Investigating Changes in Ozone Formation Chemistry during Summertime Pollution Events over the Northeastern United States

[Image: see text] Understanding the local-scale spatial and temporal variability of ozone formation is crucial for effective mitigation. We combine tropospheric vertical column densities (VCD(Trop)) of formaldehyde (HCHO) and nitrogen dioxide (NO(2)), referred to as HCHO-VCD(Trop) and NO(2)-VCD(Trop), retrieved from airborne remote sensing and the TROPOspheric Monitoring Instrument (TROPOMI) with ground-based measurements to investigate changes in ozone precursors and the inferred chemical production regime on high-ozone days in May–August 2018 over two Northeast urban domains. Over New York City (NYC) and Baltimore/Washington D.C. (BAL/DC), HCHO-VCD(Trop) increases across the domain, but higher NO(2)-VCD(Trop) occurs mainly in urban centers on ozone exceedance days (when maximum daily 8 h average (MDA8) ozone exceeds 70 ppb at any monitor in the region). The ratio of HCHO-VCD(Trop) to NO(2)-VCD(Trop), proposed as an indicator of the sensitivity of local surface ozone production rates to its precursors, generally increases on ozone exceedance days, implying a transition toward a more NO(x)-sensitive ozone production regime that should lead to higher efficacy of NO(x) controls on the highest ozone days in NYC and BAL/DC. Warmer temperatures and enhanced influence from emissions in the local boundary layer on the high-ozone days are accompanied by slower wind speeds in BAL/DC but stronger, southwesterly winds in NYC.