The dataset consists of a set of semi-quantitative surface ozone observations (hereinafter O3), together with relative humidity observations at the same temporal scales. The dataset consists of monthly mean observations from 23 observatories, listed on Table 1. The datasets are from the period 1872 to 1888 at 22 of the observatories. They're extensions in times range between 17 and 3 years for monthly mean observations. The other dataset from the Observatory Infante D. Luiz, the longest semi-quantitative surface ozone observations data series known to date, spans 58 years for monthly data (1855 to 1913). For more details see Tables 2a and 2b in Añel et al., 2024. The O3 observations were recovered and manually digitalized from the Annaes do Observatory Infante D. Luiz published between 1863 and 1915 (Fradesso da Silveira, 1863; De Almeida, 1915). The O3 observations were part of the regular meteorological observations conducted at Portugal and former colonies network of meteorological observatories and outstations controlled by the Infante D. Luis Meteorological Observatory (Lisbon), created in 1857 (Raposo, 2017). Geographically the dataset consists of O3 observations at the 23 observatories listed on Table 1, between 42°N and 9°S in latitude and 27°W and 15°E. Eighteen observatories are located at the Iberian Peninsula and five in Africa. Three of those observatories are insular, two at the Azores islands and one at Madeira Island. The O3 observations were conducted using Schönbein test-paper method, also called "ozonoscope". It was based on the color-change of an indicator strip of blotting paper coated with starched potassium iodide. The paper strip was exposed to air between 8 and 24 hours protected from solar radiation and rain. The method applied in the Portuguese observatories was the one from Berigny (1958) who defined the operating procedure, introducing a more precise chromatic scale graduated from 0 to 21 and selected he best quality of impregnated paper. That on was the Berzelius paper manufactured by James a chemist at Sédan (Marenco et al, 1994) that is often referred in logbooks containing measurements as the cale of "James de Sédan". (Añel et al., 2024).

The semi-quantitative method for the ozone measurement is uniteless. There were 2 chromatic scales for the same range of color from the original color of the paper to different tonalities of purple. This range of tonalities (function of the Ozone concentration) was 1 to 10 in the Schönbein scale (1) and 1 to 21 in the Berigny scale (2). In the XXth century, laboratory research was conducted on the chemical reaction used in the method and deriving equations relating the ozone unitless scale values to ozone magnitudes in ppmv or ppbv. Those equations include additional variables such as the humidity and others.See also: Schönbein, Ch. F., Über das Ozon. J. Praktische Chimie, 51,321 (1850); Bérigny, A., Gamme chromatique pour l'ozonomètre (séance du 9 Mars 1858), Ann. Soc. Meteorol. Paris, 6, 25-29 (1858) and Bojkov, R. D., Surface Ozone During the Second Half of the Nineteenth Century. J. Clim. Appl. Meteorol., 25:343-352 (1986).