Search for: All records

Motor vehicles are among the major sources of pollutants and greenhouse gases in urban areas and a transition to “zero emission vehicles” is underway worldwide. However, emissions associated with brake and tire wear will remain. We show here that previously unrecognized volatile and semi-volatile organic compounds, which have a similarity to biomass burning emissions are emitted during braking. These include greenhouse gases or, these classified as Hazardous Air Pollutants, as well as nitrogencontaining organics, nitrogen oxides and ammonia. The distribution and reactivity of these gaseous emissions are such that they can react in air to form ozone and other secondary pollutants with adverse health and climate consequences. Some of the compounds may prove to be unique markers of brake emissions. At higher temperatures, nucleation and growth of nanoparticles is also observed. Regions with high traffic, which are often disadvantaged communities, as well as commuters can be impacted by these emissions even after combustion-powered vehicles are phased out. 

Non-exhaust emissions (e.g., automotive brake and tire wear) are quickly replacing exhaust emissions as the dominant traffic particulate pollutant. A significant fraction of the emissions are complex mixtures of organic compounds whose composition is not well known. Due to their unique health implications, knowledge of the composition of ultrafine particles (<100 nm in diameter) is of particular interest. Here we report on the size-selected organic composition of ultrafine particles nucleated during high brake temperature conditions generated using a custom brake dynamometer system and two common brake pad types. Using high resolution mass spectrometry, we find that the organic composition of these particles is dominated by species containing oxygen (CHO) and nitrogen (CHN/CHON). Many of these compounds are unsaturated and are attributed to the thermal degradation of resin material used in the pad formulation. Other abundant compounds include various glycols and amines, several of which are unequivocally identified and discussed as potential marker compounds for brake wear emissions. A significant fraction of highly oxidized, low volatility species observed in ultrafine particles could not be conclusively attributed to the thermal degradation of the brake material, indicating the presence of chemical pathways unique to the frictional heating process. This emphasizes the importance of using a brake dynamometer to generate brake wear particles as opposed to other strategies.