Trace gas variability and ozone formation regimes in urban and sub-urban landscape over Greater Washington D.C. area

Atmospheric trace gases regulate air quality, climate forcing, and public health, yet their vertical distribution and chemical sensitivity remain complex, particularly in heterogeneous urban environments. This study uses four years (2021–2024) of Pandora spectrometer total column and tropospheric observations at urban (McMillan), sub-urban (Goddard), and sub-rural (Beltsville) sites in the Washington, D.C. region to characterise spatiotemporal variability in nitrogen dioxide (NO2), formaldehyde (HCHO), sulphur dioxide (SO2), and ozone (O3). Seasonal behaviour reveals distinct drivers of column variability: NO2 peaks in winter due to a shallow boundary layer and enhanced lifetime, while HCHO peaks in summer and correlates strongly with water vapour (r > 0.8), reflecting photochemical production. Comparisons with surface in-situ measurements show that Pandora robustly represents surface NO2 (R2 > 0.60 during high-loading events), whereas total column ozone poorly reflects surface variability (R2 < 0.26) due to stratospheric dominance. Formaldehyde-to-NO2 ratios (FNR) indicate predominantly NOx-limited ozone production at the urban site (FNR > 2), with increasing FNR during more severe ozone events. Anomalously low FNR during high-ozone episodes in 2023 is attributed to enhanced NO2 from Canadian wildfire smoke rather than reduced VOC reactivity. These findings demonstrate the value of column-integrated observations for diagnosing nonlinear ozone chemistry and validating satellite-based air quality regimes.