Automotive braking is a source of highly charged aerosol particles.

Although the last several decades have seen a dramatic reduction in emissions from vehicular exhaust, nonexhaust emissions (e.g., brake and tire wear) represent an increasingly significant class of traffic-related particulate pollution. Aerosol particles emitted from the wear of automotive brake pads contribute roughly half of the particle mass attributed to nonexhaust sources, while their relative contribution to urban air pollution overall will almost certainly grow coinciding with vehicle fleet electrification and the transition to alternative fuels. To better understand the implications of this growing prominence, a more thorough understanding of the physicochemical properties of brake wear particles (BWPs) is needed. Here, we investigate the electrical properties of BWPs as emitted from ceramic and semi-metallic brake pads. We show that up to 80% of BWPs emitted are electrically charged and that this fraction is strongly dependent on the specific brake pad material used. A dependence of the number of charges per particle on charge polarity and particle size is also demonstrated. We find that brake wear produces both positive and negative charged particles that can hold in excess of 30 elementary charges and show evidence that more negative charges are produced than positive. Our results will provide insights into the currently limited understanding of BWPs and their charging mechanisms, which potentially have significant implications on their atmospheric lifetimes and thus their relevance to climate and air quality. In addition, our study will inform future efforts to remove BWP emissions before entering the atmosphere by taking advantage of their electric charge.

Formation of Highly Oxidized Organic Compounds and Secondary Organic Aerosol from α-Thujene Ozonolysis.

We conducted laboratory chamber experiments to probe the gas- and particle-phase composition of oxidized organics and secondary organic aerosol (SOA) formed from α-thujene ozonolysis under different chemical regimes. The formation of low-volatility compounds was observed using chemical ionization mass spectrometry with nitrate (NO3-) and iodide (I-) reagent ions. The contribution of measured low-volatility compounds to particle growth was predicted using a simple condensational growth model and found to underpredict the measured growth rates in our chamber (on the order of several nm min-1). The yields of low-volatility compounds and SOA mass were similar to those of other monoterpene ozonolysis systems. While semivolatile compounds C10H14-16O3-7 were measured most abundantly with I- reagent ion, a large fraction of products measured with NO3- were C5-7 fragments with predicted intermediate volatility. Additionally, particle composition was measured with ultrahigh-performance liquid chromatography with high-resolution mass spectrometry and compared to particle composition from α-pinene ozonolysis. Structural isomers were identified from tandem mass spectrometry analysis of two abundant product ions (C8H13O5-, C19H27O7-). Our results indicate that although this system efficiently generates low-volatility organics and SOA under the conditions studied, fragmentation pathways that produce more highly volatile products effectively compete with these processes.