The Light Optical Aerosol Counter (LOAC) instrument provides aerosol particle concentration measurements within 19 size classes in the 0.2–100 μm diameter size range, and an estimate of the typology of aerosols based on dual-angle measurements. LOAC can be carried by all kinds of balloons (Renard et al., 2016). The gondola weight, including the instrument, batteries (alkaline or lithium), and telemetry system, is about 1.0 kg, for an electric consumption of 3 W. Aerosols are sucked in by a small pump in order to pass through a red laser diode beam. In general, the light scattered by the particles depends on both the size and refractive index of the particles. To separate these two parameters, LOAC uses an original concept described in Renard et al. (2016a). Measurements are performed at two scattering angles: the first one is close to forward scattering at around 12, where the light scattered (diffracted) by non-spherical particles is controlled by the size of the particles (Lurton et al., 2014); the second one is around 60, where the scattered light is strongly dependent on the refractive index of the particles (e.g. Weiss-Wrana, 1983; Renard et al., 2010; Francis et al., 2011). The 12 channel is used to retrieve the size distribution indepen dently of the nature of the particles, and the combination of the 12 and 60 channels is used to derive the “LOAC speciation index” that informs on the typology or dominant nature of aerosol particles in each size range, based on a laboratory calibration conducted with particles of well-known nature.