Changes in the Chemical Composition of the Global Troposphere

The IMAGES model has been used to assess the impact of human activities on the chemical composition of the global troposphere. To quantify past, current, and future changes in tropospheric composition, IMAGES was used to simulate the preindustrial atmosphere (year 1850), a contemporary atmosphere (year 1990), and a future atmosphere (year 2050), respectively. As expected, the largest increase in ozone occurs in the lower troposphere in the Northern Hemisphere with changes of more than 70% at mid-

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FIGURE 1 The calculated change (%) in the July zonally averaged concentration of tropospheric ozone between 1850 and 1990 simulated by the IMAGES model using the IS92a emission scenario.

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0 9 18 27 36 45 54 63 72 Calculated change in January surface ozone concentration between 1990 and 2050 [%] FIGURE 2 The calculated change (%) in July surface ozone concentrations between 1850 and 1990 simulated by IMAGES using the IS92a emission scenario.

and high latitudes (Fig. 1). In the Southern Hemisphere, the estimated ozone increase is typically 10-20%, and in the tropics 30-50%. The change is most intense near the pollution sources and becomes more uniform with height in the atmosphere: the distribution of the change is relatively uniform with longitude in the upper troposphere. Examination of the surface ozone concentrations shows increases of more than 120% over the east and west coasts of the United States, over Europe, and over China and Japan (Fig. 2). These estimated changes are relatively consistent with the limited information available on the evolution of ozone in Europe during the 20th century (Fishman and Brackett, 1997; Hudson and Thompson, 1998; Logan, 1994).

Future ozone changes are difficult to predict because they depend directly on the future evolution of emissions and hence of population growth and economic development. Such predictions must therefore be based on a series of scenarios. In the present study, we simply adopt the IS92a scenario developed by IPCC (1996) as well as the NASA estimates of future growth in aviation. The projected increase in the zonally averaged ozone concentration (July conditions) is highest in the tropics at all altitudes and low in the vicinity of the tropopause in the Northern Hemisphere (data not shown). The remarkable increase in surface ozone predicted for tropical and subtropical regions, ranging from 10 to 75% (Fig. 3), is associated with rapid economic development in the region. In the upper troposphere, the projected increase in air craft traffic over the next 50 years has a large impact on upper tro-pospheric ozone (Brasseur et ai, 1998; data not shown). When expressed in absolute O, concentrations, rather than in percentages, the projected increase in ozone is highest near the mid- and high latitude tropopause (data not shown). It is interesting to note that during January (Southern Hemisphere summer; see Fig. 3), large increases in surface ozone are predicted not only in the tropics but also in Brazil, South Africa, and Southern Asia. These predictions suggest that ozone pollution events are likely to become more frequent in the populated areas of the Southern Hemisphere during the next decades.

Over the past hundred years, changes in sulfate concentrations have been greatest in Central and Eastern Europe and in China (Fig. 4). The percentages of change in sulfate concentrations over the United States and Canada have been relatively smaller. Changes in sulfate concentrations are also expected to increase in the future, as suggested by Figure 5, especially as a result of massive coal burning. The effect is predicted to be most intense in China and northern India where coal is widely used. Thus, anthropogenic pollution will not only enhance the ozone concentration in these regions, but also increase the aerosol load. Note that, based on the IS92a scenario, the change in sulfate concentration is greater than 400% near the surface over the Asian continent. Little change is expected in the industrialized Northern Hemisphere, where the use of coal has dramatically decreased, and new tech-

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0 9 18 27 36 45 54 63 72 Calculated change in January surface ozone concentration between 1990 and 2050 [%]

FIGURE 3 The calculated change (%) in the July surface ozone concentrations between 1990 and 2050 simulated by IMAGES. January was chosen to better represent the dramatic changes in the Southern Hemisphere.

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Calculated change in July surface sulfate concentration between 1850 and 1990 [%]

FIGURE 4 The calculated change (%) in July surface sulfite concentrations between 1850 and 1990 atmospheres simulated by IMAGES using the IS92a emission scenario.

FIGURE 4 The calculated change (%) in July surface sulfite concentrations between 1850 and 1990 atmospheres simulated by IMAGES using the IS92a emission scenario.

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-70 0 70 140 210 280 350 420 490 Calculated change in July surface sulfate concentration between 1990 and 2050 [%]

FIGURE 5 The calculated change (%) in the July surface sulfate concentrations between 1990 and 2050 simulated by IMAGES using the IS92a emission scenario.

3 Absolute change In total ozone column abundance between 1850 and 1990

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3 Absolute change In total ozone column abundance between 1850 and 1990

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3 Calculated radiative forcing from changes in ozone column abundance (Fig.6a)

3 Calculated radiative forcing from changes in ozone column abundance (Fig.6a)

Ozone Column Abundance

FIGURE 6 (a) Absolute change in the zonally averaged total ozone column abundance (Dobson units, DU) from 1850 to 1990 simulated by IMAGES, (b) Calculated radiative forcing resulting from the changes in ozone column abundance as shown in (a).

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FIGURE 6 (a) Absolute change in the zonally averaged total ozone column abundance (Dobson units, DU) from 1850 to 1990 simulated by IMAGES, (b) Calculated radiative forcing resulting from the changes in ozone column abundance as shown in (a).

nologies have been developed to reduce the emissions of sulfur compounds.

Several attempts have been made to assess the climatic impact of chemical compounds. In the case of long-lived greenhouse gases, 1PCC (1996) has estimated the radiative forcing to be ap proximately 2.5 W m""2. The climate impact of ozone and sulfate is more difficult to quantify due to the nonuniform nature of the perturbation and its seasonal variability. Figures 6a and 6b show the estimated change in the tropospheric ozone column (estimated by IMAGES in October) and the corresponding change

Monthly mean direct anthropogenic sulfate forcing in July

Monthly mean direct anthropogenic sulfate forcing in July

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Monthly mean indirect anthropogenic sulfate forcing in July

Monthly mean indirect anthropogenic sulfate forcing in July

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FIGURE 7 The radiative forcing for contemporary production of sulfate aerosols simulated by ECHAM (Roeckner et ai, 1999).

in radiative forcing. These graphs show an increase in ozone of typically 10 Dobson units in the Northern Hemisphere (resulting primarily from fossil fuel combustion) and of typically 8-13 Dobson units in the tropics (resulting primarily from biomass burning effects). The corresponding radiative forcing varies from about 0.4 to 0.8 W m~2 in the Northern Hemisphere. Changes are small in the Southern Hemisphere. The globally averaged radiative forcing is estimated to be 0.37 W m~2.

In the case of sulfate aerosol particles, the cooling between the preindustrial era and the present period is estimated to be on the order of 1 -3 W m~2 (IPCC, 1995; Santner et al„ 1995). The values are highest over the eastern portion of North America and the southeastern part of Europe and Asia. The global cooling due to the direct sulfate effect is estimated by Roeckner et al. (1999) to be 0.35 W m-2 and thus on the same order (but with opposite sign) of the mean warming by ozone. Note, however, that Roeckner et al., (1999) estimate the indirect cooling effect to be close to 1 W m~2 (Fig. 7). The uncertainty of these numbers is close to a factor of 2-5.

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