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

Abstract Atmospheric nitrous acid (HONO) chemistry is of critical importance to air quality during polluted haze events. However, current air quality models generally underestimate the concentration of HONO, indicating that a fundamental understanding of haze pollution is lacking. Here, by combining field observations during haze events in Beijing and modeling results, we developed the new parameterization scheme for heterogeneous nitrogen dioxide (NO2) reaction on aerosol surfaces [with the synergistic effects of relative humidity, ammonia, and sulfur dioxide], which has not been considered in existing air quality models. Including NO2 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 NO2→HONO chemistry.