Evaluation of MODIS Collection 6.1 Aerosol Products Using Ground-Based Sunphotometer Observations: Seasonal Aerosol Characteristics and Radiative Effects over Southeast India

This study investigates the seasonal variability, source characteristics, radiative impacts, and satellite validation of aerosols over a rural site in Southeast India using ground-based MICROTOPS-II Sunphotometer observations during 2021-2023. The annual mean aerosol optical depth at 500 nm (AOD₅₀₀) was 0.56 ± 0.22, with higher values during the pre-monsoon period (0.66 ± 0.19) and winter (0.64 ± 0.23) and lower values during the monsoon (0.49 ± 0.21). Enhanced aerosol loading during dry seasons was associated with local emissions and long-range continental transport, whereas monsoon conditions favored marine influence, atmospheric ventilation, and wet scavenging, supported by PSCF and CWT trajectory analyses. Higher Ångström exponent values during winter and the pre-monsoon period (1.30 ± 0.24) indicated the dominance of fine-mode aerosols, while lower monsoon values (0.83 ± 0.37) reflected increased contributions of coarse-mode aerosols. Negative spectral curvature further confirmed fine-mode dominance during dry seasons. Precipitable water vapor increased from 2.00 ± 0.37 cm in winter to 4.35 ± 0.35 cm during the monsoon, influencing aerosol properties through hygroscopic growth. Meteorological conditions significantly affected aerosol loading and size distribution. AOD-AE relationships revealed the predominance of anthropogenic aerosols during all seasons except the monsoon, while aerosol classification indicated substantial fine-mode contributions under turbid conditions. Ground-based AOD observations were validated against MODIS Collection 6.1 AOD products (Dark Target, Deep Blue, and combined DTB), showing strong agreement. The DTB product exhibited the best performance (r = 0.80, RMSE = 0.121), confirming the reliability of MODIS retrievals for aerosol monitoring over India, particularly where ground observations are limited. OPAC-SBDART simulations indicated strong aerosol-induced surface cooling (-41 to -42 W m^-2) and atmospheric warming (38-41 W m^-2) under high aerosol loading, producing heating rates of 1.1-1.2 K day^-1, whereas lower aerosol loading reduced heating rates to 0.3-0.4 K day^-1. These findings highlight the influence of monsoon variability and meteorological conditions on aerosol characteristics and radiative forcing, while demonstrating the value of integrating ground-based and satellite observations for long-term aerosol assessment over India.