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 D. , Brooks James , Flynn Michael J. , Coe Hugh 
Published: April 2020

Abstract. 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) (∼1.94±1.58 µg m−3), when the prevailing air masses were
mostly of IGP origin, followed by post-monsoon (October–November) (mean
∼1.34±1.40 µg m−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
(∼ 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 (∼0.82±0.84 µg 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 crucial
regional climate implications. During the summer monsoon (July–September),
BC size distributions were dominated by smaller cores (MMD ≤ 0.185 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.