Abstract
ABSTRACT
Traffic related sources are a significant contributor of particulate matter (PM) in urban environments. Traffic related particles originate either from vehicles’ exhaust, as a result of incomplete fuel combustion and lubricant volatilization during the combustion procedure or from non-exhaust traffic related sources such as brake, tyre, clutch and road surface wear. It is estimated that exhaust and non-exhaust sources contribute almost equally to total traffic-related PM10 emissions. However, as exhaust emissions control become stricter the relative contribution of non-exhaust sources to traffic related emissions will become more significant. The aim of the present study is to discuss the state-of-the-art of the different aspects regarding particulate emissions resulting from brake wear. For that reason an extensive literature review was conducted and the most important open issues were identified. According to the existing literature brake wear (BW) is estimated to be 16-55% by mass of non-exhaust traffic related PM10 emissions at urban environments. Significantly lower contributions (~3% by mass) have been reported at freeways. Most researchers find that almost 40-50% by mass of generated brake wear is emitted as PM10, while mass size distributions of brake wear usually appear to be unimodal. Most studies show bimodal PN distribution with one peak lying within the ultrafine mode (<80 nm) and the other within the fine particle mode. Regarding the chemical composition of wear particles, brake wear PM10 is characterized by the presence of high concentrations of some heavy metals (Fe, Cu, Zn, Sn, Sb), as well as of S, in both fine and coarse fractions. Additionally, noticeable concentrations of organic substances and elemental carbon have been reported in the PM2.5 fraction. Both road simulation and receptor modelling studies agree in PM10 brake wear EFs of 2.0-8.8 mg km-1veh-1 for LDVs, with the average value of 6-7 mg km-1veh-1 being very close to the corresponding exhaust PM10 EF of modern (Euro 5/6) diesel vehicles. The corresponding EFs of HDVs are approximately one order of magnitude higher compared to passenger cars. All the above mentioned factors need to be taken into account in order to assess the possible need for regulating brake wear emissions.