Formation features of nitrous acid in the offshore area of the East China Sea

Nitrous acid (HONO) is an important precursor of hydroxyl radical (OH), which plays a key role in atmospheric chemistry. In this study, a shipboard-based measurement of HONO and related species in the offshore area of the East China Sea (ECS) was performed during June 2017. The HONO concentration ranged from 35 ppt to 1.95 ppb, with an average value of 0.44 ± 0.25 ppb during the entire campaign. HONO displayed a relatively higher level (0.48 ± 0.21 ppb) in the area within 30 km from the coastline (S1), whereas a lower level (0.40 ± 0.18 ppb) in the area between 30 km and 100 km from the coastline (S2). Five distinct hotspots of HONO were identified, including Ningbo Port, Yangshan Port, the Yangtze River estuary, northwest of the Zhoushan city, and the area adjacent to Jinshan Chemical Industry Park, suggesting the impact of local vessel emissions and land industrial emissions on HONO formation. During the nighttime, the direct vessel emissions contributed on average 18% of the HONO concentration. The averaged conversion frequency of NO₂-to-HONO (k_het) estimated from six nighttime cases was 1.18 × 10⁻² h⁻¹. Daytime budget analysis showed that the unknown HONO production rate (P_unknown) in S1 and S2 was 1.52 ppb h⁻¹ and 1.14 ppb h⁻¹, respectively. P_unknown was related to a light-induced HONO source from NO₂ on the sea surface and particulate nitrate. During the cruise campaign, the averaged daytime OH production rate from HONO photolysis was 1.35 ± 0.69 ppb h⁻¹, about 1.6 times higher than that from the O₃ photolysis (0.87 ± 0.55 ppb h⁻¹), which suggested an important role of HONO in the atmospheric chemistry of the offshore area of ECS.