Seasonal contrast in size distributions and mixing state of black carbon and its association with PM<sub>1.0</sub> chemical composition from the eastern coast of India

Abstract

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.

Mixing state of refractory black carbon aerosol in the South Asian outflow over the northern Indian Ocean during winter

“…Recently, Ko et al (2020) compared the MMD and NMD values of rBC size distributions from different dominant sources. Previous studies report that the MMD (and NMD) values over the regions dominated by fresh fossil fuel emissions are smaller (MMD ∼ 100 to 178 nm and NMD ∼ 30 to 80 nm) compared to the areas with dominant solid-fuel sources (biomass, biofuel, coal-burning) (MMD ∼ 130 to 210 nm and NMD ∼ 100 to 140 nm), whereas well-aged and background BC particles in outflow regions have MMD values in between (MMD ∼ 180 to 225 nm and NMD ∼ 90 to 120 nm) (McMeeking et al, 2010;Liu et al, 2010Liu et al, , 2014Kondo et al, 2011;Cappa et al, 2012;Sahu et al, 2012;Metcalf et al, 2012;Laborde et al, 2013;Reddington et al, 2013;Gong et al, 2016;Raatikainen et al, 2017;Krasowsky et al, 2018;Brooks et al, 2019;Kompalli et al, 2020b;Ko et al, 2020). The spatial distribution of mass median diameter and number median diameters during the ICARB-2018 shown in Fig.…”

Chemical Characteristics, Size Distributions, and Aerosol Liquid Water in Size-Resolved Coastal Urban Aerosols Allied with Distinct Air Masses over Tropical Peninsular India

“…It is well-known that EC is directly emitted into the atmosphere as fine particle via incomplete combustion of fossil fuel and biomass burning whereas coarse mode EC may be associated with adsorption of water molecule to the surface of EC particles or coating by organic/inorganic acids during their long-range transport in the atmosphere, leading to substantial increase in the particle diameter. 57 A robust correlation (0.96) was found between EC and nss-K + in fine mode, implying that biomass burning is a major source in fine mode. It is also worthy to note that EC was strongly correlated with OC in fine mode (0.90), however no such correlation was observed in the coarse mode (−0.02).…”

“…In contrast, EC showed unimodal during April 2018 and Sept. 2018 and bimodal size distributions during Nov. 2017, Jan. 2017, and March 2019, although the prominent peak was observed in fine mode. It is well-known that EC is directly emitted into the atmosphere as fine particle via incomplete combustion of fossil fuel and biomass burning whereas coarse mode EC may be associated with adsorption of water molecule to the surface of EC particles or coating by organic/inorganic acids during their long-range transport in the atmosphere, leading to substantial increase in the particle diameter . A robust correlation (0.96) was found between EC and nss-K + in fine mode, implying that biomass burning is a major source in fine mode.…”

Cloud condensation nuclei characteristics during the Indian summer monsoon over a rain-shadow region

“…The association of BC with accumulation mode APs during continental conditions suggests that the carbonaceous APs existing in this size range can act as CCN. From the eastern coast of India, Kompalli et al (2020) have reported highly coated larger BC particles (> 110 nm) in dry conditions under the continental influence, while nascent BC particles (∼ 80 nm) with less coating were found during ISM due to wet scavenging. This finding is in line with the current marine-2 observations.…”