Comparison of nitrogen dioxide tropospheric columns retrieved by TEMPO and Pandora

Monitoring emissions of nitrogen dioxide is crucial for understanding the atmospheric composition and its impacts on air quality and climate. This study aims to evaluate the accuracy of retrievals of nitrogen dioxide tropospheric column by the Tropospheric Emissions: Monitoring of Pollution (TEMPO) by comparing them against retrievals of the ground-based Pandora instruments.

The TEMPO is a visible and ultraviolet spectrometer flying aboard of a commercial telecommunications satellite, Intelsat 40e, in geostationary orbit over 91˚ W longitude, thus maintaining a continuous view of North America. High resolution measurements of radiance reflected by the Earth's back to the instrument's detectors enable retrievals of columns of nitrogen dioxide involved in the chemical dynamics of Earth’s atmosphere. TEMPO V03 Level 1, 2, and 3 data were recently made available from the Atmospheric Science Data Center (ASDC) via NASA EarthData Search.

Direct-Sun Pandora spectrometer is used to retrieve columnar amounts of trace gases in the atmosphere by the means of differential optical absorption spectroscopy at numerous locations around the globe.

ASDC has developed a set of Jupyter notebooks dedicated to TEMPO vs. Pandora comparisons of the columns of individual trace gases including one dealing with NO₂ tropospheric column. The notebooks allow a user to select a specific Pandora station and a timeframe of interest. The code downloads all relevant TEMPO L2 granules as well as the Pandora dataset. The latter is sub-set to the selected timeframe. Time series of the gas column retrievals along with their uncertainties are then derived with accounting for the quality flags from both datasets. Since Pandora measurements are significantly more frequent, a procedure computing weighted averages of them at the times of TEMPO retrievals was incorporated to the notebooks allowing direct comparison of gaseous columns from two sensors against each other.

The results derived by the ASDC tool show only qualitative agreement between the TEMPO and Pandora retrievals of nitrogen dioxide tropospheric column. it was also found that the discrepancies between the two are site dependent which may point to a potential problem with Pandora quality flags. Two attempts were made to improve comparison. Since TEMPO algorithm allows for negative NO₂ tropospheric columns, such retrievals were removed from consideration. There are also multiple TEMPO retrievals accompanied by uncertainty greater that the retrieved column. Removal of such retrievals constitutes another approach to improve comparison.

The findings of this study will contribute to the understanding of the reliability and applicability of space-based trace gases monitoring for air quality applications. The results will enhance our understanding of atmospheric processes related to tropospheric NO₂.