Atmospheric pollution is a major factor affecting the quality of life and the sustainability of the Earth System. Various trace gases control the energy budget of Earth and impact the purity of air, so that their changes have major impacts on the environment. As more space-based observations of trace gases become available, their assimilation into numerical models is increasing our understanding of their cycling through the Earth System. This paper will review the present status of such assimilation systems and discuss how the NPOESS mission will sustain and extend our capabilities. The focus will be on ozone and carbon monoxide.

Various instruments on the NPOESS platform will provide information about atmospheric trace gases. When properly used, this information can contribute to our understanding of long-term changes in atmospheric composition, as well as to factors such as regional and local distribution of pollutants and their regional and intercontinental transport. This is true for ozone and carbon monoxide, which have different atmospheric distributions and are observed using different techniques. CO is emitted by combustion; it has obvious links with CO2 and CH4 in the climate system, yet is also a pollutant in its own right. Infrared nadir sounders, such as CrIS, have channels that yield information about tropospheric partial column abundance: AIRS and other instruments provide similar information. Ozone can be deduced with some accuracy from space-based instruments, including stratospheric profiles from infrared and microwave limb sounders and total columns from backscattered ultraviolet measurements; infrared nadir sounders give information on ozone, but it is frequently contaminated by clouds and by variations in surface emissions.

This paper will review different aspects of ozone and CO assimilation using present observations in the Goddard Earth Observing System, Version 5 (GEOS-5) assimilation system. The focus will be on the accuracy of the analyses and on their suitability for air-quality forecasting. These results will be discussed in the context of the NPOESS instruments, outlining key requirements and how they are met now and will be met with NPOESS. Some discussion will also be given to continuity of greenhouse-gas monitoring during the NPOESS timeframe. In all discussion, the balance between model skill and the observation quality will be emphasized.