Human industrial activities including the combustion of fossil fuels, and land-use activities including biomass burning and agriculture, lead to the emission of gases and particles that perturb atmospheric composition in numerous ways. One such perturbation is the build-up of long-lived greenhouse gases including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs). The scientific community has made considerable efforts to document the long-term atmospheric trends for these species and to assess how these changes will affect global climate in the coming years. Less well documented are changes in the atmospheric concentration and distribution of shorter-lived species including nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon monoxide (CO), sulfur dioxide (SO2), ozone (O3, which is formed in the troposphere from chemical reactions involving NOx and VOCs) and airborne particulate matter (PM, which encompasses a diverse class of chemical species including sulfates, nitrates, soot, organics, and mineral dust).
Air pollution is generally studied in terms of immediate local concerns rather than as a long-term “global change” issue. In the coming decades, however, rapid population growth and urbanization in many regions of the world, as well as changing climatic conditions, may expand the scope of air quality concerns by significantly altering atmospheric composition over broad regional and even global scales. 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. Our ability to understand observed changes in global air quality and to accurately 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 connects all the countries of the world to varying degrees and can raise “background” 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 other regions of the world, and to develop realistic and effective air quality management plans for the coming decades.
Addressing these complex questions about global air quality change will require a comprehensive research strategy that integrates atmospheric observations covering a wide range of spatial and temporal scales together with diagnostic, global, and regional models. Other key elements in this research framework include inventories of pollutant emissions, meteorological data to describe atmospheric conditions and transport, laboratory measurements to characterize important chemical reactions, and process studies to provide detailed understanding of complex chemical and dynamical phenomena.
Some components of this research framework are in a more mature state than others. For instance, significant progress is being made in the development of regional/global chemical transport models and their integration with global climate models. Likewise, in recent years the atmospheric chemistry community has organized numerous field campaigns that combine model analyses with in-
tensive observations from a variety of platforms, which has enhanced our understanding of some complex chemical and radiative processes.
In contrast, we currently do not have the capacity to observe many important medium- and long-term changes (that is, changes occurring over the course of years to decades) in the chemistry and composition of the lower atmosphere. If these observational capabilities are not strengthened, this will greatly limit our ability to document the evolution of the atmosphere in the coming decades. This also limits the value of the developments cited above, since a strong observational base is needed to test and improve model predictions, and to provide a longer-term context for the observational “snapshots” obtained through intensive field campaigns. A range of observational platforms and techniques will be needed to provide measurements at the earth's surface and in the free troposphere several kilometers above the surface. 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 and other in situ observations will require that similar measurements be made by numerous international scientific groups, with careful calibration and intercomparison of different measurement systems.
The following are the committee's conclusions and recommendations:
Current observational systems are not adequate for characterizing many important medium- and long-term global air quality changes. Some particularly notable weaknesses in our current observational capabilities include the lack of (i) long-term measurements of reactive compounds and PM, (ii) methods for obtaining vertical profile data, and (iii) measurement sites that allow for a meaningful examination of long-range transport and trends in background concentrations.
The global air quality issues discussed in this report intersect with the concerns of several federal agencies, yet none of these agencies have a clear mandate to lead U.S. research efforts or maintain the long-term observational programs that are needed to address these issues.
Maintain and strengthen the existing measurement programs that are essential for detecting and understanding global air quality changes. High priority should be given to programs that aid in assessing long-term trends of background ozone and PM.
Establish new capabilities to provide long-term measurements and vertical profiles of reactive compounds and PM that will allow meaningful examination of long-range transport and trends in background concentrations.
These two recommendations will require providing support to:
develop uniform and traceable standards, on a global basis, for calibration of both gas-phase and aerosol measurements;
improve measurement technologies for use in current observational platforms (such as ground-based air quality monitoring networks, commercial aircraft, and balloons/sondes), and in new potential platforms such as “supersites” for measuring a comprehensive suite of compounds in remote locations and unmanned aerial vehicles for long-duration sampling of the atmosphere over a wide range of altitudes;
integrate measurements obtained from different observational programs and platforms, with a particular focus on integrating remotely-sensed satellite observations with in situ aircraft and ground-based observations;
promote observational programs specifically designed to address chemical and meteorological data requirements for the development and evaluation of models.
Responsibility for carrying out this work should be clearly assigned to a U.S. federal agency (or interagency) research program, and the United States should play a leadership role in fostering international cooperative research and observational activities to enhance our understanding of global air quality changes.