Bridging GEO and LEO Perspectives on Warm-Season NO2 and HCHO Variability over the Northeastern United States

<jats:p> During the 2024 ozone (O <jats:sub>3</jats:sub> ) season, the first complete year with Tropospheric Emissions: Monitoring of Pollution (TEMPO) observations over the northeastern United States, TEMPO provided hourly daytime retrievals of formaldehyde (HCHO), a proxy for volatile organic compound reactivity, and nitrogen dioxide (NO <jats:sub>2</jats:sub> ), an O <jats:sub>3</jats:sub> precursor. Matched TEMPO Version 03 (V03) and profile-shape-adjusted Tropospheric Monitoring Instrument (TROPOMI) Version 02.06.00 early-afternoon (~13:30 LT) retrievals show similar spatial patterns and changes across April, the temperature-defined early-season period (1 May-15 June), and midsummer (16 June-5 August). For 2024 O <jats:sub>3</jats:sub> season daily regional means, TROPOMI relative to TEMPO shows normalized relative difference (NRD) of -2% for tropospheric NO <jats:sub>2</jats:sub> (r=0.60) and -44% for total HCHO (r=0.85) vertical column density. Both instruments capture seasonal shifts in O <jats:sub>3</jats:sub> photochemistry from April to midsummer: period-median early-afternoon HCHO increases by 110% in TEMPO and 103% in TROPOMI as biogenic emissions increase, while NO <jats:sub>2</jats:sub> declines by 15% and 19%, respectively, as photochemistry intensifies. TEMPO reveals a steeper daytime HCHO column build-up from early season to midsummer (+0.79 to +1.01&amp;times;10 <jats:sup>15</jats:sup> molec cm <jats:sup>-2</jats:sup> h <jats:sup>-1</jats:sup> , 08:30-14:30 LT), alongside faster morning surface O <jats:sub>3</jats:sub> build-up (+2.7 to +4.0 ppb h <jats:sup>-1</jats:sup> ) and an earlier daily O <jats:sub>3</jats:sub> peak (15:00 to 12:00 LT). On high-O <jats:sub>3</jats:sub> days, TEMPO tropospheric NO <jats:sub>2</jats:sub> is enhanced in the morning relative to period-median conditions, while total HCHO is elevated throughout daytime. Early-season events are characterized by a suppressed nocturnal boundary layer that favors overnight precursor accumulation followed by morning NO <jats:sub>2</jats:sub> column enhancements, whereas midsummer events show larger absolute HCHO column anomalies and persistently warmer conditions. </jats:p>