Environmental pollution become more and more serious due to todays development
all around the world. In order to ensure this situation is under control, Malaysias
government has already established a network throughout Malaysia to monitor
this situation. Aerosols scatter and absorb solar radiation. The scattering
and absorption features depend on their chemical and physical properties (Guzzi
et al., 2001). AOT, τ, is a measure of aerosol loading in the
atmosphere (Christopher et al., 2002). Many studies
have been carried out to retrieve AOT values (Nemuc et
al., 2010; Jiang et al., 2010; Guo
et al., 2009). For this purpose, we used a handheld spectroradiometer
to measure the transmittance values at the surface. The transmittance is related
to the AOT according to Angstrom expression. So we retrieved the AOT value from
these measurements. AOT, τ, is a measure of aerosol loading in the atmosphere
(Christopher et al., 2002). Retrieval of Aerosol
Optical Thickness (AOT) over land from satellite platforms has been limited
in comparison to the number of retrievals over oceanic areas (Knapp
and Stowe, 2000). The objective of this study was to retrieval AOT values
over land using atmospheric radiation measurements. Besides, we also to explore
the relationship between Aerosol Optical Thickness (AOT) derived from the spectroradiometer
transmittance measurements and the correlation of particulate matter less than
10 micron (PM10).
Many researchers have conducted satellite monitoring of the AOT (Husar
et al., 1997; Liu et al., 2002). In
this study, we present a method for air quality mapping from ground-based measurements.
Typically, ground based measurements were made with a device of sun photometer.
In this study, our purpose is to generate AOT and PM10 maps over Penang Island,
Malaysia by using a handheld spectroradiometer. The study area of Penang Island,
Malaysia is located in equatorial region and enjoys a warm equatorial weather
the whole year, within latitudes 5° 9' N to 5° 33" N and longitudes
100° 09' E to 100° 30' E. The transmittance values were measured using
a handheld spectraradiometer between 9 a.m. to 11 p.m. on 11th April 2004. The
sampling locations were determined using a handheld GPS. The sensitivity of
this type of spectroradiometer is between the spectral wavelengths from 350
to 1050 nm. In order to derive AOT from spectroradiometer readings, we use the
Beer-Lambert-Bouguer law. In this method, the sky radiance values in the first
directly to be retrieval. Then the sky transmittance values were retrieved from
the sky radiance values. A total of 32-point transmittance measurements were
measured around Penang Island, Malaysia. The PM10 measurements were taken simultaneously
with the acquired transmittance measurements. Generally, a higher AOT value
indicates higher column of aerosol loading and therefore low visibility (Wang
and Christopher, 2003). The aim of this study carried out at the Penang
Island, Malaysia is to a set of measurements of sky transmittance. An AOT and
PM10 maps were generated using Kriging interpolating model. Interpolation methods
were used here to calculate the unknown value of interested points by referring
to the neighboring points within the same area or region. A linear relationship
between AOT and PM10 was found in this study. Finally, AOT was compute from
the atmospheric transmittance values. PM10 values were collected simultaneously
with the atmospheric transmittance using a DustTrak Aerosol Monitor 8520. This
study showed that there was a good correlation between the derived AOT and the
MATERIALS AND METHODS
Study area and data acquisition: The study area is the Penang Island, Malaysia, located within latitudes 5° 9' N to 5° 33' N and longitudes 100° 09' E to 100° 30' E (Fig. 1). The map of the study area is shown in Fig. 1. Penang Island is located in equatorial region and enjoys a warm equatorial weather the whole year. Therefore, it is impossible to get the 100% cloud free satellite image over Penang Island. But, the satellite image chosen is less than 10% of cloud coverage over the study area. Penang Island located on the northwest coast of Peninsular Malaysia.
Penang is one of the 13 states of the Malaysia and the second smallest state
in Malaysia after Perlis. The state is geographically divided into two different
entities - Penang Island (or Pulau Pinang in Malay Language) and a portion of
mainland called Seberang Perai in Malay Language. Penang Island is an island
of 293 square kilometres located in the Straits of Malacca and Seberang Perai
is a narrow hinterland of 753 square kilometres (Penang-Wikipedia,
|| The location of the study area
The island and the mainland are linked by the 13.5 km long Penang Bridge and
Penang Island is predominantly hilly terrain, the highest point being Western
Hill (part of Penang Hill) at 830 metres above sea level. The terrain consists
of coastal plains, hills and mountains. The coastal plains are narrow, the most
extensive of which is in the northeast which forms a triangular promontory where
George Town, the state capital, is situated. The topography of Seberang Perai
is mostly flat. Butterworth, the main town in Seberang Perai, lies along the
Perai River estuary and faces George Town at a distance of 3 km (2 miles) across
the channel to the east (Penang-Wikipedia, 2009).
The Penang Island climate is tropical and it is hot and humid throughout the
year. with the average mean daily temperature of about 27°C and mean daily
maximum and minimum temperature ranging between 31.4 and 23.5°C respectively.
However, the individual extremes are 35.7 and 23.5°C, respectively. The
mean daily humidity varies between 60.9 and 96.8%. The average annual rainfall
is about 267 cm and can be as high as 624 cm (Ahmad et
al., 2006). The corresponding transmittance and PM10 measurements were
collected at the several selected locations around the Penang Island between
9.00 to 11.00 a.m.
A handheld spectroradiometer was used to collect the sky transmittance data over Penang Island, Malaysia campus on 11th April 2004 from 32 stations in the USM campus (Fig. 2). The AOT was calculated based on the Beer-Lambert-Bouguer law.
Methodology: The transmittance data over Penang Island on 11th April 2004 was used in this study. The observation site is located in the northern region of the Peninsular Malaysia. The spectroradiometer measure the atmospheric transmittance from 350 to 1050 nm. The data that have been used in this study were collected at 32 locations in Penang, Malaysia.
The AOT is calculated based on the Beer-Lambert-Bouguer law. The AOT is related
to the transmission by the expression (Vermote et al.,
||transmittance for direct irradiance at wavelength, λ
||cosines (θ), θ is the zenith angle
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 Π and then divided by the reflectance of the Spectralon
||Second, we measured the diffuse irradiance. This step was performed with
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 handheld spectroradiometer
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 Π and then
dividing by the reflectance of the Spectralon panel
||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 Spancer (1971) as:
The day angle, φ, in radians is represented by:
where, d is the day number of a year (1-365)
||Finally, we computed the 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 Eq. 1
RESULTS AND DISCUSSION
In this study, we derived AOT values in the 550 nm only (Fig.
3). The AOT and PM10 maps over Penang Islands were created using a Kriging
interpolation technique for estimating aerosol values to be associated to their
distribution patterns (Fig. 4). PM10 was used as an aerosol
concentration in this study. We obtained a linear relationship between PM10
and AOT and correlation coefficient (R) of 0.81 and root-mean-squares error
(RMS) of 0.0544 μg m-3 (Fig. 5). The linear
correlation coefficient (R) is 0.81 which suggests that the PM10 concentration
measured at the earth surface is still linearly related with the AOT data (Fig.
5). It seems that the AOT values above the sampling sites increased as PM10
increased. Several studies also found this linear relationship (Wang
and Christopher, 2003).
From our experimental data set, we can see the PM10 increases as the AOT increases.
This mean that as the AOT values increases these is a increase of the concentration
|| Extract of a map of AOT
|| Extract of a map of PM10 (μg m-3)
||A linear correlation between AOT and PM10 (μg m-3)
Therefore, we obtained a positive correlation between AOT and PM10.
From the AOT and PM10 maps, we can see that the high concentration of the AOT and PM10 values was located near the Bayan Lepas and Prai industrial areas (Black circle) and Georgetown urban areas (Red circle) (Fig. 3, 4). This was due to the high density of population area and industrial area respectively. The air pollution caused by industry has even more grave effects than vehicle exhaust fumes
AOT retrieval was produced by the handheld spectroradiometer from the atmospheric
transmittance measurements. The measured sky radiance can be re-processed and
new results of the AOT values can be obtained. The interpolation PM10 map shown
by a visualization tool discloses that spectroradiometer is able to measure
AOT values. A linear relationship between PM10 and AOT was found by this study.
Findings can be used to analyze the air pollution levels over Penang Island.
This indicates that air quality can be retrieved from the spectroradiometer
The authors gratefully acknowledge the financial support from the Simulation and Modeling of the Atmospheric Radiative Transfer of Aerosols in Penang, account number: 203/PFZIK/671166 and Universiti Sains Malaysia-Short term grant. We would like to thank the technical staff who participated in this project. Special thanks are extended to the Brian Curtiss that provides me the methods of measure the sky transmittance using spectroradiometer. Thanks are also extended to USM for support and encouragement.