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Abstract:
This study introduces a new approach to represent and analyse particle number size distributions (PNSD) of atmospheric aerosols. Amazonian aerosol data, measured from May 2021 to April 2022 at the Amazon Tall Tower Observatory (ATTO), were fitted by a trimodal lognormal function and the outputs were evaluated by means of the N-Dg-σ phase space. This is a 3D space defined by the three fit parameters of the lognormal function, which represents, for a given mode i, the number concentration (Ni), the geometric median diameter (Dg,i), and the geometric standard deviation (σi). Each state of a PNSD is represented by a single dot in this phase space, while a collection of dots shows the delimitation of all PNSD states under given conditions. The connections in ensembles of data points show trajectories caused by pseudo-forces, such as precipitation regimes and vertical movement. Characteristic patterns of the Amazonian PNSDs were found in the N-Dg-σ phase space, including the sub-50 nm mode appearing as a curved cone, the Aitken mode as a semi-sphere, and the accumulation mode as a cylinder. The trajectories of the data points as a function of seasonal and diel trends occur as well-defined paths. An ellipsoid pattern describes all possible seasonal states PNSDs of the accumulation mode. The diurnal cycle of sub-50 nm particles in the dry season shows a positive linear slope as a function of all three fit parameters. For wet and dry seasons, the diurnal cycle in the accumulation mode is mainly driven by changes in N. As an effect of precipitation on the PNSDs and vice-versa, N and Dg were found to increase for the sub-50 nm mode and to decrease for the Aitken and accumulation modes after the precipitation peak. While afternoons with precipitation were preceded by mornings with larger particles of the accumulation mode, whose mean geometric diameter was ~10 nm larger than in days without precipitation. Nevertheless, only in the wet season both concentration and diameter seem to influence further rainfall. Observed patterns of the PNSDs in the N-Dg-σ phase space can support the characterization of atmospheric aerosols e.g. in comparisons of different measurement sites, contribute to our understanding of the main processes in aerosol-cloud interactions, and open new perspectives on aerosol parametrizations. This study introduced a first glance of Amazonian aerosols in an N-Dg-σ phase space.
