Evaluating the Potential of Satellite Measurements in Air Quality Monitoring: A Project for the Finnish Ministry of the Environment

We present the results of a project for the Finnish Ministry of the Environment that aimed to assess the potential of satellite measurements in complementing traditional in situ air quality measurements. Co-located NO₂ measurements from the TROPOspheric Monitoring Instrument (TROPOMI) and several traditional air quality stations (measuring in µg/m³) in Finland and Europe between April 2018 and June 2019 are compared to determine their correlation. We find that the correlation of individual air quality stations with TROPOMI is dependent on the location of the station, but is more reliable when all stations in a city centre are studied as a group. This is expected due to the spatial averaging of satellite measurements. We also find that NO₂ measurements between different cities in Finland and Europe in general correlate well.

We also analyse TROPOMI’s and the Ozone Monitoring Instrument’s (OMI) ability to study the spatial distribution of NO₂ over Finland and the Helsinki metropolitan area using gridded maps. Oversampled TROPOMI measurements are able to distinguish relatively small sources such as roads and airports, and the difference in concentrations between weekdays and weekends. TROPOMI is also able to detect emissions from different sources of NO₂ such as cities, mining sites and industrial areas. Long time series measurements from OMI show decreasing NO₂ levels over Finland between 2005 and 2018.

Finally, we convert air quality station measurements to vertical column densities using boundary layer height data, and study the effect that this has on their correlation with TROPOMI measurements.

This study was conducted on behalf of the Finnish Ministry of the Environment, and showcases how satellite measurements can be used reliably alongside traditional air quality measurements to provide a better picture of current pollution levels. Launched in 2017, TROPOMI is currently the highest-resolution air quality sensing satellite, and its societal uses are only beginning to be realized. Future Sentinel missions, especially the geosynchronous Sentinel-4, will provide an even more comprehensive view of the daily air quality situation.