Evaluating the impact of spatial resolution on tropospheric NO 2 column comparisons within urban areas using high-resolution airborne data

Judd, Laura M.; Al-Saadi, Jassim A.; Janz, Scott J.; Kowalewski, Matthew G.; Pierce, R. Bradley; Szykman, James J.; Valin, Lukas C.; Swap, Robert; Cede, Alexander; Mueller, Moritz; Tiefengraber, Martin; Abuhassan, Nader; Williams, David

NASA deployed the GeoTASO airborne UV–visible spectrometer in May–June 2017 to produce high-resolution (approximately inline-formula250 m×250 m) gapless inline-formulaNO2 datasets over the western shore of Lake Michigan and over the Los Angeles Basin. The results collected show that the airborne tropospheric vertical column retrievals compare well with ground-based Pandora spectrometer column inline-formulaNO2 observations (inline-formular2=0.91 and slope of 1.03). Apparent disagreements between the two measurements can be sensitive to the coincidence criteria and are often associated with large local variability, including rapid temporal changes and spatial heterogeneity that may be observed differently by the sunward-viewing Pandora observations. The gapless mapping strategy executed during the 2017 GeoTASO flights provides data suitable for averaging to coarser areal resolutions to simulate satellite retrievals. As simulated satellite pixel area increases to values typical of TEMPO (Tropospheric Emissions: Monitoring Pollution), TROPOMI (TROPOspheric Monitoring Instrument), and OMI (Ozone Monitoring Instrument), the agreement with Pandora measurements degraded, particularly for the most polluted columns as localized large pollution enhancements observed by Pandora and GeoTASO are spatially averaged with nearby less-polluted locations within the larger area representative of the satellite spatial resolutions (aircraft-to-Pandora slope: TEMPO scale inline-formula=0.88; TROPOMI scale inline-formula=0.77; OMI scale inline-formula=0.57). In these two regions, Pandora and TEMPO or TROPOMI have the potential to compare well at least up to pollution scales of inline-formula30×1015 molecules cminline-formula−2. Two publicly available OMI tropospheric inline-formulaNO2 retrievals are found to be biased low with respect to these Pandora observations. However, the agreement improves when higher-resolution a priori inputs are used for the tropospheric air mass factor calculation (NASA V3 standard product slope inline-formula=0.18 and Berkeley High Resolution product slope inline-formula=0.30). Overall, this work explores best practices for satellite validation strategies with Pandora direct-sun observations by showing the sensitivity to product spatial resolution and demonstrating how the high-spatial-resolution inline-formulaNO2 data retrieved from airborne spectrometers, such as GeoTASO, can be used with high-temporal-resolution ground-based column observationspage6092 to evaluate the influence of spatial heterogeneity on validation results.

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Judd, Laura M. / Al-Saadi, Jassim A. / Janz, Scott J. / et al: Evaluating the impact of spatial resolution on tropospheric NO2 column comparisons within urban areas using high-resolution airborne data. 2019. Copernicus Publications.

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