The Evolution of the Seasonal Variation and the Summer Diurnal Variation of Primary and Secondary Photochemical Air Pollution in Athens †

: The city of Athens has faced air pollution problems over the last few decades due to the high population density associated with an intense emission load constrained by the local topography causing poor ventilation. In addition, the high levels of solar irradiation, in combination with emissions of nitrogen oxides and hydrocarbons, have led to the formation of photochemical pollutants, especially ozone. In this study, the evolution of the seasonal variation of primary and secondary photochemical urban air pollution in Athens was examined for the 2001–2021 period. For this purpose, the monthly NO x (NO+NO 2 ), ozone, and O x (O 3 +NO 2 ) averages for the Athens air pollution monitoring stations of Geoponiki, Liossia, Ag. Paraskevi, and Thrakomakedones in the selected periods of 2001–2004, 2010–2013, 2016–2019, and 2020–2021 were plotted. In addition, summer diurnal NO x , ozone, and O x variations were examined. There was a clear reduction in NO x levels at all four examined stations, especially at the urban background site of Geoponiki, as well as at the most polluted peripheral site of Liossia, following a reduction in urban emissions, mainly from car trafﬁc during the examined period. The ozone and O x curves did not show the same patterns due to the complicated nature of local ozone photochemical production, in combination with the rural surface ozone background, which was particularly high over the area.

In this study, the evolution of the interannual and seasonal variation of primary and secondary photochemical urban air pollution in Athens has been examined for the 2001-2021 period.For this purpose, the annual and monthly NO x (NO+NO 2 ), ozone, and O x (O 3 +NO 2 ) averages for the urban background station of Geoponiki, as well as for the peripheral stations of Geoponiki, Liossia, Ag.Paraskevi, and Thrakomakedones, during the selected periods of 2001-2004, 2010-2013, 2016-2019, and 2020-2021 were plotted.

Results and Discussion
Figure 1 shows the average annual ozone and NO x concentrations at the stations of the Athens air pollution monitoring network, as approximately located at the NW-NE axis of the basin for the 2001-2021 period.The most polluted stations, specifically regarding primary pollution, were Patission (PAT), Athinas (ATH), and Peireas (PEI), while they showed the lowest ozone mean values.At the same time, the O x concentrations were comparable at all stations, expectedly due to NO titration by ozone [7].From the 11 presented stations, the peripheral stations of Liossia (LIO), Ag.Paraskevi (AGP), and Thrakomakedones (THR), as well as the urban background station of Geoponiki (GEO), were selected for further analysis.
and 2020-2021, as well as the corresponding NO2 concentrations for the same time peri-ods, are presented.Significant reductions in primary NOx pollution were observed at the urban background station of GEO, as well as at the peripheral station of LIO, located to the north of the basin, especially during the autumn and winter months and between the periods 2001-2004 and 2010-2013.During the summer months, the reductions were less significant, especially during the more recent periods, although an extra reduction was observed during the pandemic period (2020-2021) relatively to the previous period (2016-2019) during both autumn and winter months.Similar NO2 variations between the examined periods were observed, showing a smoother seasonal variation in summer.
In Figure 4, seasonal variations in monthly average ozone concentrations at the stations of GEO, LIO, AGP, and THR during the periods of 2001-2004, 2010-2013, 2016-2019, and 2020-2021, as well as the corresponding Ox variations for the same time periods, are presented.Significant ozone and Ox reductions were observed at the AGP and THR stations, while at the LIO station, Ox shows the same pattern.On the contrary, significant ozone increases were observed at the GEO station.In Figure 2, the evolution of annual NO x , O 3 , and O x pollutant concentrations at the GEO, LIO, AGP, and THR stations of the Athens air pollution monitoring network is presented.There was a significant reduction in NO x concentrations at GEO and LIO stations during the 2001-2011 period (around 40-50%), while there was an increase in ozone concentrations at the same stations.For the AGP and THR stations, the NO x reductions and ozone increases were less significant.Finally, the annual O x concentrations remained relatively stable throughout the examined period.
In Figure 3, seasonal variations in monthly average NO x concentrations at the stations of GEO, LIO, AGP, and THR during the periods of 2001-2004, 2010-2013, 2016-2019, and and 2020-2021, as well as the corresponding NO 2 concentrations for the same time periods, are presented.Significant reductions in primary NO x pollution were observed at the urban background station of GEO, as well as at the peripheral station of LIO, located to the north of the basin, especially during the autumn and winter months and between the periods 2001-2004 and 2010-2013.During the summer months, the reductions were less significant, especially during the more recent periods, although an extra reduction was observed during the pandemic period (2020-2021) relatively to the previous period (2016-2019) during both autumn and winter months.Similar NO 2 variations between the examined periods were observed, showing a smoother seasonal variation in summer. In

Conclusions
Substantial reductions in NOx pollution levels (about 50%) occurred at the stations of GEO and LIO during the last two decades and weaker reductions in the stations of AGP and THR, which were even enhanced during the pandemic period, especially during the autumn and winter months.Consequently, ozone levels increased at GEO and LIO stations due to the reduced NO titration effect.On the other hand, at the peripheral stations of AGP and THR, ozone average levels showed a clear decrease, especially during the spring and summer months, indicating a corresponding decrease in regional ozone background levels.Nevertheless, there were still exceedances of air quality standards in some

Conclusions
Substantial reductions in NO x pollution levels (about 50%) occurred at the stations of GEO and LIO during the last two decades and weaker reductions in the stations of AGP and THR, which were even enhanced during the pandemic period, especially during the autumn and winter months.Consequently, ozone levels increased at GEO and LIO stations due to the reduced NO titration effect.On the other hand, at the peripheral stations of AGP and THR, ozone average levels showed a clear decrease, especially during the spring and summer months, indicating a corresponding decrease in regional ozone background levels.Nevertheless, there were still exceedances of air quality standards in some stations of the Athens air quality monitoring network, specifically regarding the photochemical NO 2 and ozone pollutants.

Figure 1 .
Figure 1.Average annual ozone and NOx concentrations at Athens air pollution monitoring network stations, as approximately located at the NW-NE axis of the basin for the 2001-2021 period.

Figure 1 .
Figure 1.Average annual ozone and NO x concentrations at Athens air pollution monitoring network stations, as approximately located at the NW-NE axis of the basin for the 2001-2021 period.

Figure 4 ,
seasonal variations in monthly average ozone concentrations at the stations of GEO, LIO, AGP, and THR during the periods of 2001-2004, 2010-2013, 2016-2019, and 2020-2021, as well as the corresponding O x variations for the same time periods, are presented.Significant ozone and O x reductions were observed at the AGP and THR stations, while at the LIO station, O x shows the same pattern.On the contrary, significant ozone increases were observed at the GEO station.

Figure 2 .
Figure 2. Evolution of annual pollutant concentrations at the GEO, LIO, AGP, and THR stations of the Athens air pollution monitoring network: NOx (upper panel), O3 (middle panel), and Ox (lower panel).

Figure 2 .
Figure 2. Evolution of annual pollutant concentrations at the GEO, LIO, AGP, and THR stations of the Athens air pollution monitoring network: NO x (upper panel), O 3 (middle panel), and O x (lower panel).

Figure 2 . 6 Figure 3 .
Figure 2. Evolution of annual pollutant concentrations at the GEO, LIO, AGP, and THR stations of the Athens air pollution monitoring network: NOx (upper panel), O3 (middle panel), and Ox (lower panel).