A new insight of the vertical differences of NO₂ heterogeneous reaction to produce HONO over inland and marginal seas

Abstract. Ship based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements were carried out along the marginal seas of China from 19 April to 16 May 2018, to measures the vertical profiles of aerosol, NO2 and HONO. Five hot spots of tropospheric NO2 VCDs were found in Yangtze River Delta, Taiwan straits, Guangzhou-Hong Kong-Macao Greater Bay areas, Zhanjiang Port, and Qingdao port with averaged NO2 VCDs of 1.07×1016, 1.30×1016, 7.27×1015, 5.34×1015, and 3.12×1015 molec. cm-2, respectively. HONO performs similar spatial distribution characteristics as NO2 with averaged HONO VCDs in above five hot-spot areas of 1.01×1015, 7.91×1014, 6.02×1014, 5.36×1014, and 5.17×1014 molec. cm-2, respectively. The averaged near-surface NO2 concentrations were 8.46 and 11.31 ppb, and the averaged near-surface HONO concentrations were 0.23 and 0.27 ppb under viewing sea and viewing land observation azimuths during the whole campaign, respectively. The Chinese Academy of Meteorological Sciences (CAMS) and Southern University of Science and Technology (SUST) MAX-DOAS stations were selected as inland and coastal cases to further understand the impacts of relative humidity (RH), temperature, and solar radiation intensity (SRI) on the heterogeneous reaction of NO2 to form HONO in different scenes. The emission ratios of ΔHONO / ΔNOx in sea, CAMS and SUST were 0.46±0.31 %, 0.82±0.34 %, and 0.79±0.31 %, respectively. The RH turning points in CAMS and SUST cases were all ~65 % (60–70 %), however, two turning peaks (~60 % and ~85 %) of RH were found in sea cases. The HONO/NO2 decrease along with the increase of temperature, and with peak values on ~12.5 °C in CAMS. The HONO/NO2 increase along with increasing temperature, and with peak values on ~31.5 °C in SUST. In sea case, the HONO/NO2 increase along with the increase of temperature with a peak value on ~25.0 °C under the temperature being larger than 18.0 °C. That means the high temperature could contribute to the secondary formation of HONO in sea atmosphere. The correlation analysis between HONO and aerosol in near-surface layer illustrated the ground surface plays a more important role than aerosol surface during the HONO formation process from the heterogeneous reaction of NO2 under inland case, however, aerosol surface plays a more important role during above process under coastal and sea cases. Moreover, we found the HONO generation rate from NO2 heterogeneous reaction in sea case is larger than that in inland case in higher atmospheric layers above 600 m.