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Earth Science and Applications from Space 14 GACM Global Atmospheric Composition Mission Launch : 2016â2020 Mission size: Large Orbit: LEO, SSO Agency: NASA Estimated cost: $600 million Areas of interest: Ecosystems, Health, Water, Weather Instruments: Ultraviolet spectrometer, infrared spectrometer, microwave limb sounder Benefits: Identification of sources and sinks of harmful pollutants Better forecasts of ozone and surface radiation Better forecasts of dangerous pollution events The composition and chemistry of ourÂ atmosphereÂ must be observed, mod- eled, and predicted in order to understand andÂ mitigate potentially harmful impacts on society and ecosystems. Current satellite instruments provide critical data at low resolution. However, to obtain the more precise data needed to improve models and predictions, a new generation of instruments and observing strategies must be developed. The suite of sensors aboard GACM will advance understanding of chemical weather processes on regional to global scales and help improve models and predictions of air pollution and ground- level ultraviolet radiation. GACM includes a unique combination of passive sensors gathering data on the altitudes and concentrations of key trace gases and aerosols (airborne particles) that are related to ozone formation or that serve as tracers of pollution and airflow. These sensors include a spectrometer in the ultraviolet/visible range that will collect daytime measurements of ozone (O3), nitrogen dioxide, sulfur dioxide, formaldehyde, and aerosols. An infrared spectrometer will sample carbon monoxide through the atmosphere by day and in the mid-troposphere at night, while an advanced microwave spectrometer will sample gases and aerosols in the upper troposphere and lower stratosphere. The committee also recommends focused tech- nology development for a second-phase mis- sion featuring an active differential absorption lidar (DIAL) system that would be launched in 2022 or thereafter. It would measure O3 with a vertical resolution better than 2 kilometers and aerosols and atmospheric structure at resolutions better than 150 meters.