Gaseous nitrous acid (HONO) and nitrogen oxides (NO<sub>x</sub>) emission from gasoline and diesel vehicles under real-world driving test cycles
Creators: name: Trinh, Ha T.
name: Katsuma Imanishi
name: Morikawa, Tazuko
name: Hagino, Hiroyuki
name: Norimichi Takenaka
Publisher: name: Taylor & Francis
Year: 2017
Subjects: subject: Biochemistry
subject: Space Science
subject: Medicine
subject: Pharmacology
subject: Biotechnology
subject: 59999 Environmental Sciences not elsewhere classified
subject: FOS: Earth and related environmental sciences
subject: FOS: Earth and related environmental sciences
subject: 39999 Chemical Sciences not elsewhere classified
subject: FOS: Chemical sciences
subject: FOS: Chemical sciences
subject: Sociology
subject: FOS: Sociology
subject: FOS: Sociology
subject: Science Policy

description: Reactive nitrogen species emission from the exhausts of gasoline and diesel vehicles, including nitrogen oxides (NO<sub>x</sub>) and nitrous acid (HONO), contributes as a significant source of photochemical oxidant precursors in the ambient air. Multiple laboratory and on-road exhaust measurements have been performed to estimate the NO<sub>x</sub> emission factors from various vehicles and their contribution to atmospheric pollution. Meanwhile, HONO emission from vehicle exhaust has been under-measured despite the fact that HONO can contribute up to 60% of the total hydroxyl budget during daytime and its formation pathway is not fully understood. A profound traffic-induced HONO to NO<sub>x</sub> ratio of 0.8%, established by Kurtenbach et al. since 2001, has been widely applied in various simulation studies and possibly linked to under-estimation of HONO mixing ratios and OH radical budget in the morning. The HONO/NO<sub>x</sub> ratios from direct traffic emission have become debatable when it lacks measurements for direct HONO emission from vehicles upon the fast-changing emission reduction technology. Several recent studies have reported updated values for this ratio. This study has reported the measurement of HONO and NO<sub>x</sub> emission as well as the estimation of exhaust-induced HONO/NO<sub>x</sub> ratios from gasoline and diesel vehicles using different chassis dynamometer tests under various real-world driving cycles. For the tested gasoline vehicle, which was equipped with three-way catalyst after-treatment device, HONO/NO<sub>x</sub> ratios ranged from 0 to 0.95 % with very low average HONO concentrations. For the tested diesel vehicle equipped with diesel particulate active reduction device, HONO/NO<sub>x</sub> ratios varied from 0.16 to 1.00 %. The HONO/NO<sub>x</sub> ratios in diesel exhaust were inversely proportional to the average speeds of the tested vehicles. <i>Implications</i>: Photolysis of HONO is a dominant source of morning OH radicals. Conventional traffic-induced HONO/NO<sub>x</sub> ratio of 0.8% has possibly linked to underestimation of the total HONO budget and consequently underestimation of OH radical budget. The recently reported HONO/NO<sub>x</sub> ratio of ~1.6% was used to stimulate HONO emission, which resulted in increased HONO concentrations during morning peak hours and its impact of 14% OH increment in the morning. However, the results were still lower than the measured concentrations. More studies should be conducted to establish an updated traffic-induced HONO/NO<sub>x</sub> ratio.