2. Heterogeneity in aerosol characteristics over the Indo-Gangetic Basin: Types and implications to radiative forcing
Though the level of scientific understanding of aerosols has been increased by incorporating different in-situ measurements along with various satellite measurements, still it is far less than that of greenhouse gases (IPCC, 2007). Heterogeneity in various aerosol characteristics over a wide range of spatial and temporal scales could be one of the major causes of difficulties in reducing the level of scientific understanding due to aerosols. Also, this is also one of the causes of enhanced uncertainty in estimation of radiative forcing. Thus, it is important to improve aerosol characterization on regional basis with high spatial and temporal resolutions; particularly over the region, where large heterogeneity may take place.
The Indo-Gangetic Basin (IGB) is one such region situated in northern part of India where aerosols from different sources exhibit large spatial and temporal variability in their loading and their various characteristics such as chemical, optical, microphysical and radiative. The region is one of the densely populated and heavily polluted regions of Indian subcontinent, surrounded by the variety of natural and man-made emission sources. It is of great research interest due to its unique topography surrounded by the Himalayas to the north, moderate hills to the south, Thar Desert and Arabian Sea in the west, and Bay of Bengal in the east. The mean aerosol optical depth (AOD) over the region was observed to be ~50% higher than any other regions in India, with relative abundance of fine-mode aerosol loading at the eastern part compared to the other parts of the IGB.
Although, satellites are proved to be the good tool to understand the broad
they are unable to provide an in-depth view of aerosol characterizations on local scale and pose higher uncertainties as compared to the ground-based instruments (Tripathi et al., 2005). As a result, aerosol properties change may lead to even larger uncertainty in satellite retrievals, as this is not considered in the aerosol retrieval algorithm (Kahn et al., 2009).
Fig. 1 The IGB region, illustrating unique topography surrounded by the variety of emission sources. (Adopted after modification from personal presentation of William K. M. Lau)
Significant heterogeneity optical and microphysical properties of aerosols and associated radiative impacts have been studied for the first time using ground-based automatic sun/sky radiometer measurements at different locations over IGB during the pre-monsoon period (April-June) (Srivastava et al., 2011; Tiwari et al., 2013), which is hypothesized to affect the Indian summer monsoon circulation and also the global climate system. The sun/sky radiometers were deployed world-wide (including few stations in India, particularly in north India) under the Aerosol Robotic Network (AERONET) program of NASA, USA (Holben et al., 1998). The pre-monsoon period is of particular interest because this is the key period when locally generated and regionally transported aerosol loading peaks over the IGB region and spread up to climatically sensitive regions of Himalayan foothills (Devi et al., 2011; Gautam et al., 2011; Srivastava et al., 2012a), which has been linked to influence
the monsoon circulation activities and rainfall distributions, particularly in India (Lau et al., 2006, 2010). Further, due to high level of anthropogenic emissions over the IGB, aerosol distribution in terms of types and loading undergo strong variability associated with the episodic yet strong influence of dust transport and biomass burning aerosols, particularly during the pre-monsoon period (Gautam et al., 2011; Srivastava et al., 2012b).
In a recent study, Srivastava et al. (2012b) have inferred different aerosol types over the central and eastern parts of IGB using multi-year ground-based sun/sky radiometer measured aerosol products associated with the size and radiation absorptivity of aerosols during pre-monsoon period. High dust enriched aerosols (i.e. polluted dust, PD) were found to contribute more over the central IGB region at Kanpur (~62%) as compared to the eastern IGB region at Gandhi College (~31%) whereas vice-versa was observed for polluted continental (PC) aerosols, which contain high anthropogenic and less dust particles. Contributions of carbonaceous particles having high absorbing (mostly black carbon, MBC) and low absorbing (mostly organic carbon, MOC) aerosols were found to be ~11% and 10%, respectively at Gandhi College, which was ~ 46% and 62% higher than the observed contributions at Kanpur; however, very less contribution of non-absorbing (NA) aerosols was observed only at Gandhi College (~2%). Further, Srivastava et al. (2013) in another study have also inferred similar aerosol types over the western IGB region at Delhi. The inferred aerosol types, e.g. PD, PC, MBC and MOC over Delhi were identified to be contributed ~48%, 32%, 11% and 9%, respectively to the total aerosols. The measured optical properties for the above inferred aerosol types differed considerably, which may influence the overall radiation budget and thereby climate change over the wide region.
