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The propagation of light radiation in the atmosphere is a topic that needs to be properly analysed to mitigate its negative influence on astronomical observations. This work describes a novel approach for evaluating atmospheric propagation of artificial light at night (ALAN), emphasizing the dependence on altitude and aerosols. It is based on an innovative experiment using a sounding balloon equipped with two sky-quality meters (SQM): one vertically pointed at 30° (SQM-V) and the other horizontally at 90° (SQM-H) from the zenith. The system was launched during astronomical night conditions from an area of the Italian Apennines with low ground light emission and crossed the Tuscan sky, observing the vertical and horizontal ALAN propagation. The data analysis of the two SQMs and their georeferentiation through altitude and trajectory reconstruction allows us to model the propagation of light in the experimental field of view from a few hundred metres up to an altitude of about 30 km. In this work, the main focus is given to the tropospheric part of the atmosphere up to 12 500 m: the processed data are used to validate a theoretical model taking into account the altitude, course of the balloon, atmospheric composition, and population of the cities overflown by the balloon, obtaining a correlation of 0.85 with SQM-H and 0.91 with SQM-V. The magnitude values of close to 21.5 |$\mathrm{mag\, arcsec}^{-2}$| measured by SQM-V at 2000 m are an important experimental result for evaluating the influence of aerosols and altitude on ALAN propagation. [ABSTRACT FROM AUTHOR]