## 3 Results and Discussion
From MS5 site, BEMS period data was collected somewhat earlier from 6 to 23 February 2020, while the EMS period mostly coincides with the EMS period for the remaining sites, 18 to 26 March 2020. Figure 4: Normalized pressure, temperature and PM\({}_{2.5}\) concentrations in different hours of the day (min-max normalization for the whole period) (**a**) before emergency state 1–15 March (**b**) during emergency state 16–31 March, Aralkum episode excluded (**c**) during emergency state 1–15 April. The median of PM\({}_{2.5}\) concentration was similar in the BEMS and EMS period for all sites, a slight decrease in the median was observed only for MS3. Maximum values of PM\({}_{2.5}\) (98 th percentile) increased for MS2 and MS3 sites, remained the same for MS1, and decreased for MS5 site. Another interesting aspect is the standard deviation of the pollution, which may signal change in pollution sources and their distribution: it remained the same at MS1 and MS3 sites but showed a noticeable decrease at MS5 site. More insight into PM\({}_{2.5}\) pollution and change that occurred during EMS at these sites is possible via time series and 24 h boxplots shown in further text. Changes in Diurnal Variation of PM\({}_{2.5}\) at Sampling Site MS1, MS2, MS3 and MS5 before and after Measures of the State Emergency  
The reduced movement of vehicles as well as reduced industrial activities during lockdown in Novi Sad, leading to the reduction of the exhaust emissions, were a probable cause for a reduction (as seen from reduced interquartile range of hourly boxplots in Figure 5) and a more uniform profile of the PM\({}_{2.5}\) levels approximately during typical working hours (8 AM-5 PM). According to recently published studies, similar findings were identified in several cities in various parts of the world [22]. In the first month of lockdown, period 15 March-15 April 2020, the official heating session was still ongoing. For the most part, mean daily temperatures in the week before and in the first weeks of lockdown, during which the sampling campaign was conducted, were below 10 \({}^{\circ}\)C. Residential heating sources caused higher concentration in the morning and during the night at almost all sampling sites and can be clearly recognized from the boxplot representing 1 h variations in the week before the EMS and period at the EMS beginning (Figure 5). Changes in Diurnal Variation of Gaseous Pollutants and PM\({}_{2.5}\) at NN1 before and after Measures of the State Emergency  
For the identification of changes of main air pollutants levels, it was necessary to create diurnal variation whenever it was possible, i.e., data have been collected using monitors with high temporal resolution. Figure 6 depicts diurnal variations of measured main gaseous pollutants at site that belongs to the national monitoring network: Rumenacka SEPA, NN1 before and after measures of the state emergency. At NN1 the following was observed:  
Figure 5: Daily profile of PM\({}_{2.5}\) measured before measures of the state of emergency (**left**) and during measures of the state of emergency (**right**) at (**a**) MS1 (**b**) MS2 (**c**) MS3 (**d**) MS5. * Daily profiles of SO2, the main pollutant, depicted in the form of a boxplot (Figure 6a), show that, on average, there was no noticeable change a week before and 10 days during the emergency state. For most hours of day, the SO2 concentration was below 20 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\), with occasional \"outliers\" in the range from 35-40 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) in the morning and afternoon pollution peaks. * Daily profiles for NOx show a clear change in daily patterns (Figure 6b). Because boxplot color corresponds to the value of median, it is evident that the median of these main pollutants was reduced during the first 10 days of the emergency state during most hours of the day. NOx boxplots show clear morning and afternoon pollution peaks, both before and during the emergency state, but the median of these peaks is reduced during the emergency state.