Diurnal, Temporal and Spatial Variations of Main Air Pollutants Before and during Emergency Lockdown in the City of Novi Sad (Serbia)  
[PERSON]  
1 VINCA Institute of Nuclear Sciences--National Institute of the Republic of Serbia, University of Belgrade, 11000 Belgrade, Serbia; [EMAIL_ADDRESS](M.J.); [EMAIL_ADDRESS](M.J.-S.)
2 Faculty of Technical Sciences, Department of Environmental Engineering and Occupational Safety and Health, University of Novi Sad, Tge Dostiej Oraborolova 6, 21000 Novi Sad, Serbia; [EMAIL_ADDRESS](S.D.); [EMAIL_ADDRESS](J.K.)  
[PERSON].  
The lockdown interventions led to a reduction in population-weighted PM\({}_{2.5}\) of 14.5 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) across China (\(-29.7\%\)) and 2.2 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) across Europe (\(-17.1\%\)), with mean reduction in PM\({}_{2.5}\) concentrations during the lockdown period of around 20% in Europe when comparing the lockdown period in 2020 to previous years [6,7]. A study led by French National Institute for Industrial Environment and Risks (INERIS) [8], which analyzed short-term influence of COVID lockdowns on PM\({}_{10}\) and nitrogen dioxide (NO\({}_{2}\)), determined significant decreases in NO\({}_{2}\) and nitrogen oxides throughout Europe. Regarding PM\({}_{10}\) concentration, the situation is less uniform and European cities selected in the study [8] experienced a range of different outcomes depending on the city region. At Iberian Peninsula, reductions in PM\({}_{10}\) were observed. In Western Europe, PM\({}_{10}\) increase (or only a slight decrease) after lockdown was observed due to dominating effect of emission advections from anthropogenic sources (e.g., agricultural, industrial). At Scandinavian Peninsula, only a small decrease of PM\({}_{10}\) levels was identified due to considerable effect of road dust due to use of studded tires, road dusting and salting, i.e., high road dust emissions, which caused enhancement of PM\({}_{10}\) effectively masking the lockdown reduction effect. At Central and Eastern Europe mixed decrease/increase of the average PM\({}_{10}\) concentrations was identified as the effect of lockdown emission reductions was disturbed by a large natural dust episode on 26-29 March. In addition to PM, the main gaseous air pollutants NO\({}_{2}\), sulfur dioxide (SO\({}_{2}\)), carbon monoxide (CO), and ozone (O\({}_{3}\)) that are usually collected at monitoring sites were analyzed in different areas all over Europe [8]. From these preliminary studies and reports [7,8], it seems evident that nitrogen dioxide (NO\({}_{2}\)), nitrogen oxide (NO) and nitrogen oxides (NO\({}_{\mathrm{X}}\)) noticeably decrease while O\({}_{3}\) increases.  
This study is devoted to short-term air quality changes in the city of Novi Sad, Serbia (Lat/Long: 45.26714\({}^{\circ}\), 19.83355\({}^{\circ}\)), focused on the period immediately before and during the COVID-19 lockdown. In order to create data sets, online and off-line data from national [9], regional [10], and local monitoring networks [11] were combined with newly acquired data from an ongoing measurement campaigns utilizing PM low-cost sensors (LCS). The aim of this study was to investigate the impacts of lockdown and local meteorology on the level of selected main air pollutants [12]. The analysis was focused on the period of approximately three months, 1 February-30 April, six weeks before and six weeks during lockdown, with additional attention focused to period of about one week immediately before and during the COVID-19 lockdown, for which high-resolution LCS data were available. Selected air pollution data was supplemented (underpinned) with available local meteorological parameters in order to better understand and explain the interactions between pollution and meteorology. Depending on the temporal resolution of data provided by used monitoring tools, variations of main pollutants at the daily level were analyzed in cases when data with sufficient temporal resolution were available.