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 (NOx) and volatile organic compounds (VOCs), is a critical secondary pollutant and oxidant. Therefore, understanding whether the formation of O3 is NOx- 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 NOx 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 NOx-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.