Engine, aftertreatment, fuel quality and non-tailpipe achievements to lower gasoline vehicle PM emissions: Literature review and future prospects

Physical and chemical characteristics of soot particles from GDI engines: Roles of fuels, lubricants, and transient driving cycles

Particulate matter emissions from gasoline direct injection (GDI) engines have become a growing environmental and health concern due to their ultrafine particle sizes, high reactivity, and potential toxicity. As global emission regulations become increasingly stringent, a deeper understanding of the physical and chemical characteristics of GDI soot is critical for developing effective control strategies, particularly in optimizing gasoline particulate filter (GPF) performance. This review investigates the physical and chemical properties of soot particles emitted from GDI engines, focusing on the roles of fuel composition, lubricating oil, and transient driving conditions. GDI soot exhibits unique characteristics, including smaller particle sizes and higher oxidative reactivity, which are critical for understanding its environmental and health impacts. The study examines the influence of oxygenated fuels, such as ethanol and dimethyl carbonate, on soot morphology, nanostructure, and oxidation reactivity, highlighting their potential to enhance particulate filter regeneration. Additionally, the impact of lubricating oils on soot formation and the interaction between soot particles and oil additives are analyzed, revealing significant effects on soot morphology and chemical composition. The review also explores soot properties under transient driving cycles, emphasizing variations in particle size, surface chemistry, and polycyclic aromatic hydrocarbon content. These findings provide essential insights into the physical and chemical characteristics of GDI soot, offering a scientific basis for improving soot oxidation processes and optimizing GPF regeneration strategies.

Unveiling the mechanism secret of abrasion emissions of particulate matter and microplastics

Recent research highlights that non-exhaust emissions from the abrasion of tires and other organic materials have emerged as a substantial source of airborne particulate matter and marine microplastics. Despite their growing impact, the underlying mechanisms driving these abrasion emissions have remained largely unexplored. In this study, we uncover that abrasion emissions from organic materials are fundamentally governed by a fatigue fracture process, wherein particles are progressively detached from the material surface under cyclic abrasion loads. Our findings demonstrate that these emissions increase significantly only when the applied abrasion loads surpass the material's toughness threshold. We establish a scaling relationship between the concentration of emitted particulate matter and the measurable crack propagation rate of the organic material, offering a robust quantitative method to estimate abrasion emissions. This work not only introduces a novel mechanistic framework for understanding particulate matter pollution from organic material abrasion but also provides a scientific basis for developing strategies to mitigate emissions of airborne particulates and marine microplastics.

Regulation-driven changes in PM2.5 sources in China from 2013 to 2019, a critical review and trend analysis

Identifying changes in source-specific fine particles (PM2.5) over time is essential for evaluating the effectiveness of regulatory measures and informing future policy decisions. After the extreme haze events in China during 2013-14, more comprehensive and stringent policies were implemented to combat PM2.5 pollution. To determine the effectiveness of these policies, it is necessary to assess the changes in the specific source types to which the regulations pertain. Multiple studies have been conducted over the past decade to apportion PM2.5. The purpose of this study was to explore the available literature and conduct a critical review of the reliable results. In total, 5008 articles were screened, but only 48 studies were included for further analysis given our inclusion criteria including covering a monitoring period of ≥1 year and having enough speciation data to provide mass closure. Using these studies, we analyzed temporal and spatial trends across China from 2013 to 2019. We observed the overall decrease in the concentration contributions from all main source categories. The reductions from industry, coal and heavy oil combustion, and the related secondary sulfate were more notable, especially from 2013 to 2016-17. The contributions from biomass burning initially decreased but then increased slightly after 2016 in some locations despite new constraints on agricultural and household burning practices. Although the contributions from vehicle emissions and related secondary nitrate decreased, they gradually became the primary contributors to PM2.5 by ∼2017. Despite the substantial improvements achieved by the air pollution regulation implementations, further improvements in air quality will require additional aggressive actions, especially those targeting vehicular emissions. Ultimately, source apportionment studies based on extended duration, fixed-site sampling are recommended to provide a more thorough understanding of the sources impacting areas and transformations in PM2.5 sources prompted by regulatory actions.

Direct measurement of brake and tire wear particles based on real-world driving conditions

With implementing vehicle emission control policies, tailpipe particulate emissions have been gradually controlled, and the relative contribution of non-tailpipe particulate emissions, such as brake and tire wear, has further increased. A unified and scientific method for sampling non-tailpipe particulate matter (PM) emissions is essential to improve the accuracy of the emission characteristics and factors. This study proposes a novel sampling method based on real-world driving conditions to obtain information on emissions and extract characteristic conditions for tire and brake pad wear. We extracted 200 representative braking segments for simulation experiments based on road type, initial and final velocities, temperature, and deceleration rate. Two standard test cycles to simulate the tire wear conditions of the front and rear wheels were constructed based on velocity, lateral, and vertical forces. Under the real-world driving condition test cycle, the emission factors of PM2.5 and PM10 for brake wear particles of passenger vehicles were 2.66 mg/km and 11.65 mg/km, respectively. In contrast, the emission factors of PM2.5 and PM10 for tire wear particles were 0.21 mg/km and 1.27 mg/km, respectively. Moreover, this study provides insights and basic data for localizing and improving the emission model, which can enhance its applicability and accuracy.

Biological effects of brake wear particles in mammalian models: A systematic review

Road traffic is a major contributor to air pollution through aerosols both from exhaust emissions (EE) and non-exhaust emissions (NEE). NEE result from mechanical abrasion of brakes and tires, erosion of road surfaces and resuspension of road dust into the atmosphere by passing traffic. EE have been thoroughly studied and have decreased over time due to a stricter control. On the other hand, NEE have not received such attention and there is currently no legislation to specifically reduce NEE particles. Consequently, NEE relative part has become prevalent, potentially making of these emissions a major human health concern. The aim of this systematic review was to provide an overview of the current state of knowledge on the biological effects of brake wear particles, a type of NEE. To this end, we conducted a bibliographic search of two databases (PubMed and Web of Science) on June 1, 2023, focusing on the toxicological effects of brake wear particles induced in vitro and in vivo. We excluded reviews (no original experimental data), papers not written in English, studies performed in non-mammalian models and papers where no toxicity data were reported. Of the 291 papers, 19 were found to be relevant and included in our analysis, confirming that the assessment of the brake wear particles toxicity in mammalian models is still limited. This review also reports that brake wear particles can induce oxidative stress, proinflammatory response and DNA damage. Finally, some perspectives for further research and measures to mitigate the risk of brake wear emissions are discussed.

Particle Debris Generated from Passenger Tires Induces Morphological and Gene Expression Alterations in the Macrophages Cell Line RAW 264.7

Air pollution in the urban environment is a topical subject. Aero-suspended particles can cause respiratory diseases in humans, ranging from inflammation to asthma and cancer. One of the components that is most prevalent in particulate matter (PM) in urban areas is the set of tire microparticles (1-20 μm) and nanoparticles (<1 μm) that are formed due to the friction of wheels with asphalt and are increased in slow-moving areas that involve a lot of braking actions. In this work, we studied the effect that microparticles generated from passenger tires (PTWP, passenger tire wear particles) have in vitro on murine macrophages cells RAW 264.7 at two concentrations of 25 and 100 μg/mL, for 24 and 48 h. In addition to the chemical characterization of the material and morphological characterization of the treated cells by transmission electron microscopy, gene expression analysis with RT-PCR and active protein analysis with Western blotting were performed. Growth curves were obtained, and the genotoxic effect was evaluated with a comet assay. The results indicate that initially, an induction of the apoptotic process is observable, but this is subsequently reversed by Bcl2. No genotoxic damage is present, but mild cellular abnormalities were observed in the treated cells.