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Abstract. Over the Indian region, aerosol absorption is considered to have a potentialimpact on the regional climate, monsoon and hydrological cycle. Black carbon(BC) is the dominant absorbing aerosol, whose absorption potential isdetermined mainly by its microphysical properties, including itsconcentration, size and mixing state with other aerosol components. TheIndo-Gangetic Plain (IGP) is one of the regional aerosol hot spots withdiverse sources, both natural and anthropogenic, but still the informationon the mixing state of the IGP aerosols, especially BC, is limited and asignificant source of uncertainty in understanding their climaticimplications. In this context, we present the results from intensivemeasurements of refractory BC ( rBC ) carried out over Bhubaneswar, an urbansite in the eastern coast of India, which experiences contrasting air masses(the IGP outflow or coastal/marine air masses) in different seasons. Thisstudy helps to elucidate the microphysical characteristics of BC over thisregion and delineates the IGP outflow from the other air masses. Theobservations were carried out as part of South West Asian Aerosol MonsoonInteractions (SWAAMI) collaborative field experiment during July 2016–May 2017, using a single-particle soot photometer (SP2) that uses alaser-induced incandescence technique to measure the mass and mixing stateof individual BC particles and an aerosol chemical speciation monitor (ACSM)to infer the possible coating material. Results highlighted the distinctivenessin aerosol microphysical properties in the IGP air masses. BC massconcentration was highest during winter (December–February) ( ∼ 1.94 ± 1.58 µ g m −3 ), when the prevailing air masses weremostly of IGP origin, followed by post-monsoon (October–November) (mean ∼ 1.34 ± 1.40 µ g m −3 ). The mass median diameter(MMD) of the BC mass size distributions was in the range 0.190–0.195  µ m, suggesting mixed sources of BC, and, further, higher values( ∼  1.3–1.8) of bulk relative coating thickness (RCT) (ratio ofoptical and core diameters) were seen, indicating a significant fraction ofhighly coated BC aerosols in the IGP outflow. During the pre-monsoon(March–May), when marine/coastal air masses prevailed, BC mass concentrationwas lowest ( ∼ 0.82 ± 0.84 µ g m −3 ), and largerBC cores (MMD  >  0.210  µ m) were seen, suggesting distinctsource processes, while RCT was ∼  1.2–1.3, which may translateinto higher extent of absolute coating on BC cores, which may have crucialregional climate implications. During the summer monsoon (July–September),BC size distributions were dominated by smaller cores (MMD  ≤  0.185  µ m), with the lowest coating indicating fresher BC, likely fromfossil fuel sources. A clear diurnal variation pattern of BC and RCT wasnoticed in all the seasons, and daytime peak in RCT suggested enhancedcoating on BC due to the condensable coating material originating fromphotochemistry. Examination of submicrometre aerosol chemical compositionhighlighted that the IGP outflow was dominated by organics (47 %–49 %), andmarine/coastal air masses contained higher amounts of sulfate (41 %–47 %),while ammonium and nitrate were seen in minor amounts, with significantconcentrations only during the IGP air mass periods. The diurnal pattern ofsulfate resembled that of the RCT of rBC particles, whereas organic massshowed a pattern similar to that of the rBC mass concentration. Seasonally,the coating on BC showed a negative association with the mass concentrationof sulfate during the pre-monsoon season and with organics during thepost-monsoon season. These are the first experimental data on the mixing stateof BC from a long time series over the Indian region and include newinformation on black carbon in the IGP outflow region. These data help inimproving the understanding of regional BC microphysical characteristics andtheir climate implications.

atmospheric chemistry and physics

Seasonal contrast in size distributions and mixing state of black carbon and its association with PM&amp;lt;sub&amp;gt;1.0&amp;lt;/sub&amp;gt; chemical composition from the eastern coast of India