Augmentation of fine-mode anthropogenic aerosols over a tropical coastal site in India adjoining Eastern Arabian Sea: Potential sources and direct radiative effects

The first comprehensive long-term observation of the variability in columnar aerosol optical depth (AOD) and ambient Black Carbon mass concentrations (M_B) was conducted at a tropical coastal location neighboring the Arabian Sea (Goa; 15.45°N, 73.83°E) between December and May in two phases: 2008–2011 (Phase I) and 2017–2021 (Phase II). Inter-seasonal (winter monsoon season (WMS): December–February; spring inter-monsoon season (SIMS): March–April; and MAY) and interphase variability in aerosol types, potential source regions, aerosol direct radiative effects (ADRE), and heating rate (HR) were investigated. The slope of spectral AOD was steeper during WMS and SIMS than MAY in both phases. Relatively flat AOD spectra with low Ångström exponent (α < 1) prevailed during all seasons in Phase I and MAY in Phase II, implying the predominance of coarse-mode aerosols. However, increasing fine-mode aerosol dominance was observed during WMS and SIMS in Phase II (mean α $\sim$1.5). The highest and lowest mean M_B were recorded during WMS of Phase I (2904.68 ± 787.20 ng m⁻³), and MAY of Phase II (531.12 ± 163.95 ng m⁻³), respectively. Further, urban/industrial aerosols increased over 3-fold during WMS and SIMS from Phase I to Phase II. Strong potential sources of fine-mode aerosols were interspersed across the Deccan Plateau, central India, and the east coast of India during WMS of Phase II. An investigation into the sources showed that the enhancement in power generation capacities of thermal power plants was a major contributor to fine-mode anthropogenic aerosols, along with increased vehicular density and agricultural activity at upwind locations in Phase II. The sharp rise in single scattering albedo (SSA) in Phase II implied a substantial increase in scattering aerosols. ADRE in the atmosphere (ADRE_ATM) and HR were the highest during SIMS (63.76 ± 12.99 W m⁻²; 1.79 ± 0.36 K day⁻¹) in Phase I. Low ADRE_ATM and HR were recorded during SIMS (28.20 ± 13.84 W m⁻²; 0.79 ± 0.39 K day⁻¹) and MAY (36.15 ± 9.15 W m⁻²; 1.06 ± 0.31 K day⁻¹) in Phase II, which can be attributed to the rapid decline in absorbing aerosols during SIMS and MAY of 2020 and 2021, coinciding with the countrywide COVID-19 lockdown.