Seasonal variations in the optical properties of near-surface aerosols and the delineation of aerosol types at a tropical coastal site, Goa, adjoining the Eastern Arabian Sea

Abstract

A comprehensive study characterizing the extensive (scattering (sigma sca) and absorption (sigma abs) coefficients) and intensive (scattering Angstrom exponent (SAE), absorption Angstrom exponent (AAE), and single scattering albedo (SSA)) optical properties of near-surface aerosols, was conducted at a tropical coastal site adjoining the Eastern Arabian Sea (Goa, India) for the period from December 2019 to November 2020. Temporal (seasonal/diurnal) variations in aerosol extensive and intensive properties, and the roles of continental/marine air masses (long-range transport and land-sea-breeze circulation), boundary-layer evolution, and emission patterns in modulating the aerosol population have been examined. This is the first study at a coastal site in India to probe the SAE-AAE relationship for aerosol-type classification. Distinct seasonal variability was observed in sigma sca and sigma abs, with elevated aerosol concentrations during winter and post-monsoon, attributable to the influence of continental air masses and stagnant meteorological conditions, and reduced aerosol concentrations during monsoon owing to wet scavenging and marine air intrusion. Bimodal peaks in sigma sca and sigma abs during morning (08:00-09:00 h) and evening (20:00-22:00 h) indicated diurnal variations in aerosol concentrations driven by boundary-layer evolution and anthropogenic emissions, particularly during winter. Intensive optical properties revealed a seasonal transition from fine-mode, absorbing aerosols, associated with urban pollution during winter (SAE=1.19 plus-minus 0.15, AAE=1.02 plus-minus 0.18, SSA=0.86 plus-minus 0.04) and post-monsoon (SAE=1.30 plus-minus 0.19, AAE=0.99 plus-minus 0.19, SSA=0.90 plus-minus 0.04), to coarse-mode, scattering types in monsoon (SAE=0.40 plus-minus 0.43, AAE=0.94 plus-minus 0.26, SSA=0.93 plus-minus 0.04). The aerosol-type classification revealed low-absorbing small particles (SPLA), black carbon-dominated (BC-dominated), large particles and BC mixture (LP/BC mix), and low-absorbing large particles (LPLA) as the dominant near-surface aerosol types across seasons. Their seasonal distributions reflected a shift from continental to marine influence, with SPLA plus BC-dominated aerosol types prevalent during post-monsoon (92.2 percent) and winter (65.6 percent), and LPLA plus LP/BC mix (89.3 percent) during monsoon. The lowest mean SSA was observed for the BC-dominated aerosol type during winter (0.84), and the highest for LPLA (0.94) during monsoon. Diurnal variations were consistent across all seasons, with fine-mode (coarse-mode) aerosol types dominating during cooler nocturnal (warmer daytime) hours, indicating land-sea-breeze circulation as the primary driver. These findings underscore the complex interplay among local and upwind emission sources and atmospheric processes in shaping the aerosol regimes over tropical coastal environments.