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H. S. Lim
School of Physics
Universiti Sains Malaysia
Minden 11800 Penang, Malaysia.
Tel: +604-6577888, Fax: +604-6579150
hslim111@yahoo.com.sg
M. Z. MatJafri
School of Physics
Universiti Sains Malaysia
Minden 11800 Penang, Malaysia.
Tel: +604-6577888, Fax: +604-6579150
mjafri@usm.my
K. Abdullah
School of Physics
Universiti Sains Malaysia
Minden 11800 Penang, Malaysia.
Tel: +604-6577888, Fax: +604-6579150
khirudd@usm.my
N. M. Saleh
School of Physics
Universiti Sains Malaysia
Minden 11800 Penang, Malaysia.
Tel: +604-6577888, Fax: +604-6579150
C. J. Wong
School of Physics
Universiti Sains Malaysia
Minden 11800 Penang, Malaysia.
Tel: +604-6577888, Fax: +604-6579150
Sultan AlSultan
Remote Sensing Center of Envir. Counsultant
ISPRS, commission 7 WG VII/7, Middle East Coordinator
Malaz, Al Nurii St., P.O.Box.92038
Riaydh City 11653, Saudi Arabia.
Tel: +966504890977 Fax: +96614767828
rsensing_2004@yahoo.com
ABSTRACT
A study has been carried out for the estimation of aerosol optical thickness (AOT) using a handheld spectroradiometer measurements over Penang Island, Malaysia. The AOT values were retrieved from the atmospheric transmittance measurements in the spectral region centred at 550nm. The concentrations of particulate matters of less than 10 micron (PM10) were measured simultaneously with the atmospheric transmittance measurements using a DustTrak Aerosol Monitor 8520 at several selected locations around Penang Island. The AOT is calculated using the expression given by the Beer-Lambert-Bouguer law. Its locations determined with a Global Positioning System (GPS) receiver. The relation between AOT and PM10 has been investigated in this study. Finally, a PM10 map was generated using Kriging interpolation technique. This study proposed an alternative method for the retrieval of AOT values with a reasonable accuracy.
INTRODUCTION
Aerosols perturb the earths energy budget directly by scattering and absorbing radiation and indirectly by acting as cloud condensation nuclei and in doing so changing cloud properties. Their effects on temperature, hence atmospheric stratification, further influence clouds. The overall cooling by anthropogenic aerosols may be comparable to the warming of 2.43 Wm-2 by greenhouse gases [1].
A study was performed by using handheld spectroradiometer measurements for AOT mapping. The spectroradiometer was used to measure the atmospheric transmittances. The well known Beer-Lambert law was used in this study to retrieve AOT values from the measured transmittance values. The PM10 measurements were also collected simultaneously during the acquisition of the atmospheric transmittance measurements using DustTrak Aerosol Monitor 8520. In this study, we found a linear relationship between AOT and PM10. Finally, a PM10 map was generated using Kriging interpolation technique.
STUDY AREA
The study area is the Penang Island, Malaysia which is situated within latitudes 5o 12 N to 5o 30 N and longitudes 100o 09 E to 100o 26 E (Figure 1). The corresponding PM10 measurements were collected at the several selected locations.
Figure 1. The location of the study area
DATA 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
Modelling of atmospheric optical thickness from spectroradiometer measurements in Penang Island
H. S. Lim
School of Physics
Universiti Sains Malaysia
Minden 11800 Penang, Malaysia.
Tel: +604-6577888, Fax: +604-6579150
hslim111@yahoo.com.sg
M. Z. MatJafri
School of Physics
Universiti Sains Malaysia
Minden 11800 Penang, Malaysia.
Tel: +604-6577888, Fax: +604-6579150
mjafri@usm.my
K. Abdullah
School of Physics
Universiti Sains Malaysia
Minden 11800 Penang, Malaysia.
Tel: +604-6577888, Fax: +604-6579150
khirudd@usm.my
N. M. Saleh
School of Physics
Universiti Sains Malaysia
Minden 11800 Penang, Malaysia.
Tel: +604-6577888, Fax: +604-6579150
C. J. Wong
School of Physics
Universiti Sains Malaysia
Minden 11800 Penang, Malaysia.
Tel: +604-6577888, Fax: +604-6579150
Sultan AlSultan
Remote Sensing Center of Envir. Counsultant
ISPRS, commission 7 WG VII/7, Middle East Coordinator
Malaz, Al Nurii St., P.O.Box.92038
Riaydh City 11653, Saudi Arabia.
Tel: +966504890977 Fax: +96614767828
rsensing_2004@yahoo.com
ABSTRACT
A study has been carried out for the estimation of aerosol optical thickness (AOT) using a handheld spectroradiometer measurements over Penang Island, Malaysia. The AOT values were retrieved from the atmospheric transmittance measurements in the spectral region centred at 550nm. The concentrations of particulate matters of less than 10 micron (PM10) were measured simultaneously with the atmospheric transmittance measurements using a DustTrak Aerosol Monitor 8520 at several selected locations around Penang Island. The AOT is calculated using the expression given by the Beer-Lambert-Bouguer law. Its locations determined with a Global Positioning System (GPS) receiver. The relation between AOT and PM10 has been investigated in this study. Finally, a PM10 map was generated using Kriging interpolation technique. This study proposed an alternative method for the retrieval of AOT values with a reasonable accuracy.
INTRODUCTION
Aerosols perturb the earths energy budget directly by scattering and absorbing radiation and indirectly by acting as cloud condensation nuclei and in doing so changing cloud properties. Their effects on temperature, hence atmospheric stratification, further influence clouds. The overall cooling by anthropogenic aerosols may be comparable to the warming of 2.43 Wm-2 by greenhouse gases [1].
A study was performed by using handheld spectroradiometer measurements for AOT mapping. The spectroradiometer was used to measure the atmospheric transmittances. The well known Beer-Lambert law was used in this study to retrieve AOT values from the measured transmittance values. The PM10 measurements were also collected simultaneously during the acquisition of the atmospheric transmittance measurements using DustTrak Aerosol Monitor 8520. In this study, we found a linear relationship between AOT and PM10. Finally, a PM10 map was generated using Kriging interpolation technique.
STUDY AREA
The study area is the Penang Island, Malaysia which is situated within latitudes 5o 12 N to 5o 30 N and longitudes 100o 09 E to 100o 26 E (Figure 1). The corresponding PM10 measurements were collected at the several selected locations.
Figure 1. The location of the study area
DATA 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
- H. B. Yu, R. E. Dickinson, M. Chin, Y. J. Kaufman, M. Zhou, L. Zhou, Y. Tian, O. Dubovik and B. N. Holben, Direct radiative effect of aerosols as determined from a combination of MODIS retrievals and GOCART simulations, Journal of Geophysics Research, V 109 D03206, 2004.
- E. Vermote, D. Tanre, J. L. Deuze, M. Herman and J. J. Morcrette, Second Simulation of the satellite signal in the solar spectrum (6S), [Online] available: http://www.geog.tamu.edu/klein/geog661/handouts/6s/6smanv2.0_P1.pdf, 1997.
- A. Ung, L. Wald, T. Ranchin, C. Weber, J. Hirsch, G. Perron and J. Kleinpeter, Satellite data for Air Pollution Mapping Over A City- Virtual Stations, Proceeding of the 21th EARSeL Symposium, Observing Our Environment From Space: New Solutions For A New Millenium, Paris, France, 14 16 May 2001, Gerard Begni Editor, A., A., Balkema, Lisse, Abingdon, Exton (PA), Tokyo, pp. 147 151, [Online] available: http://www-cenerg.cma.fr/Public/themes_de_recherche/teledetection/title_tele_air/title_tele_air_pub/satellite_data_for_t, 2001.
- U. Patil, S. Ravan and A. Kaushal, GIS based air pollution surface modeling, The asian GIS montly, 7 (8), 2003.