Vertical distributions of atmospheric HONO and the corresponding OH radical production by photolysis at the suburb area of Shanghai, China

Nitrous acid (HONO) is considered as one of the main sources of the hydroxyl radical (OH), the most relevant oxidant in the atmosphere. Multi-AXis-Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were con-ducted to obtain the vertical profiles of aerosol and HONO from November 1, 2020 to January 31, 2021 at a suburb site of Shanghai, China. HONO was mainly distributed near the surface, but high values HONO occasionally occurred around 0.7 km, indicating an unaccounted source of daytime HONO at high altitudes. The positive correlation between HONO and aerosols suggested that the photo-enhanced heterogeneous reactions on the aerosol surface were an impor-tant source of daytime HONO at high altitudes. To obtain the vertical distribution of OH production by HONO photol-ysis (P(OH)HONO), the vertical profiles of photolysis rate of HONO (JHONO) were calculated by establishing a method of combining observations with empirical relationship based on heterogeneous atmospheric and radiative transfer models. The JHONO increased approximately linearly with increasing altitudes and the noontime averages value of JHONO near the ground were 6.68 x 10-4 s-1, which was strongly negatively affected by aerosols in the morning and afternoon. The P(OH)HONO profile varied in different months (November, December, January) that the changes were mainly affected by HONO and JHONO. P(OH)HONO was more positively affected by JHONO at high altitude and noon but greatly influenced by HONO concentrations in the morning and afternoon.