Automotive braking is a source of highly charged aerosol particles

Electric forces can enhance the emission of microplastics into air

Microplastics (MPs) are problematic pollutants in various environmental contexts. Dust storms, blowing dusts, and dust devils can detach significant amounts of surface MPs into the atmosphere and migrate them far from their original sources via atmospheric transport. Studies have shown that strong electrostatic fields that exceed 150 kV/m can be observed during dust storm events. In this study, we perform theoretical calculations and laboratory experiments that demonstrate that MPs can be directly lifted under a strong electric field, and the threshold electric field (Ee) required for the lifting of MPs is closely related to the material composition and morphology of the MPs themselves and the air humidity. The electric forces generated by these electric fields can decrease the threshold friction velocity required to initiate the lifting of MPs. Specifically, electric fields exceeding 200 kV/m can directly lift surface particles, while fields above 120 kV/m significantly reduce the threshold friction velocity required for wind-driven particle movement by 10 %. These effects are most pronounced for particles with diameters ranging from 80 to 250 μm. We concluded that electric forces enhance MPs lifting, playing a key role in their motion at the particle scale and atmospheric transport at the regional scale. The enhanced loading of MPs into the atmosphere increases their transport distance, This long-range transport not only exacerbates global microplastic pollution but also leads to the deposition of MPs in remote ecosystems, such as polar regions, oceans, and mountainous areas, affecting local biodiversity. Meanwhile, humans may face potential health risks by inhaling or ingesting air, water, and food contaminated with MPs, such as inflammatory responses or the accumulation of harmful chemicals. Additionally, the distribution of MPs in the atmosphere may impact the climate system, for example, by altering cloud condensation nuclei formation, which in turn affects precipitation patterns and Earth's radiation balance.

Emissions of Nitrous Acid, Nitryl Chloride, and Dinitrogen Pentoxide Associated with Automotive Braking

As worldwide trends move toward replacing combustion transportation modes with electric vehicles, characterizing non-tailpipe emissions, such as those from brake wear, becomes increasingly important. Nitrous acid (HONO), nitryl chloride (ClNO2), and dinitrogen pentoxide (N2O5) are important sources of radical oxidants (e.g., •OH, •Cl, •NO3) and nitrogen oxides (NOx) in the atmosphere, driving the chemistry that leads to air quality degradation. Discrepancies between measurements and model predictions indicate that there are significant unknown sources of these species, particularly HONO, where the contributions of different formation processes have been controversial since the first ambient observations in the 1970s. We report the generation of these reactive nitrogen species during automotive braking using chemical ionization mass spectrometry configured with iodide reagent ion. Substantial HONO levels are observed from ceramic and semi-metallic brake pads, and smaller quantities of ClNO2 and N2O5 were also detected. We propose that HONO is formed in the hot plume emanating from the brake rotor via abstraction by NO2 of allylic and aldehyde hydrogen atoms found in the complex mixture of volatile organic compounds emitted simultaneously. These results suggest that emissions from automotive braking must be taken into account in urban oxidation chemistry.

Laboratory measurement and machine learning-based analysis of driving factors for brake wear particle emissions from light-duty electric vehicles and heavy-duty vehicles

This study investigates brake wear particle (BWP) emissions from light-duty electric vehicles (EVs) and heavy-duty vehicles (HDVs) using a self-developed whole-vehicle testing system and a modified brake dynamometer. The results show that regenerative braking significantly reduces emissions: weak and strong regenerative braking modes reduce brake wear PM2.5 by 75 % and 87 %, and brake wear PM10 by 90 % and 95 %, respectively. HDVs with drum brakes produce lower emissions and higher PM2.5/PM10 ratios than those with disc brakes. A machine learning model (XGBoost) was developed to analyze the relationship between BWP emissions and factors (11 for light-duty EVs and 8 for HDVs, based on kinematic, vehicle, and braking parameters). SHapley Additive exPlanations (SHAP) were used for model interpretation. For light-duty EVs, reducing high kinetic energy losses (Ike > 6500 J) and initial speeds (V > 45 km/h) braking events significantly lowers emissions. Additionally, the emission reduction effect of regenerative braking intensity (BI) stabilizes when BI exceeds 900 J. For HDVs, controlling braking temperature (Avg.T < 200°C) and initial speed (V < 50 km/h) effectively reduces emissions. Our findings provide new insights into the emission characteristics and control strategies for BWPs. SYNOPSIS: The construction and interpretation of a machine learning based model of brake wear emissions provides new insights into the refined assessment and control of non-exhaust emissions.

Mechanism of microplastic and nanoplastic emission from tire wear

Tire and brake-wear emissions, in particular nanoparticulate aerosols, can potentially impact human health and the environment adversely. While there is considerable phenomenological data on tire wear, the creation and environmental persistence of particulate pollutants is not well understood. Here, we unequivocally show that normal mechanical tire wear results in two distinct micro and nanoplastic (MNP) populations: a smaller, aerosolized fraction (<10 μm), and larger microplastics. Nanoplastic emissions follow a power law distribution that we show is consistent with the classical arguments of Archard, and Griffiths. Nanoplastic pollution increases dramatically with vehicle speed and weight, as the power law distribution characterizing these gets steeper. Charge stabilization of the tire wear nanoparticles keeps them suspended, while microplastics settle due to gravity. Larger microplastics are formed by sequential wear processes and show a log-normal distribution, as anticipated by Kolmogorov. Thus, the particle size distribution provides mechanistic insights to tire fragmentation: the aerosolized fraction is determined by power input to the tire while the larger microplastics are determined by sequential wear processes due to tire-road surface interactions, independent of vehicle weight and speed.

Unrecognized volatile and semi-volatile organic compounds from brake wear

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 nitrogen-containing 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.