## 4 Summary--Discussion of the Results and Conclusions  
The following conclusions can be drawn from the results of the present study:  
1. Even with the ongoing considerable intensification of road transport in Poland, national annual emissions of hazardous pollutants, both total and from combustion engines under warm-up conditions, show evident decreasing trends in the case of carbon monoxide and volatile organic compounds. The annual emissions of nitrogen oxides and particulate matter have remained stable (Figures 1-5) [7; 8]. Emission reductions are associated with the technological advancement of the motor industry, and particularly with the use of modern solutions to reduce pollutant emissions from vehicles. The upward trend observed in annual carbon dioxide emission results from the intensification of road transport and, as a consequence, increased fuel consumption. The transport sector has strived hard to reduce fuel use and carbon dioxide emission; however the effectiveness of the efforts undertaken is limited due to the laws of nature that affect the thermal efficiency of internal combustion engine circuits and, first of all, by the second law of thermodynamics. 2. The share of annual pollutant emissions from car engines under cold-start conditions in total annual emissions remains almost constant. Emissions of carbon monoxide and volatile organic compounds constitute the largest shares--about 50%, whereas the particulate matter share is less than 15%, carbon dioxide less than 10% and nitrogen oxides less than 5%. 3. The additional emission of pollutants from passenger cars' combustion engines during engine warm-up and vehicle movement under urban conditions (Figures 7-11) shows, with some exceptions, a decreasing trend, which results from the technological advancements of the motor industry. This does not apply to the emissions of carbon dioxide (Conclusion 1) and of nitrogen oxides from passenger cars designated to the Euro 1 pollutant emission category and equipped with spark-ignition engines which use multifunctional catalytic converters, which are efficient in reducing pollutants in exhaust gases but sensitive to operating temperatures in terms of reducing nitrogen oxides. The reduction in additional emissions as a result of the warm-up of internal combustion engines shows the strongest trend in the case of compounds with reducing properties in oxygen-containing environments: carbon monoxide and non-methane volatile organic compounds. 4. The reduction in the specific distance emission of pollutants from combustion engines of passenger cars designated to the explicit pollutant emission categories is very clear--the strongest for carbon monoxide and non-methane volatile organic compounds (Figures 12-16). 5. In total emissions, there was observed a very high share of additional emissions from combustion engines during warm-up under the conditions of passenger car urban driving (Figures 17-21), especially for spark-ignition engines: for carbon monoxide and non-methane volatile organic compounds, it was about 90%, and lesser for nitrogen oxides, especially in the case of compression-ignition engines, as well as for particulate matter in the case of compression-ignition engines. The carbon dioxide share constituted about 30%. In general, the results obtained allow for the conclusion that, on a global scale, there is possible substantial progress in the reduction in pollutant emissions from automotive combustion engines, especially during engine warm-up. Therefore, further works towards the improvement of internal combustion engines are necessary, e.g., on the acceleration of the heating of catalytic exhaust after-treatment systems and on the long-term maintenance of the thermal status of the engine after stopping with the use of thermal accumulators [29; 30; 31; 32].