**5. Time variations in aerosol optical depth AOD**

8 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

East, which is probably due to increased volcanic activity on Kamchatka .

Ukraine, and Kazakhstan. Fig. 3 shows that the AOD over Russia decreases from the southwest to the northeast. The increased values of aerosol haziness in the southeast and southwest are most likely caused by an advective arrival of air masses from the regions with high aerosol content in the atmosphere: from Ukraine and Kazakhstan in the southwest and from southeastern Asia and China in the southeast. Fig. 3 (upper part) shows the localizations of regional tropospheric aerosol sources (western and eastern Siberia and Primorskii Krai). In the last 15 years (Fig. 3, lower part), in the absence of powerful volcanic eruptions and under conditions the atmosphere being purified of the stratospheric aerosol layer, the sources of aerosol arriving in the troposphere have become more pronounced. In addition, in the last decade, the AOD has noticeably increased for a few stations in the Far

**Figure 4.** Spatiotemporal variations in AOD: (a) multiyear variations in the annual values of AOD for all 53 stations under consideration and (b) mean seasonal variations in AOD for all 53 stations under

The spatiotemporal inhomogeneities of the AOD annual values clearly reflect their causes (Fig. 4a): the peaks of the volcanic eruptions (El Chichon, 1982, and Pinatubo, 1991) and the tundra fires of the last decade in eastern Siberia, the frequency and intensity of which have increased due to climate changes. Fig. 4b shows variations in the mean annual cycle of AOD. The features of the AOD mean annual cycle for each concrete station are formed under the influence of seasonal variations in the character of air-mass transport to a given point from regions with different aerosol contents (synoptic processes) and seasonal variations in air temperature, humidity, and in the state of the underlying surface, in combination with an industrial load of some regions. The AOD maxima are, as a rule, observed in April and July– August, but the summer maximum is more pronounced at stations (No 4, 8, 9, 10, and 11) located in the south of European Russia. First of all, this is related to the fact that, in

consideration.

Fig. 5a gives some examples of time variations in the annual means of AOD for stations with negative and positive trends. In Fig. 7b, the examples of the time trends of the AOD annual values are supplemented by the corresponding variations in the flux of direct solar radiation (for the Sun's height *h*= 30°), which reach 100 W/m2 over the course of 35 years (3 W/m2 per year); estimates were obtained for two stations with the maximum and minimum means of AOD. Thus, the influence of a decreased aerosol load on the flux of direct solar radiation incident upon the land surface under clear skies is empirically estimated. For total radiation, this influence is less pronounced. And our estimate of the rate of a decrease in direct solar radiation does not contradict the satellite data (IPCC, Climate Change 2007) on the rate of a decrease in the flux of the total reflected (upward) solar radiation ( –0.18 ± 0.11) W/m2 per year (the ISCCP project) and (–0.13 ± 0.08) W/m2 per year (the ERBS project)) over the course of 1984–1999 and the assumption made in (IPCC, Climate Change 2007), that this is caused by a global decrease in stratospheric aerosol (the so-called phenomenon of "aerosol dimming").

At most observation sites, the atmosphere was purified of aerosol within the period under consideration. On the whole, for Russia, the trend of AOD variations is negative (Fig. 6); the absolute value of the trend (over 10 years) varies from (–0.05) to (+ 0.02) and increases generally from the south-west to the north-east of Russia. The mean of the relative trend accounts for (–14%) over 10 years, its maximum is 21% over 10 years, and its minimum is (– 35%) over 10 years at a determination coefficient of no more than 0. 5. (See also Table 1). It is evident that, in this case, a decrease in the AOD mean must be observed during the last 15 years of the whole region. The largest negative trends are observed at the Solyanka station (in the south of the Krasnoyarsk Krai), in Chita (Transbaikalia), Khabarovsk (Primorskii Krai), and in the south of European Russia. The combination of the two factors—global purification of the atmosphere from transformed volcanic aerosol and decreased anthropogenic forcing—forms the negative trends in these regions. Positive trends are observed in Arkhangelsk and the Far East (Kamchatka and Okhotsk), and almost zero trends are observed in western (station nos. 18, 19, and 20) Siberia. The positive (Arkhangelsk) and decreased negative (the indicated Siberian stations) trends may be caused by increased industrial emissions in these regions, an increase in the number and intensity of fires, and comparatively low-power volcanic eruptions (for example, in Kamchatka). The estimates of the AOD trends and integral transparency obtained by other authors (for example, Ohmura, 2006) were compared with our estimates earlier in (Plakhina et al., 2007). This comparison shows an agreement with the results presented in this paper.

Variations in the Aerosol Optical Depth Above the Russia

from the Data Obtained at the Russian Actinometric Network in 1976–2010 Years 11

**Figure 6.** Spatial distributions of the multiyear variability of AOD: trends of the time variations over the period 1976-2010 years (in absolute values over 10 years) and trends of the time variations over the

Fig. 7 gives a "long" (45 years) series of annual means of AOD for the Ust' Vym station (62.2°N, 50.4°E), which demonstrates a characteristic multiyear trend of variations in the annual values of AOD and its response to stratospheric disturbances. The four powerful volcanic episodes— Agung ( 8°S, 116°E, 1963), Fuego (14°N, 91°W, 1974), El Chichon (17°N, 93°W, 1982), and Pinatubo (15°N, 120°W, 1991)—are clearly pronounced and quantitatively estimated. In particular, the maximum effect observed a year after the eruptions is 100% (in deviations from the multiyear norm); throughout the year, its attenuation occurs with the dissipation and transformation of the stratospheric aerosol layer. A decrease in the AOD values for 1995–2006 is also clearly manifested. Such a character of multiyear variations in the annual values of AOD is characteristic of most stations and is, to a great extent, determined by the four powerful volcanic eruptions in the latter half of the 20th century, because seasonal and local disturbances caused by the effects of tropospheric aerosol, when annually averaged, become leveled and have almost no influence on the distribution of the

period 1995-2010 years (in absolute values over 10 years)

**6. Effects of the volcano eruptions** 

multiyear values of AOD.

**6.1. Influence of the volcano eruptionson AOD**

**Figure 5.** Time variations in the annual values of AOD and in the flux of direct solar radiation for the Sun's height 30°: (a) multiyear variations in the annual values of AOD for three stations (Krasnodar (1), Chita (2), and Okhotsk (3)) and (b) multiyear variations in the annual values of AOD and in the annual mean of direct solar radiation flux at the Sun's height 30° for the two stations with the maximum and minimum means of AOD. For both graphs, the period under analysis is 1976– 2010. Krasnodar(*1*  corresponds to AOD and 3corresponds to direct radiation), Solyanka ( *2* corresponds to AOD and *4*  corresponds to direct radiation)

**Figure 6.** Spatial distributions of the multiyear variability of AOD: trends of the time variations over the period 1976-2010 years (in absolute values over 10 years) and trends of the time variations over the period 1995-2010 years (in absolute values over 10 years)
