Investigation on spatiotemporal distribution of aerosol optical properties over two oceanic regions surrounding Indian subcontinent during summer monsoon season.

Columnar spectral aerosol optical depths (AODs) and total suspended particulate matter (TSPM) concentrations were collected on board the Oceanographic Research Vessel (ORV) of Sagar Kanya (SK) during 7-21 June 2014 (SK-313) and 31 July-14 August 2015 (SK-323) over the Arabian Sea (AS) and Bay of Bengal (BoB), respectively, for the two successive years during summer monsoon season. AOD measured at 500 nm (AOD500) varied significantly from 0.08 to 0.66 (0.07 to 0.60), with a mean of 0.48 ± 0.13 (0.34 ± 0.13) over the BoB (AS) during SK-313 (SK-323). It simply implies that aerosol load was higher over BoB, not variability as the standard deviations of AOD over both oceans are identical (0.13). Daily AOD500 ranged between 0.15 and 0.60 accounted for 70-75% of the total occurrences over two oceanic regions. Mean Ångström exponent (α or alpha) and Ångström turbidity coefficient (β or beta) were found to be 0.43 ± 0.17 (0.39 ± 0.19) and 0.37 ± 0.15 (0.27 ± 0.13), respectively, which are higher over the AS during SK-323 (SK-313) that indicate predominance of coarse-relative to fine-mode particles. On the other hand, the spectral curvature and second derivative of alpha (α') also showed significant contribution of coarse-mode particles over fine during the two campaigns. Further, column aerosol size distribution (CSD) derived from the King's inversion also exhibited bimodal distribution with a predominant peak observed in the coarse mode (~1.0 μm) compared to the fine mode at a geometric mean radius at ~0.1 μm over two oceans. The observed data showed that the two marine regions are significantly influenced by various types of aerosols with a predominance of mixed type (MT) of aerosols. From the morphological study, it is inferred that the particles are a flake, spherical, irregular, and in flower and aggregated shapes conducted for the TSPM samples collected during SK-323 over the AS. Finally, the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model is used to study the impact of long-distance transported aerosols and identify their sources.