# Impact of COVID-19 Mobility Changes on Air Quality in Warsaw
## 3 Results
### Changes in Air Quality in Warsaw  
Over the past 15 years, nitrogen dioxide concentrations have slightly decreased compared to the beginning of the analysed period. There were, however, some fluctuations between the years. At the traffic station (Figure 4: blue), the average annual concentration was above 60 \(\upmu\)g/m\({}^{3}\) in 2006-2008 and 2010. Declines occurred in 2009 and 2011-2013. Since 2016, the concentrations have decreased, and in 2018 and 2019 they stabilised at a similar level close to 50 \(\upmu\)g/m\({}^{3}\). The minimum value was 45.4 \(\upmu\)g/m\({}^{3}\) in 2012, and the maximum value was 65.3 \(\upmu\)g/m\({}^{3}\) in 2006. Concentrations registered at the station located in the Targowek district (Figure 4: orange) ranged from 20 to 30 \(\upmu\)g/m\({}^{3}\). Only in 2006 was the concentration higher and exceeded 30 \(\upmu\)g/m\({}^{3}\). The minimum value was 22.9 \(\upmu\)g/m\({}^{3}\) and was registered in 2010, and the maximum value was 35.1 \(\upmu\)g/m\({}^{3}\) in 2006. The smallest differences between the years occurred at the station located in the district of Ursynow (Figure 4: grey). The minimum value was 20.7 \(\upmu\)g/m\({}^{3}\) in 2011, and the maximum was 27.1 \(\upmu\)g/m\({}^{3}\) in 2006. As can be seen, the highest value of the average annual concentration was recorded at all stations in the following same year: 2006.  
The \(t\)-Student and ANOVA tests results confirm that statistically significantly higher values (\(p<0.05\)) are recorded at the station measuring impacts directly from transport in the centre of the capital city (Warsaw-Al. Niepodleglosci) compared with the situation at urban background stations. Due to the location of the station in the immediate vicinity of a busy arterial road (al. Niepodleglosci, two roadways with three lanes each), the limit level (40 \(\upmu\)g/m\({}^{3}\) for the annual average) is exceeded every year. At other measurement locations, the share of transport emissions is lower and hence the measured NO\({}_{2}\) concentrations are also lower.  
Readings were taken at the air quality monitoring station located in the centre of Warsaw near the intersection of al. Niepodleglosci and ul. Nowowiejska, which showed a fairly pronounced reduction in NO\({}_{2}\) concentrations in the air during the pandemic (2020) compared to earlier years (2017-2019). Mean NO\({}_{2}\) concentrations in consecutive months between March and June in 2017-2019 were in the range of 50-57 \(\upmu\)g/m\({}^{3}\), while in 2020 they were significantly lower (by 28-35%) and were in the range of 31-37 \(\upmu\)g/m\({}^{3}\) (Figure 5).  
At the same air quality monitoring station, comparisons of NO\({}_{2}\) concentration data only on weekdays (all Tuesdays, Wednesdays, Thursdays), i.e., when the transport system is dominated by daily trips related to work and school, confirmed a decrease in air pollution in 2020 compared to the 2017-2019 average. However, during peak traffic hours, the difference between the periods compared was similar, with NO\({}_{2}\) concentrations being 25-38% lower (Figure 6).  
Figure 5: Mean NO\({}_{2}\) concentrations in al. Niepodleglosci in 2020 compared to the mean concentrations from 2017 to 2020. Calculation based on all days of the year (developed by the author).  
Figure 6: Mean morning peak hour NO\({}_{2}\) concentrations in al. Niepodleglosci for the period March–June 2020 compared to the averages for the same period 2017–2020. Calculation based on all Tuesdays, Wednesdays and Thursdays (developed by the author).