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Based on the archival data of the Institute of Natural and Technical Systems for the period from 1956 to 1996 the decomposition of the time series of the pH value of the surface layer of the deep-water part of the Black Sea was carried out with subsequent extrapolation until 2010. The analysis shows that the reconstructed time series is divided into two time interval trends with different sign and data quality: from 1956 to 1976 and from 1977 to 1996. The presence of these two fundamentally different time periods is largely due to the method for determining the pH value. Until about the mid-1970s the pH value in the Black Sea was determined mainly by the colorimetric method with an accuracy of tenths of pH, i.e. with a high error. In subsequent years, the potentiometric method began to be widely used, the accuracy of which is an order of magnitude higher. Therefore, further in the work, the second time period is analyzed – since 1977. In the time course of pH, a regular seasonal variation, interannual quasicyclic components of variability with periods of about 6.7 and 2.5 years, and a significant negative linear trend are identified. It is most likely that the main reason for the quasiperiodic variability of pH in the upper layer of the open part of the Black Sea on an interannual scale is related to variations in atmospheric precipitation caused by changes in cyclonic activity in the Atlantic-European sector, including the Black Sea region, in different phases of climatic modes in the ocean-atmosphere system (such as, in particular, NAO). The highlighted tendency of the pH value since 1977 indicates the acidification of the upper layer of the Black Sea waters, with an average rate of about 0.013pH units / 10 years. This corresponds to the rate of increase in acidity of surface waters in other regions of the World Ocean. Expedition data from 2010 confirm the validity of the extrapolation performed.

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Decomposition of the time series of the pH values of the surface water of the deep Black Sea according to archival data of the second half of the XXth century