Disentangling the source and loss effects of ship-related HONO on coastal ozone over China

Tropospheric ozone (O₃) pollution has become a major air quality challenge in China's coastal regions, where intensive shipping activities substantially perturb nitrogen oxides (NO_x) and volatile organic compounds (VOCs). Nitrous acid (HONO) has been increasingly recognized as an important regulator of atmospheric oxidation and ozone formation; however, its role in mediating the impacts of ship emissions on coastal O₃ remains insufficiently quantified. In this study, we employed the GEOS-Chem model to quantify the influence of ship-related HONO on surface O₃ over coastal China during June 2017. A suite of sensitivity simulations was designed to explicitly isolate the effects of HONO production, HONO loss, and their combined influence associated with ship emissions. Our results show pronounced north-south heterogeneity in ship-related O₃ contributions, with substantial enhancements in the daily maximum 8-h average (MDA8) O₃ (4.4-8.8 ppb) over southern coastal regions and minimal impacts over northern coasts. In VOC-limited regions, HONO loss associated with ship emissions suppresses ozone formation by enhancing NO titration, whereas HONO production enhances O₃ by alleviating NO_x-rich conditions. In contrast, in NO_x-limited regions, HONO loss promotes O₃ formation while HONO production suppresses it, consistent with conventional photochemical expectations. These findings demonstrate that ship-related HONO acts as a bidirectional regulator of coastal ozone, with its impacts governed by the balance between HONO source and loss processes and the local O₃ sensitivity regime.