Human activities are changing the composition of the earth's atmosphere in numerous ways. For instance, since the beginning of the industrial era, the atmospheric abundances of the gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) have been increasing due to the combustion of fossil fuels and other industrial activities, and due to land-use activities such as biomass burning and agriculture. Once released, these gases remain in the atmosphere up to many decades, and as greenhouse gases (GHGs), their increasing abundances contribute to global climate change.
Significant increases in the emissions of some short-lived gases have also occurred in many regions. Important examples include nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon monoxide (CO), and sulfur dioxide (SO2). The background concentration of tropospheric ozone (O3), which is formed in the lower atmosphere from chemical reactions involving NOx and VOCs, has approximately doubled over the past century. Human activities have also increased airborne particulate matter (PM), which encompasses a diverse class of chemical species including sulfates, nitrates, soot, organics, and mineral dust. These gases and particles are relatively short-lived, remaining in the atmosphere for only days to months near the surface. Ozone and PM are of particular concern because their atmospheric residence times are long enough to influence air quality in regions far from their sources and because they also contribute to climate change.
Changes in global air quality are determined by the release of key chemical compounds from source regions, and by the subsequent accumulation and interaction of these species throughout the earth's atmosphere. Our ability to under
stand observed changes in global air quality and to predict future changes will depend strongly on answering two important questions:
How can global air quality change affect, and in turn be affected by, global climate change?
Although air quality and climate are generally treated as separate issues, they are closely coupled through atmospheric chemical, radiative, and dynamical processes. The accumulation of pollutants in the atmosphere can affect climate through direct and indirect contributions to earth's radiative balance, and through chemical reactions that alter the lifetime of certain greenhouse gases. In turn, meteorological parameters such as temperature, humidity, and precipitation can affect the sources, chemical transformations, transport, and deposition of air pollutants. Our understanding of many of these climate-chemistry linkages is in its infancy. A better understanding is needed in order to make accurate estimates of future changes in climate and air quality and to evaluate options for mitigating harmful changes.
How is global air quality affected by the international and intercontinental transport of air pollutants?
Total global emissions of species including NOx, VOCs, and CO may rise dramatically in the coming decades due to increasing population and industrialization, and in particular, the growth of “megacities” in many regions of the world. The transport of pollutants such as ozone and PM across national boundaries and between continents will increase in importance as total emissions rise. Such pollutant transport interconnects all the countries of the world to varying degrees and can raise “background” 1 pollution levels over large regions of the globe. Quantifying this long-range transport is essential in order to understand what future changes may occur in U.S. air quality, to assess how U.S. pollutant emissions affect global air quality, and to develop realistic and effective air quality management plans for the coming decades.
Answering the questions posed above requires a research strategy that integrates atmospheric observations covering a wide range of parameters and spatial and temporal scales together with diagnostic, global, and regional models (see Figure 1-1 ). In particular, a renewed and comprehensive observational strategy with global cooperation is required in order to fill the significant gaps and short-falls in our current observational capabilities. If we do not maintain existing
1“Background” is used here in a narrow sense as the concentration of a pollutant that would prevail in the absence of local anthropogenic emissions.
~ enlarge ~
observational programs and develop strategies to enhance our observational capabilities, we will lose critical information about the state of the atmosphere as it evolves in this period of rapid population growth and industrialization. Predictions about air quality changes in the 21st century are highly uncertain, in part because of the limited observational record of changes in the atmospheric concentration of short-lived gases and aerosols since the pre-industrial era. Recent technical advances have greatly enhanced the possibilities for obtaining needed observations; the challenge now is to make a national commitment to developing and maintaining comprehensive observational programs that will utilize these technologies.
To address these global air quality issues, a range of observational platforms and techniques will likely be needed, including measurements at the earth's surface and in the free troposphere several kilometers above the surface (which can be accessed through platforms such as aircraft, balloons, and satellites). Satellite measurements ultimately hold the greatest promise for comprehensive global observations in the lower atmosphere, but these observational techniques are still largely in the developmental stage. Obtaining global coverage through ground-based observations will require that similar measurements be made by a wide variety of international scientific groups. Careful calibration and intercom-
Global Air Pollution Concerns
As urbanization and industrialization have intensified during the last few decades, urban air quality has become an increasingly pressing public health concern throughout many regions of the world, particularly in developing countries. More than 3 billion people—about half the world's population—are now concentrated in urban areas, and by 2010 the global urban population is expected to swell to more than 4 billion (UNEP, 1999).
By one estimate, more than 1 billion people are currently exposed to harmful levels of air pollution (Schwele, 1995). Severe particulate air pollution is a chronic problem in much of Asia, primarily as a result of coal combustion in factories and power plants, and the use of coal and wood for cooking and home heating. Motor vehicles are an increasingly important contributor to air pollution in much of the world, with more than 600 million vehicles in use, a number that could double within the next 25 years (Dunn, 1996). Automobiles are the dominant source of air pollution in many Latin American cities, including Sao Paulo, Santiago, and Mexico City, where they have had to restrict automobile use in an effort to manage severe air pollution episodes (UNEP, 1999).
U.S. air quality, as measured at thousands of monitoring stations across the country, has shown steady improvement over the past 20 years due in part to the implementation of air quality regulatory programs and cleaner technologies for motor vehicles and stationary pollution sources. There are, however, still many areas of the country that are not in compliance with ambient air quality standards for pollutants such as ozone and PM (EPA, 1999).
parison of measurement systems will be required to ensure the value of these combined data sets.
Atmospheric models are also a key element in the research framework for addressing global air quality changes, as they represent the primary tool for forecasting the future state of the atmosphere. Critical sources of input for models include inventories of pollutant emissions, meteorological data to describe atmospheric conditions and transport, laboratory measurements to identify and describe important chemical reactions, and process studies that provide detailed understanding of specific aspects of the atmosphere's complex and interactive chemistry and transport. Observational data sets are necessary for judging whether model simulations are representative of current or past atmospheric conditions. Without this type of evaluation, our understanding will be incomplete and the value of model predictions will remain highly uncertain.
For all of these reasons, a commitment to long-term observational strategies emerges as imperative for understanding global air quality changes in the 21st century. The United States needs to play a leadership role in such a commitment because of our advanced technological capabilities and large contributions to global pollution emissions. Without such a commitment, we will critically limit
our understanding of the current state of the atmosphere and our ability to assess possible future changes. The remainder of this report focuses on issues related to the key research objectives discussed above, specifically:
Chapter 2 contains a review of the interactions between climate change and atmospheric chemical change.
Chapter 3 contains a review of observational and modeling studies of intercontinental pollution transport.
Chapter 4 contains an analysis of our current capabilities for observing atmospheric chemical changes related to global air quality concerns.
Chapter 5 contains the committee's key findings and recommendations.
Throughout these chapters, a series of text boxes are included to provide a broader context for the scientific concerns and imperatives discussed in the main body of the report.