## 2 Materials and Methods  
The tourist reception areas analysed in the study include 23 national parks, 126 landscape parks, 21 tourist regions, 46 health resorts, and 16 provincial capitals. The spatial distribution of these areas is shown in Figure 1. They are concentrated in the southern (health resorts and national parks) and northern (tourist regions) parts of Poland. They constitute reception areas for mass tourism (tourist regions), nature and cognitive tourism (national and landscape parks), cultural tourism (cities), and spa tourism (health resorts). A total of more than 22 million tourists used registered accommodation facilities in Poland in 2021. Ten million tourists visit national parks in Poland every year. Health resort services are used by around 850,000 people per annum, including circa 50,000 foreign tourists. It should be noted that national parks, landscape parks, and health resorts are located within tourist regions. The source of the data used in this study were the observations made using the TROPOMI spectrometer (TROPOspheric Monitoring Instrument, Airbus Defence and Space Nederland, Leiden, The Netherlands) of the Sentinel-5P satellite, launched on 13 October 2017 as part of the Copernicus Programme. Its task is to measure air quality as well as monitor and forecast climate change. To do so, it uses passive remote sensing techniques, mainly by measuring reflected solar radiation in the ultraviolet and visible light (270-500 nm), near-infrared (675-775 nm), and short-wave infrared (2305-2385 nm) ranges. Sentinel-5P products are made available by the European Space Agency. Level-2 data products encompass: (a) columns of total ozone (O\({}_{3}\)), sulphur dioxide columns (SO\({}_{2}\)), nitrogen dioxide columns (NO\({}_{2}\)), carbon monoxide (CO), formaldehyde (HCHO), and methane (CH\({}_{4}\)); (b) tropospheric ozone columns (O\({}_{3}\)) and nitrogen dioxide columns (NO\({}_{2}\)); and (c) cloud cover and aerosols. Studies using the data collected via the TROPOMI sensor on NO2 have been conducted in many places around the world and show good correlation between the satellite data and the ground-based measurements [26; 27; 28; 29]. In the present study, annual and monthly means for the tropospheric nitrogen column were used, based on the data downloaded from the provider's website [30] ([https://s5 phub.copernicus.eu/dhus](https://s5 phub.copernicus.eu/dhus)). The borders of protected areas were retrieved from the Georgul of the General Directorate for Environmental Protection [31]. Health resort data and death statistics were obtained from the Local Data Bank [32]. Tropospheric NO\({}_{2}\) levels for the individual areas were calculated by averaging the values occurring within their boundaries (the horizontal resolution of the data was 5.5 \(\times\) 3.5 km). Single measurement data with the highest quality (QA value of more than 0.75) were extracted, processed to a newly defined 0.01\({}^{\circ}\)\(\times\) 0.01\({}^{\circ}\) grid and visualised in a GIS environment (QGIS). Annual means (2019, 2020, 2021) and monthly means were calculated for the following months: February 2019, April 2019, July 2019, and October 2019. Due to the small amount of data available (weather conditions) for January, the data for February were used. No nitrogen dioxide threshold values have yet been developed for units in which the satellite data are available. WHO provides values in \(\upmu\)g/m\({}^{3}\) while the satellite data are given in \(\upmu\)mol/m\({}^{2}\). Therefore, we recalculated the values based on the calculated relationship (linear regression) between the terrestrial (x) and satellite (y) data for the same points. Measurement data from 37 ground stations were used, and calculations were carried out for the annual means in 2019 and 2020. Analyses were performed without taking into account stations close to sources of local NO\({}_{2}\) emissions (cities and roads). The correlation obtained in both cases was very similar (Figure 2).