Statistical Analysis of Ozone Weekend Effect in the Largest Cities in Poland

This study examines O3, NOx, NO2 and NO data from five large cities in Poland (Warszawa, Krakow, Łodź, Wroclaw and Poznan) over a three-year period (2015-2017) to quantify the phenomenon of ozone weekend effect. The seasonal and diurnal variations of O3 and NOx species, showing the interdependence these two gaseous species, were presented. The number of 8-hour running average values above the threshold of 60 ppb and 1-hour above 90 ppb shows that ozone exceeded amounts are more frequent on Saturday and Sunday compared to other days of the week. The analysis of day of the week variations of O3 indicates distinct, temporal pattern with maximum O3 concentrations during weekend (especially on Sunday) and minimum noted on Wednesday, Thursday and Friday (depending on the station). The analysis of existence of the ozone weekend effect was performed on the basis of average O3 concentration at the weekend and on the day of the lowest O3 concentration during the week. Calculations were performed for the period of the whole year and for individual seasons of the year. The results of performance the non-parametric U-Mann-Whitney test indicate that differences of O3 concentration between weekend and a specific day of the week were statistically significant for most cases, despite the significantly lower concentration of ozone precursors (NOx). The analysis of Ox concentrations indicates that limited processes of O3 titration by NO (ozone quenching hypothesis) are the main cause of the ozone weekend effect in the Polish cities.