## 4 Conclusions  
The present study was carried out based on data derived from the inventory of pollutant emissions from road transport in Poland in 2018. Essentially, this study aimed to assess the makeup of particulate matter emissions according to the characteristics of motor vehicles, such as vehicle purpose and conventional size; ecological quality of internal combustion engine in terms of pollutant emissions; and traffic conditions, defined as traffic inside and outside cities and on highways. The conducted research confirms the research hypotheses considered in the introduction. It is confirmed that motor vehicle traffic has a significant impact on particulate emissions. A significant share of particulate matter emissions from sources other than the exhaust systems of internal combustion engines was also confirmed. Based on the study results, the following conclusions can be drawn:  
1. The highest relative annual emission of PM10 particles (78%) and PM10 from sources other than engine exhaust gases was 41%. The relative annual emission of PM2.5 from sources other than exhaust gases was the lowest (22%). 2. The total particulate matter emissions from sources other than exhaust gases constituted 62% of PM total emissions. 3. The highest relative national annual emissions of particulate matter from sources other than engine exhaust gases were observed for urban and non-urban vehicle traffic (40-50%), and the lowest was observed on highways (10%). The most significant finding is that the main source of particulate matter emission was not engine exhaust gases but other sources, foremost vehicle braking systems [5; 14; 21; 33] followed by the contact between vehicle tires and road surfaces [5]. A substantial progress in the construction of internal combustion engines and the development of methods for reducing pollutant emissions from these engines have largely contributed to a considerable decrease in pollutant emissions from motor vehicles. This is evident in the limitations on exhaust emissions that have been enforced for motor vehicles and their engines, as well as for light vehicles (passenger cars, light trucks, microbuses, motorcycles and mopeds) and heavy vehicles (trucks and buses) [30; 31]. Particulate matter emissions from vehicle braking systems consist of very small particles with an equivalent diameter of less than 1 \(\upmu\)m. Additionally, these emissions contain mainly heavy metals in their compounds, especially iron [14; 21; 33]. On the other hand, particulate matter produced during contact between vehicle tires and the surface of a road is  
Figure 15: Relative national annual emissions (Er) of particulate matter from road transport under different traffic conditions: urban (U), rural (R), highway (H) and total (T). characterized by particles of larger dimensions containing very harmful substances, including organic compounds such as polycyclic aromatic hydrocarbons and their derivatives [5] (the substances that promote the formation of cancer). The risk associated with particulate matter emissions from sources other than engine exhaust gases is especially high in cities (i.e., in densely populated areas), where many people are exposed to emissions. Furthermore, the certain types of vehicle in city traffic can result in high-PM emissions from sources other than vehicle engines because of the need for braking and increasing velocity in an alternating sequence. In view of these considerations, a practical conclusion can be drawn. While emissions of pollutants from engine exhaust gases are considerably limited by type-approval regulations, there are no limits in regards to particulate matter emissions from sources other than internal combustion engines. For road transport, PM emissions from vehicle engines are exclusively included in the emission inventory, and the concentrations of PM dimension fractions (and other substances) in the air as well as their effects are monitored.