Multiannual changes in atmospheric column transparency based on measurements of direct solar radiation allow us to assess various tendencies in climatic changes. Variability of the atmospheric integral (broadband) transparency coefficient, calculated according to the Bouguer‐Lambert law and transformed to a solar elevation of 30°, is used for two Russian locations, Pavlovsk and Moscow, one Ukrainian location, Feodosiya, and three Estonian locations, Tartu, Tõravere, and Tiirikoja, covering together a 102‐year period, 1906–2007. The comparison of time series revealed significant parallelism. Multiannual trends demonstrate decrease in transparency during the postwar period until 1983/1984. The trend ends with a steep decline of transparency after a series of four volcanic eruptions of Soufriere (1979), Saint Helens (1980), Alaid (1981), and El Chichón (1982). From 1984/1985 to 1990 the atmosphere remarkably restored its clarity, which almost reached again the level of the 1960s. Following the eruption of Mount Pinatubo (June 1991), there was the most significant reduction in column transparency of the postwar period. However, from the end of 1990s, the atmosphere in all considered locations is characterized with high values of transparency. The clearing of the atmosphere (from 1993) evidently indicates a decrease in the content of aerosol particles and, besides the decline of volcanic activity, may therefore be also traced to environmentally oriented changes in technology (pollution prevention), to general industrial and agricultural decline in the territory of the former USSR and Eastern Europe after deep political changes in 1991, and in part to migration of some industries out of Europe.

We have used the NASA Global Energy and Water Cycle Experiment (GEWEX) Global Aerosol Climatology Project data set of retrieved aerosol optical thickness and Ångström exponent to construct and analyze regional aerosol climatologies for a number of areas affected by different aerosol types (such as dust, biomass burning, anthropogenic, or clear maritime aerosols) that exemplify the range of natural aerosol variation. We have found that variations in the number of individual pixels used to calculate monthly means associated with short‐ and long‐term satellite orbit changes and instrument degradation have little effect on global and hemispherical values of the aerosol optical thickness and Ångström exponent. Aerosol loads are found to be higher, and aerosol particles smaller, over the northern Atlantic Ocean off the coast of Europe and the eastern United States, rather than off the west coast, thereby indicating a significant impact of anthropogenic aerosols. The smallest background levels of maritime aerosols are found in the southern Pacific Ocean, with seasonal mean optical thicknesses as low as 0.1. We analyze time series of the aerosol optical thickness in the regions affected by dust outflows from the Sahara and Asian deserts and by biomass burning. An influence of anthropogenic aerosols associated with the high regional industrial activity is apparent in the eastern China Sea. Two distinct periods are identified in satellite and Sun photometer aerosol time series over the Black Sea with transition time around 1993. During the first period the aerosol loads in the region significantly exceeded the hemispherical mean, whereas in the second period they became very close. This change is linked to the reduction of the industrial output in that region.

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[1] The proposed method of atmospheric total optical depth (TOD) determination is based on the assumption that TOD is an arbitrary smooth function of time. Any small section of this function can be approximated by a linear relationship. A redundant system of Bouguer equations corresponding to n solar observations (where n > 3) can be written for each linear section. This procedure is successively applied to each section of the TOD curve and yields (N À n + 1) systems of Bouguer equations, where N is the total number of solar observations during a day. Solving these systems of Bouguer equations gives an extraatmospheric value of the solar flux and TOD curve parameters. The accuracy of the method depends on the accuracy of TOD curve approximation, i.e., on the character of TOD variation with time and a total number of observations during a day. The new method (named approximation by linear sections (APLS)) has been used for TOD investigations in the Crimea (a peninsula in Ukraine) since 1996. In comparison with the classical Bouguer-Langley method the new method allows us to achieve higher accuracy, when the conditions of astroclimate at the observational site are not very good and especially when the observations are carried out for a half day, i.e., only before noon or after.

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