Intensive photochemical oxidation in the marine atmosphere: Evidence from direct radical measurements

Measurement of OH and HO₂ radicals was conducted at a coastal site in the Pearl River Delta in October 2019. The mixing of air masses of continental and marine origins can lead to more variability in radical concentrations. In the ocean-atmosphere period (OCM), the observed OH and HO₂ radicals could be reflected by the RACM2-LIM1 chemical mechanism. However, the heterogeneous uptake process has a certain effect on the HOx radical chemistry, but the influence of the halogen mechanism is limited due to the NOx level. Land mass (LAM) was associated with a higher net ozone generation rate (5.52 ppb/h), and the daily maximum OH and HO₂ concentrations were 7.1 × 10⁶ cm⁻³ and 5.2 × 10⁸ cm⁻³, respectively. Rapid oxidation process was accompanied by a higher diurnal nitrous acid (HONO) concentration (> 400 ppt). The particularity of HONO chemistry increases the ozone generation rate of the coastal atmosphere by ~40%. Without HONO constraint, simulated ozone concentration drops from ~75 ppb to a global background (~35 ppb). Therefore, the promotion of oxidation by elevated precursors deserves a lot of attention when aiding pollution mitigation policies.