More than 35 million people live along the New York–Washington corridor and breathe the region’s air. While air quality has improved significantly in recent decades, outbreaks of ground-level ozone remain common, particularly in the warm summer months, when the chemical reactions that produce the pollutant accelerate and stagnant air allows ozone to accumulate.
A reminder of this seasonal phenomenon came earlier than usual in 2026, when a mid-May heat wave prompted the New York State Department of Health and the New York Department of Environmental Conservation to issue a health advisory on May 17 over concerns about ozone. The code orange advisory warned young people, older adults, and those working or exercising outdoors to limit activity due to ozone’s respiratory and cardiovascular health impacts.
As expected, ground-based air-quality sensors operated by state and federal agencies showed ozone reaching unhealthy levels for sensitive groups on May 18, something that typically happens several times per year. Meanwhile, NASA’s TEMPO (Tropospheric Emissions: Monitoring of Pollution) instrument observed the event from geostationary orbit 22,000 miles (35,000 kilometers) above the equator, a unique vantage point that allows the sensor to collect frequent observations of air pollution.
TEMPO detects nitrogen dioxide (NO2), a gas emitted by burning fuels, particularly by motor vehicles, that contributes to ozone formation. “There’s often a clear and interesting pattern in TEMPO’s nitrogen dioxide data during ozone alert days,” said Hazem Mahmoud, an atmospheric scientist at NASA’s Atmospheric Science Data Center at Langley Research Center. “We see high concentrations of nitrogen dioxide during the early morning commute that drop off sharply in the late afternoon as ozone increases.”
The decline occurs as sunlight fuels photochemical reactions involving nitrogen dioxide, volatile organic compounds, and oxygen that lead to ozone formation. By late afternoon, these reactions deplete much of the available nitrogen dioxide, slowing ozone production until the cycle begins again the next day.
The pair of images above underscores the pattern. The image on the left was acquired at 7:05 a.m. local time when nitrogen dioxide concentrations were high during the morning commute. By 3:05 p.m. (right), most of the nitrogen dioxide had declined substantially, and surface ozone levels were elevated (below). Meanwhile, afternoon sea breezes appear to have transported the remaining nitrogen dioxide slightly to the west. Note that the data shown is provisional, and processing methods are still being refined.
Sensors on earlier polar-orbiting satellites, such as OMI (Ozone Monitoring Instrument) and TROPOMI (Tropospheric Monitoring Instrument), sampled nitrogen dioxide over New York once per day. After its launch in 2023, TEMPO began providing data every hour, allowing researchers to track the evolution and dispersion of air pollution at much finer time scales.
“TEMPO is helping fill data gaps between ground stations and allowing us to ask new questions,” Mahmoud said. The mission provides data that can improve not only air quality forecasts during crisis situations, such as wildfires, but also the atmospheric models used to forecast the daily rhythms of urban pollution. Such models help researchers understand how natural factors such as winds, humidity levels, and air temperatures influence pollution plumes over the course of a day.
TEMPO also detects ozone directly, but determining how much of that ozone is near the surface versus higher in the atmosphere can be challenging. Most of Earth’s ozone resides in the stratosphere, well above the troposphere, where people live and breathe. At times, however, stratospheric ozone can be transported downward into the troposphere. During events known as stratospheric intrusions, it can even descend far enough to affect air quality at the surface and add to the ozone produced at ground level.
By combining TEMPO observations with other sources of information, researchers are studying the processes that influence the distribution of ozone vertically in the atmosphere. On May 18, NASA’s ground-based tropospheric lidar network (TOLNet) in New York City recorded high concentrations of ozone near the surface, indicating that TEMPO was detecting mostly surface-level ozone associated with urban emissions and not ozone aloft, said Mahmoud.
However, on May 19, the same sensor observed a layer of ozone descending from above 5 kilometers (3 miles), he added, a clue that some of the ozone TEMPO detected that day may have originated in the stratosphere. “This is the type of information that leads to better air quality forecast models and more accurate alerts,” Mahmoud said. “Alerts can affect tens of millions of people and lead to disruptions in school, sports, and other activities, so it’s essential that they be as accurate as possible.”
On June 6, New York authorities issued another health advisory for ozone. People interested in following the event can access daily near-real-time TEMPO observations of ozone, nitrogen dioxide, and other gases on NASA’s Worldview browser, on an interactive Harvard & Smithsonian Center for Astrophysics browsing tool, and on NASA’s Earthdata portal.
NASA Earth Observatory images by Michala Garrison, using TEMPO data from NASA Earthdata. Story by Adam Voiland.
Nitrogen Dioxide – May 18, 2026 7:05 AM EDT
Nitrogen Dioxide – May 18, 2026 3:05 PM EDT
Ozone – May 18, 2026 5:05 PM EDT
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