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
Authors: Kompalli Sobhan Kumar , Suresh Babu Surendran Nair , Satheesh Sreedharan Krishnakumari , Krishna Moorthy Krishnaswamy , Das Trupti , Boopathy Ramasamy , Liu Dantong , Darbyshire Eoghan , Allan James , Brooks James , Flynn Michael , Coe Hugh 
Published: May 2019

Abstract. Over the Indian region, aerosol absorption is considered to have potential impact on regional climate, monsoon and hydrological cycle. Black carbon (BC) is the dominant absorbing aerosol, whose absorption potential is largely determined by its microphysical properties, including its concentration, size and mixing state with other aerosol components. The Indo-Gangetic Plains (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 major 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 airmasses (the IGP outflow or coastal/marine airmasses) in different seasons. This study helps to elucidate the microphysical characteristics of BC over this region and delineates the IGP outflow from the other airmasses. 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). Results highlighted the distinctiveness in aerosol microphysical properties in the IGP airmasses. BC mass concentration was highest during winter (~ 1935 ± 1578 ng m−3), when the prevailing air masses were mostly of IGP origin, followed by post-monsoon (mean ~1338 ± 1396 ng m−3). Mass median diameter (MMD) of the BC mass size distributions were in the range 0.190–0.195 µm suggesting mixed sources of BC, and further, higher values (~ 1.3–1.8) of bulk relative coating thickness (RCT) (ratio of optical and core diameters) were seen indicating a large fraction of highly coated BC aerosols in the IGP outflow. During the pre-monsoon, when marine/coastal airmasses prevailed, BC mass concentration was lowest (~ 816 ± 835 ng m−3) and larger BC cores (MMD > 0.210 µm) were seen suggesting distinct source processes, while RCT was ~ 1.2–1.3, which may translate into higher extent of absolute coating on BC cores which may have important regional climate implications. During the summer monsoon, BC size distributions were dominated by smaller cores (MMD ≤ 0.185 µm) with 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 day time peak in RCT suggested enhanced coating on BC due to the condensable coating material originated from photochemistry. Examination of sub-micron aerosol chemical composition highlighted that the IGP outflow is dominated by organics (47–49 %) and marine/coastal airmasses contained greater amounts of sulphate (41–47 %), while ammonium and nitrate were seen in minor amounts with significant concentrations only during the IGP airmass periods. The diurnal pattern of sulphate resembled that of the RCT of rBC particles, whereas organic mass showed a pattern similar to that of the rBC mass concentration. Though the pre-monsoon is sulphate dominated, the coating on BC showed a negative association with sulphate and same is true for organic mass during the post-monsoon, suggesting preferential coating and importance of source processes (and co-emitted species) on the mixing state of BC. This is the first experimental data on the mixing state of BC from a long time series over the Indian region, and includes new information on black carbon in the IGP outflow region. This data helps in improving the understanding of regional BC microphysical characteristics and their climate implications.