I. Introduction
Fine particle concentrations have been associated in numerous studies with increased risk of cardiovascular and respiratory disease [1]. Aerosol remote sensing measurements with boundary layer sensitivity can greatly enhance society's ability to detect pollutants, issue health alerts and monitor trends in air quality. Trace gases can also contribute to poor air quality and several can be monitored from space, including , , CO, , and . Some of these gases, and , are also precursors to aerosol formation. Particulates and the precursors to secondary particle formation affect human health and the environment near emission sources. Furthermore, particulates can travel over relatively long distances and affect air quality downwind over regions that may or may not have in situ surface monitoring available. Because of their greater spatial coverage compared to in situ observations, satellite aerosol observations are particularly well suited for monitoring long range transport of pollution.