“Out of sight, out of mind” goes the common saying, yet what people can’t see can have an important impact on their well-being. Invisible gases including nitrogen dioxide, sulfur dioxide, carbon monoxide, and ozone negatively affect human health, and satellites offer the best means of tracking those atmospheric pollutants. NASA maintains the world’s largest constellation of Earth-observing satellites, the data from which are made freely available to the public.
Despite the power of those eyes in the sky, many potential users of NASA’s satellite data don’t know how to employ them or may not even be aware of their existence. To build a wider community of data users, NASA recently launched a three-year initiative called the Health and Air Quality Applied Sciences Team (HAQAST), led by Tracey Holloway of the University of Wisconsin–Madison. The goal is to get relevant NASA satellite tools and data into the hands of the public-health and air-quality communities.
Building an air-monitoring network
The era of remote sensing began with a photograph taken from a hot-air balloon in 1858. As humans took to the skies, so did the science of monitoring the planet. By the dawn of the space age, it was common practice to include a host of atmosphere-sensing instruments in a rocket’s payload. In 1960 NASA launched TIROS 1, the world’s first weather satellite. Eighteen years later, the agency deployed Nimbus 7, which included the Total Ozone Mapping Spectrometer, the first satellite instrument for tracking atmospheric chemistry.
The need for comprehensive air monitoring escalated in 1970 when Congress passed amendments strengthening the Clean Air Act. Since then the US has built up a large network of ground-based air pollution monitors. Those instruments have tracked huge improvements in air quality over the past five decades, especially for the six pollutants regulated under the National Ambient Air Quality Standards (NAAQS): SO2, NO2, CO, O3, particulate matter, and lead.
Ground-based monitors remain the gold standard for detecting human exposure to the NAAQS pollutants, particularly because they are located at “nose level” and so can trace exactly what is in the air people breathe. But the instruments are expensive and spatially limited, as shown in figure 1. As a result, much of the geographic US—not to mention the world—goes unobserved. For example, although NO2, which aggravates breathing issues and contributes to the formation of particulate matter, acid rain, and O3, is monitored in many urban areas, especially in those at risk of exceeding the NAAQS, the entire state of Idaho has only one monitor. Nebraska has none. (To learn about other weaknesses of air-quality tracking in the US, read the commentary by Mika McKinnon, Physics Today online, 31 October 2017.) For NO2 and other pollutants, satellite data can complement ground-based monitors to offer a more complete picture of their trends and spatial variability.
Satellites cannot replace surface-level monitors. They can’t spot certain pollutants, such as ground-level O3. They typically detect the pollution in the entire column of air between the satellite and the ground, rather than the near-surface concentrations that we breathe. And because most environmental satellites are polar orbiting, they deliver only snapshot values for any single location, typically once a day or less. However, when used in combination with surface monitors and advanced models, satellites can amplify the power of existing air-quality monitoring many times over. For the past 20 years, NASA’s Earth Observing System, detailed in figure 2, has employed a suite of space-based instruments to track everything from atmospheric water vapor and global temperature profiles to smoke and oceanic phytoplankton levels.
Putting the data to good use
To help facilitate the transfer of NASA data and knowledge to those working at the intersection of public health and air quality, NASA’s HAQAST members collaborate with a variety of public stakeholders to identify the most pressing air-quality and public-health research issues that can be informed with satellite data.
At Georgia Tech, HAQAST members Ted Russell and Talat Odman are heading a project aimed at bringing satellite data to bear on wildland-fire smoke, the largest source of fine particulate matter in the southeastern US. Particulates smaller than 2.5 μm in diameter have been linked to premature death from cardiovascular diseases and can trigger or amplify the symptoms of a host of respiratory illnesses. They are easily airborne and can drift miles from a fire to a suburban or urban area. With wildland fires on the rise, the need for accurate measurements of fine particulate matter in the sparsely monitored southeastern US is urgent.
Russell and Odman are using satellite data retrieved from NASA’s MODIS, VIIRS, and GOES instruments, shown in figure 3, to help pinpoint the exact locations of fires and to capture the heights of their smoke plumes. Those data feed Russell and Odman’s models to improve smoke forecasts, both for cities and for rural areas that do not have their own system of ground monitors.
On the other side of the country, San Jose State University’s Frank Freedman and colleagues are combining daily satellite data from NASA’s MODIS instrument with ground monitoring data and powerful computer models to provide retrospective estimates of air pollution at fine spatial detail. The team is developing a Web-based app that will reconstruct a searchable history of air pollution at a fine scale. For instance, the app will be able to provide daily or even hourly averages of historic air pollution fields surrounding major roadways.
The team’s work is also forward looking, as the system will be adaptable enough to accommodate the increasingly detailed satellite and monitoring data that will come from ever-improving equipment. Freedman projects that a beta version of the app will be available to the public by early 2019, with a finalized first version debuting early in the next decade.
Flexibility is also central to the HAQAST mission. The partner organizations that HAQAST serves, including state air agencies, city health departments, and fire-response teams, need information to support targeted, immediate demands. Many projects require expertise from multiple members of HAQAST, who can join forces for maximum impact. Such collaborations are called Tiger Teams.
Arlene Fiore of Columbia University’s Lamont–Doherty Earth Observatory, in partnership with three state air-quality agencies, is leading a Tiger Team that will help states incorporate satellite data into the air-quality planning process that is required of regions that exceed NAAQS limits. Although some air agencies are routine users of satellite data, others lack the resources needed to train employees to evaluate the suitability of satellite products. Questions frequently arise as to the meteorological conditions and scales for which satellite data are most useful, as well as the data’s general accuracy. Fiore’s team will collaborate with states that already use satellite data to create an accessible set of frequently asked questions, best practices, and case studies that can guide other potential users as they learn the basics of applying satellite data to air-quality issues.
Projects such as Fiore’s illustrate the potential of tapping into the satellite data that NASA has collected for decades. The hope is that in coming years, HAQAST partners will put that data to good use to solve important problems.
Daegan Miller is an environmental historian and a member of the NASA Health and Air Quality Applied Sciences Team at the University of Wisconsin–Madison.