Evaluating the\nApplicability of a Real-Time Highly\nOxygenated Organic Molecule (HOM)-Based Indicator for Ozone Formation\nSensitivity at a Boreal Forest Station

Ground-level ozone (O<sub>3</sub>), formed via complex\nphotochemistry\ninvolving nitrogen oxides (NO<sub><i>x</i></sub>) and volatile\norganic compounds (VOCs), is a critical secondary pollutant and oxidant.\nTherefore, understanding whether the formation of O<sub>3</sub> is\nNO<sub><i>x</i></sub>- or VOC-limited is essential for effective\nair quality management. Recently, we demonstrated that the distribution\nof highly oxygenated organic molecules (HOMs) can be used as a real-time\nindicator to determine the sensitivity of the O<sub>3</sub> formation\nregime in monoterpene oxidation chamber studies. The key reactions\nbetween peroxy radicals and NO that form O<sub>3</sub> also determine\nthe branching between nitrate-containing and non-nitrate HOM species.\nHowever, validation outside laboratory conditions is lacking. This\nstudy evaluates the HOM-based indicator’s applicability at\na boreal forest station in Hyytiälä, Finland. Using\nmodel scenarios with doubled NO<sub><i>x</i></sub> or VOC\nemissions as references, we find that the monoterpene HOM-based indicator\ncan determine O<sub>3</sub> formation sensitivity regimes at this\nsite. Our results indicate that this rural background station is primarily\nNO<sub><i>x</i></sub>-limited, especially during elevated\ntemperatures with high VOC concentrations but is often in a transition\nregion between the two limit regions. Future studies are needed to\nassess the applicability of this HOM-based indicator at other sites\nand times, especially where monoterpenes are not the dominant VOC\nspecies.