Introduction

Since the preindustrial era, the chemical composition of the atmosphere has changed dramatically. For example, the concentration of carbon dioxide (C02) has increased from approximately 280 ppmv in 1850 to 367 ppnw in 1999, that of methane (CH4) from 700 to 1745 parts per billion (ppbv), and that of nitrous oxide (N20) from 270 to 314 ppbv (IPCC, 1996; WMO, 1999) . In addition, since 1950, large quantities of industrially manufactured chlorofluorocarbons (CFCs) have leaked to the atmosphere. These CFCs have been the major cause of the formation of the observed depletion of stratospheric ozone (03). Because of their long lifetimes (several decades), the effects of these gases will be felt for many years to come, despite the implementation of the Montreal Protocol and other international agreements (WMO, 1999). Fossil fuel combustion, a primary anthropogenic perturbation of the 20th century, has produced not only large amounts of carbon dioxide, but also substantial quantities of shorter lived trace gases, including nitrogen oxides (NOx), carbon monoxide, and volatile organic carbon compounds. The release of these short-lived compounds has contributed to substantial (but hard to quantify) changes in tropospheric ozone at the global scale (see, e.g., WMO, 1999).

Radiatively active gases, including CO;, CH4, N20, CFCs, and Oi; contribute to the so-called "greenhouse effect" of the atmosphere, and the observed perturbations in their atmospheric concentrations have led to significant "climate forcing." For the period 1850-2000, this forcing is estimated to be around 2.5 W m~2 (IPCC, 1996).

At the same time, large amounts of sulfur dioxide have been released to the atmosphere, primarily as a result of coal burning. These emissions are most intense in the urbanized and industrialized regions of Asia, Europe, and North America. Sulfur dioxide is rapidly converted into tiny sulfate aerosol particles (0.1-1 ¡jlm in size) which scatter a relatively large fraction of the incoming radiation back to space, resulting in a substantial cooling in the regions where the particles are particularly abundant (Erisman and Draaijeers, 1995; Roeckner et ai, 1999). The inclusion of sulfate aerosols into general circulation models has resulted in substantial improvement in these models' ability to correctly capture the spatial pattern of global increases in temperature (Kiehl and Briegleb, 1993). The presence of sulfate aerosols also tends to modify the optical properties and lifetime of clouds, providing an additional regional cooling mechanism (Santer et al, 1995). The magnitude of this indirect effect, however, is, poorly quantified.

Changes in the chemical composition of the atmosphere both affect the physical climate system and disrupt biogeochemical cycles, which are central to the "health" of the biosphere. For example, the sulfate aerosols, mentioned previously, together with the enhanced concentrations of nitrates, constitute the major sources of acid rain. Acid precipitation can destroy aquatic ecosystems and has contributed to the well-known phenomenon of waldsterben in Europe and North America (Schulze, 1989; Aber, 1989?). Clean air acts implemented in Europe and North America have been remarkably successful at reducing the sulfate content of rainwater but the regulation of nitrogen oxides has proven to be less tractable. Enhanced concentrations of nitrogen oxides constitute a second major source of acid rain. They also lead to fertilization of the biosphere, which has a direct impact on the global carbon cycle. In addition, the enhanced concentrations of nitrogen oxides in the atmosphere has led to enhanced ozone concentrations in the boundary layer and probably in the free troposphere. Surface ozone concentrations greater than 40 ppbv damage plant leaves and decrease plant productivity (Reich, 1987). Clearly, changes in the chemical composition of the atmosphere have multifaceted counteracting effects.

GLOBAL BlOGLOCHkMlCAL CACLbS IN I Hi CLIMAI L SYS ILM

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In spite of the measures taken to reduce anthropogenic emissions of chemical compounds, the impact of regional and global perturbations on atmospheric composition remains large and is expected to intensify in the next decades. Economic development, expansion of urbanization, and the accompanying rise in the emissions of greenhouse gases and of ozone precursors in Asia and South America are expected to be rapid over the next decades.

In this chapter, we focus on processes that affect the budget of ozone in the troposphere at the global scale. We use the global chemical transport model of the troposphere called IMAGES to assess the importance of various factors that influence the global ozone budget. In Section 2, we provide a synthetic overview of the chemical processes that affect O, and several of its precursors in the atmosphere. In Section 3, we provide a brief description of the IMAGES model, and in Section 4, we discuss some results obtained by using this model. Conclusions are provided in Section 5.

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