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New Study Unseen Emissions From Vehicle Brakes
Created on 10.26
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.
最近的一项研究表明，车辆制动器磨损是非废气交通排放的重要且未被充分探索的来源，释放出多种气态污染物。研究调查了不同制动材料挥发性有机化合物 （VOC） 的实时排放，从而在这些气体与有害超细颗粒的产生之间建立了直接联系。
主要亮点· 研究发现，美国常用的无石棉有机 （NAO）
· 该研究成功地从两种刹车片类型的排放中鉴定了 200 多种不同的化学和公式。
· 在超细颗粒形成之前，一直观察到气体排放明显增加，支持了这些有害颗粒从排放的挥发性有机化合物中成核的假设。
· 在模拟正常城市驾驶的条件下记录了大量排放，挑战了此类污染物仅在下坡等极端制动事件期间释放的普遍看法。
两个刹车片的故事
在使用销钉式摩擦计的受控实验室环境中，研究人员比较了两种商业上流行的制动材料的排放：NAO 和 LMCF。结果表明，两者在VOC排放方面存在重大差异。在相同的机械应力条件下，NAO材料始终比LMCF材料排放更高浓度的VOCs。
排放物的化学成分也各不相同。一个关键发现是，硅氧烷是一组硅基化合物，是 NAO 刹车片的特征发射，特别是在高应力下，但在 LMCF 排放中几乎完全不存在。虽然大多数其他挥发性有机化合物的总体范围在两种刹车片类型之间相似，但这一发现使硅氧烷成为 NAO 制动器磨损的潜在示踪剂。
气体和颗粒之间的联系
也许最关键的发现是气体排放与随后形成的超细颗粒（小于 100 纳米的颗粒）之间的直接关系。该研究首次明确表明气相排放的增加先于超细颗粒的形成。
在实验过程中，随着制动盘温度的升高，VOC 浓度持续激增，在检测到纳米颗粒之前达到峰值。这一观察结果有力地支持了制动器磨损排放的有害超细颗粒是通过这些挥发性有机化合物成核形成的理论。
环境和健康影响
这项研究提出了关于通常作为“友好”替代品销售的制动材料对环境影响的重要问题。研究发现，尽管 NAO 垫具有“绿色”声誉，但与 LMCF 垫相比，NAO 垫排放的气态污染物更多。众所周知，许多已确定的挥发性有机化合物（例如苯和甲醛）对人体健康有毒，这让人对这些材料的净效益产生怀疑。
此外，这些排放并不局限于极端制动场景。实验表明，在与典型城市驾驶相对应的温度和速度下，挥发性有机化合物的释放量显着。这表明，减少刹车片的气体排放可能是减少城市中有害超细颗粒形成的关键策略，而实验室测量的排放系数可能无法准确反映复杂的现实世界条件。
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