LOAC is light and compact enough for in situ measurements using weather balloons. It provides particles number concentrations for 19 sizes in the 0.2 – 50 µm size range, with an uncertainty of ±20% for concentrations higher than 10 particles cm-3; the uncertainty increases to about ±30% for submicronic particle concentrations higher than 1 particle cm-3, and to about ±60% for concentrations smaller than 10−2 particle cm-3. The raw LOAC concentrations are corrected in term of sampling efficiency when the measurements are conducted under weather balloon (Renard et al., 2016), the sampling being dominated by sub-isokinetic conditions and the divergence of the flow field at the inlet entrance. The size distributions have been converted to 675-nm extinction using Mie scattering theory, assuming spherical particles with a refractive index corresponding to stratospheric sulphuric acid particles. Only size classes below 1µm have been used to avoid spurious effects (i.e. local enhancements in the calculated extinction value) resulting from the transient presence of micrometric particles. As a result, only a partial extinction have been derived. LOAC V1.5 has been improved in terms of reduced stray light and laser power (65 mW) in comparison with LOAC V1.2 presented in Renard et al., AMT, 2016.