### Current State of Knowledge on Particulate Matter Emissions from Road Transport
Much less information is available on road transport particulate matter emissions from sources other than engine exhaust. The models of particulate matter emissions related to road transport that have been developed [14; 21; 22; 30] are among the models in accordance with the criterion of functional similarity (the so-called behavioral models) [41]. These models make the total particulate matter emission from both the internal combustion engine and the vehicle tribological sources dependent on properties such as driving speed, vehicle type and weight and road surface. Refs. [22; 42] present a literature review on various aspects of particulate matter emissions from sources other than exhaust, in particular from brake and tire wear. Refs. [10; 12; 13] describe an original system for reducing the emissions of particulate matters from braking systems, analyzing the dimensions and mineral and chemical composition of particulate matter generated in braking systems. The developed system enabled a reduction of particulate matter emissions from braking systems by approximately 90%. The studies found that particulate matters from braking systems are particularly dangerous to health due to their compositions and dimensions. The material of these particles are heavy metals and their compounds mainly consist of iron. The dimensions of these particles are predominantly less than 1 um. Ref. [42] concerns the emission of particulate matters from braking systems. The authors of the publication proposed an original research methodology for the analysis of particulate matter emissions and determine the volume of particulate matter in a car'sbraking system. This methodology consisted of applying the WLTP (Worldwide Harmonized Light Vehicle Test Procedure) test [32; 33] to a chassis dynamometer (i.e., under the conditions of type approval tests). Under these comparable driving conditions, the authors investigated the influence of numerous operating factors on the emissions of particulate matters from the braking system. Ref. [43] highlights the problem of information availability with respect to the properties and qualities of road transport particulates from sources other than fuel combustion. They also investigate the limited information available on particulate emissions from abrasion. Ref. [26] systematically reviews and analyzes the available results of studies on particulate matter (PM10 and PM2.5) source apportionments carried out in cities to estimate the typical source contributions of particulate matter in different countries and regions. Based on the available information, 25% of PM2.5 emissions in cities worldwide come from traffic, 15% from industrial activities, 20% from domestic combustion, 22% from unspecified sources of anthropogenic origin, and 18% from natural sources (biological and mineral particles). Ref. [44] presents the results of a study on particulate matter emissions from road transport. Most of the particles were composites of tire, road surface and brake system materials. Particulate matter varied in size distribution, composition and structure, depending on travel velocity, traffic volume and rolling stock. Ref. [18] states that particulate matter emissions from other traffic-related sources become an increasingly important issue as emissions of exhaust pollutants decrease. Among others, particulate matter emissions from tire and road surface wear may be responsible for 5-10% of microplastics deposited in the oceans. In [45], the authors indicate that statistical studies confirm a smaller share of PM10 emissions from mobile sources in cities when compared with stationary sources (53%). Ref. [39] presents the results of a study on PM2.5 emissions from tire and road surface abrasion in London, Tokyo and Los Angeles. It was found that the proportion of PM2.5 particles from tire and pavement abrasion was only 0.27% of the total PM2.5 concentration. Based on the official results of emission inventories, few publications have discussed the emission of particulate matter from road transport.