Scientists have long known the air we breathe can be hazardous to our health. The World Health Organization estimates that more than 90 percent of the world’s population breathes air containing harmful levels of pollutants. Airborne particulate matter (PM) is especially dangerous. Breathing these tiny, floating solid and/or liquid particles of organic and inorganic matter, also known as aerosols, results in more than 4 million premature deaths each year due to cardiovascular, respiratory, and other illnesses, according to a major international health study called the Global Burden of Disease.

Now a new NASA satellite mission currently in development promises to take research into the connections between PM air pollution and human health to new heights. The Multi-Angle Imager for Aerosols (MAIA) investigation, targeted for launch in 2022, will produce unique maps of PM air pollution that epidemiologists will use to study how different types of PM – mixtures of particles with different sizes, shapes, and compositions – affect our health. The three-year investigation marks the first-ever partnership between NASA, epidemiologists and health organizations to use space-based data to study human health and improve lives.

“We know exposure to airborne particles from combustion of fossil fuels, traffic, smoke, and dust is associated with various diseases and even mortality,” said MAIA Principal Investigator David Diner of NASA’s Jet Propulsion Laboratory (JPL) in Southern California. JPL is building the MAIA instrument and managing the investigation. “It is likely that infections from bacteria, fungi, or a virus such as COVID-19 can be exacerbated by air pollution-related health problems that people already have, making them more susceptible to severe illness and adverse health consequences.”

Particulate Matter Air Pollution: A Not-So-Clear but Present Danger

When it comes to PM and health, size matters: differently sized particles are associated with different health effects. The commonly regulated size classifications are PM₁₀ (inhalable particles with diameters of 10 micrometers and less) and PM_2.5 (respirable particles with diameters of 2.5 micrometers and less). Larger particles can irritate our airways, while smaller particles can penetrate deeper into our lungs and cause inflammation that affects other organs. Scientists have associated PM_2.5 with increased risk of coronary heart disease, heart attacks, and strokes. Other studies show people who breathe more PM are more likely to develop lung cancer, lower respiratory infections, chronic obstructive pulmonary disease (COPD), and problems during pregnancy and birth, such as preterm birth and low birth weight.

Less understood is which particle constituents are more harmful than others. “Particulate matter is complex,” Diner said. “Dust particles are irregularly shaped, while droplets are spherical. Particles have different chemical components and originate from various sources, many of them human-produced, like vehicle emissions and agricultural burning. Amounts also vary widely by location and season.”

Traditionally, scientists have used monitoring instruments on the ground to accurately measure air pollution exposure. One kind uses filters to capture airborne particles that are weighed and analyzed in a lab to calculate the amount of various particles. Other instruments use scattering of light or absorption of electrons to generate real-time estimates of particle mass concentration. Because PM changes from place to place, a dense array of monitors would be needed to adequately sample an entire city.

MAIA: Getting the Big Picture on Air Pollution

Enter MAIA. From space, MAIA will acquire data that will be used to generate PM air pollution maps in a globally distributed set of target areas. Epidemiologists – public health professionals who study the causes, distribution, and frequency of disease in human populations – will use the maps to conduct health studies to determine which types and sources of PM are most harmful.

Diner says there’s a growing realization among scientists of the value satellite data bring to the table for studying air pollution.

“The obvious way to monitor aerosols near the ground is to install sensors, collect particles and measure their properties,” he said. “That can quickly become daunting due to costs and logistical complexities, especially in the developing world. In contrast, a satellite can image areas around the world and fill in coverage gaps, but satellite aerosol data only provide indirect inferences about particle composition.”

To determine the concentrations and chemical composition of these tiny particles, MAIA satellite data will be combined with data from ground instruments and computer models of how chemicals are formed and transported in the atmosphere. “This complementarity gives the investigation its strength,” Diner added.

"A unique aspect of MAIA is the integration of the experiences and expectations of the public health community who will use the data into the mission's DNA from the outset," said Yang Liu, a professor in the Rollins School of Public Health at Emory University in Atlanta and a MAIA science team member. "Combining data from the satellite instrument and various ground sensor networks with atmospheric chemistry and statistical models, and generating simulated data prior to launch to engage the user community, will enable a smooth transition from data to applications and maximize MAIA’s societal benefits."

MAIA Health Studies

Among the methodologies MAIA epidemiologists will use to study the effects of particulate matter pollution measured by the MAIA instrument on human health are time series and cohort studies. In time-series studies, daily death and hospitalization records in a particular city or urban area are used and their relationships with short-term (single- or multi-day) air pollution concentrations are investigated. Cohort studies examine the impacts of long-term (one-year or longer) exposure to air pollution on health, and the health of a large group of people is tracked for several years. In designing these studies, epidemiologists must account for extraneous variables that could also cause people to contract the diseases under investigation and potentially bias their study results. After accounting for such "confounding factors" (examples of which include family history and smoking) scientists can calculate the impact of air pollution on a person’s risk of disease.

MAIA researchers will conduct different epidemiological studies in its primary target areas, depending on the type of health records available, what studies have been done in the past, and types of PM present. Three timescales of exposure will be studied: acute (short-term spikes in PM over a period of days to weeks), sub-chronic (moderate duration, focusing on exposure to PM by expectant mothers), and chronic (health impacts resulting from the accumulated effects of PM exposures over many years).

“More diseases are being associated with particulate matter exposure,” said Diner. “For example, scientists are finding connections between particulate exposure and neurological disorders, such as cognitive impairment. There’s a broad range of health outcomes people are interested in studying, and many will be driven by the sources of health data our team acquires.”

A Heritage of Space-based and Airborne Aerosol Measurements

MAIA’s heritage includes numerous space missions and instruments, including the Multi-angle Imaging SpectroRadiometer (MISR) on NASA’s Terra satellite, for which Diner is also principal investigator. Launched in 1999, MISR views Earth from nine different angles, giving scientists a better picture of Earth’s climate, including aerosols, cloud forms, and land surface covers.

“MISR demonstrated new methods of using multi-angle satellite imagery to characterize airborne particles,” Diner said. That work led to collaborations with scientists using MISR data to study links between PM and human health. “Subsequent to the launch of MISR, we obtained funding from NASA’s Earth Science Technology Office to develop technologies that expand our ability to characterize aerosols.” The Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) instrument and second-generation AirMSPI-2 are the result of these efforts and have prototyped several key technologies used in MAIA.

MAIA: A Targeted Investigation of Major Cities

While MAIA’s orbit will take it over much of the world, practical considerations limit data collection to several dozen target areas, each measuring about 147,000 square kilometers (nearly 57,000 square miles), or roughly the area of Southern California. From this set, MAIA epidemiologists will focus their health studies on a dozen primary target areas containing highly populated cities: Los Angeles, Atlanta, and Boston in the U.S.; Barcelona, Spain; Rome, Italy; Tel Aviv, Israel; Johannesburg, South Africa; Addis Ababa, Ethiopia; Delhi, India; Beijing, China; Taipei, Taiwan; and Seoul, South Korea. Primary target areas were selected based on various criteria, including population, variability in the amounts and types of PM present, cloudiness, how well monitored the area is by ground instruments, and access to public health records.

Particle measurements will also be recorded in more than 20 secondary target areas, including Mexico City, Mexico; Santiago, Chile; Accra, Ghana; Nairobi, Kenya; and Bangkok, Thailand.

MAIA measurements will be particularly useful in very large metropolitan areas and cities in countries that have not had the resources to measure particulate pollution, said MAIA science team member Bart Ostro, an environmental epidemiologist at the University of California, Davis, and former chief of air pollution epidemiology at the California Environmental Protection Agency. “Providing data to help identify specific particle sources can help decision makers prioritize control strategies and reduce costs,” he said.

The Science Behind MAIA’s Measurements

To distinguish aerosol particle types, MAIA’s specialized digital camera will capture sunlight reflecting off Earth and its atmosphere. MAIA’s coated and polished aluminum mirrors allow the camera to record light in 14 spectral bands, many more wavelengths than a typical digital camera, allowing it to capture visible light as well as ultraviolet, near-infrared, and shortwave-infrared. These wavelengths are needed because tiny particles tend to scatter light most efficiently at wavelengths similar to their own size. Shorter-wavelength visible spectral bands provide information on the smallest particles (PM_2.5 and smaller), while shortwave-infrared bands provide information on larger aerosol types, like dust and volcanic ash. Ultraviolet wavelengths are sensitive to absorption of sunlight by particles containing certain mineral and organic matter types.

While a regular digital camera captures images using a rectangular array of detectors (pixels), MAIA’s detectors are arranged in individual rows, a type of detector known as a pushbroom imager. As MAIA flies over Earth, the satellite’s motion pushes the rows of detectors over the area like a broom across a floor. Since the satellite is traveling at about 25,000 kilometers (15,500 miles) per hour, each exposure must be very rapid.

Observing the atmosphere at multiple angles from a spacecraft makes aerosols stand out more prominently against the surface background and tells us about their size and shape. MAIA’s camera is mounted on a support that rotates 60 degrees forward and backward. As the instrument flies over a target, a mechanism points the camera at it several times, capturing images in succession from different angles. The camera also points left and right, allowing it to see targets even when they’re not directly underneath the satellite. This allows the primary target areas to be observed three to four times each week.

Sunlight becomes polarized (that is, the light waves have a preferred plane of vibration) when scattered by airborne particles. By placing polarizing filters above its camera’s detectors and using a specialized device called a polarization modulator, the MAIA instrument can accurately measure the degree to which incoming light is polarized by atmospheric particles. This provides additional information about particle sizes and shapes.

MAIA is the primary instrument aboard its commercial host satellite, Orbital Test Bed (OTB)-2, provided by General Atomics Electromagnetic Systems. OTB-2’s 740-kilometer low-Earth polar orbit is “Sun-synchronous,” meaning every time it crosses the equator (about every 100 minutes), the local time is the same. This allows MAIA to see each target city at approximately the same time of day (mid-morning).

The Doctor is IN: MAIA’s Diverse Team

MAIA unites experts from around the world. In addition to scientists, engineers and technicians at JPL and other NASA centers, the MAIA science team consists of investigators from other U.S. and international institutions, including universities and government agencies. Among them, the U.S. Department of State is providing logistical support for deploying MAIA’s surface monitor equipment around the world, and the U.S. Agency for International Development is providing financial support to analyze surface monitor data and conduct capacity building activities in Africa. Many other partners are assisting in operating ground instruments and planning health effects studies.

Five of MAIA’s co-investigators are practicing epidemiologists. Diner says working with them has been a fascinating experience.

“Our team is diverse, so we had to learn how to communicate with each other and understand everyone’s roles,” he said. “Epidemiology is complex and relies on statistics developed over long time periods. I don’t pretend to understand everything about their methodologies, but I’ve learned a lot. In return, we’ve hopefully given them an appreciation for how to go about designing a satellite mission from the ground up. It’s a different world than they’ve been involved in before.”

Uses of MAIA Data

MAIA will produce free, publicly available maps of PM_2.5 and PM₁₀ concentrations for each primary target area. The PM_2.5 maps will be partitioned into several key chemical constituents: sulfate, nitrate, organic carbon, elemental carbon, and dust. MAIA epidemiologists will use these maps in conjunction with health records to explore statistical linkages. Results will be disseminated through peer-reviewed publications.

In addition to epidemiological studies, MAIA data will support other applications, including evaluations of the impact of PM pollution on natural and human environments, information for air quality regulators and policymakers, and research into aerosol and cloud interactions with Earth’s climate.

Small Mission, Big Contributions

Ultimately, Diner says MAIA will give scientists, the medical community and decision makers who regulate air quality new information that can enable cleaner air, improved public health and cost savings.

“It’s my hope MAIA will help trace particulate pollution to its sources, providing the public health and regulatory communities data they need to control particle emissions,” he said. “It’s satisfying being part of something that could contribute to reducing the burdens of disease on society.”