Climate change and air pollution are interlocking crises that threaten human health. Reducing emissions of some air pollutants can help achieve climate goals, and some climate mitigation efforts can in turn improve air quality.

One part of MIT Professor Arlene Fiore's research program is to investigate the fundamental science in understanding air pollutants - how long they persist and move through our environment to affect air quality.

"We need to understand the conditions under which pollutants, such as ozone, form. How much ozone is formed locally and how much is transported long distances?" says Fiore, who notes that Asian air pollution can be transported across the Pacific Ocean to North America. "We need to think about processes spanning local to global dimensions."

Fiore, the Peter H. Stone and Paola Malanotte Stone Professor in Earth, Atmospheric and Planetary Sciences, analyzes data from on-the-ground readings and from satellites, along with models, to better understand the chemistry and behavior of air pollutants - which ultimately can inform mitigation strategies and policy setting.

A global concern
At the United Nations' most recent climate change conference, COP26, air quality management was a topic discussed over two days of presentations.

"Breathing is vital. It's life. But for the vast majority of people on this planet right now, the air that they breathe is not giving life, but cutting it short," said Sarah Vogel, senior vice president for health at the Environmental Defense Fund, at the COP26 session.

"We need to confront this twin challenge now through both a climate and clean air lens, of targeting those pollutants that warm both the air and harm our health."

Earlier this year, the World Health Organization (WHO) updated its global air quality guidelines it had issued 15 years earlier for six key pollutants including ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide (CO). The new guidelines are more stringent based on what the WHO stated is the "quality and quantity of evidence" of how these pollutants affect human health. WHO estimates that roughly 7 million premature deaths are attributable to the joint effects of air pollution.

"We've had all these health-motivated reductions of aerosol and ozone precursor emissions. What are the implications for the climate system, both locally but also around the globe? How does air quality respond to climate change? We study these two-way interactions between air pollution and the climate system," says Fiore.

But fundamental science is still required to understand how gases, such as ozone and nitrogen dioxide, linger and move throughout the troposphere - the lowermost layer of our atmosphere, containing the air we breathe.

"We care about ozone in the air we're breathing where we live at the Earth's surface," says Fiore. "Ozone reacts with biological tissue, and can be damaging to plants and human lungs. Even if you're a healthy adult, if you're out running hard during an ozone smog event, you might feel an extra weight on your lungs."

Telltale signs from space
Ozone is not emitted directly, but instead forms through chemical reactions catalyzed by radiation from the sun interacting with nitrogen oxides - pollutants released in large part from burning fossil fuels-and volatile organic compounds. However, current satellite instruments cannot sense ground-level ozone.

"We can't retrieve surface- or even near-surface ozone from space," says Fiore of the satellite data, "although the anticipated launch of a new instrument looks promising for new advances in retrieving lower-tropospheric ozone". Instead, scientists can look at signatures from other gas emissions to get a sense of ozone formation. "Nitrogen dioxide and formaldehyde are a heavy focus of our research because they serve as proxies for two of the key ingredients that go on to form ozone in the atmosphere."

To understand ozone formation via these precursor pollutants, scientists have gathered data for more than two decades using spectrometer instruments aboard satellites that measure sunlight in ultraviolet and visible wavelengths that interact with these pollutants in the Earth's atmosphere - known as solar backscatter radiation.

Satellites, such as NASA's Aura, carry instruments like the Ozone Monitoring Instrument (OMI). OMI, along with European-launched satellites such as the Global Ozone Monitoring Experiment (GOME) and the Scanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY), and the newest generation TROPOspheric Monitoring instrument (TROPOMI), all orbit the Earth, collecting data during daylight hours when sunlight is interacting with the atmosphere over a particular location.

In a recent paper from Fiore's group, former graduate student Xiaomeng Jin (now a postdoc at the University of California at Berkeley), demonstrated that she could bring together and "beat down the noise in the data," as Fiore says, to identify trends in ozone formation chemistry over several U.S. metropolitan areas that "are consistent with our on-the-ground understanding from in situ ozone measurements."

"This finding implies that we can use these records to learn about changes in surface ozone chemistry in places where we lack on-the-ground monitoring," says Fiore. Extracting these signals by stringing together satellite data - OMI, GOME, and SCIAMACHY - to produce a two-decade record required reconciling the instruments' differing orbit days, times, and fields of view on the ground, or spatial resolutions.

Currently, spectrometer instruments aboard satellites are retrieving data once per day. However, newer instruments, such as the Geostationary Environment Monitoring Spectrometer launched in February 2020 by the National Institute of Environmental Research in the Ministry of Environment of South Korea, will monitor a particular region continuously, providing much more data in real time.

Over North America, the Tropospheric Emissions: Monitoring of Pollution Search (TEMPO) collaboration between NASA and the Smithsonian Astrophysical Observatory, led by Kelly Chance of Harvard University, will provide not only a stationary view of the atmospheric chemistry over the continent, but also a finer-resolution view - with the instrument recording pollution data from only a few square miles per pixel (with an anticipated launch in 2022).

"What we're very excited about is the opportunity to have continuous coverage where we get hourly measurements that allow us to follow pollution from morning rush hour through the course of the day and see how plumes of pollution are evolving in real time," says Fiore.

Data for the people
Providing Earth-observing data to people in addition to scientists - namely environmental managers, city planners, and other government officials - is the goal for the NASA Health and Air Quality Applied Sciences Team (HAQAST).

Since 2016, Fiore has been part of HAQAST, including collaborative "tiger teams" - projects that bring together scientists, nongovernment entities, and government officials - to bring data to bear on real issues.

For example, in 2017, Fiore led a tiger team that provided guidance to state air management agencies on how satellite data can be incorporated into state implementation plans (SIPs). "Submission of a SIP is required for any state with a region in non-attainment of U.S. National Ambient Air Quality Standards to demonstrate their approach to achieving compliance with the standard," says Fiore. "What we found is that small tweaks in, for example, the metrics we use to convey the science findings, can go a long way to making the science more usable, especially when there are detailed policy frameworks in place that must be followed."

Now, in 2021, Fiore is part of two tiger teams announced by HAQAST in late September. One team is looking at data to address environmental justice issues, by providing data to assess communities disproportionately affected by environmental health risks. Such information can be used to estimate the benefits of governmental investments in environmental improvements for disproportionately burdened communities. The other team is looking at urban emissions of nitrogen oxides to try to better quantify and communicate uncertainties in the estimates of anthropogenic sources of pollution.

"For our HAQAST work, we're looking at not just the estimate of the exposure to air pollutants, or in other words their concentrations," says Fiore, "but how confident are we in our exposure estimates, which in turn affect our understanding of the public health burden due to exposure. We have stakeholder partners at the New York Department of Health who will pair exposure datasets with health data to help prioritize decisions around public health.

"I enjoy working with stakeholders who have questions that require science to answer and can make a difference in their decisions." Fiore says.

Research Report: Inferring Changes in Summertime Surface Ozone-NOx-VOC Chemistry over U.S. Urban Areas from Two Decades of Satellite and Ground-Based Observations

Related Links
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