• Korean Journal of Remote Sensing
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• v.23 no.1
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• pp.1-5
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• 2007

Relatively simplified method for determination of surface reflectance has been used by using the ratio between SWIR and VIS band reflectance over land surface. The surface reflectance ratios (SWIR/VIS) were estimated over land in Korea from Terra Moderate Resolution Imaging Spectre-radiometer (MODIS) L1 data. The ratios by using the minimum reflectance technique were lower than those by MODIS operational aerosol retrieval algorithm. Although the comparison between MODIS and sunphotometer Aerosol Optical Thickness (AOT) has a good correlation coefficient (R=0.84), slightly overestimated MODIS AOTs were shown with a slope of linear regression line of 0.89. The comparison between the ratio and AOT dearly exhibit that the error of MODIS AOT could be originated from the underestimated surface reflectances by MODIS operational algorithm.

• Korean Journal of Remote Sensing
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• v.24 no.4
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• pp.369-379
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• 2008

The aerosol retrieval algorithm for the Moderate Resolution Imaging Spectroradiometer (MODIS) measurements was updated recently. This paper reports on the comparison and validation of two latest versions (Collection 4 and 5, shortly C004 and C005) of the MODIS aerosol product over northeast Asian region. The differences between the aerosol optical thickness (AOT) from the C004 and C005 retrieval algorithms and the correlation with ground based AERONET sunphotometer observations are investigated. Over the study region, spatially averaged annual mean AOT retrieved from C005 algorithm $(AOT_{C005})$ is about 0.035 AOT (5%) less than the C004 counterparts. The linear correlations between MODIS and AERONET AOT also are R=0.89 (slope=0.86) for the C004 and R=0.95 (slope=1.00) for the C005. Moreover, the magnitude of the mean error in $AOT_{C005}$, difference between MODIS AOT and AERONET AOT, is 40% less than that in $AOT_{C004}$.

• Atmosphere
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• v.19 no.3
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• pp.243-253
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• 2009

In other to interpret the long-term variations of sunshine duration, cloud lifetime, and precipitation intensity observed in and around Seoul and Busan for the period from 1986 to 2005, aerosol indirect effect was employed and applied. For the identification of long-term trend of aerosol concentration, observed visibility and AOT of AERONET sunphotometer data were also used over the same regions. The result showed that the time series of visibility was decreased and those of AOT increased, especially trends were remarkable in 2000s. In both regions, occurrence frequencies of observed cloudiness (cloud amount ${\leq}6/10$) and strong precipitation (rain rate > $0.5mmhour^{-1}$) have been steadily increased while those of cloudiness (cloud amount > 7/10) and weak precipitation (rain rate ${\leq}0.2mmhour^{-1}$) decreased. These results are corresponding to the trend of both visibility and AERONET data, implying the aerosol indirect effect that makes size of cloud droplet reduce, cloud life-time longer and precipitation efficiency decreased. Our findings demonstrate that, although these phenomena are not highly significant, weather and climate system over Korean urban area have been changed toward longer lifetime of small cloudiness and increasing precipitation intensity as a result of increased aerosol indirect effect.

• Korean Journal of Remote Sensing
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• v.32 no.2
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• pp.133-139
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• 2016

Particle depolarization ratios (DPRs) at 440, 675, 870 and 1020 nm are retrieved from AERONET sun/sky radiometer observations at Gosan and Kongju in South Korea. The retrieved results show good agreement with DPRs measured by lidar at 532 nm. High DPRs are found when Asian dust passes through at the upper atmosphere over 2 km above the Earth's surface. In case of lower atmosphere less than 2 km from the ground, DPRs are relatively low due to the small amount of dust particles and mixing of dust with air pollutants.

• Korean Journal of Remote Sensing
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• v.14 no.3
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• pp.307-314
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• 1998

To examine the detectability of the yellow sand and/or aerosol from China crossing over the Yellow Sea within the range of OSMI wavelengths(400-900 nm), we have investigated the optical characteristics of aerosols in yellow sand events observed on April, 1998 in Seoul. The spectral reflectance(%) and aerosol optical thickness in the range of Visible(VIS) and near Infrared (NIR) wavelengths were derived from the measurements of solar radiation using the GER-2600 spectroradiometer and sunphotometer during the April, 1798. The average spectral reflectance for the yellow sand events is over 40% and higher around 14:30 than 12:00 LST, but that for clear days is about 20% both at 12:00 and 14:30 LST in the range of 500-900 nm. The aerosol optical thickness at 501 nm varied from 0.25 on very clear day to 1.01 during a so-called "yellow-sand" episode and that for 673 nm varied from 0.14 to 0.92, respectively.

• Korean Journal of Remote Sensing
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• v.32 no.2
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• pp.97-104
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• 2016

We present linear particle depolarization ratio at 440, 675, 870, and 1020 nm retrieved from measurements with an AERONET sun/sky radiometer at Osaka, Japan. The retrieved data were compared with lidar derived linear particle depolarization ratio at 532 nm at the same site. We find good agreement between linear particle depolarization ratios derived with Sun photometer and measured by lidar except for those at 440 nm. The coefficients of determination between lidar derived data and sun/sky radiometer derived data were 0.28, 0.81, 0.88, and 0.89 at 440, 675, 870, and 1020 nm, respectively. We find that the linear particle depolarization ratio derived with sun/sky radiometer varies by the mixing between Asian dust and pollution particles. As the mixing ratio of Asian dust and pollution particles is increased, the linear particle depolarization ratio values are lower than the values of pure Asian dust. It was confirmed by the value of single-scattering albedo and particle size distribution.

• Korean Journal of Optics and Photonics
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• v.29 no.4
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• pp.139-148
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• 2018

In this study, aerosol size distributions were retrieved from aerosol optical depth measured over a range of 10 wavelengths from 250 to 1100 nm. The 10 wavelengths were selected where there is no absorption of atmospheric gases. To obtain the solar spectrum, a home-made solar tracking system was developed and calibrated. Using this solar tracking system, total optical depths (TODs) were extracted for the 10 wavelengths using the Langley plot method, and aerosol optical depths (AODs) were obtained after removing the effects of gas absorption and Rayleigh scattering from the TODs. The algorithm for retrieving aerosol size distributions was suggested by assuming a bimodal aerosol size distribution. Aerosol size distributions were retrieved and compared under various arbitrary atmospheric conditions. Finally, we found that our solar tracking spectrometer is useful for retrieving the aerosol size distribution, even though we have little information about the aerosol's refractive index.

• Korean Journal of Remote Sensing
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• v.26 no.6
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• pp.605-615
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• 2010

This study demonstrates the feasibility of small satellite, namely PROBA platform with the compact high resolution imaging spectrometer (CHRIS), for aerosol retrieval in Hong Kong. The rationale of our technique is to estimate the aerosol reflectances by decomposing the Top of Atmosphere (TOA) reflectances from surface reflectance and Rayleigh path reflectances. For the determination of surface reflectances, the modified Minimum Reflectance Technique (MRT) is used on three winter ortho-rectified CHRIS images: Dec-18-2005, Feb-07-2006, Nov-09-2006. For validation purpose, MRT image was compared with ground based multispectral radiometer measurements and atmospherically corrected Landsat image. Results show good agreements between CHRIS-derived surface reflectance and both by ground measurement data as well as by Landsat image (r>0.84). The Root-Mean-Square Errors (RMSE) at 485, 551 and 660nm are 0.99%, 1.19%, and 1.53%, respectively. For aerosol retrieval, Look Up Tables (LUT) which are aerosol reflectances as a function of various AOT values were calculated by SBDART code with AERONET inversion products. The CHRIS derived Aerosol Optical Thickness (AOT) images were then validated with AERONET sunphotometer measurements and the differences are 0.05~0.11 (error=10~18%) at 440nm wavelength. The errors are relatively small compared to those from the operational moderate resolution imaging spectroradiometer (MODIS) Deep Blue algorithm (within 30%) and MODIS ocean algorithm (within 20%).

• Korean Journal of Remote Sensing
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• v.16 no.4
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• pp.305-313
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• 2000

A vicarious calibration method was developed for the OSMI sensor calibration. Employing measured aerosol optical thickness by a sunphotometer and a sky radiometer and water leaving radiance by ship measurements as inputs, TOA (top of the atmosphere) radiance at each OSMI band was simulated in conjunction with a radiative transfer model (Rstar5b) by Nakajima and Tanaka (1988). As a case of examining the accuracy of this method, we simulated TOA radiance based on water leaving radiance measured at NASA/MOBY site and aerosol optical thickness estimated nearby at Lanai, and compared simulated results with SeaWiFS-estimated TOA radiances. The difference falls within about $\pm$5%, suggesting that OMSI sensor can be calibrated with the suggested accuracy. In order to apply this method for the OSMI sensor calibration, ground-based sun photometry and ship measurements were carried out off the east coast of Korean peninsula on May 31, 2000. Simulations of TOA radiance by using these measured data as input to the radiative transfer model show that there are substantial differences between simulated and OSMI-estimated radiances. Such a discrepancy appears to be mainly due to the cloud contamination because satellite image indicates optically thin clouds over the experimental area. Nevertheless results suggest that sensor calibration can be achieved within 5% uncertainty range if there are ground-based measurements of aerosol optical thickness, and water leaving radiances under clear-sky and optically thin atmospheric conditions.

• Korean Journal of Remote Sensing
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• v.33 no.6_1
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• pp.961-970
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• 2017

Despite of large demand for high spatial resolution products of aerosol properties from satellite remote sensing, it has been very difficult due to the weak signal by a single pixel and higher noise from clouds. In this study, aerosol retrieval algorithm with the high spatial resolution ($500m{\times}500m$) was developed using Geostationary Ocean Color Imager (GOCI) data during the Korea-US Air Quality (KORUS-AQ) period in May-June, 2016.Currently, conventional GOCI Yonsei aerosol retrieval(YAER) algorithm provides $6km{\times}6km$ spatial resolution product. The algorithm was tested for its best possible resolution of 500 m product based on GOCI YAER version 2 algorithm. With the new additional cloud masking, aerosol optical depth (AOD) is retrieved using the inversion method, aerosol model, and lookup table as in the GOCI YAER algorithm. In some cases, 500 m AOD shows consistent horizontal distribution and magnitude of AOD compared to the 6 km AOD. However, the 500 m AOD has more retrieved pixels than 6 km AOD because of its higher spatial resolution. As a result, the 500 m AOD exists around small clouds and shows finer features of AOD. To validate the accuracy of 500 m AOD, we used dataset from ground-based Aerosol Robotic Network (AERONET) sunphotometer over Korea. Even with the spatial resolution of 500 m, 500 m AOD shows the correlation coefficient of 0.76 against AERONET, and the ratio within Expected Error (EE) of 51.1%, which are comparable to the results of 6 km AOD.