## 3 Results  
Detailed information on the evolution of the structure of vehicles used in Poland in the period under consideration, i.e., in the years 1990-2017, and the nature of their operation can be found in the official reports of KOBiZE. Selected elements of this information are also found in publications, e.g., in [5; 26; 27]. The results of the research using the results of the inventory of pollutant emissions from road transport in Poland in the years 1990-2017 with the use of COPERT 5 software are shown in Figures 1-6. Figure 1: Annual carbon monoxide emission—E\({}_{\mathrm{aCO}}\): national (total)—T, from car engines at cold-start—c, from car engines at warm-up—h and the share of additional emissions during engine heating in total emissions—(c/T). Figure 4: Annual emission of particulate matter—E\({}_{\rm aPM}\): national (total)—T, from car engines at cold-start—c, from car engines at warm-up—h and the share of additional emissions in the time of engine heating in total emissions—u(c/T). Figure 3: Annual emission of nitrogen oxides—E\({}_{\rm aNON}\): national (total)—T, from car engines at cold-start—c, from car engines at warm-up—h and the share of additional emissions during engine heating in total emissions—u(c/T). Figure 2: Annual emission of non-methane volatile organic compounds: E\({}_{\rm aNVOC}\): national (total)—T, from car engines at cold-start—c, from car engines at warm-up—h and the share of additional emissions during engine heating in total emissions—u(c/T). Figures 1-5 show annual pollutant emissions, i.e., total emission at a national level, emissions from the engines analyzed during cold-start and warm-up, and Figure 6 shows the share of the additional emission in the heating time of engines in the total emission. In the study period (1990-2017), national annual emissions of carbon monoxide and non-methane volatile organic compounds were decreasing ever after 1999, regardless of a boost in the use of motor vehicles. Emission reductions were a result of the technical advances made in the design of combustion engines and their exhaust after-treatment systems [15; 17; 24; 28], as well as beneficial changes in the composition of the vehicle population in the country towards using vehicles with modern technological solutions, including with hybrid powertrains and, above all, electric vehicles [7; 8]. The national annual emission of nitrogen oxides and particulate matter showed upward trends, with fluctuations in the years 1999-2005. The progress in the reduction in nitrogen oxides and particulate matter emissions was evidently lesser when compared to carbon monoxide and non-methane volatile organic compounds [15; 17; 24; 28]. The observed increase in the national annual carbon dioxide emission was primarily associated with a fuel consumption increase as a result of intensive development of the automotive industry in Poland, including changes in the structure of vehicles in use-trends in the use of larger vehicles [7; 8]. Figure 5: Annual emission of carbon dioxide—E\({}_{\text{aCO2}}\): national (total)—T, from car engines at cold-start—c, from car engines at warm-up—h and the share of additional emissions during engine heating in total emissions—u(c/T). Figure 6: Share of additional annual emissions of pollutants during engine warm-up in the total annual emissions averaged over all inventory years. Figure 6 shows a set of shares of additional annual emission in the time of engine heating in the total annual emission, averaged for all inventory years, for all the analyzed pollutants. Carbon monoxide and volatile organic compounds emissions are most sensitive to the thermal status of internal combustion engines. In the case of nitrogen oxides' emission from the engine during warm-up, low engine temperatures support low nitrogen oxides' emission [9; 10; 13; 15; 24]; however, then the effectiveness of the exhaust after-treatment system is decreased [10; 11; 12; 13; 16; 19; 20; 21; 23].