of desert dust from Africa or the Middle-East. Although satellite monitoring of the aerosol optical thickness (AOT) is being performed from solar radiances recorded over sea-water (Husar et al.. 1997; Moulin et al., 1998). very few data are available on aerosol optical properties in this region. Critical assumptions on aerosol size distribution and refractive index required for satellite AOT retrievals can cause large errors (Ignatov et al., 1995; Moulin et al., 1997). In this work, we present results from ground-based monitoring of AOT. Our purpose is to characterize the marine background aerosol and improve retrieval of African dust optical thickness over the Mediterranean with Meteosat.

Retrieval of atmospheric properties and surface bidirectional reflectances over land from POLDER/ADEOS

Polarization and Directionality of the Earth's Reflectances (POLDER) is a new instrument devoted to the global observation of the polarization and directionality of solar radiation reflected by the Earth‐atmosphere system. It will fly onboard the ADEOS platform in 1996. This paper outlines the improvements expected from POLDER in the description of atmospheric aerosols and water vapor over land, and of surface bidirectional reflectances. It then gives a detailed description of the operational algorithms which are implemented in the “land surface and atmosphere over land” processing line. This line is part of an effort initiated by Centre National d'Etudes Spatiales (the French Space Agency) to develop lines of products in order to facilitate the exploration of POLDER's new capabilities by the international science community. Emphasis is given in this paper to the presentation of the principles, physical rationale, and elements of validation of the algorithms of this processing line. The main products are (1) for each orbit segment, the amount and type of aerosols, the water vapor content, and bidirectional reflectances corrected for atmospheric effects, and (2) every 10 days, global maps of surface directional signatures, of hemispherical surface reflectances, and of parameters describing the statistical distribution of aerosol and water vapor content. These products will be made available to all interested investigators. The most innovative algorithms of the processing line are (1) cloud detection, based on a series of tests involving reflectance thresholds, oxygen pressure estimates, and analysis of polarized radiance in the rainbow direction, (2) retrieval of aerosol optical thickness and type from directional polarized radiance measurements, and (3) retrieval of surface directional signature through an adjustment of a time series of directional reflectance measurements with a semiempirical bidirectional reflectance model.

Modeling phase functions for dustlike tropospheric aerosols using a shape mixture of randomly oriented polydisperse spheroids

Laboratory and in situ measurements show that scattering properties of natural nonspherical particles can be significantly different from those of volume‐or surface‐equivalent spheres, thus suggesting that Mie theory may not be suitable for interpreting satellite reflectance measurements for dustlike tropospheric aerosols. In this paper we use the rigorous T‐matrix method to extensively compute light scattering by shape distributions of polydisperse, randomly oriented spheroids with refractive indices and size distributions representative of naturally occurring dust aerosols. Our calculations show that even after size and orientation averaging, a single spheroidal shape always produces a unique, shape‐specific phase function distinctly different from those produced by other spheroidal shapes. However, phase functions averaged over a wide aspect‐ratio distribution of prolate and oblate spheroids are smooth, featureless, and nearly flat at side‐scattering angles and closely resemble those measured for natural soil and dust particles. Thus, although natural dust particles are, of course, not perfect spheroids, they are always mixtures of highly variable shapes, and their phase function can be adequately modeled using a wide aspect‐ratio distribution of prolate and oblate spheroidal grains. Our comparisons of nonspherical versus projected‐area‐equivalent spherical particles show that spherical‐nonspherical differences in the scattering phase function can be large and therefore can cause significant errors in the retrieved aerosol optical thickness if Mie theory is used to analyze reflectance measurements of nonspherical aerosols. On the other hand, the differences in the total optical cross sections, single‐scattering albedo, asymmetry parameter of the phase function, and backscattered fraction are much smaller and in most cases do not exceed 10%. This may suggest that for a given aerosol optical thickness the influence of particle shape on the aerosol radiative forcing is negligibly small. Spherical‐nonspherical differences in the extinction‐to‐backscatter ratio are very large and should be explicitly taken into account in inverting lidar measurements of dustlike aerosols.

Use of sky brightness measurements from ground for remote sensing of particulate polydispersions

The software code SKYEAD.pack for retrieval of aerosol size distribution and optical thickness from data of direct and diffuse solar radiation is described; measurements are carried out with sky radiometers in the wavelength range 0.369-1.048 µm. The treatment of the radiative transfer problem concerning the optical quantities is mainly based on the IMS (improved multiple and single scattering) method, which uses the delta-M approximation for the truncation of the aerosol phase function and corrects the solution for the first- and second-order scattering. Both linear and nonlinear inversion methods can be used for retrieving the size distribution. Improved calibration methods for both direct and diffuse radiation, the data-analysis procedure, the results from the proposed code, and several connected problems are discussed. The results can be summarized as follows: (a) the SKYRAD.pack code can retrieve the columnar aerosol features with accuracy and efficiency in several environmental situations, provided the input parameters are correctly given; (b) when data of both direct and diffuse solar radiation are used, the detectable radius interval for aerosol particles is approximately from 0.03 to 10 µm; (c) besides the retrieval of the aerosol features, the data-analysis procedure also permits the determination of average values for three input parameters (real and imaginary aerosol refractive index, ground albedo) from the optical data; (d) absolute calibrations for the sky radiometer are not needed, and calibrations for direct and diffuse radiation can be carried out with field data; (e) the nonlinear inversion gives satisfactory results in a larger radius interval, without the unrealistic humps that occur with the linear inversion, but the results strongly depend on the first-guess spectrum; (f) aerosol features retrieved from simulated data showed a better agreement with the given data for the linear inversion than for the nonlinear inversion.

Retrieval of aerosol optical thickness over land using the ATSR‐2 Dual‐Look Satellite Radiometer

We present a new method of retrieving aerosol optical thickness over land that takes advantage of the dual look capability of the ATSR‐2 instrument. Unlike some methods, the approach does not require dense dark vegetation to be located, nor is it dependent upon invariant targets, nor does it assume that the underlying surface reflectance is purely Lambertian. It makes use of a new surface approximation that exploits the property of the surface which is least variable with wavelength to allow atmospheric parameters to be retrieved and, consequently, is applicable over virtually any land surface. The method is tested using simulated ATSR‐2 measurements based upon a radiative transfer model coupled with a surface bidirectional model that includes the hot‐spot effect, and is shown to be robust to the effects of differing underlying canopy types. At high optical depths, the retrieval process achieves a mean error in optical thickness of less than 2.5%, depending on surface type, with a maximum absolute error (for the same data sets) of 5% while at low optical depths errors are of the order of 5%. The effects of systematic error in the simulated radiances are also shown. The method may be readily extended to other multiple look satellite radiometers.

Estimation of aerosol columnar size distribution and optical thickness from the angular distribution of radiance exiting the atmosphere: simulations

We report the results of simulations in which an algorithm developed for estimation of aerosol optical properties from the angular distribution of radiance exiting the top of the atmosphere over the oceans [Appl. Opt. 33, 4042 (1994)] is combined with a technique for carrying out radiative transfer computations by synthesis of the radiance produced by individual components of the aerosol-size distribution [Appl. Opt. 33, 7088 (1994)], to estimate the aerosol-size distribution by retrieval of the total aerosol optical thickness and the mixing ratios for a set of candidate component aerosol-size distributions. The simulations suggest that in situations in which the true size-refractive-index distribution can actually be synthesized from a combination of the candidate components, excellent retrievals of the aerosol optical thickness and the component mixing ratios are possible. An exception is the presence of strongly absorbing aerosols. The angular distribution of radiance in a single spectral band does not appear to contain sufficient information to separate weakly from strongly absorbing aerosols. However, when two spectral bands are used in the algorithm, retrievals in the case of strongly absorbing aerosols are improved. When pseudodata were simulated with an aerosol-size distribution that differed in functional form from the candidate components, excellent retrievals were still obtained as long as the refractive indices of the actual aerosol model and the candidate components were similar. This underscores the importance of component candidates having realistic indices of refraction in the various size ranges for application of the method. The examples presented all focus on the multiangle imaging spectroradiometer; however, the results should be as valid for data obtained by the use of high-altitude airborne sensors.

Nonsphericity of dust‐like tropospheric aerosols: Implications for aerosol remote sensing and climate modeling

The nonsphericity of dust‐like tropospheric aerosols causes us to question the applicability of using conventional Mie theory to compute their radiative properties. In this paper we compare T‐matrix computations of light scattering by polydispersions of randomly oriented nonspherical aerosols and Mie computations for equivalent spheres. We demonstrate that even moderate nonsphericity results in substantial errors in the retrieved aerosol optical thickness if satellite reflectance measurements are analyzed using Mie theory. On the other hand, the use of Mie theory for nonspherical aerosols produces negligible errors in the computation of albedo and flux related quantities, provided that the aerosol size distribution and optical thickness are known beforehand. The first result can be explained by large nonspherical‐spherical differences in scattering phase function, while the second result follows from small nonspherical‐spherical differences in single‐scattering albedo and asymmetry parameter. Our results demonstrate that no cancellation of errors occurs if one consistently uses Mie theory in the retrieval algorithm and then in computing the albedo for the retrieved aerosol optical thickness.

Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with SeaWiFS: a preliminary algorithm

The second generation of ocean-color-analyzing instruments requires more accurate atmospheric correction than does the Coastal Zone Color Scanner (CZCS), if one is to utilize fully their increased radiometric sensitivity. Unlike the CZCS, the new instruments possess bands in the near infrared (NIR) that are solely for aiding atmospheric correction. We show, using aerosol models, that certain assumptions regarding the spectral behavior of the aerosol reflectance employed in the standard CZCS correction algorithm are not valid over the spectral range encompassing both the visible and the NIR. Furthermore, we show that multiple-scattering effects on the algorithm depend significantly on the aerosol model. Following these observations, we propose an algorithm that utilizes the NIR bands for atmospheric correction to the required accuracy. Examples of the dependence of the error on the aerosol model, the turbidity of the atmosphere, and surface roughness (waves) are provided. The error in the retrieved phytoplankton-pigment concentration (the principal product of ocean-color sensors) induced by errors in the atmospheric correction are shown to be <20% in approximately 90% of the cases examined. Finally, the aerosol thickness (τ(α)) is estimated through a simple extension of the correction algorithm. Simulations suggest that the error in the recovered value of τ(α) should be ≲ 10%.