# Impact of COVID-19 Mobility Changes on Air Quality in Warsaw
## 3 Results
### Impact of Mobility Changes on Air Quality  
Table 3 with the results of the analysis of the 2020 data on SIM card movements on weekdays (Tuesday, Wednesday, Thursday) shows that traffic generation during the initial period of the pandemic (March-April) was significantly lower (daily average of 165,000 trips) compared to the May-June period (daily average of 203,000-206,000). It was similar during the peak traffic hours when most trips are made for work and school. During the initial pandemic period (March-April), traffic generation was significantly lower (peak hour average of 178,000-184,000 trips) compared to the May-June period (peak hour average of 220,000-226,000 trips). Against the background of the traffic generation data, the data on NO\({}_{2}\) concentrations are also interesting. A comparison was made between changes in the number of trips generated and changes in the observed nitrogen dioxide concentrations (Table 3, Figure 7). The results of the \(t\)-Student and ANOVA tests confirmed that in 2020, NO\({}_{2}\) concentrations observed in March-April (with a minimum of 34 \(\mathrm{\SIUnitSymbolMicro g}/\mathrm{m}^{3}\) in April) were statistically significantly lower (\(p<0.05\)) in comparison to those registered in May-June (with a maximum of 43 \(\mathrm{\SIUnitSymbolMicro g}/\mathrm{m}^{3}\) in May). A relationship between the increase in trips generated and NO\({}_{2}\) concentrations became apparent. An increase of approximately 25% in the number of trips generated (between April and May) was observed. Similarly, NO\({}_{2}\) concentrations increased by 25%. The relationship between the increase in the number of trips and the increase in NO\({}_{2}\) concentrations was not as clearly revealed during peak traffic hours (Figure 8). In 2020, similar NO\({}_{2}\) concentrations were observed in the morning peak during March-May (range 49-51 \(\mathrm{\SIUnitSymbolMicro g}/\mathrm{m}^{3}\)), i.e., differences in concentrations did not exceed 4%. Changes in NO\({}_{2}\) concentrations were therefore significantly different to changes in the number of trips, which increased by 23% between March and May. This was different from the afternoon peak. The level of hourly NO\({}_{2}\) concentrations in the March-May period increased from 37 to 59 \(\mathrm{\SIUnitSymbolMicro g}/\mathrm{m}^{3}\), which means an increase of 51%, while the number of trips increased by 28%. Thus, in the afternoon peak, the effect of the increase in the number of trips generated on the increase in NO\({}_{2}\) concentrations was confirmed, with air pollution even higher (compared to the increase in traffic) than for the daily data. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Period**} & \multirow{2}{*}{**24 h**} & **Average of Morning** & **Average of Afternoon** \\  & & **Peak Hours** & **Peak Hours** \\ \hline \multicolumn{4}{c}{Number of trips [vehicle/h]} \\ \hline March & 164,985 & 184,094 & 242,969 \\ \hline April & 164,654 & 178,000 & 240,858 \\ \hline May & 205,918 & 226,009 & 310,330 \\ \hline June & 202,834 & 220,276 & 303,436 \\ \hline \multicolumn{4}{c}{NO\({}_{2}\) concentration [\(\mathrm{\SIUnitSymbolMicro g}/\mathrm{m}^{3}\)]} \\ \hline March & 37 & 49 & 39 \\ \hline April & 34 & 49 & 37 \\ \hline May & 43 & 51 & 59 \\ \hline June & 38 & 41 & 44 \\ \hline \end{tabular}
\end{table} 
Table 3: Number of trips generated and NO\({}_{2}\) concentration averaged over the 2020 peak hour (working days). A weak correlation between NO\({}_{2}\) concentrations and the volume of traffic generated during the morning peak hours may be explained by the fact that the concentration of nitrogen dioxide in downtown Warsaw is lowest during the night hours (Figure 9) and only begins to rise during the morning peak hours, creating a sort of transition phase between the periods of lowest and highest concentrations. Concentrations begin to rise after 12 o'clock, reaching a peak in the afternoon and evening. Weak correlations most likely result from the fact that the air quality monitoring station at al.