## 2 Materials and Methods
### Ongoing Sampling Campaign with Low-Cost Sensors and Reference Pumps  
An ongoing measuring campaign with low-cost sensors and reference gravimetric pumps in the city of Novi Sad started in February 2020 and was conducted during mid-February and March. Our monitoring campaign utilizing low-cost sensors was already underway in Novi Sad when Emergency State Measures were introduced on 16 March. The more, or less severe lockdown was used as a unique opportunity to conduct this additional experiment, in which results about the possible relation between massive change of general population daily habits and change in traffic and industrial emissions could be obtained. At selected sampling sites, besides low-cost sensors that measured PM\({}_{2.5}\) with a temporal resolution of several seconds, PM\({}_{2.5}\) was collected on Quartz fiber filters by reference gravimetric pumps set to sample air for 48 h. A note about sampling duration is in place here. Timing and duration of the sampling can be adapted for specific purpose in order to use the available resources most effectively. For example, within the ESCAPE project sampling schedule (for purposes of Europe-wide LUR modeling) was two-week sampling, with timers which were set to sample for 15 min every 2 h so that effectively a 42-h sample was collected over 14 days [15]. This compromise must encompass frequency of accessing device in the field conditions on one hand and quality and quantity of samples necessary for calibration of low-cost sensor on the other hand. In this way \"best of both worlds\" are obtained: the low-cost sensors provide high temporal resolution, while use of reference gravimetric pumps minimizes the errors. Our sampling schedule produces approximately 8 samples over period of 16 days. Additional details are given in Appendix A. Particulate matter low-cost sensor suite (LCS), _ekoNET_ was made by Dunavnet [16]. LCS platform was equipped with a PMS7003 particulate matter sensor. Plantower PMS7003 has counting efficiency of 50% at 0.3 \(\upmu\)m and 98% at 0.5 \(\upmu\)m, and maximum consistency error \(\pm\)10% at 100-500 \(\upmu\)g/m\({}^{3}\) and \(\pm\)10 \(\upmu\)g/m\({}^{3}\) at 0-100 \(\upmu\)g/m\({}^{3}\) range [17; 18]. Reference gravimetric pump that was collocated with each LCS was Leckel Model LVS3 [19], with standard PM\({}_{2.5}\) inlet and 2.3 lpm flowrate. In the period before the Emergency State Measures, the sampling campaign was performed at 4 measuring sites (MS), namely: MS1 (Hajduk Veljkova 4), MS2 ([PERSON] put 20), MS3 ([PERSON] 2), MS5 ([PERSON] 78). All four localities can be characterized as traffic sites since they are situated in the vicinity of traffic intersections and streets with a high or medium volume of traffic (Figure 1). After lockdown, the campaign was continued during the following 8-10 days at 3 sites (MS1, MS2, MS3) and repeated at MS5 where sampling was performed previously during February. The instruments were situated within 1m of each other, and no additional inlets were used in order to minimize particle losses. At measuring sites, the instrumentation was at about 2-3 m above the ground. The sampling campaign was carried out at all 4 measuring sites continuously, 7 days before the state of emergency (abbreviation BEMS in further text) and 8 days during the state of emergency (abbreviation EMS in further text) that was declared due to COVID-19 in Serbia. Measuring site 1 (MS1, Lat/Long: 45.24968\({}^{\circ}\), 19.82467\({}^{\circ}\)) was located in a narrow city center, at a distance of 50 m from the intersection of Futoski put, Hajduk Veljkova, and Cara Dusana Street. Possible sources of PM\({}_{2.5}\) in addition to traffic may include residential heating, small boilers and stoves for individual households in the surrounding area. Types of vehicles passing through the intersection are light vehicles and public transportation (buses). A small heating plant is placed within close proximity (about 1 km).