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IASTA e-Bulltein  | Vol. 3, No. 1 Home
Remote Sensing of Aerosols: Research Gaps and Needs in India

Rationale

Atmospheric aerosols exhibit large temporal and spatial variability due to a variety of production, removal and transport processes. The diversified sources (both natural and anthropogenic) and short life time result in the aerosol characterization a real challenge. The interesting phenomenon such as "Atmospheric Brown Cloud (ABC)", occurring during the winter period and its impacts on health, agriculture and climate on both regional and global scales, has led to considerable concern and further intense research. Besides the paucity of sufficient experimental data in the region, study of the variability in natural aerosols is noticed to be one of the critical issues that aids for better understanding and analysis of this complex problem. Aerosol radiative forcing is recognized to be one of the largest sources of uncertainties in assessing future climate change. The direct radiative forcing of anthropogenic aerosols from sulphates, fossil fuel soot, and organic aerosols range from -0.25 to -1.0 Wm-2 while the indirect radiative forcing estimates range from 0 to -1.5 Wm-2. The radiative forcing of greenhouse gases on the other hand ranges from +2.1 to + 2.8 Wm-2. These estimates show that the magnitude of aerosol radiative forcing is almost equal to that of greenhouse gases but opposite in sign. Thus the issue underlying potential aerosol-climate interactions needs thorough understanding of the presence of aerosols and their associated radiative effects, both in the context of the present-day climate and climate change scenarios. Long-term measurement of aerosol properties is an essential requisite for such exercises including seamless prediction and verification of the presence of aerosols in climate modification.

The aerosol optical depth, which may be derived from measurements of attenuated direct solar radiation, is a measure of aerosol radiative forcing at all the temporal and spatial scales. To better understand the direct and indirect aerosol effects, it is necessary to know not only the aerosol optical depth,
but also the aerosol scattering and absorption properties separately, and their vertical profiles. The radiative effects of aerosols on regional climate forcing can be better understood by isolating the local influences. Moreover, there is a lack of knowledge about the distributions of aerosols in the free troposphere, particularly in the tropical zone - one of the less-studied regions of the Planet. The aerosols in the boundary layer are directly produced from the natural and anthropogenic processes while those in the troposphere and aloft are largely due to gas-to-particle conversion processes. Thus the boundary layer aerosol system is different from that of the troposphere. As a result, removal of boundary layer aerosol component from that of the columnar (in quiescent conditions), grossly represents free tropospheric aerosol component. In the last few years, the interest in the local/ regional/ continental/ hemi-spherical/ global distributions and associated large-scale effects of free tropospheric aerosols has increased. Most such observations are sporadic, for short periods of time and for only a few places on Earth.

Currently, aerosol forcing estimates are obtained primarily from models. In order to obtain information on wider scale and global level, the satellite-based remote sensing data need to be used. For better characterization of aerosol properties, multi-angle and polarization remote sensing data are also required. For this purpose, suitable methods for retrieving aerosol parameters over large areas need to be developed by way of both the experimental and modeling approaches. Besides more campaign-mode experiments that are needed to establish the results, there is also a necessity to establish statistical significance of the results by making model runs for several decades and using ensemble technique to isolate the model variability in the retrieval of aerosol parameters. While the retrieval of the spatial and temporal distributions of aerosols on a global scale is a task for satellite measurements, the increased emphasis on satellite aerosol retrievals has created the need for accurate ground-based, ground-truth aerosol measurements with which to validate satellite aerosol retrievals over land. In addition, measurements from ground-based lidars and sun photometers
can be used to produce land-based aerosol climatology and that is complementary to the satellite retrievals.

Background

Aerosols affect the climate in several ways. They can influence the radiation budget and hence dynamics directly by interacting with solar and terrestrial radiation, and indirectly by their effect on cloud microphysics, precipitation and albedo. In addition, the aerosol residence time and transport mechanisms are different in different regions of the atmosphere. While a great deal of attention has been directed towards the influence of stratospheric aerosols on climate, the treatment of tropospheric aerosols, particularly in the boundary-layer where the natural and anthropogenic contributions are significant, has become a difficult task mainly because of their shorter residence time.

Albeit the parameters such as columnar aerosol optical depth (AOD) and corresponding size distribution are useful for the study of seasonal and long-term trends. Apart from the regular measurements of aerosols using lidars and solar radiometers at different research and academic institutions, several field programs mainly aimed at "characterization of aerosols" have been carried out in India; most of them have been conducted in campaign-mode. With the initiation of the MONsoon EXperiment (MONEX-79); Indian Middle Atmosphere Program (IMAP); Geosphere Biosphere Program (GBP); INDian Ocean Experiment (INDOEX); Indian Climate Research Program (ICRP, and now ICRB, Indian Climate Research Board) and with the establishment of scientific bodies such as Indian Aerosol Science and Technology Association (IASTA), scientists are able to coordinate their research work more effectively in advancing the knowledge in the field of Aerosol Science and Technology.


Future Scope and Needs

Considering the continuation of aerosol observations from the ongoing research programs, and more new proposal for making detailed aerosol measurements, using portable systems,
© 2015 Indian Aerosol Science and Technology Association