Characterisation of Aerosols over Indo-Gangetic Basin during Winter Season

ByHiren Jethva

Ph.D. StudentCentre for Atmospheric & Oceanic Sciences

Indian Institute of ScienceBangalore-12

Plan of Talk

● Why Aerosol Study???

● Introduction & Data Specifications

● Satellite and In-situ observations

● AERONET Vs. MODIS

● Discussion

● Conclusions

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.

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.

AERONET Technical Specification

Ground-based aerosol measurement network

Goal Assessment of aerosol optical properties and validation of satellite retrievals of aerosol properties

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

Aerosol Parameters

● Aerosol Optical Depth:

● Angstrom Exponent :

slope

● Aerosol Number Size Distribution:

Log-normal distribution

● Single-scattering Albedo:

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

We use data (1ºx1º) retrieved from first 7 spectral bands that are dedicated to Aerosol properties retrieval

1.AOD2.Angstrom Exponent

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

14 Jan 2002

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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Punjab

Kanpur

Aerosol Fine Fraction Mode

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 ???

AERONET Observations

● Persistent high Aerosol Optical Depth (>0.5).

● Angstrom Exponent varies between 1 and 1.5 through out winter

season!!!

Nov 2001

AOD

Ang. Exp.

AOD

Ang. Exp.

Dec 2001

Nov 2002

AOD

Ang. Exp.

● 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

Single-scattering Albedo

Fine

Coarse

Total

Lower single-scattering albedo (coarse particles) during winter season.

Internal Aerosol Mixing ???

Aerosol Volume Size Distribution

Bi-modal Aerosol Size Distribution

● 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

Dust episodes are well resolved but not biomass burning!

Monthly Climatology Winter

Monthly Climatology Summer

Monthly Climatology Monsoon

NCEP Monthly Wind Vector 850 mb

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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

Dark Target ApproachShort wave surface reflection Vs mid-IR surface reflection

Assumed Regions of Aerosol Models in MODIS Retrieval

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.

• 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

Thank You!!!