Carbonaceous aerosols and their light absorption properties over the Bay of Bengal during continental outflow

Marine atmosphere of the Bay of Bengal (BoB) is prone to get impacted by anthropogenic aerosols from the Indo-Gangetic Plain (IGP) and Southeast Asia (SEA), particularly during the Northeast Monsoon (NEM). In this study, we quantify and characterize carbonaceous aerosols and their absorption properties collected in two cruise campaigns onboard ORV Sindhu Sadhana during the continental outflow period over the BoB. Aerosol samples were classified based on the air mass back trajectory analyses, wherein samples were impacted by continental air parcel (CAP), marine air parcel (MAP), and mix of both (CAP+MAP). Significant variability in PM10 mass concentration (in μg m-3) is found with maximum value for MAP samples (75.5±36.4) followed by CAP+MAP (58.5±27.3) and CAP (58.5±27.3). The OC/EC ratio (> 2) and diagnostic tracers i.e. nss-K+/EC (0.2-0.96), nss-K+/OC (0.11-1.32) along with absorption angstrom exponent (AAE: 4.31-6.02) and MODIS (Moderate Resolution Imaging Spectroradiometer) derived fire counts suggest the dominance of biomass burning emission sources. A positive correlation between OC and EC (i.e. r = 0.86, 0.70, 0.42 for CAP, MAP, and CAP+MAP, respectively) further confirmed the similar emission sources of carbonaceous species. Similarly, a significant correlation between estimated secondary organic carbon (SOC) and water-soluble organic carbon (WSOC; r = 0.99, 0.96, and 0.97 for CAP, MAP, and CAP+MAP, respectively) indicating their similar chemical nature as well as dominant contribution of SOC to WSOC. Absorption coefficient (b_'abs-365' ) and mass absorption efficiency (MAEBrC-365) of soluble fraction were estimated at 365 nm wherein, b_'abs-365' showed a linear relationship with WSOC and nss-K+, signifying contribution of water soluble brown carbon from biomass burning emissions. The estimated MAEBrC-365 (0.30-0.93 m2 g-1), during this study, were consistent with the earlier observations over the BoB, particularly during continental outflow season.