In the range of 5-6 Mt/y tire wear particles (TWP) are emitted from vehicles in both developed and emerging countries. In an attempt to reduce these emissions, new regulations will come into force in the EU and USA, although currently no oversight methods are actually in place. This study proposes a method for assessing direct TWP emissions (TWPD) from vehicles. The method entails labeling the tire with mercury, then collecting and fractionating the particles emitted at the rear of the wheel (RoWP), and lastly analyzing their Hg content in the laboratory using atomic absorption spectroscopy. It provides access to the magnitude, size distribution and factors affecting TWPD emissions under actual driving conditions. Furthermore, the implementation of desorption and dispersion models allows evaluation of the proportion of TWPD embedded in tire and road wear particles (TRWP) and estimating the TWPD contribution to the lower atmosphere PM1, PM2.5 and PM10 pollution within the EU-27. A key finding of this research is that ultrafine TWPD (accounting for 33-260 mg/g of abraded front tire material) account for 30-70 % of total TWPD emissions (with >93 % being in the form of inclusions), although they make up 0.5-5.7 % of RoWP mass emissions. Our data also draw attention to the magnitude and lower TRWP-embedment of TWPD emissions in urban areas.