Aerosol variability in the Río de la Plata estuary: Multimodal analysis of four years of AERONET observations

Abstract. We analyze four years (2020–2023) of AERONET observations from Montevideo_FING, an urban-estuarine site at the Río de la Plata, to characterize aerosol optical properties and to assess whether inversion-retrieved particle-volume size distributions (PVSDs) are better represented by a multimodal than by a standard bimodal form. The record has a median aerosol optical depth at 500 nm (AOD₅₀₀) of 0.067 and weak total-load seasonality, but the fine-coarse balance varies markedly: the fine mode fraction (FMF) increases from 0.42 in summer to 0.72 in winter, and wind-resolved analysis shows stronger fine mode contribution under northerly and westerly flow than under southerly and easterly flow.

In the almucantar inversion subset, a modified Gaussian Mixture Model (mGMM), adapted here for inversion-retrieved 22-bin PVSDs, classifies 68 % of cases as multimodal, indicating that strict bimodality is not the dominant state under inversion-qualified conditions. In a spectral-consistency test based on Mie calculations and the Ångström exponent of AOD, the median signed relative difference decreases from 17 % with the standard bimodal PVSD representation to 3.4 % with the multimodal representation obtained with mGMM.

In a multisite comparison, Montevideo occupies an intermediate position between marine and urban-continental reference ranges, consistent with a mixed coastal-urban setting. TROPOMI NO₂ columns are also higher in the overland quadrants, providing independent contextual support for stronger lower-tropospheric anthropogenic influence under overland flow. Overall, the 2020–2023 record shows a clear seasonal and wind-dependent redistribution between fine and coarse aerosol contributions.