Elucidating HONO formation mechanism and its essential contribution to OH during haze events

Atmospheric nitrous acid (HONO) chemistry is of critical importance to air quality during polluted haze events, especially in China. However, current air quality models (such as WRF-CHEM, WRF-CMAQ, Box-MCM) generally underestimate the concentration of HONO, leading to a lack of fundamental understanding of haze pollution. Here, by combining field observations during haze events in Beijing and modeling results, we developed the new parameterization scheme for heterogeneous nitrogen dioxide (NO 2 ) reaction on aerosol surfaces with the synergistic effects of relative humidity and ammonia, which has not been considered in existing air quality models. Including NO 2 heterogeneous reactions into modeling significantly improves the estimation accuracy of HONO and OH levels, with the contribution reaching up to 91% and 78% during pollution episodes. The OH derived by HONO can partly explain high concentrations of particulate matter. Together, our work provides a new approach to illustrate the formation of HONO, OH, and haze with the consideration of heterogeneous NO 2 → HONO chemistry.