Current and Future Directions in Satellite Remote Sensing of Airborne Particulate Matter and Assessment of Human Health Impacts (Core Science Keynote)

Airborne particulate matter (PM) is a well-known cause of heart disease, cardiovascular and respiratory illness, low birth weight, and lung cancer. The Global Burden of Disease (GBD) Study ranks PM as a major environmental risk factor worldwide, causing more than 3 million premature deaths per year. Nearly 90% of city dwellers breathe air violating World Health Organization (WHO) guidelines for outdoor PM. Global maps of concentrations of particles with diameters <2.5 micrometers (PM2.5) derived from satellites, including the Multi-angle Imaging SpectroRadiometer (MISR), MODerate resolution Imaging Spectroradiometer (MODIS), and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) have provided key contributions to many health-related investigations, including GBD. Yet our understanding of the relative toxicity of specific PM types—mixtures having different size distributions and compositions—is relatively poor. Surface PM monitors alone cannot solve this problem because they are too sparsely distributed, expensive to install and maintain, and non-existent in many parts of the world where air pollution health impacts are greatest. Aerosol concentrations can vary over spatial scales smaller than the distances between monitors, leading to inaccurate exposure estimates.

Observation from space offers the only practical means of acquiring frequent, high spatial resolution maps of PM concentrations in major population centers around the world. To investigate associations between airborne particle types and human health impacts, the Multi-Angle Imager for Aerosols (MAIA) investigation was proposed to NASA’s third Earth Venture Instrument (EVI-3) solicitation, and NASA selected it for funding in March 2016. The satellite-based MAIA instrument obtains its sensitivity to particle size and composition by building upon the legacies of many satellite instruments; observing in the ultraviolet, visible, near-infrared, and shortwave-infrared regions of the electromagnetic spectrum; acquiring images at multiple angles of view; determining the degree to which the scattered light is polarized; and integrating these capabilities at moderately high spatial resolution.

MAIA incorporates a pair of cameras on a two-axis gimbal to provide multiangle observations of selected target areas distributed around the world. The two cameras are placed side-by-side to double the swath width achievable with a single camera. A set of Primary Target Areas (PTAs) on five continents includes major population centers covering a range of PM concentrations and particle types, surface-based aerosol sunphotometers, PM size discrimination and chemical speciation monitors, and access to geocoded health datasets. The MAIA investigation will combine Weather Research and Forecasting coupled with Chemistry (WRF-Chem) transport model estimates of the abundances of different aerosol types with MAIA instrument data to reduce model biases. Geostatistical models derived from collocated surface and MAIA retrievals will then be used to relate retrieved fractional column aerosol optical depths to near-surface concentrations of major PM constituents, including sulfate, nitrate, organic carbon, black carbon, and dust. Epidemiological analyses of geocoded birth, death, and hospital records will be used to associate exposure to PM types with adverse health outcomes, capitalizing on the statistical power obtained by targeting high-population cities.

In addition to observations of the PTAs, MAIA will also collect aerosol and cloud observations over regions of interest to the radiation science, climate, and environmental science communities, including major aerosol source regions and cloud regimes affected by aerosol pollution, and targets of opportunity such as large wildfires, dust storms, or volcanic eruptions. MAIA launch is planned for early in the next decade, and the baseline mission life in orbit is 3 years, with an additional 2 years allocated to carrying out the epidemiological studies. The investigation brings together an international team of researchers and policy specialists with expertise in remote sensing, aerosol science, air quality, epidemiology, and public health.