## 3 Results and Discussion
EMS\({}^{*}\) denotes EMS data with Araklum Desert episode excluded from data. Before and during the emergency state lowest PM\({}_{2.5}\) concentrations were observed when the wind was coming from the southeast direction and to a lesser extent north-east direction (Figure 3 upper row), while most of the highest PM\({}_{2.5}\) concentrations were observed for low wind speeds within the 1.5 m/s, (Figure 3 upper and middle row). Having in mind the position and type (traffic) of Rumenaka monitoring station, this indicates the presence of strong local sources of air pollution, mainly related to traffic and residential heating, whose influence is minimized mainly by favorable meteorological conditions. These conditions may come in form of stronger winds, for example over 5 m/s, as can be seen from low PM\({}_{2.5}\) concentrations in Figure 3b,c middle row. This is also confirmed by lower row in Figure 3, depicting PM\({}_{2.5}\)-PM\({}_{10}\) robust slope. Different types of sources have different size distribution signature, for example combustion sources emit a large proportion of PM\({}_{2.5}\), while on the other hand material used during construction, geological matter, polen and similar have larger coarse fraction. As can be seen from Figure 3b,c, within wind speeds smaller than 1 m/s there is an indication of strong local  
Figure 3: PM\({}_{2.5}\) [\(\mu\)g/m\({}^{3}\)] counts by wind direction (upper row), PM\({}_{2.5}\) [\(\mu\)g/m\({}^{3}\)] pollution rose (middle row) and PM\({}_{2.5}\)–PM\({}_{10}\) robust slope (lower row) (**a**) before emergency state 1–15 March (**b**) during emergency state 16–31 March, Aralkum episode excluded (**c**) during emergency state 1–15 April. combustion processes, indicated by high PM\({}_{2.5}\)-PM\({}_{10}\) ratio. Stronger wind noticeably change local air pollution composition and reduce PM\({}_{2.5}\) concentration, thus making PM\({}_{2.5}\)-PM\({}_{10}\) ratio very small. Figure 4 shows the mean values and 95% confidence interval for the normalized pressure, temperature and PM\({}_{2.5}\) measured at station Rumenacka, before and during emergency state. All of the air quality variables are min-max normalized taking into account the whole period 1 March to 15 April. Min-max values for the pressure, temperature and PM\({}_{2.5}\) concentrations are 989.0-1024.0 mbar, \(-\)2.06-28.4 \({}^{\circ}\)C, and 0.0-91.97 \(\upmu\)g/m\({}^{3}\), respectively. The period during the emergency state had a lower mean temperature in 0-6 h period of the day, as evident from the plots. This is also accompanied by increased PM\({}_{2.5}\) concentrations in the 0-6 h period of the day and higher pressure throughout the day compared to the period before the emergency state. In the city of Novi Sad, residential heating is realized either as individual boilers or via district heating facilities, which are a substantial contributor to PM concentrations. Residential heating had to be used more since citizens have been instructed to stay at home as much as possible [21]. This is evident in Figure 4b, where lower temperatures correspond to higher PM\({}_{2.5}\) pollution. On the other hand, as evident from Figure 4c, higher temperatures, at least in part, can be associated with reduction in PM\({}_{2.5}\) pollution. Descriptive Statistic of PM\({}_{2.5}\) Levels before and at the Beginning of Emergency Measure State Measured with Low-Cost Sensors  
In the current and following section, we will try to gain a bit more insight into periods before and during COVID lockdown by utilizing higher temporal resolution of low-cost sensors. Table 3 shows descriptive statistics for data collected via low-cost sensors at 4 different locations in Novi Sad. For each measuring spot, two columns are given, first referring to the period before the emergency state, and second referring to the period after the emergency state. These periods are identical for measuring sites MS1, MS2 and MS3, going from 8 to 15 March 2020 for the BEMS period, and from 16 to 24 March 2020 for the EMS period.