Estimating surface-level nitrogen dioxide concentrations from Sentinel-5P/TROPOMI observations in Finland

Abstract

In recent decades, satellite instruments have been providing observations of air pollutants such as nitrogen dioxide (NO2) with global coverage. Since late 2018, the TROPOspheric Monitoring Instrument (TROPOMI) on-board the Copernicus Sentinel-5 Precursor satellite has produced NO2 vertical column densities (VCDs) with the best spatial resolution (currently 5.5 × 3.5 km2 at nadir). In this paper, we test and adapt two previously published methods to estimate surface-level NO2 concentrations from TROPOMI VCD retrievals in the specific conditions of Finland. Satellite-based estimates show good correlation with co-located surface NO2 measurements from the Finnish AQ network, although not accounting for the level of NO2 mixing within the boundary layer leads to underestimation. We also use a linear relation between in situ measurements and surface-level estimates to correct for the observed negative bias in the estimates. Finally, we use chemical transport model simulations to estimate the ratio of the annual mean surface concentrations to the values sampled according to TROPOMI observation times, and use it to correct our surface-level NO2 estimates for compatibility with annual limit values defined in AQ legislation. Overall, the results provide new information to complement traditional ground-based AQ measurements, especially over areas where the surface AQ networks are sparse, and to support national environmental authorities in air quality assessment and reporting.