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 ni …