Emissions of Nitrous Acid, Nitryl Chloride, and Dinitrogen Pentoxide Associated with Automotive Braking

As worldwide trends move toward replacing combustion transportation modes with electric vehicles, characterizing non-tailpipe emissions, such as those from brake wear, becomes increasingly important. Nitrous acid (HONO), nitryl chloride (ClNO₂), and dinitrogen pentoxide (N₂O₅) are important sources of radical oxidants (e.g., ^•OH, ^•Cl, ^•NO₃) and nitrogen oxides (NO) in the atmosphere, driving the chemistry that leads to air quality degradation. Discrepancies between measurements and model predictions indicate that there are significant unknown sources of these species, particularly HONO, where the contributions of different formation processes have been controversial since the first ambient observations in the 1970s. We report the generation of these reactive nitrogen species during automotive braking using chemical ionization mass spectrometry configured with iodide reagent ion. Substantial HONO levels are observed from ceramic and semi-metallic brake pads, and smaller quantities of ClNO₂ and N₂O₅ were also detected. We propose that HONO is formed in the hot plume emanating from the brake rotor via abstraction by NO₂ of allylic and aldehyde hydrogen atoms found in the complex mixture of volatile organic compounds emitted simultaneously. These results suggest that emissions from automotive braking must be taken into account in urban oxidation chemistry.