Atmospheric Chemistry in a Changing World

The story of the importance of atmospheric chemistry begins with the origin and evolution of life on Earth. The accumulation of greenhouse gases in Earth’s atmosphere allowed surface temperatures to be maintained above the freezing point of water. Reactions involving carbon, hydrogen, and nitrogen compounds in the primeval soup led to the formation of self-replicating molecules, and, about 500 million years ago, the rise of atmospheric oxygen led to the formation of the stratospheric ozone layer, which protects life on Earth from extremely harmful levels of solar ultraviolet radiation. Running in the shadow of these major events has been a full suite of atmospheric chemistry processes affecting and being affected by the evolving nature of both terrestrial and oceanic life. Some of these processes include biogeochemical cycling of elements, long-range transport of nutrients, regulation of temperature, and exposure to air pollution.

The contemporary atmosphere was created as a result of biological activity some two billion years ago. To this day, its natural composition is supported and modified, mostly through biological processes of trace gas production and destruction, while also involving physical and chemical degradation processes. The biosphere has a major influence on present environmental conditions, both on a regional and global scale. One of the bestdocumented and most important indicators of global change is the progressive increase of a number of trace gases in the atmosphere, among them carbon dioxide (CO₂) , methane (CH₄) , and nitrous oxide (N₂O), all of which are of biospheric origin. There is considerable uncertainty, however, regarding the processes that determine the concentration and distribution of trace gases and aerosols in the atmosphere and the causes and consequences of atmospheric change (Andreae and Schimel 1989). To improve our understanding IGAC created an environment for multi-disciplinary collaboration among biologists,chemists, and atmospheric scientists. This was essential to develop analytical methods, to characterise ecosystems, to investigate physiological controls, to develop and validate micrometeorological theory, and to design and develop diagnostic and predictive models (Matson and Ojima 1990).

Earth’s atmosphere is made up of 78% nitrogen and 21% oxygen. It is therefore highly oxidising but the oxygen only acts as the main source of the more reactive molecules and free radicals that provide the atmosphere’s real oxidising power. These are formed almost entirely by photochemistry and many involve coupling between ozone and water vapour, the influence of which on the overall oxidising power is often underestimated. The oxidants to be dealt with here therefore include ozone (O₃), hydroxyl radicals (OH), peroxy radicals (both inorganic (HO₂) and organic (RO₂)), and peroxides (H₂O₂ and RO₂H). Other oxidants include nitrate radicals (NO₃) and halogen atoms; however, these play a subsidiary role and probably are unimportant over large parts of the atmosphere.

Between 1970 and 1990 the major advances in atmospheric chemistry were made in gas-phase photochemistry, except perhaps for a brief intermezzo of “nuclear winter” studies. This focus is now shifting, as it is recognised that natural and anthropogenic aerosols play a substantial role in the radiative properties of the atmosphere and Earth’s climate. In addition, studies on the causes of the Antarctic ozone hole have demonstrated the large role of reactions that take place on ice and particulate surfaces. If such reactions occur in the stratosphere, they must take place also in the troposphere, with its abundance of various types of aerosol. Considering these factors, and especially because of various break-throughs in experimental techniques, it is likely that aerosol research will be prominent in atmospheric chemistry in the coming decades. This research will involve process studies both in the atmosphere and in laboratories, studies on the sources and sinks of aerosols, chemical analyses of the particulate matter (PM), modelling, and especially regional (campaigns) and global (satellites) observations on the distribution of the atmospheric aerosol. This is all the more important because climate models, which in most cases currently consider only sulphur chemistry, cannot be tested sufficiently for want of data, despite the potentially great climate effects of aerosols. Aerosol particles may already be significantly counteracting the radiative forcing by the greenhouse gases (Ramaswamy et al. 2001).

The global distribution and the regional and temporal variations of chemical compounds in the atmosphere, both gases and aerosols, are still not well known. A full understanding of the atmospheric system can only be achieved through an integrated use of field measurements, modelling, and laboratory measurements. The past ten years have seen an impressive increase in the number of field measuring systems in use, while laboratory studies of reaction rate coefficients and mechanisms have benefited from advances in technology. The advances are often synergistic, with field instruments frequently being based on laboratory systems, and new versatile detectors brought into the lab to increase the capabilities there.

The chemistry of the atmosphere is controlled by a large number of complex chemical and physical processes. The study of such a complex system requires the use of numerical models, which have improved substantially over the past ten years. These models are mathematical representations of the main physical and chemical processes controlling the spatial and temporal distributions of trace gases and aerosol particles. The models have been developed to test our understanding of the atmospheric processes, to identify key variables and important interactions, and to interpret local, regional, and global observations. Additionally, they can be used to simulate global distributions of tropospheric compounds, predict the evolution of the chemical state of the atmosphere and of the radiative forcing of climate in response to natural or anthropogenic perturbations, provide an optimal use of satellite data through data assimilation, and help policymakers define emission reduction policies.

The preceding chapters have presented a topical synthesis of results obtained by the international scientific community after a decade of intensive research efforts focussing on the chemistry of the global atmosphere. Areas highlighted in this book are the relations between atmospheric composition and biospheric processes, the budget of atmospheric photooxidants including tropospheric ozone, and the importance of aerosols for the chemical composition of the atmosphere and for the climate system. The perturbing role of human activities has been stressed.