## 3 Results and Discussion  
As an initial analysis step, we have plotted particulate matter concentration at available stations that belong to local and national monitoring networks before and after entering the state of emergency (Figure 2) in the period from 1 February to 30 April. Local and regional monitoring stations publicly report daily average concentrations, while data collected at national monitoring station are available with 1 h resolution. A large peak in both PM\({}_{2.5}\) (Figure 2b) and PM\({}_{10}\) (Figure 2a) concentration is clearly visible on 26, 27 and 28 March, and is similar in magnitude for all monitoring stations. For these several days, the daily average levels of PM\({}_{10}\) and PM\({}_{2.5}\) were up to 250 and 100 \(\mathrm{\SIUnitSymbolMicro m}\)\({}^{3}\), respectively. Extremely high concentrations of particulate matter were recorded at automatic monitoring stations in the whole of Serbia and surrounding countries. The level of PM\({}_{10}\) at the higher temporal resolution (e.g., 1 min) in some cities in Serbia, including Novi Sad, reached up to 600 \(\mathrm{\SIUnitSymbolMicro m}\)\({}^{3}\). No similar peaks exist for PM\({}_{10}\) for other days, while for PM\({}_{2.5}\) similar magnitude peaks do exist, albeit only locally (i.e., for only one of the observed monitoring stations). This kind of pattern indicates some isolated regional event that happened in the period on 26-28 March. Figure 2 shows the back trajectory that was calculated using Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) transport and dispersion model. The air mass back trajectory was calculated at 270 m above the ground, on 27 March, when the maximal concentration of PM\({}_{10}\) was measured at Novi Sad's national and local network sites. Based on the back-trajectory analysis, the presence of a possible source of non-local air pollution is evident. This source of dust can be associated via back trajectory tracing to the Aralkum Desert that is located at the Kazakhstan and Uzbekistan border. Back trajectory for surrounding days is also given in Appendix B. Tables 1 and 2 show descriptive statistics for fine and coarse particulate matter data collected at stations that belong to the national and local monitoring network in the area  
Figure 2: Daily concentrations at sites belonging to national and local monitoring networks before and after entering the state of emergency (dashed gray line) (**a**) PM\({}_{10}\) (**b**) PM\({}_{2.5}\). of Novi Sad for the period from 1 February to 30 April, split into periods before and after lockdown measures introduced due to COVID-19 pandemic. Tables also include descriptive statistic when 3-day Araklum Desert episode is excluded from data. Looking into data from EMS and EMS\({}^{*}\) columns in Tables 1 and 2, it becomes evident that natural distant sources may be very dominant in total particulate matter pollution and thus effectively mask local changes originating from emergency state, strongly swaying mean and maximum values of PM pollution. When comparing BEMS (before emergency state) and EMS* (during emergency state) columns in Table 1, it can be seen that the median slightly increased for LN1 and NN1 (both stations are urban traffic), strongly increased for LN3 (urban background), and decreased for LN2 and LN4 (LN2 and LN4 are suburban traffic and suburban background stations respectively). When comparing BEMS and EMS\({}^{*}\) columns in Table 2, it can be seen that the median decreased for LN2, LN4 and NN1 (suburban traffic, suburban background and urban traffic) and, again, strongly increased for LN3 (urban background). While it is expected that traffic sites experienced decrease in PM\({}_{2.5}\) pollution, a strong increase in both PM\({}_{10}\) and PM\({}_{2.5}\) for LN3 is somewhat surprising and may indicate presence of source which emits both fine and coarse particles that was active during EMS. However, LN3 had lowest concentration of all stations in BEMS period.