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Seasonal contrast in size distributions and mixing state of black carbon and its association with PM1.0 chemical composition from the eastern coast of India

Kompalli, SK and Babu, SNS and Krishnakumari Satheesh, S and Moorthy, KK and Das, T and Boopathy, R and Liu, D and Darbyshire, E and Allan, JD and Brooks, J and Flynn, MJ and Coe, H (2020) Seasonal contrast in size distributions and mixing state of black carbon and its association with PM1.0 chemical composition from the eastern coast of India. In: Atmospheric Chemistry and Physics, 20 (6). pp. 3965-3985.

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Official URL: https://dx.doi.org/10.5194/acp-20-3965-2020


Over the Indian region, aerosol absorption is considered to have a potential impact on the regional climate, monsoon and hydrological cycle. Black carbon (BC) is the dominant absorbing aerosol, whose absorption potential is determined mainly by its microphysical properties, including its concentration, size and mixing state with other aerosol components. The Indo-Gangetic Plain (IGP) is one of the regional aerosol hot spots with diverse sources, both natural and anthropogenic, but still the information on the mixing state of the IGP aerosols, especially BC, is limited and a significant source of uncertainty in understanding their climatic implications. In this context, we present the results from intensive measurements of refractory BC (rBC) carried out over Bhubaneswar, an urban site in the eastern coast of India, which experiences contrasting air masses (the IGP outflow or coastal/marine air masses) in different seasons. This study helps to elucidate the microphysical characteristics of BC over this region and delineates the IGP outflow from the other air masses. The observations were carried out as part of South West Asian Aerosol Monsoon Interactions (SWAAMI) collaborative field experiment during July 2016-May 2017, using a single-particle soot photometer (SP2) that uses a laser-induced incandescence technique to measure the mass and mixing state of individual BC particles and an aerosol chemical speciation monitor (ACSM) to infer the possible coating material. Results highlighted the distinctiveness in aerosol microphysical properties in the IGP air masses. BC mass concentration was highest during winter (December-February) ( &tild; 1:94±1:58 μgm-3), when the prevailing air masses were mostly of IGP origin, followed by post-monsoon (October- November) (mean &tild; 1:34±1:40 μgm-3). The mass median diameter (MMD) of the BC mass size distributions was in the range 0.190-0.195 μm, suggesting mixed sources of BC, and, further, higher values (&tild;1.3-1.8) of bulk relative coating thickness (RCT) (ratio of optical and core diameters) were seen, indicating a significant fraction of highly coated BC aerosols in the IGP outflow. During the pre-monsoon (March-May), when marine/coastal air masses prevailed, BC mass concentration was lowest (&tild; 0:82±0:84 μgm-3), and larger BC cores (MMD>0.210 μm) were seen, suggesting distinct source processes, while RCT was &tild;1.2- 1.3, which may translate into higher extent of absolute coating on BC cores, which may have crucial regional climate implications. During the summer monsoon (July- September), BC size distributions were dominated by smaller cores (MMD0.185 μm), with the lowest coating indicating fresher BC, likely from fossil fuel sources. A clear diurnal variation pattern of BC and RCT was noticed in all the seasons, and daytime peak in RCT suggested enhanced coating on BC due to the condensable coating material originating from photochemistry. Examination of submicrometre aerosol chemical composition highlighted that the IGP outflow was dominated by organics (47 -49 ), and marine/coastal air masses contained higher amounts of sulfate (41 -47 ), while ammonium and nitrate were seen in minor amounts, with significant concentrations only during the IGP air mass periods. The diurnal pattern of sulfate resembled that of the RCT of rBC particles, whereas organic mass showed a pattern similar to that of the rBC mass concentration. Seasonally, the coating on BC showed a negative association with the mass concentration of sulfate during the pre-monsoon season and with organics during the post-monsoon season. These are the first experimental data on the mixing state of BC from a long time series over the Indian region and include new information on black carbon in the IGP outflow region. These data help in improving the understanding of regional BC microphysical characteristics and their climate implications. © Author(s) 2020.

Item Type: Journal Article
Publication: Atmospheric Chemistry and Physics
Publisher: Copernicus GmbH
Additional Information: Copyright to this article belongs to Copernicus GmbH
Keywords: absorptionaerosolair massblack carbonchemical compositionclimate effectclimate forcingmixingparticulate matterphotometerseasonal variationsize distribution
Department/Centre: Division of Mechanical Sciences > Divecha Centre for Climate Change
Division of Mechanical Sciences > Centre for Atmospheric & Oceanic Sciences
Date Deposited: 29 Oct 2020 07:02
Last Modified: 29 Oct 2020 07:02
URI: http://eprints.iisc.ac.in/id/eprint/65223

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