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Characterisation of Aerosols over Indo-Gangetic Basin during Winter Season
ByHiren Jethva
Ph.D. StudentCentre for Atmospheric & Oceanic Sciences
Indian Institute of ScienceBangalore-12
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Plan of Talk
● Why Aerosol Study???
● Introduction & Data Specifications
● Satellite and In-situ observations
● AERONET Vs. MODIS
● Discussion
● Conclusions
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Why Aerosol Study ???
● Aerosols are tiny particles of liquid and solids suspended in the atmosphere.
● It plays a vital role in the earth's atmosphere through its ability to alter
the radiation budget by scattering and absorption of incoming shortwave
and outgoing longwave radiation.
● It is believed to be a major source of uncertainty in weather and climate
prediction models due to its complex interaction with incoming
shortwave and outgoing longwave radiation.
● In the present industrial era, aerosols become prime concern for causing
air pollution and health related diseases.
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Introduction
● In the recent past, the issue has been raised regarding the persistent and widespread air pollution over Indo-Gangetic basin during winter season.
● Human activities (biomass burning) in this region have been blamed for
causing high air pollution.
● In the present study, the efforts has been made to characterize the aerosols over
Indo-Gangetic basin using different data sources.
● The comparison of satellite retrieved products (MODIS) with in-situ
measurements has been carried out to evaluate the performance of satellite
retrieval.
● The possible reasons for the discrepancy between satellite retrieval and in-situ
measurements has been discussed.
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AERONET Technical Specification
Ground-based aerosol measurement network
Goal Assessment of aerosol optical properties and validation of satellite retrievals of aerosol properties
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AERONET Technical Specification
● Standard Sunphotometer (CIMEL : 7 CE 318N VBS5)
● Polarized Sunphotometer (CIMEL CE 318N VPS8)
● Advanced Sunphotometer (CIMEL CE 318N EBS9)
● Aerosol Parameters (340 nm, 380 nm, 440 nm, 500 nm, 670 nm, 870nm, 1020 nm)
Aerosol Optical Depth
Abs. AOD
Angstrom Exponent
Single Scattering Albedo
Aerosol Volume Size Distribution
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Aerosol Parameters
● Aerosol Optical Depth:
● Angstrom Exponent :
slope
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● Aerosol Number Size Distribution:
Log-normal distribution
● Single-scattering Albedo:
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MODIS Technical Specification
● Orbit: 705 km, 10:30 a.m. descending node (Terra) 1:30 p.m. ascending node (Aqua), sun-synchronous, near-polar, circular , cross track
● Swath Dimensions: 2330 km (cross track) by 10 km (along track at nadir)
● Spectral Bands : 36
● Spatial Resolution: 250 m (bands 1-2)
500 m (bands 3-7)
1000 m (bands 8-36)
● Design Life: 6 years
Moderate Resolution Imaging Spectroradiometer
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We use data (1ºx1º) retrieved from first 7 spectral bands that are dedicated to Aerosol properties retrieval
1.AOD2.Angstrom Exponent
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Satellite Evidence of Aerosol Loading● A large Aerosol Optical Depth (AOD) over Indo-Gangetic region as well over
Bangladesh and north of Bay of Bengal from satellite MODIS.
4th Dec 2001
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14 Jan 2002
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High AOD (550 nm) over Thar Desert and Punjab (High Mass Concentration and Low Ang. Exp. 0.3) that decreases in SE direction
Ang. Exp. About 1.2 has been observed over southern edge of Himalaya
Monthly Climatology from MODIS
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Continued...
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Continued...
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Continued...
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Punjab
Kanpur
Aerosol Fine Fraction Mode
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Satellite Observations
● A high Aerosol Optical Depth and Mass Concentration over Thar Desert, Punjab and Indo-Gangetic basin.
● A belt of high AOD (0.5-0.6) moves from NW direction to SE from Oct to Jan
period.
● Angstrom Exponent of 1.2 on southern edge of Himalaya and not over Indo-
Gangetic basin!!!
In-Situ Measurements ???
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AERONET Observations
● Persistent high Aerosol Optical Depth (>0.5).
● Angstrom Exponent varies between 1 and 1.5 through out winter
season!!!
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Nov 2001
AOD
Ang. Exp.
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AOD
Ang. Exp.
Dec 2001
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Nov 2002
AOD
Ang. Exp.
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● A large difference in magnitude of Angstrom Exponent between AERONET and MODIS.
Why ???
● AOD at shorter wavelength (470 nm) is found to be higher in AERONET than
in MODIS.
AERONET
MODIS
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Single-scattering Albedo
Fine
Coarse
Total
Lower single-scattering albedo (coarse particles) during winter season.
Internal Aerosol Mixing ???
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Aerosol Volume Size Distribution
Bi-modal Aerosol Size Distribution
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● Basis: The presence of UV absorbing particles reduces the spectral dependence of top of the atmosphere radiance from pure molecular atmosphere ( dependence)
● Advantages: Low surface reflectivity in UV spectrum
Retrieve absorbing aerosols (dust and BC)
A quarter of century data (from 1979 to present)
● Disadvantages: Low resolution (1.25 x 1 degree)
Sub-pixel cloud contamination AI does not have physical meaning
TOMS Aerosol Index
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Dust episodes are well resolved but not biomass burning!
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Monthly Climatology Winter
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Monthly Climatology Summer
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Monthly Climatology Monsoon
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NCEP Monthly Wind Vector 850 mb
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Continued...
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Continued...
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Satellite Retrieval of Aerosol Properties
Satellite measured spectral radiance
Filtering of aerosol back-scattered signalby estimating surface reflection
Create a Look-Up tables withassumed (expected) aerosol models
Pre-radiative transfer calculations for assumed (expected) aerosol models
Match the satellite measuredspectral radiances with Look-Up table
Retrieval of Aerosol Model
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Dark Target ApproachShort wave surface reflection Vs mid-IR surface reflection
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Assumed Regions of Aerosol Models in MODIS Retrieval
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Conclusions
● A belt of aerosol loading has been observed over Indo-Gangetic basin through out the winter season from satellite MODIS images.
● In-situ measurements (AERONET) confirm high aerosol opacity over Kanpur
station.
● The angstrom exponent is found to be underestimated by MODIS algorithm
due to lower optical depth at shorter wavelength (470 nm).
● Coarser particles induces lower single-scattering albedo that gives the hint of
internal aerosol mixing.
● Bi-modal aerosol volume size distribution indicates the presence of sub-micron
as well as larger size particles over Kanpur station.
● TOMS Aerosol Index found to be a strong indicator of dust events and not
biomass burning.
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• The persistent wind pattern in the basin may be responsible for widespread of aerosols.
• The inappropriate aerosol models and uncertainty in surface reflection estimation are
believed to be the possible reason for the discrepancy between MODIS and
AERONET
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Thank You!!!