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 (NOx) 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 NOx-rich conditions. In contrast, in NOx-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.