A new insight into the vertical differences in NO2 heterogeneousreaction to produce HONO over inland and marginal seas

Ship-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements were conducted along the marginal seas of China from 19 April to 16 May 2018 to measure the vertical profiles of aerosol, nitrogen dioxide (NO2), and nitrous acid (HONO). Along the cruise route, we found five hot spots with enhanced tropospheric NO2 vertical column densities (VCDs) in the Yangtze River Delta, Taiwan Strait, Guangzhou-Hong Kong-Macau Greater Bay Area, port of Zhanjiang, and port of Qingdao. Enhanced HONO concentrations could usually be observed under high-level aerosol and NO2 conditions, whereas the reverse was not always the case. To understand the impacts of relative humidity (RH), temperature, and aerosol on the heterogeneous reaction of NO2 to form HONO in different scenarios, the Chinese Academy of Meteorological Sciences (CAMS) and Southern University of Science and Technology (SUST) MAX-DOAS stations were selected as the inland and coastal cases, respectively. The RH turning points in CAMS and SUST cases were both similar to 65% (60 %-70 %), whereas two turning peaks ( similar to 60% and similar to 85 %) of RH were found in the sea cases. As temperature increased, the HONO / NO2 ratio decreased with peak values appearing at similar to 12.5 degrees C in CAMS, whereas the HONO / NO2 gradually increased and reached peak values at similar to 31.5 degrees C in SUST. In the sea cases, when the temperature exceeded 18.0 degrees C, the HONO / NO2 ratio rose with increasing temperature and achieved its peak at similar to 25.0 degrees C. This indicated that high temperature can contribute to the secondary formation of HONO in the sea atmosphere. In the inland cases, the correlation analysis between HONO and aerosol in the near-surface layer showed that the ground surface is more crucial to the formation of HONO via the heterogeneous reaction of NO2; however, in the coastal and sea cases, the aerosol surface contributed more. Furthermore, we discovered that the conversion rate of NO2 to HONO through heterogeneous reactions in the sea cases is larger than that in the inland cases in higher atmospheric layers (> 600 m). Three typical events were selected to demonstrate three potential contributing factors of HONO production under marine conditions (i.e., transport, NO2 heterogeneous reaction, and unknown HONO source). This study elucidates the sea-land and vertical differences in the forming mechanism of HONO via the NO2 heterogeneous reaction and provides deep insights into tropospheric HONO distribution, transforming process, and environmental effects.