Records of surface ozone data collected twice a day by means of the Sch¿nbein method at Villa Colon, Montevideo, Uruguay, during the years 1883--1885 and at Cordoba, Argentina, during the years 1886--1892 have been converted to present-day concentrations. Most of the uncertainties suffered from the original technique are removed by a two-step procedure, in particular a conversion algorithm for humidity and a linear regression constructed by simultaneous readings of Sch¿nbein and a quantitative method. The relative error of this procedure, previously applied to the Moncalieri data series, is 33%. Within these limitations we can state that one century ago the surface ozone levels at that latitude of the southern hemisphere were comparable to those of Moncalieri and Montsouris. Furthermore, the recent levels observed in remote areas at that latitude are approximately twice as large as a century ago. ¿ American Geophysical Union 1992