Measuring and Mapping Urban NO2 Pollution Heterogeneity Using a Custom Pandora Spectrometer Experiment and Observations From TEMPO

Nitrogen dioxide (NO₂) is a primary pollutant present at high concentrations in urban areas. Ground based measurements remain sparse and satellite observations rely on retrievals, so understanding intraurban, diurnal-scale changes in NO₂ is challenging. We present a novel methodology leveraging the sky scan capabilities of four Pandora spectrometers deployed around the Boston area to improve our understanding of intra-urban NO₂ variability. We collected sky scans in up to four directions per instrument per hour in addition to direct sun observations, yielding high-quality data across all site-azimuth pairs. The observations were applied to understand the role of wind conditions on driving spatial heterogeneity in column NO₂ abundance and local-scale diurnal patterns, finding that each wind type influences NO₂ differently. We leverage our experiment to understand the capabilities of the Tropospheric Emissions: Monitoring of Pollution (TEMPO) satellite instrument for tracking changes in NO₂ on intra-urban spatial scales and individual days, something that has not yet been examined in prior validation studies. Due to its value for measuring spatiotemporal variability, this approach can be used to more robustly constrain TEMPO and its uncertainties. Our comparisons with TEMPO in Boston advise caution when using data before 13 UTC as a bias persists, but in the afternoon we show that TEMPO captures similar dynamic range to the Pandoras, with differences likely attributable to spatial sampling mismatches. Our findings support the viability of a multi-azimuth sky scan experiment, and potentially broader adoption of such methodology across the global network of Pandoras.