Year-round aerosol characteristics and radiative effects in the South Asian pollution outflow over a background site in the Maldives

Aerosol radiative properties using recently available high-quality columnar aerosol data collected at several AERONET sites in South Asia, with a focus on pollution outflow from continental South Asia observed over Hanimaadhoo in Maldives, a small island in northern Indian Ocean are quantified. The seasonal mean aerosol optical depth (AOD) over Hanimaadhoo is ≥ 0.3 (except ca. 0.2 during monsoon season), and single scattering albedo (SSA) is > 0.90 in all seasons. Fine mode aerosols contribute dominantly to AOD. SSA decreases as a function of wavelength due to influence of continental aerosols, except during the monsoon season when its spectral trend reverses due to increase in dust. Carbonaceous aerosols dominate (>90%) contribution to absorption AOD (AAOD). Black carbon (BC) and brown carbon (BrC) contribute >75% and <25%, respectively, to AAOD due to carbonaceous aerosols. The observed seasonal mean aerosol radiative forcing at the surface (ARFSFC), at the top of the atmosphere (ARFTOP) and in the atmosphere (ARFATM) is > −25 Wm-2, >−20 Wm-2 and ~+20 Wm-2, respectively. Aerosol loading and atmospheric heating have increased over this background site over the last decade. A regional-scale analysis of aerosol properties and radiative effects across and surrounding the Indo-Gangetic Plain (IGP) shows that AOD is ≥ 0.3 over entire region, and aerosols reduce seasonally 30–50 Wm-2 of solar radiation reaching the surface, contributing significantly to solar dimming effect. The atmospheric solar heating rate due to aerosols (HR) is ≥ 1 K day−1 across IGP. These high ARFs, ARFESFC and HR, and increasing trends have significant implications to climate and hydrological cycle over South Asia and beyond.