Evaluating the Applicability of a Real-Time Highly Oxygenated Organic Molecule (HOM)-Based Indicator for Ozone Formation Sensitivity at a Boreal Forest Station

Ground-level ozone (O₃), formed via complex photochemistry involving nitrogen oxides (NO_x) and volatile organic compounds (VOCs), is a critical secondary pollutant and oxidant. Therefore, understanding whether the formation of O₃ is NO_x- or VOC-limited is essential for effective air quality management. Recently, we demonstrated that the distribution of highly oxygenated organic molecules (HOMs) can be used as a real-time indicator to determine the sensitivity of the O₃ formation regime in monoterpene oxidation chamber studies. The key reactions between peroxy radicals and NO that form O₃ also determine the branching between nitrate-containing and non-nitrate HOM species. However, validation outside laboratory conditions is lacking. This study evaluates the HOM-based indicator’s applicability at a boreal forest station in Hyytiälä, Finland. Using model scenarios with doubled NO_x or VOC emissions as references, we find that the monoterpene HOM-based indicator can determine O₃ formation sensitivity regimes at this site. Our results indicate that this rural background station is primarily NO_x-limited, especially during elevated temperatures with high VOC concentrations but is often in a transition region between the two limit regions. Future studies are needed to assess the applicability of this HOM-based indicator at other sites and times, especially where monoterpenes are not the dominant VOC species.