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Notes and data sources: The observations are from the EPA AirNow program and were made by over 120 federal, state, local, and tribal air quality agencies. The AirNow observations are preliminary and subject to change. The GEOS-CF system is a product of the NASA Global Monitoring and Analysis Office (GMAO). It provides daily, global forecasts of atmospheric composition at a spatial resolution of 25 km, using meteorological analyses from other GEOS systems. These forecasts are intended as research datasets for use by NASA researchers and the broader community.