Remote sensing of particulate pollution from space: Have we reached the promised land?

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This paper follows presentation and publication of the Hoff and Christopher 2009 Critical Review and represents an integration of discussants’ views of their paper. The discussants’ comments are organized beginning with a note on the history of satellite sensing, followed with a brief discussion of the strengths and weaknesses of satellite observations relative to air quality management needs. The commentary is completed with a summary of the applications of remote sensing to air quality. The authors of the review chose not to provide a response to the comments from the discussants.

Hoff and Christopher’s review presents a thorough and balanced summary of the satellite observations and processing methods for inferring aerosol optical depth (AOD) from space on the basis of measurements. Hoff and Christopher highlight 50 years of progress from the basic imagery of the early Television Infrared Observation Satellite (TIROS) to the recent satellite instrumentation capability to image trace gas and especially aerosol distributions in the atmosphere. The review serves as a bridge between more general discussions of satellite-based applications to air quality issues and a large body of specialized research papers in the atmospheric science literature. The authors have done an admirable job of organizing material from current literature for the satellite retrieval of optical depth data and reliability of investigators’ interpretation of AOD measurements compared with groundbased measurements of sun photometry and particle mass concentrations. Hoff and Christopher1 assess the viability of AOD as a measure of aerosol “column burden” relevant to the needs the air quality community, which range from determining compliance with ambient standards to input for air quality forecasting and tracking of trends. They ask the question, “Have we reached the promised land [relative to satellite measurements and air quality applications]?”— Can space-based AOD observations be used in regulatory applications for air quality determinations? The authors conclude that the two communities have not yet reached the “promised land.” Improvements in satellite AOD and pollutant gas observations are still needed relative to surface-based reference concentrations. But more importantly, satellite observations are integral to a system of observations and models providing resources to consistently address air quality issues not only in the United States and Canada, but also across the world.

The review explains the basic principles and assumptions of space-based remote sensing for particles, defines specialized terms and acronyms, gives details on different satellite orbits and their viewing frequencies, summarizes atmospheric observables for different instruments and platforms, and identifies future capabilities. The measurables relevant to air quality include the criteria pollutants, sulfur dioxide (SO2), carbon monoxide (CO), nitrogen dioxide (NO2) ozone (O3), and fine particles (fine particulate matter [PM2.5] mass concentration inferred from AOD). Additional measurements of interest include formaldehyde (an air toxic,), glyoxal (a potential secondary particle precursor), and bromine oxide (BrO—an indicator of halogen concentrations [particularly in the upper atmosphere]). In broad terms, satellite measurements with few exceptions serve as complements to surfacebased and aircraft measurements and to air quality model projections of ambient concentrations.

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