Modelling of atmospheric optical thickness from spectroradiometer measurements in Penang IslandDATA ANALYSIS AND RESULTS A total of 42 atmospheric transmittance data over Penang Island were collected on 13th November 2004. The AOT values were calculated using the Beer-Lambert-Bouguer law from the atmospheric measurements. We selected the spectral wavelength in this study centred at 500.5 nm. The AOT is related to the transmittance by the expression. Tdλ = e-τλ/us [2] (1) where Tdλ =transmittance for direct irradiance at wavelength, λ us = cosines (θ), θ is the zenith angle The AOT values were obtained after performing the sequence of the following calculations; First, we measured the total solar irradiance. This was done by measuring the radiance reflected from a Spectralon panel placed perpendicular to the direction of the Sun. The measured radiance was converted into irradiance by multiplying by ? and then divided by the reflectance value of the Spectralon panel to obtain the total solar irradiance values. Second, we measured the diffuse irradiance. This step was performed using the same Spectralon panel maintained in the same orientation as in step 1. The panel was shaded from direct sunlight using a disk of black painted cardboard mounted on a stick. The size of the disk and distance to the panel should be such that the shadow of the disk on the panel is just sufficient to fully shade the area viewed by the FieldSpec HH. As in step 1, we measured the reflected radiance. Again, the measured radiance is converted to irradiance by multiplying by ? and then dividing by the reflectance value of the Spectralon panel to obtain the diffuse irradiance values. Third, we computed the direct solar irradiance by subtracting the diffuse irradiance from the measured total solar irradiance. Fourth, we computed the top-of-atmosphere (TOA) solar irradiance values corresponding to the wavelengths of the spectra measured by the FieldSpec HH. In this study, Wehrli 1985 AM0 Spectrum was chosen to calculate the TOA spectra and interpolate it to the FieldSpec HH wavelengths, multiplying by cosines solar angle and then correct for the exact Earth-Sun distance factor, D, given by Spencer, 1971 as  The day angle, , in radians is represented by  where d is the day number of a year (1-365) Finally, we computed the direct atmospheric transmission by dividing the direct solar irradiance computed in step 3 by the top of atmosphere values calculated in the fourth step. This was done by first exporting the FieldSpec HH spectrum to a text file, importing the text file into a program like MS Excel and then performing the calculations in Excel. Then the AOT values were computed using Equation (1). The plot of the relationship between the AOT and PM10 is shown in Figure 2. The data shows positive linear correlation between AOT and PM10. Map of PM10 were generated using Kriging interpolation technique (Figure 3). [3] and [4] also applied interpolation technique in their study for air quality mapping.  Figure 2. A linear correlation between AOT and PM10 (μg/m3) values  Figure 3. A map of PM10 (μg/m3) CONCLUSION A handheld spectroradiometer can be used as a sensor in remote sensing studies to provide a very useful information for the air quality estimation. Further research will be carried out to validate the proposed technique. Finally, the proposed technique can monitor the air quality in Penang Island effectively. ACKNOWLEDGEMENTS This project was carried out using a short term research grants from Universiti Sains Malaysia. We would like to thank the technical staff who participated in this project. Special thanks are extended to Brian Curtiss who provided me the methods of measuring the sky transmittance using a spectroradiometer. Thanks are also extended to USM for support and encouragement. REFERENCES
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