Pathways to Predicting Atmospheric Composition

With advances in our observational, modeling and data assimilation capabilities, accurate atmospheric composition forecasts can be achieved with significant benefits to society. This paper will highlight some of what is needed to realize this goal, such as expanded scientific knowledge, advanced remote sensing capabilities and modeling. I. INTRODUCTION The atmosphere serves as a global "commons," linking the other components of the Earth system, including the oceans, land, terrestrial and marine plants and animals, and the frozen regions. Indeed, the atmosphere links the natural system and all elements of human societies of all nations. Because of these linkages, the atmosphere is a conduit of change. For example, natural events and human activities can change atmospheric composition and hence the Earth's radiative (energy) balance. Subsequent responses by the climate system and the stratospheric ozone layer can influence the well being of human and natural systems. Global change research and observations have shown that changes in the composition and chemistry of the atmosphere spread over very large areas very quickly—that the atmosphere is the "fast mixer" in the Earth system. Because of the rapid and often global dispersal of chemical emissions, observations of changes in the atmosphere are among the very earliest harbingers of global changes. And the very long atmospheric residence times of some emissions imply virtually irreversible global changes over decades, centuries, and millennia—for all countries and populations, not just the pollutant emitters. Finally, the emerging capability of modeling the composition of global atmosphere as a whole is pointing to the relationships between issues once considered separately and independently, such as continental air quality and climate change. Prediction of the evolution of atmospheric trace constituent composition is intimately linked to that of the meteorological conditions under which chemical and transport processes occur and the ability of models to capture the essential physical and chemical processes, which transform chemical species. The composition of the atmosphere is also affected by biological processes and can affect the biosphere in ways that impact human well being. Atmospheric changes which have had the greatest impact on human health are air pollution (gases and aerosols) and increases of ultraviolet radiation. The chemical constituent of greatest interest is ozone. In the stratosphere, ozone protects the Earth from biologically damaging solar ultraviolet radiation. In the troposphere ozone is an active chemical pollutant that affects both plant and animal life. Ozone responds (for both production and destruction) to the concentration of many precursor species coming from both natural and anthropogenic sources. Accurate modeling of atmospheric composition requires knowing or forecasting the future evolution of these chemical forcings, as well as relevant changes in climatic conditions. In the case of sufficiently large changes in atmospheric composition, interactions with resulting changes in atmospheric circulation and physical properties cannot be ignored (e.g. the chemistry of the polar stratosphere affects the atmospheric transport). Processes of particular importance for assessments of potential atmospheric chemical composition impacts include the long range transport of material from natural and man made sources; the formation of aerosols and cloud particles and their interactions with constituents. With advances in our observational, modeling and data assimilation capabilities, accurate atmospheric composition forecasts can be achieved with significant benefits to society. This paper will highlight some of what is needed to realize this goal, such as expanded scientific knowledge, advanced remote sensing capabilities, and modeling.