# Impact of COVID-19 Mobility Changes on Air Quality in Warsaw
## 4 Discussion  
The four main sources of air pollutant emissions are the municipal and household sector (so-called surface sources or low-stack emissions), road transport, industry and energy production and distribution. In addition, agriculture can also be taken into account, but mostly in the case of ammonia emissions [6]) and the waste management sector, which do not play a dominant role in the emissions of the main types of pollutants. The energy production and distribution sector contributed to Poland in 2018 primarily to emissions of sulphur oxides (SOx), nitrogen oxides (NOx) and mercury (Hg), while industry (comprising heavy and light industry) was responsible mostly for the emissions of non-methane volatile organic compound (NMVOC) emissions, mercury (Hg) PM2.5, PM10 and sulphur oxides (SOx) [6]. More than 44% of PM2.5 and 41% of PM10 emissions in Poland in 2018 came from the municipal and household sector. It is also a very significant source of polycyclic aromatic hydrocarbon (PAH) emissions-85.05%, including 89.39% of benzo(a)pyrene emissions, carbon monoxide (CO) emissions-59.81% and dioxins and furrans-58.71% [6]. In the municipal and household sectors, the main sources of air pollutant emissions are processes related to domestic heating. In its report, the Polish Smog Alert presented the results of its 2020 study, where it estimates that there are still nearly 3 million single-family houses heated with boilers and stoves that emit significant amounts of pollutants and do not meet the requirements of the anti-smog resolutions. Moreover, Poles in their houses burn more than 80% of the coal that is used in all households across the European Union [31]. Another problem is the thermal insulation of houses, as one-third of single-family buildings do not have any insulation (a slight decrease to 33% from 38% in 2014) [32]. Analysing the data for Warsaw, in 2020, in the first months of the pandemic (March-April), an increased impact of PM2.5 and PM10 emissions was noted despite the above-described significantly lower number of trips generated compared to 2019. The situation identified could therefore be an increased burning of solid fuels in households. The shift to remote learning and work has contributed to people staying at home. The need to ensure adequate thermal comfort in buildings throughout the day required burning more fuel than when the inhabitants of the buildings were at their places of work and study during  
Figure 9: Summary of the mean NO\({}_{2}\) concentrations [\(\mathrm{\mu g/m^{3}}\)] at the air quality monitoring station at al. Niepodleglösci in Warsaw. Calculation for all days in the period 2017–2020. the day. This, in turn, may have resulted in increased emissions of particulate pollutants and their higher concentrations in the air. This situation was analysed in detail in the vicinity of the air quality monitoring station in Wawer in Warsaw, a district with predominantly single-family housing. In the case of PM2.5 and PM10 (Figure 10), in March and April 2020, concentrations on as many as 24 out of 61 days were higher compared to the corresponding period in 2019. This is particularly true in March, when outdoor air temperatures, under Polish climatic conditions, usually still require the heating of buildings. Figure 10: Cont. A comparison of the data also indicates that in subsequent months (May, June), as the weather improved and the ambient air temperature rose, there was a reduction in concentrations of PM2.5 and PM10, with a concomitant increase in the volume of traffic generated (as lockdown restrictions were relaxed) (Figure 11). A comparison of data on PM2.5 and PM10 concentrations during the initial period of the spring 2020 pandemic in relation to the situation in 2020 and in relation to the volume of traffic generated indicates that a reduction (in March and April) and then an increase (in May and June) in the mobility of people did not directly affect changes in particulate concentrations. However, it had an indirect impact.