Ozone measurements in Zagreb, Croatia, at the end of 19th century compared to the present data

Surface ozone measurements at the Zagreb-Gric Meteorological Observatory (founded in 1861), applying the Schonbein colorimetric method, were introduced at the end of the 19(th) century (1889-1900), at the time of the city's most intensive development. The data measured in 11 (0-10) grade Schonbein scale were published in the Observatory annual journals. The well-known geophysicist, Andrija Mohorovicic, then a leader of the Observatory, supervised the surface ozone measurements. The ozone data, converted into quantitative units (ppb (the symbol ppb in the text relates to volume ratios (ppbv), which differ from mass ratios (ppbm), but as only volume ratios are used in the article, the simple unit symbol ppb is used.) after applying several statistical tests for the data quality check, were compared with recent ozone data. These first results, including the conversion method used, were published. In our study some additional quality data tests and ozone data comparisons were made, and a description of significant environmental conditions surrounding the measuring site in Zagreb was presented. A comparison between the multiannual data from the pre-industrial period and the ones from the recent multiannual period was made, based on annual and monthly averages. The average value of the surface ozone at the end of the 20(th) century (1989-1994) in Zagreb is by 24 % higher compared to the end of the 19(th) century. The rise relates to daily maxima in both time series. A bi-modal shape of the annual run of the surface ozone monthly average was found in the older as well as in the more recent data sets. The position of the primary maximum in the cold part of the year (winter/spring) during the last decade of the 19(th) century points at the rural surface-air characteristics. The annual primary maximum during the recent observation period was found in the summer months, and it demonstrates an increase in air pollution, mostly of anthropogenic origin. The old Zagreb pre-industrial ozone data show the expected sensitivity to the basic climatic factors, daily and annually.