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

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 5 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 10 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 15 (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 20 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 25 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 sig...

Black carbon physical and optical properties across northern India during pre-monsoon and monsoon seasons

“…Despite monsoon arrival, the IGP BC characteristics remained similar despite some BC removal, with moderately coated BC particles prevailing. This is consistent with the fuel use patterns (including forest fires/biomass-burning episodes) across the IGP at this time of year (Kumar et al, 2011;Vadrevu et al, 2012;Kaskaoutis et al, 2014).…”

“…The IGP BC mass concentrations are elevated somewhat compared to other locations across northern India due to the strength of the emissions sources as well as being in close proximity to the local emissions (Brooks et al, 2019). Our BC mass concentrations are comparable to mean values reported for urban regions, by Kompalli et al (2019) in north-east India (∼ 0.8 µg m −3 ) and Liu et al (2014) in London (∼ 1.3 µg m −3 ), and greater than values in the city of Paris (∼ 0.9 µg m −3 ) in Laborde et al (2013). However, they are much lower than those reported in Chinese cities, such as Beijing (∼ 5.5 µg m −3 ; Wu et al, 2016) and Shenzhen (∼ 4.1 µg m −3 ; Huang et al, 2012), though the earlier studies are from urban surface sites and not regional aircraft measurements.…”

“…It has been found that BC mass concentrations and radiative forcing estimates need significant adjustments in model work (Bond et al, 2013), and aerosol optical depth (AOD) values are often underestimated by models (Govardhan et al, 2019;Xian et al, 2019) and when compared to remote sensing studies (Pan et al, 2015). The work of this paper builds upon recent experimental data on the mixing state of BC from the eastern coast of India from Kompalli et al (2019) and includes new information on BC throughout northern India. By providing high-temporal-and high-spatial-resolution data for the pre-monsoon and monsoon seasons, future model work will have better informed mass concentrations and mixing state information.…”

“…Prior to our study, only a few surface-based measurements of BC using a single-particle soot photometer (SP2) had been made (Raatikainen et al, 2017;Thamban et al, 2017;Kompalli et al, 2019). The data presented in this work represent the first vertically resolved measurements of BC mass and mixing state using an SP2 during the pre-monsoon and monsoon seasons over northern India.…”

“…The data presented in this work represent the first vertically resolved measurements of BC mass and mixing state using an SP2 during the pre-monsoon and monsoon seasons over northern India. Previous studies (Rana et al, 2019) have been unable to identify particular sources of BC from ambient measurements, though some insight into larger BC particles has been possible (Kumar et al, 2011;Vadrevu et al, 2012;Kaskaoutis et al, 2014). Previously SP2 data from urban environments such as London and China have been used to determine contributions to BC from different sources based on the core size and coating thickness, the latter being represented as scattering enhancement (Liu et al, 2014(Liu et al, , 2019.…”