Remote Sensing of Particulate Pollution from Space: Have We Reached the Promised Land? (original) (raw)

The recent literature on satellite remote sensing of air quality is reviewed. 2009 is the 50th anniversary of the first satellite atmospheric observations. For the first 40 of those years, atmospheric composition measurements, meteorology, and atmospheric structure and dynamics dominated the missions launched. Since 1995, 42 instruments relevant to air quality measurements have been put into orbit. Trace gases such as ozone, nitric oxide, nitrogen dioxide, water, oxygen/tetraoxygen, bromine oxide, sulfur dioxide, formaldehyde, glyoxal, chlorine dioxide, chlorine monoxide, and nitrate radical have been measured in the stratosphere and troposphere in column measurements. Aerosol optical depth (AOD) is a focus of this review and a significant body of literature exists that shows that ground-level fine particulate matter (PM 2.5) can be estimated from columnar AOD. Precision of the measurement of AOD is Ϯ20% and the prediction of PM 2.5 from AOD is order Ϯ30% in the most careful studies. The air quality needs that can use such predictions are examined. Satellite measurements are important to event detection, transport and model prediction, and emission estimation. It is suggested that ground-based measurements, models, and satellite measurements should be viewed as a system, each component of which is necessary to better understand air quality. BACKGROUND On Explorer VII, which was launched October 13, 1959, 1 Suomi 2 assessed infrared (IR) radiative heat balance measured from an orbiting satellite as a forcing agent for atmospheric circulation. This year marks one-half century of space-borne observations of the Earth's atmosphere. In 1961, meteorologists were first presented iconic images of the Earth from the first TIROS satellites. 3 By showing clouds and weather systems that visually identified features only seen on synoptic weather charts, satellite meteorology was born and developed as a natural tool to identify present and future weather. This critical review discusses the measurement of airquality-related gases and aerosols from monitors orbiting above the atmosphere. The National Academy of Sciences (NAS) National Research Council "Decadal Survey" 4 identifies a need to further involve satellite measurements in decision-making and applications for societal benefit. The NAS expectation of such observations to be integrated into routine monitoring and assessment for tropospheric pollution deserves a critical examination of the quality and utility of such measurements. There are several important constraints on the ability of satellite instruments to measure atmospheric composition. Orbit, atmospheric transparency, wavelength of observation, molecular spectroscopy, scattering, and absorption are among the variables that define whether a measurement can be made. This review begins with the physical principles underlying satellite observations. SATELLITE ORBITS The promise of space-borne atmospheric measurements has encouraged the launch of thousands of Earthobserving sensors in low (Ͻ2000 km, LEO [note that unfamiliar terms and abbreviations are listed in the glossary]; Table 1), medium (2000-25,000 km, MEO) and geosynchronous (35,786 km, GEO) orbits. Approximately 900 satellites are currently being tracked by the R.M. Hoff S.A. Christopher IMPLICATIONS Satellite measurements are going to be an integral part of the Global Earth Observing System of Systems. Satellite measurements by themselves have a role in air quality studies but cannot stand alone as an observing system. Data assimilation of satellite and ground-based measurements into forecast models has synergy that aids all of these air quality tools.