Working Group I: The Scientific Basis

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5.2.5 Trends in Aerosols

The regional nature of aerosols makes tropospheric aerosol trends more difficult to determine than trends in long-lived trace gases. Moreover, there are few long-term records of tropospheric aerosols (SAR). Ice cores provide records of species relevant to aerosols at a few locations. As shown in Figure 5.4, ice cores from both Greenland and the Alps display the strong anthropogenic influence of sulphate deposited during this century (Döscher et al., 1995; Fischer et al., 1998). Carbonaceous aerosols also show long-term trends (Lavanchy et al., 1999). Sulphate in Antarctic ice cores shows no such trend (Dai et al., 1995) since its source in the Southern Hemisphere is primarily natural.

Aerosols have been measured from balloon sondes at Wyoming since 1971. For the number of aerosols larger than 0.15 mm, decreasing trends of -1.8 ± 1.4%/yr and -1.6 ± 1.8%/yr (90% confidence limits) were found in the 2.5 to 5 and 5 to 10 km altitude ranges, respectively (Hofmann, 1993). The total number of particles increased by 0.7 ± 0.1%/yr at Cape Grim from 1977 to 1991 but the number of particles large enough to nucleate cloud droplets (CCN) decreased by 1.5 ± 0.3%/yr from 1981 to 1991 (Gras, 1995). There are also some long-term data on visibility and turbidity. For example, summertime visibility in the eastern United States was worst in the 1970s, which was also a time of maximum SO2 emissions (Husar and Wilson, 1993).

Figure 5.3: (a) Aerosol optical depth and (b) Ångström exponent from POLDER satellite data for May 1997 (Deuzé et al., 1999). The largest optical depths over the Atlantic Ocean are from north African dust. The Ångström exponent expresses the wavelength dependence of scattered light between 670 and 865 nm. The African dust plume has a small Ångström exponent due to the importance of coarse mode aerosols whereas the larger Ångström exponents around the continents show the importance of accumulation mode aerosols in those locations.

Figure 5.4: (a) Sulphate concentrations in several Greenland ice cores and an Alpine ice core (Fischer et al., 1998; Döscher et al., 1995). Also shown are the total SO2 emissions from sources from the US and Europe (Gschwandtner et al., 1986; Mylona, 1996). The inset shows how peaks due to major volcanic eruptions have been removed by a robust running median method followed by singular spectrum analysis.
(b) Black carbon and organic carbon concentrations in alpine ice cores (Lavanchy et al., 1999).

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