Unveiling the HONO Offsetting Effect: Rethinking NOx Emission Controls

Strategic Insights -

Research has shed light on the overlooked role of nitrous acid (HONO) in ozone (O) control, particularly during high-pollution episodes in Shenzhen, China. A team from Peking University Shenzhen Graduate School used machine learning (ML) to integrate HONO-NO reduction relationships into a process-based photochemical model (OBM-MCM), revealing a significant impact of HONO on O production rates and NO-driven effects.

Key Takeaways:

  • The research found a 95% increase in daytime net O production rates compared to the conventional unconstrained case through enhanced OH radical formation and accelerated VOC oxidation and HO/RO + NO pathways.
  • The relative incremental reactivity (RIR) of HONO exhibits a strong anticorrelation with NO (r = 0.86), indicating that a greater NO-driven increase in O corresponds to a greater HONO-driven decrease in O.
  • The study predicts that a 10% reduction in NO synchronically results in reducing atmospheric HONO and TVOCs by 7.6 and 3%, respectively, leading to a shift in O from a maximum of 28% increase to a 14% decrease.

The study highlights the importance of considering HONO responses in NOx emission controls, challenging traditional EKMA frameworks that NO control brings adverse effects under VOC-limited regimes.

Statistics:

  • The research observed a 95% increase in daytime net O production rates through enhanced OH radical formation and accelerated VOC oxidation and HO/RO + NO pathways.
  • The RIR of HONO exhibits a strong anticorrelation with NO (r = 0.86).
  • A 10% reduction in NO results in reducing atmospheric HONO and TVOCs by 7.6 and 3%, respectively.
  • The O production rate shift from a maximum of 28% increase to a 14% decrease highlights the potential of rethinking NOx emission controls.

Sources:

  • Unveiling the HONO Offsetting Effect: Rethinking NOx Emission Controls during Urban Ozone Pollution Episodes. Environmental Science & Technology, 2025.
  • Peking University Shenzhen Graduate School, Key Laboratory for Urban Habitat Environmental Science and Technology, School of Environment and Energy.
  • Amer Chemical Soc, 1155 16TH St, NW, Washington, DC 20036, USA.