Springtime High Ozone Events in the Northeast United States: Placing 2024 in a Climatological Context

Ground-level ozone (O₃) pollution intensifies the risk of respiratory and cardiovascular diseases. Its mitigation is challenging due to nonlinear photochemical reactions involving precursor trace gases, nitrogen oxides (NO_x) and volatile organic compounds (VOCs), raising the question of which species to control to most effectively reduce O₃. While the overall and mid-summer number of high-O₃ days (defined as any monitor recording hourly surface O₃ exceeding 70 ppb) in the Northeast United States has decreased over the past two decades, the relative importance of these days occurring in the early O₃ season (April-June) has increased from 40% of 119 total high-O₃ days in 2005 to 60% of 27 days in 2023. We investigate which factors contribute to these early season high-O₃ events and compare the conditions on these days with those in mid-summer. Formaldehyde (HCHO, a proxy for VOC reactivity) and nitrogen dioxide (NO₂, the dominant component of NO_x) have been routinely measured on the ground and retrieved once daily from space by the Ozone Monitoring Instrument (OMI; data available since October 2004) and the TROPOspheric Monitoring Instrument (TROPOMI; since May 2018) aboard low Earth orbit satellites. We select 2024 as a case study year for two reasons: (1) fifteen early season high-O₃ days were reported in at least one Northeast U.S. state, and (2) it is the first complete O₃ season with hourly HCHO and NO₂ retrievals during daylight hours from NASA’s Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument, the first trace gas measuring instrument aboard a geostationary satellite focused on North America. We integrate available in situ measurements, column retrievals, and model simulations from EPA’s Air Quality Time Series Project (EQUATES, publicly available from 2002 to 2019) and the Multi-Scale Infrastructure for Chemistry and Aerosols (MUSICA, covering 2018 to 2024) to provide a climatological perspective on variations in O₃ and its precursors during early-season events from 2005 to 2024. Early in the O₃ season, the relative importance of anthropogenic VOCs, such as those from volatile chemical products (VCPs), may increase relative to mid-summer events as leaves from deciduous isoprene-emitting plants are not fully grown, implying local O₃ formation may be more sensitive to VOCs. We also examine meteorological factors, as high-O₃ events may be associated with rapidly rising temperatures and long-range transport of upwind fire emissions. Understanding these dynamics will provide insights into whether NO_x or VOC control would be more effective for O₃ mitigation and the role meteorology plays in the early O₃ season.