Estimation Of Aerosol Optical Depth Research Articles

Radiative effects of aerosols in sub‐Sahel Africa: Dust and biomass burning

Information on radiative fluxes that reach the ground is needed in numerous areas of climate research. On a global scale, such information is obtainable only from satellites. Top of the atmosphere satellite observations during clear and cloudy sky conditions have been found useful for inferring, among others, information on aerosol optical depth (AOD) and cloud optical depth (COD). These are important elements for estimating surface radiative fluxes. Satellite retrievals of AOD are based on the assumption that the aerosols are of a specific type. In certain climatic regions, dust aerosols and those from biomass burning are injected into the atmosphere simultaneously and are distributed distinctly in the vertical. In this study, it is demonstrated that in such scenarios inaccurate assumption on the vertical distribution of the aerosols can lead to errors in AOD estimates from satellites and, subsequently, in the inferred shortwave (SW) radiative fluxes that reach the surface. The magnitude of such errors can be as high as 80 W m−2. The frequency of these mixed situations has not as yet been documented, and it is possible that on global scale the impact is small. Using available records of observed SW radiative fluxes, their reduction (“dimming”) at numerous locations has been reported. Some relate this reduction to aerosol effects. The findings of this study have implications for the ability to assess such “dimming” from satellites in areas where the vertical structure of aerosol distribution needs to be accounted for.

Estimate of Aerosol Optical Depth Using Broadband Direct Normal Observations at Highest Polluted Area in the World

Problem statement: The measurement of different components of direct solar radiation for this research has been carried out in Helwan, Egypt. Helwan (Latitude 29°52 N, Longitude 31°20 E). This is a considered as the largest polluted region in the world. The level of pollution in Helwan region is higher, compared to the international limit by about 7 or 10 times in industrial and populated region respectively. Approach: The daily variation for different components of solar radiation bands as global, direct (total, yellow, red and infrared) and diffuse solar radiation had been studied and discussed. The data measurements have been taken for nine years (1991-2000). Hourly data of solar irradiance on a horizontal plane had also been recorded simultaneously together with relative humidity and wind speed along with some traditional techniques for selecting the clear sky days of the period examined. Results: The results clearly showed seasonal dynamics in aerosol loading, type and perceptible water. Conclusion: Background levels of aerosols, which we defined as yellow color less than 0.10, had observed at almost all sites but varying frequencies.

Assessment and comparison of three years of Terra and Aqua MODIS Aerosol Optical Depth Retrieval (C005) in Chinese terrestrial regions

The latest MODIS aerosol optical depth (AOD) retrieval algorithm (C005) based on analyses of global AERONET data for both Terra and Aqua satellites has gradually supplanted the older algorithm (C004) since 2006. However, due to few AERONET sites in China, assessing applications of MODIS products over Chinese terrestrial regions for both Terra and Aqua is very necessary, including evaluating and comparing differences between Terra and Aqua. In this study, three years of Terra and Aqua AODs (C005) were analyzed over Chinese terrestrial regions using long-term and large-scale ground-based network observation data, from the Chinese Sun Hazemeter Network (CSHNET). The results show that C005 MODIS aerosol products (C005) have enough accuracy over China, with the same correlation coefficient ( R) ∼ 0.86, the slope ∼ 1.006 and 0.990, the offset ∼−0.043 and − 0.038, 53% and 50% of collocated data within the expected errors for Terra and Aqua, respectively, though there existed differences in different ecosystems and geographic locations. Overall, the MODIS products underestimated by 0.03 or 9% for Terra, 0.02 or 6% for Aqua, and more underestimation (11–16%) was noted in summer and autumn, which is based on annual and seasonal means of all sites. For different Angstrom exponent ( α) ranges, the MODIS underestimated by 0.03 or 8% at the moderate α, and 0.07 or 27% for the fine ( α > 1.5) and coarse ( α < 0.5) modes, which dominated for both Terra and Aqua. Remarkably, there are no very significant differences between Terra and Aqua AODs based on long-term statistical Terra and Aqua AODs for different Angstrom exponent ( α) ranges and regional monthly averages Over China, which confirms that either the Terra or Aqua estimate of AOD over China is a valid representation of the daily average, though there may be a diurnal cycle near aerosol emission regions. Moreover, the magnitude of the differences of regional monthly mean Terra and Aqua AODs varied from region to region and increased with aerosol loading, but there was no significantly consistent negative or positive bias between Terra and Aqua AODs in each region over China.

Improved MODIS Aerosol Retrieval Using Modified VIS/SWIR Surface Albedo Ratio Over Urban Scenes

In this paper, we demonstrate a regional approach to Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) retrieval over urban scenes. By using simultaneous MODIS and AErosol RObotic NETwork (AERONET) sky radiometer data, we show that for urban scenes, the surface reflectance ratios between the visible (VIS) (0.47- and 0.66-?m) and shortwave-infrared (2.12-?m) channels used in the MODIS Collection 5 (C005) retrieval are significantly underestimated. This leads to an underestimate in the VIS ground albedos resulting in a significant overestimate of the AOD in the VIS channels. Since the dependence on scattering geometry is weak, revised surface reflectance ratio maps at spatial resolutions down to 1.5 km can be developed, permitting aerosol retrievals at these resolutions. Applying this procedure results in significant reductions in MODIS-retrieved AOD bias in comparison to AERONET. As an example, for New York City, at the operational 10-km resolution, the MODIS-AERONET regression using C005 is ?MODIS = 1.24 ?AERONET + 0.11, R 2 = 0.85 , while for the regional method, we apply results in ?MODIS = 1.02 ?AERONET + 0.04, R 2 = 0.92 . This improved performance is maintained even for high-resolution retrieval down to 1.5 km with a regression of ?MODIS = 0.96 ?AERONET + 0.02, R 2 = 0.88. In addition, we show that this method can be extended to other urban scenes such as Mexico City with similar results and discuss the implications of C005 AOD overestimates on PM2.5 monitoring. Furthermore, comparisons with distributed PM2.5 sensors in the region confirm that the high estimates of AOD from C005 cannot be explained by excess urban emissions.

The GRAPE aerosol retrieval algorithm

Abstract. The aerosol component of the Oxford-Rutherford Aerosol and Cloud (ORAC) combined cloud and aerosol retrieval scheme is described and the theoretical performance of the algorithm is analysed. ORAC is an optimal estimation retrieval scheme for deriving cloud and aerosol properties from measurements made by imaging satellite radiometers and, when applied to cloud free radiances, provides estimates of aerosol optical depth at a wavelength of 550 nm, aerosol effective radius and surface reflectance at 550 nm. The aerosol retrieval component of ORAC has several incarnations – this paper addresses the version which operates in conjunction with the cloud retrieval component of ORAC (described by Watts et al., 1998), as applied in producing the Global Retrieval of ATSR Cloud Parameters and Evaluation (GRAPE) data-set. The algorithm is described in detail and its performance examined. This includes a discussion of errors resulting from the formulation of the forward model, sensitivity of the retrieval to the measurements and a priori constraints, and errors resulting from assumptions made about the atmospheric/surface state.

Assessment of Dust Aerosol Optical Depth and Shortwave Radiative Forcing over the Northwest Pacific Ocean in Spring Based on Satellite Observations

AbstractDust aerosol optical depth (AOD) and its accompanying shortwave radiative forcing (RF) are usually simulated by numerical models. Here, by using 9 months of Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol product data in combination with Clouds and the Earth's Radiant Energy System Single Scanner Footprint (CERES/SSF) data, dust AOD and its shortwave RF were estimated over the cloud—free north—West (NW) Pacific Ocean in the springs of 2004, 2005, and 2006. The results showed that in this region, the mean dust AOD and its shortwave RF were 0.10 and –5.51 W m-2, respectively. In order to validate the dust AOD derived by MODIS, results from the Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model were also used here. The correlation coefficient between the monthly averaged dust AOD derived by MODIS measurements and the model simulation results was approximately 0.53. Since the estimates of the dust AOD and its shortwave RF obtained in this study are based mainly...

Improved algorithm for MODIS satellite retrievals of aerosol optical depths over western North America

Quantitative evaluation of chemical transport models (CTMs) with aerosol optical depth (AOD) products retrieved from satellite backscattered reflectances can be compromised by inconsistent assumptions of aerosol optical properties and errors in surface reflectance estimates. We present an improved AOD retrieval algorithm for the MODIS satellite instrument using locally derived surface reflectances and CTM aerosol optical properties for the 0.47, 0.65, and 2.13 μm MODIS channels. Assuming negligible atmospheric reflectance at 2.13 μm in cloud‐free conditions, we derive 0.65/2.13 surface reflectance ratios at 1° × 1.25° horizontal resolution for the continental United States in summer 2004 from the subset of top‐of‐atmosphere (TOA) reflectance data with minimal aerosol reflectance. We obtain a mean ratio of 0.57 ± 0.10 for the continental United States, with high values over arid regions and low values over the Midwest prairies. The higher surface reflectance ratios explain the high AOD bias over arid regions found in previous MODIS retrievals. We calculate TOA reflectances for each MODIS scene using local aerosol optical properties from the GEOS‐Chem CTM, and fit these reflectances to the observed MODIS TOA reflectances for a best estimate of AODs for that scene. Comparison with coincident ground‐based (AERONET) AOD observations at 16 sites in the western and central United States in summer 2004 shows poor correlation in the daily data but the correlation improves as averaging time increases. Averaging over the available coincident observations (n = 11–44 days) results in strong correlations (R0.47μm = 0.90, R0.65μm = 0.67) and a 19% low bias, representing considerable improvement over the operational MODIS AOD products in this region.

Synthesis of information on aerosol optical properties

In a previous study (Liu et al., 2005) obtained are global scale estimates of aerosol optical depth at 0.55 μm based on spatial and temporal variation patterns from models and satellite observations, regulated by the Aerosol Robotic Network (AERONET) measurements. In this study an approach is developed to obtain information on global distribution of the single scattering albedo (ω0), the asymmetry parameter (g), and the normalized extinction coefficient over shortwave (SW) spectrum. Since space observations of ω0 are in early stages of development and none are available for g, first an approach was developed to infer them from relevant information from the Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model, Moderate Resolution Imaging Spectroradiometer (MODIS) and AERONET retrievals. The single scattering albedo is generated by extending GOCART ω0 at 0.55 μm to the entire SW spectrum using spectral dependence derived from AERONET retrievals. The asymmetry parameter over the solar spectrum is derived from the MODIS Ångström wavelength exponent, utilizing a relationship based on AERONET almucantar observations. The normalized extinction coefficient is estimated from the MODIS Ångström wavelength exponent. The methodology was implemented as a “proof of concept” with one year of data. The approach described here is a step in preparedness for utilizing information from new observing systems (e.g., MISR, A‐Train constellation) when available. The impact of the newly derived information on the quality of satellite based estimates of surface radiative fluxes was evaluated and is presented by Liu and Pinker (2008).

An approach to compute Photosynthetically Active Radiation using IRS P4 OCM

An approach was developed using the IRS P4 Ocean Colour Monitor (OCM) for the estimation of Photosynthetically Active Radiation (PAR). Bird's clear sky model will be implemented using IRS P4 OCM data on an operational basis. IRS P4 OCM data (bands 6 and 8) were used in the present study to provide an instantaneous estimation of aerosol optical depth using the exponential approach. The transmittance coefficient required for aerosol optical depth was computed as a next step. This was further incorporated in Bird's clear sky model with other atmospheric transmittance coefficients due to absorption from water vapour, ozone, and uniformly mixed gases. Spectral irradiance was integrated over photosynthetically active wavelengths (400–700 nm), which provides instantaneous photosynthetically available radiation (IPAR) at the sea surface. Ship cruise data (SK 171 and SK 186) for PAR using Satalantic radiometer was compared with IRS P4 OCM derived IPAR within 5.2% accuracy on average for two different monsoon seasons in the Arabian sea. The root mean square error between the observed PAR (in situ) and estimated (model‐derived) PAR was found to be 21.28 W m−2. This IPAR value is an important parameter for primary productivity estimation and will be used to compute primary productivity on an operational basis using IRS P4 OCM data.

A comparison of CMAQ‐based aerosol properties with IMPROVE, MODIS, and AERONET data

Evaluation of concentrations predicted by air quality models is needed to ensure that model results are compatible with observations. In this study aerosol properties derived from the Community Multiscale Air Quality (CMAQ) model‐simulated aerosol mass concentrations are compared with routine data from NASA satellite‐borne Moderate Resolution Imaging Spectroradiometer (MODIS) sensor aboard the Sun‐synchronous Terra satellite, NASA's ground‐based Aerosol Robotic Network (AERONET), and the ground‐based Interagency Monitoring of Protected Visual Environment (IMPROVE) network. The motivation for this analysis is to determine how best to use these parameters in evaluating model‐predicted PM2.5 concentrations. CMAQ surface extinction estimates due to scattering at 550 nm wavelength are compared with the IMPROVE nephelometer data obtained from 25 sites within the United States. It is found that model‐predicted surface extinctions bear high correlations with nephelometer measured data. Sulfate fractional aerosol optical depth (AOD) is found to dominate in the northeastern part of the United States; hence ground‐based measurement of sulfate concentrations have been compared with time series of columnar AOD as observed by the MODIS instrument and also with the CMAQ‐predicted tropospheric column values obtained during the June–August period of 2001. CMAQ surface extinctions are found to be relatively higher than the IMPROVE nephelometer observations; however, there is a good agreement between CMAQ AOD trends and AERONET and MODIS data, obtained at the seven AERONET sites located in the eastern United States. CMAQ is also found to capture the day‐to‐day variability in the spatial AOD patterns. Monthly average satellite AOD estimates are found to be higher than the AOD data obtained using the CMAQ‐predicted aerosol concentrations. Seasonal variation of satellite‐measured aerosol intensive property “Angstrom exponent” (a gross indicator of the aerosol size distribution) is presented for four selected sites: one each in the eastern and central parts, and two in the western part of the continental United States. Variability of Angstrom exponent at these four selected sites is analyzed in conjunction with the variation of summertime AOD (observed and modeled), mass concentration (observed and modeled) and modeled SO4 average concentrations during the summer (June–August) period of the year 2001. Annual time series of Angstrom exponent data at the four selected sites show a large east‐west variation.

Aerosol optical depth and land surface reflectance from multiangle AATSR measurements: global validation and intersensor comparisons

This paper presents the results and satellite intercomparisons for the retrieval of aerosol optical depth (AOD) and land surface bidirectional reflectance using the Multiangle Advanced Along-Track Scanning Radiometer (AATSR). The algorithm developed is based on inversion of a physical model of light scattering that requires no a priori knowledge of the land surface. The model is evaluated for a number of sites around the world to test its operation over a range of aerosol types and land covers including dark and bright surfaces. Validation is performed using Aerosol Robotic Network ground-based sun-photometer measurements and by intercomparison with independent estimates of AOD derived from spaceborne instruments including Multiangle Imaging Spectroradiometer (MISR), Moderate Resolution Imaging Spectroradiometer (MODIS), and Total Ozone Mapping Spectrometer (TOMS) aerosol products. Results show good agreement (Pearson's correlation coefficient r2=0.70 for all sites combined) between the AATSR-derived estimates of AOD and the sun-photometer measurements. There is also a high correlation (r2=0.84) between the AATSR- and MISR-derived AOD estimates, but the correlations of the AATSR-derived AOD with MODIS-derived AOD and TOMS aerosol index are lower. In addition, the ability of the sensor to discriminate between different aerosol types is evaluated. Moreover, the estimates of the aerosol properties are used for atmospheric correction of the top-of-atmosphere reflectance. The AATSR surface reflectances are compared with the MODIS bidirectional reflectance distribution function/Albedo and MISR surface products and are shown to correspond with root-mean-square errors of 0.03 and 0.06 or better, respectively. The retrieval method is applied on an image basis resulting in an image of surface reflectance and a separate map of AOD. A map of AOD at 550 nm covering the Sahel and southern Sahara region is presented to demonstrate operation at regional and potentially global scales

Computationally efficient method for retrieving aerosol optical depth from ATSR-2 and AATSR data

We present a robust and computationally efficient method for retrieving aerosol optical depth (AOD) from top-of-atmosphere ATSR-2 (Along-Track Scanning Radiometer) and AATSR (Advanced ATSR) reflectance data that is formulated to allow retrieval of the AOD from the 11 year archive of (A)ATSR data on the global scale. The approach uses a physical model of light scattering that requires no a priori information on the land surface. Computational efficiency is achieved by using precalculated lookup tables (LUTs) for the numerical inversion of a radiative-transfer model of the atmosphere. Estimates of AOD retrieved by the LUT approach are tested on AATSR data for a range of global land surfaces and are shown to be highly correlated with sunphotometer measurements of the AOD at 550 nm. (Pearson's correlation coefficient r(2) is 0.71.).

Comparison between experimental data and modeling estimates of aerosol optical depth over Athens, Greece

Ground-based spectroradiometric measurements taken in the atmosphere of Athens during May 1995 have been used to investigate various atmospheric parameters. From the direct-beam irradiance measurements the spectral aerosol optical depths as well as the Ångström parameters are derived using the least-squares method applied to measured spectral irradiance data. In this way the errors associated with the turbidity coefficient, β, are found to be 0.0029 on average and with the wavelength exponent, α, 0.031. In this paper, a comparative study between the aerosol optical depth derived by the spectral direct-beam irradiance measurements and those estimated through the Ångström's formula is made. The correlation coefficient between the measured and computed optical depth is higher than 0.93 in most cases, while their greatest differences take place at the shorter wavelengths. A further analysis of the above differences at 500 and 1000 nm shows very low values, especially at 500 nm, indicating the accuracy of the fitting. The differences at 1000 nm are much higher due to uncertainties in the water-vapor optical depth determination. The errors in Ångström parameters as well as in the Ångström's formula fitting to the derived spectral aerosol optical depth depend strongly on atmospheric turbidity, reaching to higher values under low atmospheric turbidity conditions.

Impact of 3-D Clouds on Clear-Sky Reflectance and Aerosol Retrieval in a Biomass Burning Region of Brazil

Three-dimensional (3-D) cloud radiative effects on clear-sky reflectances and associated aerosol optical depth retrievals are quantified for a cumulus cloud field in a biomass burning region in Brazil through a Monte Carlo simulation. In this study the 1-km Moderate Resolution Imaging Spectroradiometer cloud optical depth and surface reflectance datasets are used to compute the 3-D radiation fields with ambient aerosol optical thickness of 0.1 at a wavelength of 0.66 /spl mu/m. The 3-D radiative effects range from -0.015 to 0.018 with an average of 0.004 and standard deviation of 0.006. The 3-D effects are most pronounced and variable for cloud neighboring pixels, where both large negative effects over shadows and positive effects near sunlit cloud edges are found. The clear next-to-cloud pixels, that contain /spl sim/83% of the clear pixel population, are affected in the most complex way and not reliable for aerosol retrieval. In the area 2 km away from clouds, the 3-D effects enhance the reflectance in clear patches. The average and variability of enhancements gradually decrease as a function of the cloud-free distance, resulting in a systematically higher aerosol optical depth estimates for pixels closer to clouds in one-dimensional (1-D) retrieval. At a distance of 3 km away from clouds, the 3-D effect is still appreciable with the average enhancement slightly less than 0.004. This enhancement will lead to an over estimate of aerosol optical thickness of /spl sim/0.04 in 1-D retrieval, which is significant for an ambient atmosphere with aerosol optical thickness of 0.1.

Cases of aerosol optical depth estimation in the Athens area, Greece

This work estimates the Ångström turbidity coefficients and investigates the variation of the aerosol optical depth (AOD) in the Athens area, during different atmospheric conditions. The AOD is estimated in the wavelength band of 400–670 nm from direct-beam spectral irradiance measurements using ground-based instrumentation, during an experimental campaign performed in the period 22 September–1 October 2002. All data were collected under clear-sky conditions near the city center of Athens; the AODs were estimated relative to the local zenith to avoid the influence of the optical air mass. The study shows that the AOD is influenced by wind regime and traffic in the Athens area. The Angstrom's turbidity coefficients and the AOD values were found to be higher under the influence of South-sector winds compared to those from the North-sector. Under South-sector low winds, the pollutants are accumulated in the Athens basin. On the contrary, the North-sector winds clean the atmosphere.

Comparison and Uncertainty of Aerosol Optical Depth Estimates Derived from Spectral and Broadband Measurements

Abstract An experimental comparison of spectral aerosol optical depth τa,λ derived from measurements by two spectral radiometers [a LI-COR, Inc., LI-1800 spectroradiometer and a Centre Suisse d'Electronique et de Microtechnique (CSEM) SPM2000 sun photometer] and a broadband field pyrheliometer has been made. The study was limited to three wavelengths (368, 500, and 778 nm), using operational calibration and optical depth calculation procedures. For measurements taken on 32 days spread over 1 yr, the rms difference in τa,λ derived from the two spectral radiometers was less than 0.01 at 500 and 778 nm. For wavelengths shorter than 500 nm and longer than 950 nm, the performance of the LI-1800 in its current configuration did not permit accurate determinations of τa,λ. Estimates of spectral aerosol optical depth from broadband pyrheliometer measurements using two models of the Angstrom turbidity coefficient were examined. For the broadband method that was closest to the sun photometer results, the mean (rms) ...

Aerosol spectral optical depths over the Bay of Bengal: Role of transport

Recent experiments have shown the potential role of air masses in transporting aerosols to locations far away from source regions. Despite the importance of the Bay of Bengal to Indian climate and monsoon, no serious aerosol observations are available for this region. Extensive aerosol optical depth estimates, made for the first time from an island location, Port Blair (11.63°N; 92.71°E) in the Bay of Bengal, during the Indian winter of 2002, are used to examine the impact of air trajectories in modifying the optical depths and their spectral dependences. The results are examined for their distinctiveness with respect to the origin as well as transport. It is seen that the trajectories arriving from the regions east of the station (South China, Thailand, Laos, Cambodia, Vietnam, Burma) are richer in aerosol abundance, more in the sub micron size range, than those arriving from the west, across the Indian landmass.

Aerosol correction for remotely sensed sea surface temperatures from the National Oceanic and Atmospheric Administration advanced very high resolution radiometer

This research presents the first‐phase derivation and implementation of daytime aerosol correction algorithms for remotely sensed sea surface temperature (SST) from the advanced very high resolution radiometer (AVHRR) instrument flown onboard NOAA polar orbiting satellites. To accomplish this, a long‐term (1990–1998), global AVHRR‐buoy match‐up database was created by merging the NOAA/NASA Pathfinder Atmospheres and Pathfinder Oceans data sets. The merged data set is unique in that it includes daytime estimates of aerosol optical depth (AOD) derived from AVHRR channel 1 (0.63 μm) under global conditions of significant aerosol loading. Histograms of retrieved AOD reveal monomodal, lognormal distributions for both tropospheric and stratospheric aerosol modes. It is then shown empirically that the SST depression caused under each aerosol mode can be expressed as a linear function in two predictors, these being the slant path AOD retrieved from AVHRR channel 1 along with the ratio of channels 1 and 2 normalized reflectances. On the basis of these relationships, parametric equations are derived to provide an aerosol correction for retrievals from the daytime NOAA operational multichannel and nonlinear SST algorithms. Separate sets of coefficients are utilized for two aerosol modes: tropospheric (i.e., dust, smoke, haze) and stratospheric/tropospheric (i.e., following a major volcanic eruption). The equations are shown to significantly reduce retrieved SST bias using an independent set of match‐ups. Eliminating aerosol‐induced bias in both real‐time and retrospective processing will enhance the utility of the AVHRR SST for the general user community and in climate research.

Estimation of aerosol opacity and land surface bidirectional reflectance from ATSR‐2 dual‐angle imagery: Operational method and validation

Imagery from the second Along Track Scanning Radiometer (ATSR‐2) is analyzed to reveal that a separation of the atmospheric from the land surface signal is possible given two near‐simultaneous images at differing slant paths. The method is based on the inversion of a simple physical model of light scattering by the land surface. The result is an image of the land surface reflectance that is relatively uncontaminated by atmospheric scattering and a separate image of the atmospheric aerosol loading. Unlike single‐image satellite methods for aerosol estimation, no assumptions are required about the land surface spectral properties, allowing for quantitative mapping of aerosol over an arbitrary land surface. Validation is performed over boreal forest by comparison with data from a network of Sun photometers. Satellite estimates of aerosol optical depth agree with field measurements to within 0.02 at 550 nm. An important consequence is that measurements can be compared over time, and small changes in land surface reflectance can be separated from atmospheric changes.

Estimation of aerosol optical depth by applying the optimal distance number to NOAA AVHRR data

In order to correctly interpret the surface information with remotely sensed data, the extinction effects of the atmosphere must be appropriately removed. The aerosol optical depth (AOD) serves as an important parameter in describing this effect. Several factors, such as the satellite observation geometry, terrain geometry, temporal variation of canopy, and so forth, produce errors in the AOD estimation. This study strives to avoid such errors by introducing an “optimal distance number” into the structure function method that has been developed previously in estimating the AOD. The results of our experiment measured by a ground-based sunphotometer showed that the average errors of the estimated AOD values was reduced from 40% to 11% after the “optimal distance number” was used, indicating a significant improvement.