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

Abstract

The 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 the continental air parcel (CAP), marine air parcel (MAP), and mix of both (CAP + MAP). Significant variability in the PM₁₀ mass concentration (in μg m⁻³) is found with a 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) and nss-K⁺/OC (0.11–1.32) along with the 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, and 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) indicate their similar chemical nature as well as dominant contribution of SOC to WSOC. The absorption coefficient (b_abs-365) and mass absorption efficiency (MAE_BrC-365) of the soluble fraction were estimated at 365 nm wherein, b_abs-365 showed a linear relationship with WSOC and nss-K⁺, signifying the contribution of water soluble brown carbon from biomass burning emissions. The estimated MAE_BrC-365 (0.30–0.93 m² g⁻¹), during this study, was consistent with the earlier observations over the BoB, particularly during the continental outflow season.