Measurements and model comparisons suggest that HONO is not an important source of OH radical concentrations below a rural forest canopy

Abstract

The hydroxyl radical (OH) initiates the oxidation of volatile organic compounds (VOCs) in the atmosphere, leading to the production of ozone and secondary organic aerosols (SOA). While nitrous acid (HONO) can be an important OH source during the daytime, the mechanisms controlling HONO concentrations remain poorly understood, with unknown sources commonly reported. To improve our understanding of their sources and sinks, HONO and OH concentrations were measured below a forest canopy using a Laser-Photofragmentation/Laser-Induced Fluorescence instrument and were modeled using a zero-dimensional model based on the Master Chemical Mechanism. Heterogeneous conversion of NO₂ on ground surfaces dominated nighttime HONO production (95%) while the reaction between OH and NO was the dominant daytime source (77%). Deposition was a primary HONO removal process at night (99%) and during the day (64%) followed by photolysis (34%). Overall, the model was able to reproduce the HONO measurements, although it could not reproduce observed increases in HONO during individual rain events, which were likely due to elevated heterogeneous production. Measured concentrations of OH were similar to previous observations above the canopy at this site, suggesting that significant chemical oxidation was occurring in the canopy that could impact the concentration of oxygenated VOCs and SOA above the canopy. Ozonolysis of monoterpenes was a dominant source of radicals below the canopy, accounting for 70% of total radical initiation. Photolysis of HONO contributed approximately 1% to total radical initiation, suggesting that at this site HONO is not an important source of OH radicals beneath the canopy.