Monsoon–Regulated Aerosol Variability and Radiative Effects over a Rural Receptor Site in Southeast India

Abstract

This study aims to characterize the seasonal variability and optical properties of aerosols and to investigate their potential sources, transport pathways, and radiative impacts over a rural site in Southeast India. Ground–based MICROTOPS–II Sunphotometer observations during April 2021–December 2023 were integrated with trajectory–based source analysis and OPAC–SBDART radiative–transfer simulations to examine the links between aerosol characteristics, meteorological conditions, source regions, and radiative effects. The annual mean aerosol optical depth at 500 nm (AOD500) was found to be 0.56 ± 0.22, peaking during pre–monsoon (0.66 ± 0.19) and winter (0.64 ± 0.23), and lowering during the rainy monsoon (0.49 ± 0.21). Enhanced aerosol loading during the dry seasons was associated with local emissions and long–range continental transport, whereas monsoon conditions favored marine influence, atmospheric ventilation and wet scavenging, as supported by trajectory analyses using potential source contribution function (PSCF) and concentration weighted trajectory (CWT) models. Higher Ångström exponent (AE) values during winter and pre–monsoon (1.30 ± 0.24) indicated dominance of fine–mode continental aerosols, while the lower monsoon values (0.83 ± 0.37) reflected increased contribution of coarse particles. Negative values of spectral curvature further confirmed fine–mode dominance during dry seasons. The estimated precipitable water vapor increased markedly from winter (2.00 ± 0.37 cm) to monsoon (4.35 ± 0.35 cm), likely influencing aerosol optical properties through hygroscopic growth. Meteorological parameters significantly modulated aerosol loading and size distribution across seasons. AOD–AE relationships revealed predominance of fine anthropogenic aerosols in all seasons except monsoon, while aerosol classification indicated substantial fine–mode contributions under turbid atmospheric conditions. OPAC–SBDART simulations estimated significant aerosol–induced surface cooling (−41 to −42 W m−2) and atmospheric warming (38–41 W m−2) under high aerosol loading conditions, leading to atmospheric heating rates of 1.1–1.2 K day−1. However, lower aerosol loading in monsoon reduced heating rates to 0.3–0.4 K day−1. Current findings highlight the critical role of monsoon flow and meteorological dynamics in regulating aerosol characteristics and regional radiative forcing over Southeast India.