Two decades of AERONET analysis of fine and coarse-mode aerosols impacts on radiative forcing

The Amazon rainforest offers a unique experimental framework to assess aerosol effects on the radiative balance, given the interplay between a low-concentration, biogenically dominated background state and episodic, high-concentration anthropogenic perturbations or Saharan dust and smoke plume intrusions within a spatiotemporally varied aerosol population. Whereas previous studies have characterized the seasonal dynamics of the optical properties of these particles, disentangling the radiative effects of different-sized aerosols remains challenging. Our study focused on distinguishing the contributions of fine- and coarse-mode aerosols to top-of-atmosphere (TOA) radiative forcing (RF) and on evaluating a comprehensive suite of aerosol optical properties across six AERONET sites in the Amazon over 2 decades (2000-2024). This was performed by assigning labels to each data point based on the daily average of fine- and coarse-mode aerosol optical depths (AODs), using thresholds to categorize aerosol conditions as “low” (below 25th percentile) or “high” (above 75th percentile), and then evaluating each kind of event. For every site, events of low fine-mode and low coarse-mode (LL), and low fine-mode and high coarse-mode (LH) conditions usually occur during the wet season and the transition from wet to dry season. Conversely, events of high fine-mode and low coarse-mode (HL) and high fine-mode and high coarse-mode (HH) conditions usually occur during the dry season. Across all sites, under low fine-mode conditions, the AOD shows a strong dependence on the coarse-mode, with increases of approximately 131% from LL to LH. However, under high fine-mode conditions, the AOD shows a weaker dependence on the coarse-mode, with increases of approximately 12% from HL to HH. Regarding TOA RF, sites in the deforestation arc show a weak dependence on coarse-mode under both low and high fine-mode conditions (RF_LL = -3.15 W/m² to RF_LH = -3.05 W/m² and RF_HL = -31.3 W/m² to RF_HH = -33.4 W/m²). In contrast, sites in the central-north region show a stronger dependence on coarse-mode (RF_LL = -4.40 W/m² to RF_LH = -11.1 W/m² and RF_HL = -20.0 W/m² to RF_HH = -29.1 W/m²). For a multilinear regression model in the form RF = c_FM AOD_FM + c_CM AOD_CM, where c_FM and c_CM are the RF efficiencies of fine- and coarse-mode per unit of their respective AODs, we obtained c_FM = -25 W/m² and c_CM = -95 W/m² for the deforestation arc sites, and c_FM = -39 W/m² c_CM = -66 W/m² for the central-north Amazon ones. In conclusion, we have shown that coarse-mode aerosols contribute significantly to all the optical properties analyzed, particularly by increasing AOD during low fine-mode conditions and by enhancing (in magnitude) radiative forcing at sites in the central-north Amazon. Moreover, as all RF efficiencies are negative, the predominant aerosol effect in the Amazon atmosphere is always cooling, and the coarse-mode efficiency is consistently greater than the fine-mode efficiency at all sites.