Insights Into Summertime Surface Ozone Formation From Diurnal Variations in Formaldehyde and Nitrogen Dioxide Along a Transect Through New York City

Estimating tropospheric ozone (O₃) production from observations is challenging but possible given the close coupling of O₃ with formaldehyde (HCHO) and nitrogen dioxide (NO₂), two remotely sensed air pollutants. The previous reliance on once-daily satellite overpasses highlights the need to study diurnal changes and surface-column relationships. Using surface observations, Pandora spectrometer retrievals, and a high-resolution (1.33 km) air quality model (WRF-CMAQ), we characterize diurnal patterns of HCHO and NO₂ at seven locations along an upwind-downwind pathway through New York City during June–August 2018. Diurnal patterns of limited surface HCHO measurements suggest biogenic emission influence, while a bimodal surface NO₂ pattern indicates the impact of local anthropogenic nitrogen oxides emissions. Details of these patterns vary by site: an afternoon NO₂ spike at New Haven (CT) indicates traffic emissions, while a delayed daily HCHO peak at Westport (CT) relative to other sites likely reflects sea breeze dynamics. Peak column concentrations generally lag surface peaks by about four hours, occurring at 9–10 a.m. for morning NO₂ (from Pandora and WRF-CMAQ) and around 4 p.m. for midday HCHO (from WRF-CMAQ). TROPOMI overpass time at 1:30 p.m. misses peak column HCHO and NO₂ concentrations. A box model (F0AM) constrained with site-level observations and WRF-CMAQ fields indicates 1–9 ppb hr⁻¹ higher noontime local O₃ production rates on three sets of paired high- versus mid-to-low-O₃ days. F0AM sensitivity analyses on these six days suggest a predominantly transitional O₃ formation regime at urban and downwind sites, differing at some sites from the NO_x-saturated regime diagnosed for summertime average conditions via the weekday-weekend effect.