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A recent study reveals that vehicle brake wear is a significant and under-explored source of non-exhaust traffic emissions, releasing a wide array of gaseous pollutants. Research investigated the real-time emissions of Volatile Organic Compounds (VOCs) from different brake materials, establishing a direct link between these gases and the creation of hazardous ultrafine particles.
Key Highlights
- Nonasbestos Organic (NAO) brake pads, commonly used in the United States, were found to emit significantly higher concentrations of Volatile Organic Compounds (VOCs) than the Low-Metallic Copper-Free (LMCF) pads typical in the European market.
- The study successfully identified over 200 different chemical sum formulas from the emissions of both brake pad types.
- A distinct increase in gaseous emissions was consistently observed just before the formation of ultrafine particles, supporting the hypothesis that these harmful particles nucleate from the emitted VOCs.
- Significant emissions were recorded under conditions simulating normal urban driving, challenging the common perception that such pollutants are only released during extreme braking events like a hill descent.
A Tale of Two Brake Pads
In a controlled laboratory setting using a pin-on-a-disc tribometer, researchers compared the emissions from two commercially prevalent brake materials: NAO and LMCF. The results demonstrated a major difference in VOC emissions between the two. Under identical mechanical stress conditions, the NAO material consistently emitted higher concentrations of VOCs than the LMCF material.
The chemical makeup of the emissions also varied. A key finding was that siloxanes, a group of silicon-based compounds, were a characteristic emission of NAO brake pads, particularly under high stress, but were almost entirely absent in LMCF emissions. While the overall range of most other VOCs was similar between the two pad types, this discovery makes siloxanes a potential tracer for NAO brake wear.
The Link Between Gases and Particles
Perhaps the most critical finding is the direct relationship between gaseous emissions and the subsequent formation of ultrafine particles, which are particles smaller than 100 nanometers. The study is the first to definitively show that the increase in gas-phase emissions precedes the formation of ultrafine particles.
During experiments, a surge in VOC concentrations consistently occurred as brake disc temperatures rose, peaking just before the detection of nanoparticles. This observation strongly supports the theory that the hazardous ultrafine particles emitted from brake wear are formed through the nucleation of these VOCs.
Environmental and Health Implications
This research raises important questions about the environmental impact of brake materials often marketed as “friendly” alternatives. The study found that NAO pads, despite a “green” reputation, emit more gaseous pollutants compared to LMCF pads. Many of these identified VOCs, such as benzene and formaldehyde, are known to be toxic to human health, casting doubt on the net benefits of these materials.
Furthermore, these emissions are not confined to extreme braking scenarios. The experiments showed significant VOC release at temperatures and speeds that correspond to typical urban driving. This suggests that mitigating gaseous emissions from brake pads could be a crucial strategy for reducing the formation of harmful ultrafine particles in cities and that emission factors measured in labs may not accurately reflect complex real-world conditions.
View study HERE.
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