Multi-Angle Imager for Aerosols

On March 10, 2016, the National Aeronautics and Space Administration (NASA) announced that it had selected 2 proposals for its Earth Venture Instrument program, which supports highly focused science investigations that complement NASA’s larger missions. The National Research Council recommended in 2007 that NASA undertake these types of projects regularly. One such investigation, the Multi-Angle Imager for Aerosols (MAIA), is designed to study the adverse health effects of respirable particulate matter (airborne particles with an aerodynamic diameter 10 mm, or PM10) and various components of fine particulate matter (airborne particles with an aerodynamic diameter 2.5 mm, or PM2.5). The budget for the MAIA investigation (cost capped at $97 million in fiscal year 2018 dollars) supports the delivery and operation of the satellite instrument and its associated retrieval algorithms and ground dataprocessing software, as well as aerosol and epidemiologic research by the science team. The instrument launch date depends on which satellite platform NASA chooses to host the instrument, and it is scheduled for the early 2020s. Evidence linking disease risks and premature death with exposure to particulate matter is convincing. These adverse health outcomes can be observed at relatively low levels, as measured in urban areas in the United States and Canada. Worldwide, the World Health Organization estimated that in 2012, 3.7 million deaths were attributable to ambient particulate matter, about 88% of which occurs in lowand middle-income countries. This finding further highlights that particulate matter air pollution constitutes the world’s largest environmental health risk. Historically, population exposure to particulate matter has been estimated with measurements made by expensive in situ monitoring stations. However, such monitoring networks are sparse or nonexistent in many parts of the world where particulate matter concentrations are the highest. Even in the United States, the approximately 1200 stations operated by the US Environmental Protection Agency (EPA) are located in only 30% of approximately 3100 counties. Uncertainties in estimates of particulate matter exposure tend to bias the health risks toward the null hypothesis, resulting in an underestimation of public health risks. Rural and suburban populations are often excluded because of a lack of estimates on exposure. Since the launch of NASA’s Terra satellite in late 1999, satellite-retrieved aerosol optical depth (a measure of particle light extinction in the atmosphere) has emerged as a promising solution for providing worldwide estimates of PM2.5 exposure. During the past decade, various satellite-based methods have been developed to estimate ground-level PM2.5 levels to effectively augment ground-based monitoring and help fill pervasive gaps in data that impede efforts to study air pollution and protect public health. What is missing from this intriguing picture of satellitepowered studies on the health effect of particulate matter worldwide is information on particle composition. Particulate matter includes many components that may vary in their toxicity and in the degree to which they contribute to adverse health effects. One review noted that ‘‘one of the biggest gaps in our knowledge relates to what specific air pollutants, combination of pollutants, sources of pollutants, and characteristics of pollutants are most responsible for the observed health effects. ’’ Differential toxicity can have important implications for the establishment of ambient air quality standards and for more targeted pollution control strategies. Focusing regulations on the most toxic particulate matter components could protect public health at a lower total cost. So far, only a fraction of EPA’s ground stations measure PM2.5 composition, usually on a 1-in-6-days schedule. For the rest of the world, such information is even more limited.