Tracking Seasonal and Diurnal Variations in Nitrogen Dioxide and Formaldehyde from Space over the Northeast U.S.

Understanding how emissions and meteorology change air quality should underpin evaluation of air pollution controls and can guide future mitigation. Instruments aboard low-Earth orbiting satellites, such as TROPOMI, have provided global monitoring of nitrogen dioxide (NO₂, a criteria pollutant and the primary component of nitrogen oxides (NO_x)) and formaldehyde (HCHO, an air toxic and a proxy for volatile organic compound reactivity) vertical column densities (VCDs) since 1995. Understanding variations in these two pollutants also provides insights into the formation chemistry of ozone (O₃), a criteria pollutant and the major component of photochemical smog. The year 2024 marks the first full O₃ season with hourly daytime retrievals over North America from the geostationary TEMPO instrument. Across the Northeastern U.S., TROPOMI (V02.05.00) shows a clear early-afternoon seasonal decrease in NO₂ and an increase in HCHO VCDs from April to July 2024, whereas TEMPO (V03) reveals non-monotonic seasonal patterns that vary by hour. TEMPO captures a bimodal pattern in NO₂ VCDs, with elevated values in the early morning and late afternoon, and an afternoon rise in HCHO VCDs that is more pronounced during June-July. We also use harmonized multi-satellite records (GOME, SCIAMACHY, OMI, TROPOMI) to investigate how long-term changes in springtime NO₂, HCHO, and meteorology are influencing early-season O₃ events. In 2024, 17 high-O₃ days (MDA8 O₃ > 70 ppb, based on State or Local Air Monitoring Stations) occurred between April-June, the highest early-season proportion (57%) since 1996. TEMPO reveals a sharp daytime drop in NO₂ VCDs over New York City from April to May (–51%), a change not seen in TROPOMI. With similar HCHO VCDs between the two months, this NO₂ decline raises the HCHO-to-NO₂ ratio, making May conditions resemble June and suggesting an earlier shift toward more NO_x-sensitive O₃ production than previous years. April-June 2024 temperatures (based on NLDAS) were 1.4 °C above the 1996-2024 average, likely also contributing to these earlier high-O₃ events. We also consider the role of regional transport alongside local O₃ production to better understand whether NO_x or VOC controls may be effective for early-season mitigation.