Impact of Black Carbon on Climate

Aerosols influence climate indirectly by changing cloud properties and precipitation and hence can have a profound impact on the hydrological cycle. Among the various aerosol types, black carbon aerosols have greater impact because of the high absorption of solar radiation. Hence it is essential that measurements of black carbon aerosols from ground, aircraft and space are necessary to answer crucial questions related to the impact of black carbon on climate. Several studies have suggested that aerosols may modify global warming by changing the planetary albedo, but the magnitude of the impact of aerosol on climate is still uncertain says IPCC 2007. 

Measurement of Aerosols over Indian Region: Current Status  

There is a large spatial and temporal variability of aerosols in India and hence several field campaigns were undertaken to characterize the optical, physical and chemical properties of aerosols and their radiative impact. The major goals of these experiments have been the characterization of regional aerosol properties and estimation of their direct and indirect radiative forcing. In India, a systematic investigation of the physico-chemical properties of aerosols, their temporal heterogeneities, spectral characteristics, size distribution and modulation of their properties by regional mesoscale and synoptic meteorological processes have been investigated extensively since the 1980s at different  regions as  a part of the different national programs such as the I-MAP (Indian Middle Atmosphere Programme), and later the ISRO-GBP (Indian Space Research Organization’s Geosphere Biosphere Program). 

   

During the I-MAP, a project was initiated to monitor the aerosol characteristics over the Indian region at a few selected locations. This became operational in the late eighties and has been continued after the I-MAP as a part of ACE (Aerosol Climatology and Effects) project of the ISRO-GBP. A national network of multi-wavelength radiometers (MWR) was setup under the ACE project of the ISRO-GBP, to facilitate the long-term observations of aerosols over distinct geographical environments.

Measurements of Black Carbon Aerosols  

As a part of the (INDOEX) extensive measurement of black carbon was carried out over the Indian Ocean. Based on these measurements Satheesh et al. (1999) developed an aerosol model for tropical Indian Ocean, which demonstrated that black carbon contributes 11% to composite aerosol optical depth. Later, using several calibrated satellite radiation measurements and five independent surface radiometers Satheesh and Ramanathan (2000) showed that even though black carbon contributes 11% to optical depth its contribution to radiative forcing can be as much as 60%. Over continental India, Babu and Moorthy (2001) reported the anthropogenic impact of aerosol black carbon mass concentration at a tropical coastal station (Trivandrum). This is probably the first report of black carbon measurement over continental India. Thereafter, several investigators have measured  black carbon measurements at various locations in India.

A road/land campaign (LC-I) was conducted during February to March 2004 under ISRO-GBP initiative, to understand the spatial distribution of aerosol and trace gases over Central/peninsular India. Simultaneous measurements were made over spatially separated locations, using identical instruments. These measurements covered an area of more than a million square kilometres over the course of a month from land based mobile laboratories, and generated a wealth of information on black carbon as well as important aerosol parameters including size, mass concentration, optical depth, and scattering and absorption coefficients using state of the art instruments. 

As a continuation of this experiment, Land Campaign II (LC-II) was organized by the Indian Space Research Organization under ISRO-GBP during December 2004, to characterize the regional aerosol properties and trace gases across the entire Indo- Gangetic belt. All these studies showed the persistence of high aerosol optical depth and black carbon concentrations near the surface. 

The Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) was a multi-institutional, multi-instrumental, multi-platform field campaign, where integrated observation and measurements of aerosols with special emphasis on black carbon, radiation and trace gases along with other complementary measurements on boundary layers and meteorological parameters were made simultaneously. The main goal of the ICARB was to assess the regional radiative impact of aerosols and trace gases, and to quantify the effect of the long-range transport of aerosols and trace gases, involving the Indian mainland, the Arabian Sea, the Bay of Bengal, and tropical Indian Ocean during February-May period of 2006. The ICARB was conceived as an integrated campaign, comprising three segments namely the land, ocean, and aircraft segments. In each one of these segments, measurements of the optical, physical and chemical properties of atmospheric aerosols were carried out. The land segment comprised a network of ground-based observatories, representing distinct geographical features of India, and providing a time-series observation during the period when spatially resolved measurements were made using the moving platforms in the other two segments.  

Quantitative estimates of the vertical structure and the spatial gradients of aerosol extinction coefficients have been made from airborne LIDAR measurements across the coastline and around the oceanic regions along the east and west coasts of India.  

The research group at Physical Research Laboratory (PRL) lead by Prof. M.M. Sarin has made substantial efforts in characterizing organic carbon/ black carbon ratios over a few locations in India. The atmospheric abundances of elemental carbon (EC), organic carbon (OC) and water-soluble organic carbon (WSOC) have been measured by this group in aerosol samples collected during wintertime (December-March) from selected sites (urban, rural and high-altitude) in northern India. 

     

Even though all these international and national field experiment and campaigns have provided vital information on the optical, physical as well as chemical properties of aerosols, they are limited to a certain period or location. In this perspective, the long term experiments at different locations have the added advantages of understanding aerosol influences on a longer time scale thereby helping us to infer the signs of anthropogenic impact. A sufficiently long time series can also help in inferring climate change signals. 

The ISRO-GBP is maintaining 32 surface observatories covering representative locations in India. In all these sites black carbon measurements have been made. The duration of data available from these sites vary with location. In addition there have been a few field campaigns such as LC-I, LC-II and ICARB. Thus, we have information on the spatial and seasonal variation of BC at the Earth’s surface. ICARB aircraft segment carried out a few measurements of altitude profiles of black carbon aerosols. Our knowledge on the OC/BC ratios are based on isolated measurements only.

There are several aspects on which the knowledge and understanding is rather fragmented and inadequate. These include Vertical distribution of Black Carbon State of mixing of Black Carbon with other aerosols Organic Carbon / Black Carbon Ratio Effect of Black Carbon on Cloud Cover and Effect of Black Carbon on Monsoon.

A hybrid approach, which involves field experiments including network measurements as well as aircraft-based field measurements along with multi-satellite analyses is essential for the assessment of the impact of aerosol black carbon over India. Combining ground measurements with multi-satellite data can create synergy that will benefit both methods. This approach will provide new insights into the problem and new methodologies to gather information on black carbon aerosol can be formulated. Using the outcome of this project, crucial questions related to climate impact of black carbon aerosols can be addressed. 

Courtesy : PIB Features (Kalpana Palkhiwala)