Air quality (AQ) is defined by the atmospheric composition of gases and particulates near the Earth's surface. This composition depends on local emissions of pollutants, chemistry, and transport processes; it is highly variable in space and time. Key lower-tropospheric pollutants include ozone, aerosols, and the ozone precursors NOx and volatile organic compounds. Information on the transport of pollutants is provided by carbon monoxide measurements. Air quality impacts human society, because high concentrations of pollutants can have adverse effects on human health; health costs attributable to AQ are high. The ability to monitor, forecast, and manage AQ is thus crucial for human society. In this paper we identify the observational requirements needed to undertake this task, discuss the advantages of the geostationary platform for monitoring AQ from space, and indicate important challenges to overcome. We present planned geostationary missions to monitor AQ in Europe, the United States, and Asia, and advocate for the usefulness of such a constellation in addition to the current global observing system of tropospheric composition.

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*The National Center for Atmospheric Research is sponsored by the National Science Foundation.