Detailed budget analysis of HONO in Beijing, China: Implication on atmosphere oxidation capacity in polluted megacity

In this study, continuous measurements of HONO and related parameters were carried out at an urban site in Beijing during intense summer and winter campaigns. Highly variable HONO levels were observed ranging from 0.53 ppb to 2.41 ppb in summer and from 0.02 ppb to 3.18 ppb in winter. An additional unknown source with HONO production rates of 2.10 ppb h−1 and 0.87 ppb h−1 were derived, accounting for 86.4% and 70.4% of the total HONO daytime sources in summer and winter, respectively. In order to explore the potential sources and formation pathways of HONO, a zero-dimensional box model (F0AM) based on Master Chemical Mechanism (MCM3.3.1) coupled with six updated HONO sources was used to reveal the HONO chemistry and its impact on atmospheric oxidation capacity. With the additional sources, the simulation results reproduced the observations well, and suggested the HONO concentrations during the summer daytime (7:00–17:00) mainly came from the homogeneous OH + NO reaction (22.0%), photosensitized heterogeneous conversion on aerosol surfaces (19.2%) and direct emission (18.5%). Whereas, photosensitized heterogeneous conversion on aerosol surfaces (29.7%), photosensitized heterogeneous conversion on ground surfaces (24.9%) and direct emission (20.4%) became the three major sources during winter daytime. The surprisingly high OH oxidation rates simulated in the winter campaign, which was significantly underestimated in current MCM, highlighting the urgent need for full consideration of additional HONO sources in the numerical model, especially in the cases of winter haze in megacities of North China.