### Current State of Knowledge on Particulate Matter Emissions from Road Transport  
There have been numerous papers published on particulate matter emissions from engine exhaust. Publications in the field of particulate matter emissions from internal combustion engines concern both the research methodology and analysis of empirical test results, as well as the results of modeling particulate matter emissions. In [34], the procedures and methodologies of the UN/ECE PMP Working Group are presented in order to evaluate their abilities to test particulate emissions and numbers. Results of inter-laboratory tests of particulate matter emissions are presented for various types of internal combustion engines, with particular emphasis on the repeatability and reproducibility of measurement data. The modeling of particulate matter emissions from internal combustion engines is the subject of [35; 36; 37]. In [35], the available models of particulate matter emissions  
Figure 1: Limits of the specific brake emission of particulate matter using the static test WHSC (World Harmonized Stationary Cycle) for Euro V and Euro VI emission stages, and the dynamic tests ETC (European Transient Cycle) and WHTC (World Harmonized Transient Cycle) for Euro V and Euro VI, respectively (consistent with [30]). from exhaust systems are analyzed, mainly in the form of zero-dimensional emission characteristics comprising various types of emission factors. These are most often specific distance emissions of particulate matters and specific brake emissions of particulate matters, characteristics that enable the modeling of particulate matter emissions from exhaust systems depending on, for example, the nature of vehicle traffic. This article proposes an original model of the emission of particulate matters from vehicle engines, and presents the results of simulation tests of particulate matter emissions. In [36], the subject of pollutant emission inventory is studied in the context of, inter alia, testing of particulate emissions from exhaust systems of the internal combustion engines of motor vehicles. The article presents considerations of the inventory of particulate matter emissions and air quality modeling in the Pearl River Delta region--a region with particularly high environmental pollution. The main purpose of the work is to assess air quality modeling methods in order to simulate pollutant concentrations. Three modeling models were assessed under conditions typical of winter and summer periods. The best results of compliance for the concentration measurements were obtained for the EDGAR (Emission Database for Global Atmospheric Research) model [37]. Most of the works concern the analysis of the results of empirical tests of particulate matter emissions from combustion engines. Ref. [38] summarizes the evolution of particulate matter emissions from spark ignition engines marketed from the early 1990s to 2019 in different regions of the world. Significant progress has been made in this regard, mainly due to the technical progress of internal combustion engines. This applies not only to the improvement of fuel dosing and combustion systems, but above all to exhaust gas cleaning systems. The significant effect of ambient temperature on particulate matter emissions is highlighted. Specific exhaust gas cleaning systems are the subject of [31]. This paper presents the results from testing a particulate matter filter. The tests were mainly carried out under the conditions of a low engine load typical of passenger cars in cities. Ref. [39] discusses the emission of various quantities of particulate matter from compression-ignition and spark-ignition engines. The authors point out that more precursors of very harmful organic aerosol are produced as a result of petrol combustion in a spark-ignition engine. In [40], it is shown that gasoline-powered spark ignition engines emit more particulate matter compared to compression ignition engines equipped with particle filters (DPF).