Characterization of Secondary Organic Aerosol Formation from Brake Emissions
MADELINE COOKE, Adam Thomas, Véronique Perraud, Lisa Wingen, Lena Gerritz, Berenice Rojas, Paulus Bauer, Barbara Finlayson-Pitts, James Smith, University of California, Irvine
Abstract Number: 393
Working Group: Chemicals of Emerging Concern in Aerosol: Sources, Transformations, and Impacts
Abstract
Traffic emissions are a main source of particulate matter (PM) in urban environments, comprising two categories: exhaust emissions, generated from combustion in the tailpipe, and non-tailpipe emissions, generated mainly from brake and tire wear. With the national trend toward replacing combustion vehicles with electric, exhaust emissions are currently trending downwards and non-tailpipe emissions are rising in importance. Thus far, studies on brake wear have focused on primary emissions, demonstrating that braking generates PM with organic content across a wide size range, including fine and ultrafine particles. While understanding of primary brake emissions has advanced, the potential for brake emissions to undergo atmospheric oxidation and form secondary organic aerosol (SOA) has not yet been explored. In addition to PM, primary brake emissions contain organic and inorganic gases of various volatility that will be reactive in air. Herein, we investigate SOA formation from both gas and particle-phase brake emissions after reaction with ozone (O3). Through controlled chamber experiments coupled to our unique custom-built dynamometer, we investigate the reactivity of gaseous brake emissions from two common brake pads (ceramic and semi-metallic) with O3 and show that oxidation results in the nucleation of secondary ultrafine particles. Furthermore, gas-phase measurements indicate that emitted organic gases react with O3, which could lead to the formation of low volatility organic gases and explain the observed nucleation. In addition, we compare the composition of organic species in PM from brake emissions before and after O3 oxidation utilizing ultrahigh performance liquid chromatography combined with high-resolution Orbitrap mass spectrometry. We observe that the organic fraction in PM is dominated by CHO and CHON species. Several particulate species are reactive with O3, decreasing in abundance after O3 exposure. Our results can be applied to better predict how non-tailpipe emissions will impact urban air quality and to mitigate their effects moving forward.